Ly Dai
email: hunglydai@gmail.com

INFLATION STRUCTURE IN VIETNAM ECONOMY

Keywords: Inflation Rate, Economic Growth, Pandemic, Quantitative Analysis, Qualitative Analysis, Vietnam economy

Objective:

The paper investigates the structure of inflation in the Vietnam economy, as a developing economy with a strong record of low inflation and high economic growth rate over recent years, even during the recent Covid-19 pandemic.

Methodology:

The paper uses a quantitative analysis which investigates the quarterly dataset of inflation for the Vietnam economy over 1990-2021. The inflation is decomposed into three main components including core inflation, energy inflation and food inflation.

Findings:

The evidence records that the main driver of inflation in the Vietnam economy is the food inflation. Moreover, both energy and food inflation tends to have a large deviation over time. This illustrates the impact of the world commodity market on domestic inflation. The core inflation, however, is quite stable over time, reflecting the effectiveness of monetary policy in the Vietnam economy.

Implications:

The results implies that the food inflation can be a prioritized objective for the Vietnam economy to stabilize the domestic inflation. Moreover, the monetary policy, which affects the core inflation, needs to be combined with the fiscal policy, which affects the energy inflation.

INTRODUCTION

The inflation rate plays an important role on the macroeconomic stability of an economy. When the inflation is kept to be at a low rate, the macroeconomic environment can stimulate the economic activities, while there still exists an increase of price to encourage the production by firms. Currently, when there is a concern about the risk of stagflation in the world economy (World Bank, 2022), the role of inflation and especially its relationship with the economic growth is at the central point of both academic and policy discussion. During the stagflation period, an economy can have a high inflation rate combined with a low economic growth rate. Then, the public policy, including both monetary and fiscal policy, would have a less space to carry out.

The current paper aims to analyze the structure of inflation rate, and its relationship with the economic growth rate in the Vietnam economy. The choice of Vietnam as a focal case relies on the reason that this economy is a developing economy with a high economic growth rate at average of 6.4% over a long time period of 30 years from 1990 to 2020. Moreover, this economy has been successful in keeping the inflation rate to be less than 4% since 2015. The combination of high economic growth and low inflation rate raises a typical case for the analysis of inflation structure.

The evidence records that the main driver of inflation in the Vietnam economy is the food inflation. Moreover, both energy and food inflation tends to have a large deviation over time. This evidence illustrates the impact of the world commodity market on domestic inflation. The core inflation, however, is quite stable over time, reflecting the effectiveness of monetary policy in the Vietnam economy. As a result, there exist a clear divergence of core inflation with the economic growth, along with a trade-off between the energy inflation and economic growth. The food inflation exhibits both a divergence with economic growth over the 2008-2017 period but a trade-off over the 2018-2020 period.

The paper employs a quantitative analysis method which decomposes the headline inflation rate into three components, including the core inflation, energy and (iii) Food inflation rate (Fcpi). In particular, the energy inflation rate reflects the price of energy. The food inflation rate reflects the price of food. And the core inflation rate is the inflation component of the overall inflation after subtracting for the energy and food price. This core inflation reflects the effectiveness of monetary policy. The second variable is the economic growth rate (VNgdp). This variable is the growth rate of real gross domestic product (GDP) in constant 2010 national price.

The data is from the Vietnam General Statistics Office (GSO, 2022).

Table 1 shows the descriptive statistics for the variables in the data sample. For the inflation rates, the food inflation has both a highest mean and largest standard deviation while the core inflation has a lowest mean and standard deviation. Compared with these variables, the economic growth has quite high mean at 5.7% but with a low standard deviation at 1.3%. In brief, the data sample offers a quite large variation for the analysis of inflation structure in the Vietnam economy. In the Vietnam economy, the energy and food inflation rate tends to fluctuate with a larger variation than the core inflation rate. As the Figure 1 shows, the food inflation rate has a largest fluctuation and play a most important role on driving the headline inflation rate. And the energy inflation also has high deviation, contributing a crucial role on shaping the headline inflation pattern. This evidence is consistent to the fact that both the food and energy are the commodity goods whose prices are affected by the world market [START_REF] Gregorio | Commodity prices, monetary policy, and inflation[END_REF]. And among these two types of good, the food can be a part stabilized by the domestic suppliers in the Vietnam economy, especially the rice and agriculture goods. The oil price, however, depends mostly on the relative supply and demand forces in the world economy. Next, we focus on the relationship between the inflation rate and economic growth in the Vietnam economy. As a well-known result on both theory and evidence, there exists a trade-off between the inflation and economic growth, since a demand shock raises both the price level and output growth rate [START_REF] Lucas | Some international evidence on output-inflation tradeoffs[END_REF][START_REF] Hung | Output-inflation trade-off in the presence of foreign capital: Evidence for Vietnam[END_REF].

EMPIRICAL EVIDENCE

The headline inflation rate has a trade-off relationship with the economic growth rate in the Vietnam economy over the 2008-2014 period, as the Figure 2 shows. The next period of 2015-2020, however, witnesses a low inflation rate below 4% associated a high economic growth rate above 6%. Thus, the trade-off between the inflation rate and economic growth seems to be weaker since the first quarter of 2015 in the Vietnam economy.

In order to detect the source of trade-off between the inflation and economic growth, we decompose the headline inflation into three sub-components, including the core inflation, the energy inflation and food inflation rate. The core inflation rate is depicted in the Figure 3. For most of the time period from Q1/2008 to Q4/2019, there exists a divergence of core inflation and economic growth rate. In particular, the core inflation rate tends to reduce while the economic growth rate tend to improve with a slight increase. The reduction of core inflation rate can be due to the effectiveness of monetary policy in the case of Vietnam economy. In fact, the credit growth rate has been reduced to less then 20% per year since 2015. The economic growth rate, however, is still stable and even goes up. The reason can rely on the investment-based strategy which includes the stimulus of public investment and also encouragement of foreign direct investment (FDI). As the data collected from

Vietnam GSO (2022), on average, each quarter has 3.56 billion USD of disbursed FDI investment. This large investment has contributed significantly on the economic growth of Vietnam economy [START_REF] Hung | Output-inflation trade-off in the presence of foreign capital: Evidence for Vietnam[END_REF]. The energy inflation rate tends to increase along with the economic growth, as shown in the Figure 4. After a period of large fluctuation over 2008-2012, the energy inflation rate tends to be quite stable around 5% from 2013 to present. This tendency results in a clear trade-off between the inflation and economic growth rate: a high inflation rate is associated with high inflation rate.

The pattern of food inflation rate can be thought as a combination of core inflation and energy inflation. As Figure 5 records, the food inflation rate is similar to the core inflation in the sense that these two components of inflation rate tend to reduce over time from 2008 to 2017. This reduction results in the divergence of food inflation with economic growth rate. But the food inflation rate is also similar to the energy inflation in the sense that its value tends to go up against since 2018 to 2020. This tendency raises the trade-off between the food inflation and economic growth rate.

  inflation and food inflation over a period of 2008-2020. The decomposition is useful to shed a new light on the structure of inflation fluctuation over time. Moreover, the time period of 2008-2020 also witnesses the structural break during the current Covid-19 pandemic which happens in the Vietnam since 2019. The paper is structured as follows. After the first section on Introduction, the second section on Analysis Framework presents the data description. Then, the empirical evidence records the results on the structure of inflation rate and its relationship with the economic growth. The paper closes with the fourth section on Conclusion. ANALYSIS FRAMEWORK The paper employs a time-series data sample of Vietnam economy over Q2/2008-Q4/2020. This time period covers the key events of Vietnam such as the participation in the world trade organization (WTO) in 2006, the global financial crisis in 2008, and the current Covid-19 pandemic since 2020. The first variable is the inflation rate (Hcpi) is measured by the change of consumer price index (CPI) over time. The CPI is explored from the World Bank's Global Inflation Database. Then, the inflation rate is decomposed into three main categories, including (i) Core inflation rate (Ccpi); (ii) the energy inflation rate (Ecpi);

Figure 1 :

 1 Figure 1: Inflation Rate by Components

Figure 2 :

 2 Figure 2: Inflation Rate and Economic Growth

Figure 3 :

 3 Figure 3: Core Inflation Rate and Economic Growth

Figure 4 :

 4 Figure 4: Energy Inflation Rate and Economic Growth

Figure 5 :

 5 Figure 5: Food Inflation Rate and Economic Growth

Table 1 :

 1 Descriptive Statistics

	Variables	Observation Mean Std.Dev Min	Max
	Headline Inflation
	(Hcpi)	50	6.7	6.3	0.3	27.7
	Core Inflation (Ccpi)	50	1.9	1.5	-0.7	5.9
	Energy Inflation (Ecpi)	50	6.6	7.1	-3.5	26.1
	Food Inflation (Fcpi)	50	8.1	10.2	-1.9	43.8
	Economic Growth (VNgdp)	50	5.7	1.3	0.3	7.4

CONCLUSION

The paper uses a quantitative analysis which investigates the quarterly dataset of inflation for the Vietnam economy over 2008-2020. The inflation is decomposed into three main components including core inflation, energy inflation and food inflation.

The evidence records that the main driver of inflation in the Vietnam economy is the food inflation. Moreover, both energy and food inflation tends to have a large deviation over time. This illustrates the impact of the world commodity market on domestic inflation. The core inflation, however, is quite stable over time, reflecting the effectiveness of monetary policy in the Vietnam economy. As a result, there exist a clear divergence of core inflation with the economic growth, along with a trade-off between the energy inflation and economic growth. The food inflation exhibits both a divergence with economic growth over the 2008-2017 period but a trade-off over the 2018-2020 period.

The results implies that the food inflation can be a prioritized objective for the Vietnam economy to stabilize the domestic inflation. Moreover, the monetary policy, which affects the core inflation, needs to be combined with the fiscal policy, which affects the energy inflation.


Ly Dai
email: hunglydai@gmail.com

EXTERNAL DEBTS AND TRADE BALANCE: AN INTERNATIONAL EVIDENCE

Keywords: External Debts, Trade Balance, Quantitative Analysis

Objective:

The paper investigates the relationship between the external debts and the trade balance on a data sample of 190 economies over 1990-2020. In particular, the trade surplus provides the financial resource to repay the external debts. Thus, a greater trade surplus ensures a greater ability to sustain a huger external debts.

Methodology:

The research methodology is based on the qualitative analysis combined with the quantitative analysis. The qualitative method summarizes the literature review about the relationship between the external debts and the trade balance. Then, the quantitative method carries out the graphical analysis between the external debts and the trade balance.

Findings:

The qualitative analysis records that there exits mixed results on the external debts and trade balance. And the quantitative analysis illustrates that the external debts are positively correlated with the trade balance. Thus, the evidence confirms that a greater trade surplus can sustain a huger external debts.

Implications:

The empirical results uncover that the trade balance is crucial for a sustained quantity of external debts. Thus, an appropriated policy for the external debts is to stimulate the trade balance, such as with the international trade policy and associated exchange rate policy.

INTRODUCTION

The external debts play a central role on the international financial integration of an economy, both advanced and developing ones. Recent reports released in September 2022 by World Bank states the debts, including both domestic and external ones, are one of three main key drivers of economies in the east Asia & pacific. This raises the concern about an analysis of the external debts with a coverage of world economies sample.

The current paper investigates the dependence pattern of external debts on the trade balances. On theory, we employ an intertemporal approach to current account, postulated by Obstfeld and Rogoff (1995). Accordingly, the current value of external debts is equal to the present value of future trade balances. On other words, the future trade balances are crucial to sustain the repayment of current external debts.

Using a data sample of 190 economies over the 1990-2020 period, we find that a huger external debts per GDP is associated with more trade surplus. Thus, the evidence confirms the validity of intertemporal approach to current account that the trade surplus is positively related to the external debts. Moreover, compared with the external debts per GDP predicted by the cross-section regression model, the long-run external debts per GDP of United States and Vietnam are higher while that of China is lower.

The current paper makes a contribution on the literature on the external debts and related cross-border capital flows, with a survey by [START_REF] Gourinchas | External adjustment, global imbalances, valuation effects[END_REF]. The external debts are ensured by the domestic macroeconomic fundamentals, such as the economic growth, aggregate savings and by the foreign reserves orginating from trade surplus. Within this literature, our evidence adds an international evidence that supports the implication by the intertemporal approach to current account (Obstfeld and Rogoff, 1995).

The paper is structured as follows. After the first section on Introduction, the second section on Analysis Framework shows the literature review and associated data sample. Then, the third section on Empirical Evidence analyzes the external debts. And the last section on Conclusion summarizes the results and discusses the public policy.

ANALYSIS FRAMEWORK

Theory

The Intertemporal Approach to Current Account (IACA) (Obstfeld and Rogoff, 1995) is a prominent theory on the relationship between the external debts and trade balance. Formally, the current debts per GDP is equal to the net present value of future trade balance. Accordingly, the IACA states that an economy must run a surplus trade balance (or positive current account) to repay the debts. As an implication, an economy having a continuous deficit trade balance over a long time period, it needs to issue new debts to repay the old debts. But this situation is unsustainable since the economy can fall into a kind of Ponzi game, in which the value of debts increases continuously and explore at a finite time point.

The intertemporal approach to current account is consistent with both theoretical results and empirical evidence on the external debts. The debts create a channel for domestic firms to finance the future investment project, especially when there exists a tight credit constraint on the borrowing from banks (Woodford, 1990). By accumulating the public debts, one firm can save its revenue to smooth the investment demand in the future.

Another theory states that the accumulation of external debts as foreign reserves when there is a structural transformation within an economy (Song, Storesletten and Zilibotti, 2011). In particular, the movement of labor and capital from low-productivity firms to high-productivity firms results in a huger savings, which are translated into the foreign reserves and external debts accumulation.

In the international monetary system, the external debts play a crucial role on financing domestic investment. Farhi and Maggiori (2017) proves that the issuance of external debts creates a privilege benefit for the issuer. When an economy holds a monopoly position on issuing the external debts as safe assets in the global capital markets, it can receive the foreign capital from the foreign economies who wish accumulate the safe assets.

The structure of debts within an economy is also important for the current account adjustment [START_REF] Coeurdacier | Credit constraints and growth in a global economy[END_REF]. In an overlapping generations model of multi-country, the borrowing by young people can be financed by the savings by the middle-age people. When the population expands along with a tighter credit constraint, the huge savings by the middle-age people can exceed the borrowing by young people. Then, the excess savings would be translated into the accumulation of external debts as a financing resource for the borrowing by the foreign economies.

The external debts are also important for the net total capital inflows since this type of assets account for most of current account dynamics in the world economy [START_REF] Hung | International capital flows with safe assets accumulation[END_REF]. As the net total capital inflows, more net debts inflows are also associated with an economy with a higher economic growth rate. But for the public debts, which together with foreign reserves, account for the net public capital inflows, there can exist an allocation puzzle: an economy with a greater economic growth rate tends to accumulate less net public capital inflows [START_REF] Gourinchas | Capital flows to developing countries: The allocation puzzle[END_REF][START_REF] Alfaro | Sovereigns, upstream capital flows, and global imbalances[END_REF].

In summary, the external debts are important for the current account dynamics, especially the public capital flows. Moreover, their value is financed by the trade balances, since the latter provides the income denominated on foreign currency.

Data

The data is a cross-section sample of 190 economies over the time period of 1990-2020. Each variable is averaged over time so that the analysis can focus on the long-run equilibrium. Moreover, with a quite long time period of over 30 years, the average value can absorb the short-run fluctuation of each variable over time.

The dependent variable is the trade balance, denoted by (atb2y). The variable is the value of exports subtracting the imports, i.e, net exports, per gross domestic product (GDP) on percentage. The scale by the domestic output can rule out the country size effect on measuring the variable. The data is taken from the world development indicators by World Bank (2022).

The independent variable is the external debts, denoted by (xtdebt2y). This variable is the value of debts in foreign currency on the percentage of GDP. The data is from the world fiscal space, also constructed by World Bank (2022). balance per GDP has a mean of (-6.37%) with a large standard deviation of 13.30%.

Compared with this variable, the external debts per GDP has a greater mean of 4.14% but with a much lower standard deviation of 0.79%. The difference between the minimum value and maximum one is also larger for the trade balance per GDP than for the external debts. In brief, the data sample offers a quite large variation to explore the relationship between the trade balance and external debts.

EMPIRICAL EVIDENCE

According to the intertemporal approach to current account, a huger external debts per GDP requires a greater trade balance. Thus, we expect a positive correlation between the external debts and trade balance. We compare the realized value of external debts with the predicted one based on the regression line. On the vertical axis, the positive residuals mean that the actual value being higher than the predicted value by the regression model. For the United States economy, the realized external debts are higher than its predicted value. Thus, this economy has higher external debts than the corresponding value to its trade balance. The Vietnam economy follows a similar patern when the realized external debts are higher than its predicted value. For the China economy, the realized external debts are lower than their predicted values. Thus, this economy has lower external debts than the corresponding value to their trade balance.

CONCLUSION

The paper investigates the relationship between the external debts and the trade balance on a data sample of 190 economies over 1990-2020. According to the intertemporal approach to current account, the current external debts is equal to the net present value of future trade surplus. Then, the quantitative analysis illustrates that the external debts are positively correlated with the trade balance.

Thus, the evidence confirms that a greater trade surplus can sustain a huger external debts.

The empirical results uncover that the trade balance is crucial for a sustained quantity of external debts. Thus, an appropriated policy for the external debts is to stimulate the trade balance, such as with the international trade policy and associated exchange rate policy. When an economy gains a higher trade surplus, it can ensure the solvency of external debts, then contributing on the macroeconomic stability of domestic economy.

For the future research avenue, the paper can be extended to take into account some other key factors of external debts. Recent theoretical and empirical evidence records that the exchange rate fluctuation and domestic production capacity are positive for a sustained externabl debts. Thus, accounting for these variables can shed a new light on the relationship between the external debts and trade balance across economies.

Figure ( 1

 1 Figure (1) captures the external debts and trade balance. There exists a clearly upward sloping curve, showing that the external debts are positively dependent on the trade balance.In particular, more trade balance per GDP is associated with higher external debts supply. Therefore, the evidence confirms the validity of theory based on the intertemporal approach to current account.

Figure 1 :

 1 Figure 1 : External Debts and Trade Balance

Figure 2

 2 Figure 2 depicts the case study of external debts for some typical economies. We choose three economies, including United States, China and Vietnam. The United States economy is the world leading advance economy while the China economy is an emerging large open economy. And the Vietnam economy is a typical developing economy with high economic growth rate. These economies also have a quite large value of trade balance per GDP.

Figure

  Figure 2 : Case Study

Table 1 :

 1 Descriptive Statistics

	Variables	Obs	Mean	Std.Dev	Min	Max
	Trade Balance (atb2y)	190	-6.37	13.30	-49.82	37.17
	External Debts	163	4.14	0.79	2.17	6.92
	(Lnaxtdebt2y)

Table ( 1

 ( ) shows the descriptive statistics of the data sample. The trade


Ly Dai
email: hunglydai@gmail.com

ECONOMIC ROBUSTNESS UNDER CURRENT COVID-19 PANDEMIC

Keywords: Economic Robustness, Economic Growth, Pandemic, Quantitative Analysis, Qualitative Analysis, Vietnam economy

Objective:

The paper analyzes the reaction of an economy against the current covid-19 pandemic. The analysis covers many economies, both advanced and developing ones, with a focal point on the Vietnam economy.

Methodology:

The research method employs a quantitative analysis based on data analysis of Vietnam economy over 2010-2022, covering the current Covid-19 pandemic.

Moreover, the method also uses a qualitative analysis based on reviewing the recent literature on the recovery of economies after the pandemic.

Findings:

The paper shows that the pattern of robustness is different across economies. Some economies experience a sharp reduction in economic growth with a substantial recovery while others have both a less decrease in economic growth but with slow recovery. Moreover, the pandemic also results in a more vulnerable economy against unexpected shocks.

Implications:

The government needs to provide a more stimulating fiscal package to enhance the robustness of an economy against the pandemic. In particular, the package can be oriented directly to low-income households, and small business firms.

Overview.

The Covid-19 pandemic affects the economy through direct and indirect effects, with varying degrees of severity.

The Direct Effects.

The direct impact of the Covid-19 pandemic on an economy manifests itself through different channels including households, businesses, and the Government.

The impacts through these channels directly affect people's living standards both in the short and long term.

The Covid-19 pandemic led to a direct decrease in people's living standards.

Specifically, according to the household channel, the pandemic led to a decrease in people's income. According to the Vietnam General Statistics Office (GSO, 2022), Vietnam's per capita income has decreased by 1.1% in 2021 compared to 2020. In addition, the pandemic has also led to a narrowing of the number of jobs, with more than 9.1 million jobs affected in the economy (GSO, 2022).

According to the business channel, the pandemic has led to the loss of many businesses, even bankruptcy, with typical enterprises in the service sector, even large and leading enterprises. When the business is at a loss, the reduction of personnel will inevitably happen. In addition, the income of workers who keep their jobs will also have to decrease. These two factors together create a decrease in the income of workers, and hence, the standard of living of the people.

According to the public finance channel, the pandemic has led to an imbalance of state budget revenue and expenditure in the direction of reduced budget revenue and increased budget spending to respond to the pandemic with different stimulus packages. The budget deficit will lead to an increase in public debt. From there, a dilemma of public debt will be formed: increasing public debt will have more resources to fight the Covid-19 epidemic, but at the same time disrupt the stable structure of state budget safety, potentially hiding the risk of default.

Since the early days of the pandemic, this public debt dilemma has been warned by many economists, especially for developing economies (Norman Loayza, 2020). Currently, the default situation of some countries such as Sri Lanka in early 2022 can be considered as a current case, with the causes of internal weakness of the economy but also a catalytic role. effects of the recent Covid-19 pandemic.

According to this public finance channel, the increase in public debt will create more pressure on debt repayment for the citizens of the host country.

Accordingly, current and future employees will need to work harder to repay the debt created to prevent the Covid-19 pandemic.

The Indirect Effects.

The indirect impact of the Covid-19 pandemic on an economy manifests itself through various channels including gross domestic product, investment and savings, and forms of socio-cultural connectivity. The impacts through these channels indirectly affect people's living standards, and it will take time to translate into changes in people's living standards.

According to the gross domestic product (GDP) channel, the Covid-19 pandemic has changed the shape of the economic growth rates of economies around the world, even the disrupting in economic development models. Socio-economic. This fracture is shown by the sharp fluctuations in the economic growth rates of countries such as the United States and China, and at the same time, by the disaggregation of the development levels of the economies in the world.

The World Bank report (2021) notes that the Covid-19 pandemic has contributed to widening the development gap between the group of developed countries and the rest of the world. The main reason is that the total output of developed countries will decrease but will recover strongly after the Covid-19 pandemic, while the total output of other countries will fall more sharply, and recover more weakly after the Covid-19 pandemic. Therefore, the gap between these two economic groups will widen during and after the Covid-19 pandemic.

According to the investment and savings channel, the Covid-19 pandemic has both reduced investment and savings, and increased investment and saving. Within an economy, the group of individuals with high incomes will have increased investment and savings rates during the pandemic, due to the increased accumulation of assets during pandemic, based on the boom of the market of property and financial markets. The remaining group, consisting of middle-income and low-income individuals, will have to use their accumulated savings to cover their living expenses, thereby reducing both savings and investment.

On the level of the world economy, the investment flows tend to increase between developed economies, and decrease capital flows from developed countries to developing economies. Recently, the Vietnam General Statistics Office (2022) noted that the total amount of foreign direct investment capital into Vietnam in the first months of 2022 has continuously decreased. Specifically, the total registered foreign investment capital of the first 6 months of 2022, including newly registered capital, adjusted registered capital, and value of capital contribution and share purchase by foreign investors, is estimated at 14.03 billion USD, down 8.1% over the same period last year. This decrease mainly came from the decline in newly registered capital (reaching 4.94 billion USD, down 48.2% over the same period last year), while the adjusted registered capital reached 6.82 billion USD, up 65.6% over the same period last year, and the registered capital for capital contribution and share purchase by foreign investors reached 2.27 billion USD, up 41.4% over the same period last year.

According to the social-cultural connection channel, the Covid-19 pandemic has led to the formation of new socio-cultural connections, besides the traditional channel. Accordingly, people tend to get closer together in smaller and more closed communities, sharing with each other through online channels, in parallel with face-to-face communication. The formation of these small groups leads to a variety of forms of social association, and from there, creates additional values that contribute to improving people's living standards.

Economic and Social Dimensions in Recovery of Vietnam Economy.

The Covid-19 pandemic has a multidimensional impact on an economy. The decline in economic growth rates of countries around the world, even negative in Vietnam in the third quarter of 2021 (GDP decreased by 6.17% over the same period last year), shows that the pandemic caused a structural break in the growth patterns of countries. Since then, the social dimension associated with economic development has also undergone a significant change compared to the prepandemic period.

Economic Dimension.

The Covid-19 pandemic has led to a decline in economic growth globally, including in the Vietnam economy.

The Covid-19 pandemic originated in China at the end of 2019 and gradually spread to other countries from the beginning of 2020. When the pandemic occurred, the average GDP growth rate of the eight economies with the largest trade with China and the rest of the world. Vietnam (including the United States, Japan, China, South Korea, Thailand, Taiwan, Singapore, and Malaysia) for the first time reached negative values and was negative for four consecutive quarters in 2020. According to data from the Department of Statistics According to statistics of these 8 economies, in 2020, on average, these 8 economies have a growth rate of (-2.2%).

In Vietnam, the quarterly GDP growth rate also fell sharply, bottoming at 0.29% in the second quarter of 2020. This bottom is even much lower than the 3.10% bottom of the quarter. The first year of 2009 was related to the 2008 world financial crisis. At the end of 2020, Vietnam's growth rate reached 2.91%, the lowest since the economy opened in 1986. 2020.

However, the next developments of the pandemic will create an asymmetric impact between the world and Vietnam in 2021. According to the World Bank's Global Economic Outlook (June 2022), the world economy has recovered from (-3.3%) in 2020 to 5.7% in 2021. In particular, the world's leading economies have recovered rapidly to achieve impressive growth rates in 2021 such as the United States reaching 5.7%, the Eurozone reaching 5.4%, Japan reaching 1.7%, and China reaching 8.1%. In Vietnam, the economy still maintains the growth rate of 2020, even slightly decreasing to 2.58% in 2021 (GSO , 2022). Thus, while the world economy recovered strongly to surpass pre-pandemic levels, Vietnam's economy is quite stable and has not rebounded strongly to the level achieved before the pandemic occurred. This has potential risks of missing the development pace of Vietnam compared to the world.

Also accompanied by a decrease in the growth rate of economies, the total amount of international trade also decreased due to the disruption of supply chains during the pandemic.

According to World Bank (2022), total international trade grew by 1.4% in 2019, the year before the outbreak of the pandemic, but decreased to (-8.0 %) in 2020, when the pandemic broke out. By 2021, total international trade has recovered strongly, reaching 10.3%, and is forecast to maintain a growth rate of 4.0% in 2022 and 2023.

The high growth in total global trade is partly due to the fact that the base level fell during the Covid-19 pandemic in 2020. In addition, this high growth also represents a possibility of recovery. The reconnection of global supply chains also happened quite quickly when the pandemic was gradually brought under control.

In the global supply chain, the commodity group (commodity) plays an important role in developing economies like Vietnam. This commodity group includes export agricultural and mining products such as rice, wheat, coffee, and coal. Basically, the prices of these commodities fluctuate closely according to the correlation between supply and demand in the world and often tend to fluctuate strongly, thus contributing to a significant change in the trade balance of the economies.

According to World Bank data (2022), the prices of commercial goods have changed significantly from 2019 to 2021. The crude oil price decreased by 33.9% in 2020, and recovered to 66.5% in 2021. Meanwhile, the price index of non-crude goods still maintains an increase of 3.3% in 2020 and 32.7% in 2021.

Changes in the prices of these commodities also lead to different growth patterns in the two groups of economies.

Firstly, the group of developing and emerging countries exporting commercial goods (Commodity-Exporting Emerging and Developing Economies), with Russia typically, has a growth rate of 1.8% in 2019, followed by a strong decline down to (-3.8%) in 2020 and rebound to 4.8% in 2021. Meanwhile, the group of developing and emerging countries import commercial goods (Commodity-Importing Emerging) and Developing Economies), with China typically, has a growth rate of 4.9% in 2019, only slightly decreasing (-0.4%) in 2020, and rebounding strongly to 7.5 % in 2021. Thus, the group of economies importing commercial goods maintained a stable and higher growth rate than the group of economies exporting commercial goods.

Social Dimension.

The Covid-19 pandemic has created a marked change in the form of society's activities, including labor, employment, and inequality in income distribution.

Until December 2021, the whole world has more than 266.5 million cases of Covid-19; 5.3 million people died from the Covid-19 pandemic (GSO, 2022). This pandemic has forced countries to take urgent measures to socialize on a large scale within and between countries. Since the beginning of 2022, groups of countries have separated on policy to adapt to Covid-19. Currently, most countries, such as Vietnam, have switched from a blockade mode to a safe adaptation to the epidemic, gradually treating Covid-19 as a common disease. Another group of countries, typically China, continues to pursue a Zero-Covid policy to strictly control the epidemic, despite the related economic impact. In the long term, the impact of each policy will be different on economic development, especially on the health of society. In either way, the Covid-19 pandemic has created a decline in the general health background of people, weakening the immune system, and creating conditions for strong outbreaks of epidemics. The scale is much different from before, like the dengue fever epidemic in Vietnam, and the monkeypox epidemic happening in Africa.

The employment of workers is affected by the Covid-19 pandemic in terms of both size and structure. In particular, the number of people participating in the labor force has declined, mainly due to the rapid decline in demand for jobs stemming from the business decline of businesses during the pandemic. According to GSO (2022), the country's labor force aged 15 and over in the fourth quarter of 2021 was estimated at 50.7 million people, an increase of 1.7 million people compared to the previous quarter and a decrease of 1.4 million people compared to the same period last year. In general, in 2021, the labor force aged 15 and over is 50.5 million people, down 0.8 million people compared to the previous year.

In 2021, which is the second year that Vietnam's economy faces the Covid-19 pandemic, the unemployment rate of working-age workers is 3.22% on average (2.42% in the first quarter; 2.62% in the second quarter; 3.98% in the third quarter; 3.56% in the fourth quarter). In particular, the unemployment rate in urban areas is 4.42%, while the number in rural area is 2.48%. The youth unemployment rate (from 15-24 years old) in 2021 is estimated at 8.48%, of which the urban area is 11.91% and the rural area is 6.76%. The underemployment rate of working-age workers is estimated at 3.10% (2.20% in the first quarter of 2021; 2.60% in the second quarter; 4.46% in the third quarter; and 3.37% in the fourth quarter). In particular, the underemployment rate in urban areas is 3.33%, and the underemployment rate in rural areas is 2.96%.

According to the data updated to the second quarter of 2022 (GSO, 2022), when Vietnam's economy is in a state of safe adaptation to the pandemic, the country's workforce 15 years and older has employment in the second quarter of 2022 is estimated at 50.5 million people, with the national unemployment rate of working age estimated at 2.32%. The national unemployment rate for young people aged 15-24 years old is estimated at 7.63%, and the underemployment rate among the working-age population of the country is estimated at 1.96%.

Inequality in income distribution in Vietnam tends to improve during the

Covid-19 pandemic.

In the trend of inequality in income distribution in the period 2016-2020, according to data from the General Statistics Office (2022), income inequality is decreasing as the Gini coefficient reduces from 0.431 in 2016 to 0.373 in 2020. The inequality in income distribution has decreased in both rural and urban areas.

Accordingly, when the Covid-19 pandemic occurred, the inequality index of income distribution dropped quite quickly. The difference in the Gini coefficient between 2019 and 2018 is 0.002 points, but the difference between 2020 and 2019 is 0.05 points, which is 25 times. For urban areas, between 2019 and 2018, the Gini coefficient of this area did not change, but between 2020 and 2019, the Gini coefficient decreased by 0.48 points. For rural areas, between 2019 and 2018, the Gini coefficient decreased by 0.007 points, between 2020 and 2019, the Gini coefficient decreased by 0.042 points.

The gap between the highest and the lowest income quintile has narrowed from 10.2 times in 2019 to 8.0 times in 2020. At the same time, the urban rich-poor gap has decreased from 7.2 times in 2019 to 5.3 times in 2020. For rural areas, this gap decreases from 9.6 times in 2019 to 8.0 times in 2020.

Time Dimension of Economic Robustness in Vietnam Economy.

The Covid-19 pandemic creates impacts on the socio-economic development process in different directions, along the time dimension, and the level of impact.

Over time, the pandemic creates different short-term and long-term effects.

In general, the form of socio-economic development is stable over time. When the Covid-19 pandemic occurs, this form is changed, even broken, and will tend to return to its original form or transform into another form that replaces the old development form. The ability to return to this form depends on the robustness of an economy. Accordingly, the morphological break is the short-term impact of the Covid-19 pandemic, and the return to the original form or redirecting to a new form is the long-term impact of the Covid-19 pandemic.

Short-Term.

The most obvious break in the form of socio-economic development is reflected in the unprecedented change in economic growth rate and the way people interact with each other.

In particular, the economic growth rate of Vietnam is going through a period with an unprecedented growth rate since 1986 when the economy began to open up and integrate into the world economy. When considering the annual growth rate, on average in the period 2000-2019, the average growth rate of Vietnam is about 6.2%/year. However, during the period of the Covid-19 pandemic starting from 2020 to the present, the economic growth rate, as measured by the rate of change of gross domestic product (GDP) at real prices, decreased. to less than 3%, reaching 2.91% in 2020 and 2.58% in 2021, respectively. Such a large drop, from an average of 6.2% to less than 3.0%, is a change unusual change due to the Covid-19 pandemic.

When considering the quarterly growth rate, the period 2020 to date has even recorded a negative growth rate. Specifically, in the third quarter of 2021, the GDP growth rate reached (-6.17%) over the same period last year. According to the General Statistics Office (2022), this negative growth number was recorded for the first time since Vietnam collected and announced the growth results.

At the same time, human-to-human interaction also changed significantly when the Covid-19 pandemic occurred, in the direction of enhancing online ways, based on the application of new technology.

As social isolation was conducted to prevent the spread of the Covid-19 pandemic, people have gradually turned to online forms of communication. This trading platform has created an important push to promote the development of virtual spaces, especially recently, the virtual universe (Metaverse) initiated by Facebook Corporation. In this virtual universe, an individual can observe and even interact with different dimensions without changing geographical location.

Long-Term.

In the long term, the socio-economic development pattern of a country may return to the original trend, before the pandemic, or change to a new pattern. This process depends on the resilience of that economy.

According to research results of the Vietnam General Statistics Office ( 2022 An economy's response to a pandemic may follow a gradual pattern of gradual increases without a breakout, or follow an oscillating trajectory around a strong uptrend [START_REF] Brunnermeier | The Resilient Society[END_REF]. Based on recent developments, in terms of the socio-economic development of Vietnam, this economy may fall into the first state, where the development may be gradual. Recently, the economic growth rate has increased again, reaching 7.72% in the second quarter of 2022, the highest quarterly growth rate in the past 10 years (2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018)(2019)(2020)(2021). But in considering the accumulation of the first 6 months of 2022, the GDP growth rate has only reached 6.42% and has not returned to the corresponding level before the pandemic in 2019. Thus, the Vietnam economy is still in the process of recovering back to pre-pandemic levels, and will essentially continue in a gradual pattern of growth, where a breakthrough is unlikely.

Conclusion.

The paper analyzes the reaction of an economy against the current covid-19 pandemic. The analysis covers many economies, both advanced and developing ones, with a focal point on the Vietnam economy. The paper employs a quantitative analysis, based on data analysis of the Vietnam economy over 2010-2022, covering the current Covid-19 pandemic, combined with a qualitative analysis based on reviewing the recent literature on the recovery of economies after the pandemic.

The paper shows that the pattern of robustness is different across economies. Some economies experience a sharp reduction in economic growth with a substantial recovery while others have a less decrease in economic growth but with slow recovery. Moreover, the pandemic also results in a more vulnerable economy against unexpected shocks.

The government needs to provide a more stimulating fiscal package to enhance the robustness of an economy against the pandemic. In particular, the package can be oriented directly to low-income households, and small business firms.


Ly Dai
email: hunglydai@gmail.com

INTERACTION BETWEEN FINANCIAL ECONOMY AND REAL ECONOMY

Keywords: Financial Economy, Real Economy, Quantitative Analysis

Objective:

The paper analyzes the interaction between the financial economy and real economy on a data sample of 185 economies over 1990-2019. In particular, the financial economy mentions to the value of financial markets, including the bonds, debts, securities and related assets, while the real economy is based on the production capacity of an economy.

Methodology:

The research methodology is based on a quantitative analysis. This method carries out the graphical and data analysis, then, derives main princinple underlying the interaction between the financial and real economy.

Findings:

The evidence shows that the financial economy can have various correlation pattern with the real economy. There exists a non-linear dependence pattern of economic growth on the stock market value per GDP and a negative correlation of domestic credit to private sector with the economic growth.

Implications:

The empirical evidence suggests that the financial economy does not necessary illustrate the production capacity but mainly illustrates the expectation by the households and investors. Thus, an appropriated policy can be the participation of government to provide useful information to drive and stablize the financial markets, especially during the recession time period.

Introduction.

The interaction between the financial economy and real economy plays an important role on the macroeconomic stability of an economy. According to the neoclassical growth model [START_REF] Solow | A contribution to the theory of economic growth[END_REF][START_REF] Swan | Economic growth and capital accumulation[END_REF], an economy tends to converge to a long-run equilibrium, also called as a steady state. The convergence process requires the economy to accumulate the capital to build up the capital stock so that the capital per effective labor ratio can be constant at the steady state. And the financial economy serves as the main engine for the capital accumulation process, so that an economy can attain its steady state.

The role of financial economy also works through various channels, besides the capital accumulation. First, the financial economy has its cycle, which can be thought with four phases, including the trough, boom, peak and recession [START_REF] Rey | International channels of transmission of monetary policy and the Mundellian trilemma[END_REF]. This cycle can be parallel to the real economy, and exerts positive or negative to the real economy. Second, the financial economy can provide the financial assets to fulfill the demand for assets by the households. The demand for assets can be divided into the demand for the safe assets and that for the risky assets.

Recently, the demand for safe assets is far greater than their supply, resulting on a safe assets scarcity [START_REF] Caballero | The safe assets shortage conundrum[END_REF]. This safe assets conundrum reduces the effectiveness of the monetary policy even the interest rate is reduced to its zero lower bound.

The current paper proves a formal analysis of financial economy, and its interaction with the real economy. The analysis method employs a cross-section data sample which covers 185 economies, both developing and advanced economies, from 1990 to 2019. This quite long time period of over 30 years can be enough to absorb the short-run fluctuations of financial economy and real economy. This approach is also employed by recent papers such as [START_REF] Gourinchas | Capital flows to developing countries: The allocation puzzle[END_REF], [START_REF] Alfaro | Sovereigns, upstream capital flows, and global imbalances[END_REF] to analyze the long-run equilibrium of crossborder capital flows. With this sample, the evidence records that the financial economy can exerts a positive impact on the real economy if its scale is kept at an appropriated value. Otherwise, the impact of financial economy on the real economy can turn to be negative, so that, an expansion of financial economy can be harmful to the real economy.

The paper contributes on the related literature on the financial economy, with a new survey can be found in [START_REF] Rey | Global financial cycle[END_REF] Our paper contributes to the literature by an analysis of interaction between the financial and real economy. We record that the financial economy can exert a positive impact on the real economy only when the financial economy attains an appropriated scale.

The paper is structured as following. The second section presents the analysis framework with the empirical method and associated data sample. Then, the third section shows the empirical evidence on the interaction of financial economy and real economy. Finally, the paper closes with the conclusion with a summary and policy implications.

Analysis Framework.

For the method, we employ a quantitative analysis. Basically, we investigate the correlation between the financial economy and the real economy. The correlation provides a primary information on the interaction of these two economies. Then, it lays a ground for the discussion on the impact of financial economy on the real economy.

For the sample, we collect the data from 1990 to 2019. This is a quite long time period with over 30 years. The sample also covers about 185 economies, both Table 1 shows the descriptive statistics for the data sample. The economic growth rate has a mean of 3.58%, with a quite high standard deviation of 2.0%. In comparision with this variable, the stock market value per GDP has a lower mean at 1.94 but a higher standard deviation at 2.17. And the domestic credit to private sector per GDP also exerts a standard deviation at 0.90. In brief, the data sample offers a quite large variation to explore the interaction between the financial economy and the real economy. Figure 1 shows the correlation between the stock market value per GDP with the economic growth rate. The data seems to uncover an inverted-U-shaped curve on the dependence pattern of economic growth rate on the stock market value. In particular, when the stock market value per GDP is low, an expansion of stock market may go along with a higher economic growth rate. But when the stock market value per GDP is large, a greater stock market may be associated with a lower economic growth rate. The turning point of stock market value per GDP by logarit form could be at 2.4, which is equivalent to the stock market value per GDP to be 11.0 percentage on average over 1990-2019. This is equivalent to the stock market value per GDP of Mexico. For the stock market value, recent evidence records mixed results. [START_REF] Hou | The dynamic effects of banking, life insurance, and stock markets on economic growth[END_REF] records the evidence on a sample of 31 countries over the period that the effect of stock market on the economic growth varies with the time period, income level and financial development. [START_REF] Levine | Stock markets, banks, and economic growth[END_REF], by using a data sampl of 49 economies from 1976 to 1993, find that the stock market value per GDP, as a measure of stock market liquidity, is positively and significantly correlated with the current and future rates of economic growth, on controlling for the economic and political factors. This evidence is also consistent with the results by [START_REF] Cooray | Do stock markets lead to economic growth?[END_REF], who shows that the stock market, including its size, liquidity and activity, can enhance the economic growth. In another paper, [START_REF] Arestis | Financial development and economic growth: the role of stock markets[END_REF] show that both stock and banks markets may be able to promote the economoc growth. And the effect of stock market is less powerful that the latter.

Figure 2 illustrates the correlation between the domestic credit to private sector with the economic growth rate. The evidence shows that the domestic credit may be negatively correlated with the economic growth rate. This implies that an expansion of credit to private sector may go along with a lower growth rate.

For the domestic credit to private sector, the traditional literature usually considers this variale as a measure for the financial development. A higher financial development level can results in more credit to private sector per GDP ratio.

According this line of research, the domestic credit to private sector can exert a positive effect on the economic growth rate. Wold-Rufael (2009) provides an evidence, based on a time-series sample of Kenya over 1966-2005, that there exist a two-way Granger causality between the domestic credit provide by the banking sector and economic growth. This implies that the economic growth and financial development are jointly determined or they complement each other, rather than just one way positive effect from the financial development to the economic growth. [START_REF] Adu | Financial development and economic growth in Ghana: does the measure of financial development matter?[END_REF] records that the domestic credit to private sector per GDP is conductive for the economic growth in the case of Ghana. In brief, the non-linear dependence pattern of economic growth on the stock market value per GDP and the negative correlation of domestic credit to private sector with the economic growth can uncover new directions on the literature on the interaction between the financial economy and the real economy.

Conclusion.

The paper analyzes the interaction between the financial economy and real economy. In particular, the financial economy mentions to the value of financial markets, including the bonds, debts, securities and related assets, while the real economy is based on the production capacity of an economy. The research methodology is based on a quantitative analysis over a cross-section data sample of 185 economies over 1990-2019.

The evidence shows that the financial economy can have various correlation pattern with the real economy. There exists a non-linear dependence pattern of economic growth on the stock market value per GDP and a negative correlation of domestic credit to private sector with the economic growth.

For the policy implication, the empirical evidence suggests that the financial economy does not necessary go along with the expansion of production capacity but may illustrates the expectation by the households and investors. Thus, an appropriated policy can be the participation of government to provide useful information to drive and stablize the financial markets, especially during the recession time period.

  on the global financial cycle. An economy can have two related component, including the production component which produces the output, and the financial component which channels the savings from households to the capital investment by firms. While the production has its own business cycle, the financial one also has its own financial cyle. At the financial globalization, the financial cycle of a large open economy can also affect the financial and economic cycle of other economies through the exchange rate and cross-border capital flows (Miranda-Agrippino and Rey, 2020).

  developing and advanced economies. The data is from the world development indicators, published online by the World Bank.The financial economy is measured by the stock market value per gross domestic output (GDP) on percentage, and the domestic credit to private sector to GDP also on percentage. Recent studies can use various indicators such as the housing price index to capture the financial economy. Within our paper, we select only two key indicators which have available data.The real economy is measured by the growth rate of real GDP in 2011 constant national price on percentage. This variable captures the expansion of production capacity of an economy.

Figure 1 :

 1 Figure 1: Stock Market Value and Economic Growth

Figure 2 :

 2 Figure 2: Domestic Credit to Private Sector and Economic Growth

Table 1 :

 1 Descriptive Statistics

	Variables	Observations Mean	Standard	Min	Max
				Deviation
	Economic Growth Rate
	(aGDPgrowth) (%)	178	3.58	2.00	-1.17 12.72
	Stock Market Value per GDP (LnaVal2y)	81	1.94	2.17	-3.72 6.47
	Credit to Private Sector	185	3.51	0.90	0.59	5.23
	(LnaCredit)


Virginie Baudry

Jean-Marc Rousset

EXPERIMENTAL STUDY OF SHIP RESPONSE DUE TO INTERNAL VISCOUS CARGO MOTIONS

Potential liquefaction of some cargoes (Nickel ore, iron ore, …) is a major risk for the industry. The difficulties to simulate accurately the behaviour of these materials as well their interaction with a bulk carrier model lead to use highly viscous fluid. Such an experimental approach is presented in the paper. Roll responses of the ship model as well as details about the internal free surface behaviours are investigated for different loading conditions: solid cargo, fresh water and viscous fluids.

INTRODUCTION

The liquefaction is a phenomenon in which an apparent dry cargo material is abruptly transformed from a solid dry state to a fluid state. Some common materials currently transported on board bulk carriers, such as iron ore fines or Nickel ore are examples of materials that may liquefy.

On board bulk carriers, two major kinds of liquefaction may cause the vessel to list : (1) a global liquefaction phenomenon where the whole cargo suddenly begins to behave as a liquid cargo, with associated free surface consequences (metacentric height reduction), (2) a partial liquefaction phenomenon resulting in the shifting of the cargo. Both phenomena may generate a permanent list of the vessel and possible capsize.

Between 1988 and 2016, 23 bulk carrier incidentsamong which 17 vessel losses -were assigned to liquefaction problems [START_REF] Michael | Bulk Cargo liquefaction incidents during marine transportation and possible causes[END_REF].

The limitation of liquefaction risk being a topic of major concern for the security of bulk carriers, the International Maritime Organization (IMO) improved the test methods commonly used to measure the Transportation Moisture Level. The International Maritime Solid Bulk Cargo Code (IMSBC) [START_REF]IMO International Maritime Solid Bulk Cargoes code[END_REF] provides mandatory rules to limit the risk of liquefaction inception. Especially, it specifies safe practices to ensure the safe transportation of liquefiable cargoes, and also includes since 2016 the modified Proctor Fagerberg tests designed to determine the Transportable Moisture Limit of Iron Ore Fines. Some other reports are however stating that the current regulations are not always cautiously followed [START_REF] Gard As | Liquefaction of solid Bulk Cargoes[END_REF].

Mandatory rules are therefore frequently being amended to reduce the risk of liquefaction, however, the lack of comprehension of liquefaction inception onboard still limits the impact of the current regulation. Many research topics are therefore currently developed to better understand both the liquefaction inception phenomenon and the impact of ore liquefaction on ship stability [4]. Zou et al [START_REF] Zou | Numerical Simulations on the Capsizing of Bulk Carriers with Nickel Ores[END_REF] presented a numerical model to study the sloshing motion of a highly viscous fluid submitted to roll motion in a rectangular tank. In this 2D model, a finite volume method is used to solve the Navier-Stokes equations, and the level set method is implemented to describe the behaviour of the free surface. The external moment exerted on the ship due to liquid sloshing is investigated both numerically and experimentally, and an increase of the moment generated by the viscous fluid with time is reported. The authors therefore mentioned a phase lag affecting the ship roll angle due to the motions of the viscous fluid.

More recently, Zhang et al [START_REF] Zhang | Parametric studies on nickel ore slurry sloshing in a cargo hold by numerical simulations[END_REF] investigated numerically the behaviour of Nickel Ore Slurries in a cargo hold, and also investigated on the phase lag between the liquefied cargo and the ship's motion for different cargo moisture contents. Baudry et al [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF] studied experimentally the behaviour of highly viscous materials in cargo holds and also studied that phase lag depending on excitation frequency and amplitude through forced motion experiments. Recent researches have therefore been focused on the behaviour of highly viscous non-Newtonian fluids in cargo holds with characteristics close to nickel ore slurries. The next step is to investigate on the ship response submitted to internal fluid motion. In the present paper, the coupled motion of highly viscous fluid with ship is studied, the internal fluid having rheological characteristics similar to the slurries modelled by Zhang et al.

EXPERIMENTAL INVESTIGATIONS

The main objective of the present experimental campaign is to analyse the response in waves of a ship submitted to internal non-Newtonian viscous cargo motions. The analysis of internal cargo motions would also bring elements for the comprehension of the global response and would therefore give tracks to understand the loss of stability experienced by some bulk carriers.

Ship model characteristics

The ship model is chosen according the following objectives: o a global experimental set-up ensuring different noticeable ship responses with respect to the nature of the cargo, such as a solid cargo or liquid cargos of different viscosities; o a ship roll period, inner tank dimensions and geometric arrangement similar to those described in [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF]; o a loading configuration (displacement, KG) as constant as possible whatever the nature of the cargo.

The first objective can be reached by a set-up as simple as possible involving a reduced list of key parameters and reducing physical variabilities. The experiments are performed with a ship model having a unique cargo hold, without forward speed and subject to side waves. The mooring set-up is designed to restrain the drift and the yaw of the model without perturbation of the other degrees of freedom.

The second objective is leading to keep the 1:50 geometric scale and a simplified ship hold identical to the tank previously tested on the hexapod, with inner dimensions L T 0.645 m x l T 0.640m. The actual results would then enable a direct comparison with those presented in 2017.

The last objective is reached designing a box-like barge with symmetrical cylindrical bow and stern, hosting a typical prismatic bulk carrier hold in its middle. The main dimensions of the model and the corresponding ship are listed in Table 1.

The hull, without bilge keel, is made of rigid foam (density 0.65) and an internal aluminium structure supports the equipment and the ballast. The bottom and longitudinal walls of the tank are provided directly by the inner sides of the hull and the transverse walls are made of Perspex. The middle of the tank is also corresponding to the midship (FIGURE 1).

The ballast is made of lead weights. Their arrangement is adjusted with respect to the different loading configurations in order to keep the same global centre of gravity. The changes in roll periods are then only related to the nature of the cargo (liquid or solid).

Experimental set-up and instrumentation

The model was positioned transversally in the middle of the ocean engineering tank, 18m away from the wave maker. The model is restraint by four soft mooring lines (FIGURE 2).

The soft mooring lines are composed of a thin dyneema rope and a steel linear spring (k=10 N/m). The barge mooring points are located at the bow and the stern on a longitudinal axis passing through the ship centre of gravity.

Reports on liquefaction incidents have shown that such kind of phenomenon mainly occur in transverse sea states. The experiments are then performed in transverse sea states (90° heading). The mooring lines are then 45° oriented from the longitudinal axis of the ship and attached on the wave tank walls.

Each fairlead is equipped with a lightweight uniaxial force transducer to measure the line tension. A similar pretension (around 6 N) is adjusted in each line through preliminary tests.

The model attitude is measured by a Qualisys optical motion tracking system which is calibrated in the measurement volume with a position uncertainty better than 1mm.

Two resistive wave probes measure the wave elevation respectively 1m upstream and downstream the model. A third one is located in the resting longitudinal axis of the model, far enough from it to be considered as a reference.

Eight capacitive wave probes are located inside the cargo hold to measure the internal free surface evolution. However the wave run-up on the inner lateral walls of the tank is not measured because of its prismatic shape. The probes 1, 4, 3 and 6 are located at 0.15m from the walls (FIGURE 3).

Both tank and internal wave probes are calibrated before the tests with the associated fluid (fresh water and/or viscous fluid). The model motions are measured simultaneously with the wave gauges, the line tensions and the wavemaker motions.

FIGURE 2. EXPERIMENTAL SET-UP IN THE WAVE TANK FIGURE 3. CAPACITIVE WAVE PROBES LOCATIONS IN THE TANK

Preparation and experimental program

The test program involves different steps to prepare the ship model for each cargo configuration (solid, water and viscous cargo).

The preparation consists in the ballasting of the model to reach the design water line and distributing the weights onboard to obtain the required centre of gravity through inclining tests on calm water.

For both water and viscous fluid configuration, the filling ratio of the internal tank is 46%, which corresponds to a standard filling ratio of bulk carrier cargo hold. The fluid depth in the tank is 0.135 m.

Experiments are performed in regular sea states, with wave periods from 6s to 18s at prototype scale (0.8 to 2.5 s at model scale), and wave heights of 1m, 2m and 4m at real scale (0.02, 0.04 and 0.08m at model scale). The regular waves are calibrated prior to the test campaign, without the ship model in the tank.

The evolution of the fluid viscosity during the experiments is taken into account and the test matrix is pragmatically adapted to identify the resonance period in roll for each configuration.

Viscous fluid properties

In the continuity of the hexapod measurement campaign performed in 2016 at Ecole Centrale de Nantes [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF], the viscous fluid tested inside the ship tank is a cellulose-based drilling mud. This material has a high viscosity range with respect to its concentration, with an almost constant density close to fresh water. From a rheological point of view, the liquid is a non-Newtonian fluid with a pseudo-plastic behaviour.

For the present experiments, a concentration of 20g/l in fresh water is considered to comply with the forced motion tests performed with the hexapod. Unlike the previous experiments, the mixture used in this campaign is unfortunately unstable and its viscosity is changing slowly with the time. The rheometer measurements are then performed twice during the campaign, on day 1 and on day 4. Results of these measurements are provided on FIGURE 4. [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF] [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF] are giving details on the characterization methods and the authors note that the fluid motions in the tank are inducing low shear rate: the left part of the curves is the region of interest, as indicated on FIGURE 4.

The main characteristics of the fluid are listed in Table 2. Its rheological analysis shows an effective reduction of viscosity at low shear rate by a factor of 3 at 𝛾̇= 0.05. 2.

Non-Newtonian viscous fluid main characteristics Zhang et al [START_REF] Zhang | Parametric studies on nickel ore slurry sloshing in a cargo hold by numerical simulations[END_REF] present a numerical model based on CFD solver to study the sloshing of Nickel ore slurries with three different moisture contents. In the case of low shear rates, the Herschel-Bulkley model is recommended: 𝜏 = 𝜏 𝐻𝐵 + 𝑘 𝐻𝐵 𝛾̇𝑛 with 𝜏 𝐻𝐵 : yield stress, 𝑘 𝐻𝐵 : consistency index, 𝛾̇ : the shear rate, n is the power law index. The authors assume the viscosity of the slurry can be defined by its apparent viscosity 𝜇 𝑎 = 𝜏/𝛾̇ The apparent viscosities of our liquid are coherent with the Nickel ore slurry 3 for the high value and Nickel ore slurry 2 for the lowest value. Table 3.

Low shear rate rheology parameters of nickel ore slurries studied by Zhang et al

RESULTS AND ANALYSIS

The results presented in this paper are given at model scale.

Free decay tests

Free decay tests on calm water are performed for each loading configuration (solid, water and viscous fluid cargo) in order to evaluate both roll natural frequency and damping.

The typical roll motion equation of a ship, without coupling with other degrees of freedom, can be written as follows [START_REF] Zhao | Nonlinear roll damping of a barge with and without liquid cargo in spherical tanks[END_REF]:

(𝐼 𝑥 + 𝐴 𝑟 )𝜑̈+ 𝐵 𝐿 𝜑̇+ 𝐵 𝑁𝐿 𝜑̇|𝜑̇| + 𝐾𝜑 = 𝑀 𝑥 (𝑡) 𝑀 𝑥 (𝑡) is the exciting moment, equals to zero in a calm water free decay test, 𝐼 𝑥 is the inertia in roll, 𝐴 𝑟 the added mass in roll, K the restoring coefficient, and 𝐵 𝐿 and 𝐵 𝑁𝐿 respectively linear and quadratic damping coefficients.

Only linear damping is evaluated in this campaign, through decay tests with small angle of excitation. The non-dimensional roll motion equation becomes:

𝜑̈+ 2 𝑅 𝜑̇+ 𝜔 𝑅 2 𝜑 = 0 where  = 𝐵 𝐿 𝐵 𝑐
, and B c is the critical damping defined as:

𝐵 𝑐 = 2√(𝐼 𝑥 + 𝐴 𝑟 )𝐾 .
Parameters  and 𝑇 𝑅 are experimentally evaluated from the roll measurements.

The results of roll periods evaluated for the different cargo configurations are given in the roll period between the two previous values; o A decrease of 70% of the viscosity between Day-1 and Day-2 is inducing an increase of 9% of the roll period, from T=1.66s to T=1.81s; o The non-dimensional roll damping is about 10 -3 in the solid configuration, 3.10 -2 with water in the tank, and around 10 -1 in the case of viscous fluid cargo.

Table 4. Free Decay tests analysis

The analysis of the free decay tests show that the nature of the cargo has a strong effect both on the roll natural period and on the ship roll damping. An increasing viscosity for the fluid in the tank is decreasing the roll period and tends to reach a limit equivalent to a solid cargo value. On the opposite, when the fluid has a low viscosity, the natural roll period is increased, the value obtained with water being certainly an upper bound.

The presence of a moving fluid cargo induces internal free surface effects, therefore decreases the value of the transverse metacentric height GM.

The loss of GM in these decay tests can be estimated from the following formula Erreur ! Source du renvoi introuvable. : 𝑇 𝑅 =

2.35𝑅 √𝐺𝑀

, where T R is the natural ship roll period, R is the radius of gyration in roll (𝑅 = 0.34. 𝐵 , B being the breadth of the ship equals to 0.73m). The table below gives the GM value for each loading condition. Based on these values, the stability of the ship is affected by the loss of GM and the risk of capsizing would increase as well with transverse seas. It should be however mentioned that the estimation is based on linear theory with the hypothesis of a (quasi-)static cargo.

Tests in regular waves

The following figures (Figures 5, 6 and7) present typical time series of ship motions when the model is tested in a transverse regular wave of amplitude 0.02m and period of 1.27s (H=2m and T=9s at full scale). This wave period is chosen relatively far from the natural ship roll period. For each loading condition (solid, viscous, water), surge, sway then heave are shown in millimetres.

The surge motions are small values lower than 10mm confirming the model is correctly positioned transversally to the incident waves. The heave motions are also of the same order of the wave amplitude (20mm) and a stationary motion is reached after a certain number of waves (around 60).

The sway motions are shown on the middle graph of each loading condition figure. A transitory displacement (t=20s to t= 60s) is first observed then a low sway oscillation of roughly The nature of the cargo is influencing the roll motion of the model (for t>60s), with an average amplitude for the water loading approximatively a quarter of the angle measured with the solid loading (0.5° versus 1.8°). The viscous cargo is associated with a roll amplitude almost half way between the two loading conditions (around 0.8°).

The roll angles of the model at the respective natural ship roll period for each loading condition are reported in the Table 6 below and shown on the next figures. Note that to increase the visibility of the measurement, the viscous cargo results are shown for a wave amplitude twice than the one for the solid cargo.

Copyright © 2018 by ASME Regular waves with a period equivalent to the natural roll period of the ship generate resonance effects. This can be clearly for the solid cargo: higher amplitude would certainly induce capsizing. In case of a liquid cargo, the inner motion in the tank brings some damping to the ship motion. This effect is the strongest with a liquid.

Responses of the ship with regular waves

The incident wave amplitude of 20mm represents a risk of capsize for the solid loading and might be also dangerous with the fresh water. It is decided then to avoid this wave amplitude close the ship resonance period.

The FIGURE 12 below are presenting the Response Amplitude Operator (in °/cm) of the ship roll for regular waves of period from 0.8 to 2.5s and incident amplitudes of 1 and 2cm.

As seen previously, the resonance of the ship in roll is clearly reached at T=1.51s for the solid loading and T=1.86s for the water loading.

A second small peak is however seen around 1.1s in this configuration. The theoretical sloshing period would be 1.2s for a rectangular tank, with same overall dimensions and filled with 0.135m of water. As the theoretical period would be a bit smaller with a prismatic bottom, it is assumed that this experimental peak is the (water) sloshing period of the tank.

An increased incident wave amplitude is not changing the response (apart from the resonance area) for the solid and water loadings. At least for these amplitudes, the linear approach is valid.

For the viscous cargo, the behaviour is different. Globally the curve of the response is flatter without strong peak. The range of period with roll amplification is quite large, from 1.4s to 2s, while the level of response stays relatively low (<3°/cm). The fluid damping associated with the viscosity of the cargo is influencing response of the ship.

The viscous liquid is showing however a non-linear behaviour as the 10mm incident wave amplitude generates a higher response than the 20mm incident amplitude. Some more specific analysis is necessary to understand this point especially related to the inner motion in the tank.

FIGURE 12. RAO IN ROLL FOR THE 3 LOADING CONDITIONS (SOLID, VISCOUS LIQUID AND WATER)

As mentioned previously, the stability with the time of the viscous fluid is not assured. From the day 1 to the day 5, the viscosity of the liquid is decreasing, without noticeable change in density. The FIGURE 13 is summarising the roll RAO obtained during these testing days with the viscous loading.

Copyright © 2018 by ASME

The roll resonance period is increasing with the decrease of viscosity, as noticed with the decay tests results. It tends to reach the period corresponding to the fresh water, around 1.8s -1.9s.

The attenuation of the response is however increasing in the same time. This behaviour is not obvious as we would presume that the mixture disintegration should tend to a water-like behaviour with a strong peak response at a period around 1.8s. This flat response may be caused by an increase of the surface tension counterbalancing a lower viscosity and preventing the sloshing effect in the tank. The fluid free surface might be then almost still and the ship response would be similar to the solid loading condition at this period. As the surface tension was not measured during this experimental campaign, the hypothesis has to be verified.

FIGURE 13. EVOLUTION OF THE ROLL RESPONSE WITH A VISCOUS LOADING CONDITION AND A DECREASING VISCOSITY WITH TIME.

The measured viscosities of the liquid in the tank are of the same order than the Nickel ore slurry of 40% and 50% mentioned in [START_REF] Zhang | Parametric studies on nickel ore slurry sloshing in a cargo hold by numerical simulations[END_REF]. This slurry can be obtained either after a badly drained loading of the holds (situation forbidden by the IMSBC code) or after a cargo liquefaction occurring on the route. A bulk carrier filled with these types of slurry would then present some roll RAO characteristics similar to the present experiment.

In particular medium to high transverse (irregular) waves may generate a ship roll response both at periods higher than the "solid" loading resonance and with a noticeable amplification compared to a regular bulk cargo. This configuration would be at least critical for the ship and maybe dramatic for the crew if leading to capsizing.

Internal fluid behaviour

This section is dedicated to the free surface evolution measured in the tank the ship, in the water loading condition and in the viscous liquid loading (especially with the high viscosity of day 1). The different wage gauges located in the tank according to the sketch of FIGURE 3.

An illustration of the signals from the wave gauges is given by the FIGURE 14 with an incident wave of 1.8s and 0.02m amplitude, for the water loading condition.

FIGURE 14. TANK WAVE MEASUREMENTS WITH

WATER LOADING, T=1.8s A=0.02m

The FIGURE 15 shows the maximum amplitude of the wave gauges with the water loading condition. Two peaks are present. The first at a period around 1.1s is the already mentioned sloshing period of the tank. The second peak at 1.86s is the natural ship roll period.

Strong motions of the free surface are observed with wave heights of 40mm and some measurements by the wave gauges are not possible due to braking waves and ventilation.

FIGURE 15. TANK WAVE RESPONSES WITH WATER LOADING

A similar graph is given FIGURE 15 for the viscous loading condition. The highest amplitudes of the free surface are only observed at the ship roll period. The amplitudes are however almost halves those from the water loading results.

FIGURE 16. TANK WAVE RESPONSES WITH VISCOUS LOADING

FIGURE 17 presents the shape of the free water surface at 4 different instants for an incident wave period of 1.13s and 0.01m amplitude. The far left and the far right parts of the curve are at 0.15m of the respective lateral tank walls. According to the slopes, the free surface is assumed to hit the top of the tank in the upper corners.

FIGURE 17. WATER LOADING: LOCATION OF THE FREE SURFACE FOR A WAVE PERIOD T=1.13S, A=1CM

The difference with the viscous liquid is noticeable when compared with the FIGURE 18 for the same wave conditions. The cargo is almost not moving and keeps a still free surface. The inner damping is attenuating the motions of the fluid and there is no interaction with the ship hull.

FIGURE 18. FREE SURFACE PROFILE FOR VISCOUS LOADING, T=1.13S, A=1CM

Close to the viscous loading resonance, i.e. T=1.68s, the shape of the free surface given by FIGURE 19 presents a node at the centre of the tank. The shape of the liquid is similar to an oscillating flat surface. The linear standing wave is oscillating between +10° and -10° around a node which is also higher than the still elevation of the fluid.

The following Erreur ! Source du renvoi introuvable. presents the temporal evolution of the free surface measured by the wave gauges, with an incident wave of period 1.72s and 0.02m amplitude. The crests are clearly flattened and the mean level of the fluid is slightly higher than the resting level.

These observations are similar to the reported measurements from the forced motion tests reported in [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF]. Smooth free surface profile evolutions are also obtained numerically by Zhang & al for a Nickel ore slurry with a viscosity close to the present viscous fluid [START_REF] Zhang | Study on the sloshing of nickel ore slurries with three different moisture contents[END_REF]. The experiments on the hexapod [START_REF] Baudry | Experimental study of viscous cargo behavior and investigation on global loads exerted on ship tanks[END_REF] have shown phase lags between the motion of the tank one hand and the fluid motions with the associated global loads, on the other hand. The figures below are trying to describe such behaviour, i.e. focusing on the phase lag between the motion of fluid seen by the wave gauges close to the lateral tank wall (WG1 and WG4) and the roll motion of the ship.

In the case of water loading condition (FIGURE 21) and according to the definition of phases, the figure below shows that the cargo is moving in phase with the ship for periods higher than the natural ship roll period with a solid cargo (T=1.51s). For small wave periods, the motion of the free surface is opposite to the ship roll and the phase lag is null.

The FIGURE 22 is illustrating the position of the free surface in the tank related to the position of the ship. A phase lag of 180° is equivalent to have the water cargo moving naturally with the tank: the mass of the cargo is flowing towards the low side and the free surface tends to stay horizontal irrespective to the position of the ship. In the viscous loading case, the FIGURE 23 is showing the evolution of the phase lag with two viscosities. The lower viscosity presents an evolution towards the water case: low value of phase lag then almost 180° over 1.6s of wave period (square and triangle symbols).

The phase lag associated with a high viscosity (day 1) is showing an unique evolution with the wave period with values to 70° to 135°. The phase lag is also different with respect to considered wave gauge: 8 to 12° of difference are measured between WG1 and WG4 even if they are at the same distance from the portside lateral wall.

FIGURE 23. PHASE LAG WITH VISCOUS LOADING

Globally the motion of the viscous cargo is more complex than the case of water loading as there is always a phase lag between the roll and the movements of this fluid. The ship roll is therefore affected by the inner viscous motion and this one is a result of low velocities and dissipation effects (internal fluid damping). As an example, fluid accumulation may occur on a ship side and generate an additional inclining moment which may be favourable or not to the ship stability.

The FIGURE 24 below illustrates the existing phase lag between wave gauges and roll angle of the model. Further analyses are needed to understand the interaction between inner viscous motion and roll motion.

CONCLUSION

The liquefaction of some mineral ore may lead to the capsizing of a bulk carrier but the physical mechanism is not well understood. To explore the interaction between a viscous fluid simulating a liquefied cargo and a rolling ship, experiments are performed in a wave tank with a simplified model.

The responses of the model under transverse regular waves show a complex feature with a fairly large range of wave periods inducing roll motions and a strong effect of the viscosity damping on the amplitudes.

FIGURE

  FIGURE 4. RHEOLOGICAL ANALYSIS OF THE VISCOUS MATERIAL

FIGURE 9 .

 9 FIGURE 9. ROLL WITH SOLID LOADING, WAVE PERIOD 1.51S, AMPLITUDE 1CM

FIGURE 19 .FIGURE 20 .

 1920 FIGURE 19. FREE SURFACE PROFILE FOR VISCOUSLOADING, T=1.68S, A=1CM

  lag for the low periods is indicating that the mass of the fluid is moving in the opposite way: the water run-up is maximum on the elevated side of the tank (ship righting herself).

FIGURE 21 .FIGURE

 21 FIGURE 21. PHASE LAG WITH WATER LOADING

FIGURE 24 .

 24 FIGURE 24. FREE SURFACE MEASUREMENTS IN THETANK WITH VISCOUS CARGO, T=1.8S, A=0.02M

Table 1

 1

	.	Ship model main characteristics
		Scale	1:50	1:1
	Length (m)	3.65	182.5
	Breadth (m)	0.73	36.5
	Depth (m)	0.40	20.0
	Draught (m)	0.189	9.45
	 (10 3 kg)	0.465	58 125
	Including cargo mass (10 3 kg)	54.5	6 812

Item Unit Viscous fluid-Day 1 Viscous fluid-Day 4

				Day 1
				Day 4
	Type /Name
	Dilution with respect to water	g/l	20	20
	Density	kg/m 3	1000	1000
	Tank filling ratio		13.5cm (46%)	13.5cm (46%)

Herschel-Bulkley model coefficients -Extrapolated from experiments t

  From our experimental data, 𝜏 𝐻𝐵 , 𝑘 𝐻𝐵 , 𝑛 as well as the apparent viscosity are evaluated (Table2) and compared to the values ofZhang et al. reported in the flowing table.

					pseudo-plastic material
				Pa	0	0
		HB
	k	HB		kg/ms	9.63	21.69
	n HB		-	0.571	0.415
	Apparent viscosity at		𝛾̇= 0.05
				Pa.s	34.82	125.13
	Experimental data
	Measured Viscosity at	𝛾̇= 0.05	Pa.s	37	120

  Table 4 and the main observations are given below: o The highest value of the natural roll period is measured with the tank filled of water; o The lowest roll period is measured in the case of a solid cargo. The difference between both measurements is of 23%, more than 2.5 seconds at full scale; o The loading conditions with a viscous fluid are producing a

Table 5

 5

	.	Estimation of the GM
	Configuration	T R [s]	R [m]	GM [m]
	Solid	1.51	0.2482	0.149
	Viscous fluid -day 1 1.66	0.2482	0.123
	Viscous fluid -day 4 1.81	0.2482	0.104
	Water	1.86	0.2482	0.098

  The following figure is reported the roll angles of the model during the same tests, with solid cargo (top graph), viscous cargo (middle) and fresh water (bottom)

	35s period and 50mm amplitude is present related to the mooring
	stiffness. The good repeatability of displacements during the tests,	Configuration	T(s) model scale	T(s) real scale
	irrespective to the cargo nature, leads to assume the mooring is	Solid	1.51	10.68
	not affecting the rolling motion of the model.
							Water	1.86	13.15
							Viscous Fluid -Day 1	1.66	11.74
							Viscous Fluid -Day 4	1.81	12.80
	Item	Density (kg/m 3 ) t HB (Pa)	k HB (kg/ms)	n HB	Apparent Viscosity m slurry (Pa.s)
	Nickel Ore Slurry 1, 60%water	1364	0.456	0.439	0.353	12.17
	Nickel Ore Slurry 2, 50%water	1500	1.742	1.02	0.427	40.52
	Nickel Ore Slurry3, 40%water	1667	4.741	3.774	0.565	108.71
	FIGURE 5. TIME SERIES FOR SOLID LOADING
	FIGURE 6. TIMES SERIES FOR VISCOUS LOADING
							FIGURE 8. ROLL MEASUREMENT FOR (TOP) SOLID
							CARGO, (MIDDLE) VISCOUS CARGO AND
							(BOTTOM) WATER LOADING.
	FIGURE 7. TIME SERIES FOR WATER LOADING
							Copyright © 2018 by ASME

Table 6 .

 6 Roll measurements in regular waves

	Loading configuration	Troll [s]	Roll amplitude[°]
	Solid	1.51	15
	Viscous liquid	1.72	2.5
	Water	1.86	7.8

ACKNOWLEDGMENTS

The work presented in this paper has been funded by the French ministry of Ecology, Sustainable Development and Energy. We also acknowledge the members of the LiquefAction project consortium, and especially Oldendorff carriers.