A Benidir

O Flamand

G Dimitriadis

The impact of circularity defects on bridge stay cable dry galloping stability

Keywords: Circular cylinder, shape distortion, dry galloping, bi-stability, critical regime

The present work studies the influence of circularity defects, on the aerodynamic behaviour of stay cables of cable-stayed bridges. It focuses on wind tunnel tests on High-Density Polyethylene cable covers with and without helical fillets, in a range of Reynolds numbers from the sub-critical to the critical regime. The paper considers the impact of circularity defects on the aerodynamic stability of cable sheaths by testing various amplitudes of imposed ovalization. The defects are artificially applied on real sheaths whose original cross-sections are close to circular. The experiment consists in measuring surface pressures to investigate how the amplitude of ovalization influences the flow around the sheaths, especially in the critical Reynolds number regime when transition in the boundary layer occurs. The analysis is based on bifurcation diagrams and Proper Orthogonal Decomposition. The investigation demonstrates that important circularity defects can significantly increase the bistable nature of the flow around a sheath at the critical regime. Nevertheless, the introduction of a helical fillet de-correlates the flow around the sheath, causing jumps in lift that have different sign along its length.

INTRODUCTION

The underlying mechanism that causes dry galloping of bridge stay cables is still under investigation by several research teams. Dry galloping causes significant vibrations of inclined cables under dry conditions, exclusively due to wind [START_REF] Nikitas | Aerodynamic forcing characteristics of dry cable galloping at critical Reynolds numbers[END_REF]). Field observation, numerical simulation and wind tunnel tests have been used to identify the parameters that affect these vibrations. Previous studies reported that the origin of dry galloping of stay cables is related to the flow around the cable sheath at the critical flow regime [START_REF] Mctavish | An investigation of large-amplitude cable vibrations at critical and supercritical Reynolds numbers[END_REF], [START_REF] Larose | The aerodynamic forces on the stay cables of cable-stayed bridges in the critical Reynolds number range[END_REF], [START_REF] Cheng | Divergent motion of cable exposed to skewed wind[END_REF]). These high amplitude vibrations are serious concerns for bridge safety. Most cable sheaths used in the US and Europe are either smooth or feature helical fillets, while tubes with pattern-indented surfaces are mainly used in Asia [START_REF] Kleissl | Comparison of several innovative bridge cable surface modifications[END_REF]). [START_REF] Katsuchi | Wind-tunnel Study on Dry-galloping of Indented-surface Stay Cable[END_REF] investigated the mechanism of dry galloping in both smooth and pattern-indented surfaces. The authors reported large cable vibrations in models with these surfaces at different Reynolds numbers. The findings exhibited the importance of the cross-section and/or the surface parameters on triggering cable vibrations near the critical regime. Furthermore, dry galloping has been proved to occur in a critical range of yaw/inclination angle and at a specific reduced wind speed [START_REF] Vo | A wind tunnel study on control methods for cable dry-galloping[END_REF]). In wind tunnel tests, it is possible to reproduce large cable vibrations at particular orientations. In general, the damping of the models used in such tests is low. In field observation, cable vibration amplitudes up to 1.5 m have been observed in Japan in windy and dry weather conditions [START_REF] Matsumoto | Dry galloping characteristics and its mechanism of inclined/yawed cables[END_REF]). The estimated wind velocity was close to 18 m/s, which coincides with the beginning of the critical flow regime around these specific cables. In this flow regime, Karman vortex mitigation occurs and the drag crisis phenomenon is observed. The latter is related to the transition from laminar to turbulent flow in the boundary layers (Transition in Boundary Layer - [START_REF] Zdravkovich | Flow around circular cylinder -Volume 1: Fundamentals[END_REF]). The rapid drop in the drag coefficient is combined with the appearance of a non-negligible lift force, which is a consequence of the appearance of a negative pressure bubble on one side of the circular cylinder. [START_REF] Schewe | On the force fluctuations acting on a circular cylinder in cross flow from subcritical up to trans-critical Reynolds numbers[END_REF] used the term bi-stability to describe the occurrence of transition on either one side of a circular cylinder. Recent research conducted by [START_REF] Nikitas | Critical Reynolds number and galloping instabilities : experiments on circular cylinders[END_REF] and [START_REF] Benidir | Impact of circularity defect on helical fillets HDPE bridge stay covers: Analysis of bi-stability at critical Reynolds number by bifurcation diagrams[END_REF] showed that bi-stability causes abrupt jumps between the different regimes of the boundary layers at constant Reynolds number. However, as the dry galloping mechanism is referenced it has an aerodynamic origin; this consideration just sheds light on an important parameter, which is the macroscopic defect of the cable sheaths. The impact of the real circularity defect on flow around circular cylinders was investigated first by [START_REF] Flamand | A comparison between dry cylinder and rain-wind induced excitation[END_REF], then by [START_REF] Matteoni | Effects of bridge cable surface roughness and crosssectionaldistortion on aerodynamic force coefficients[END_REF]. [START_REF] Benidir | Impact of roughness and circularity defect on bridge cables stability[END_REF] measured the circularity defects on four smooth High-Density Polyethylene sheaths of different diameter. They carried out wind tunnel tests to show that this defect influences the flow around the sheaths, especially at the critical regime. [START_REF] Ma | Effect of the Reynolds number on the aerodynamic forces and galloping instability of a cylinder with semi-elliptical cross sections[END_REF] asserted that the aerodynamic forces on cables are considerably affected by the angle of attack, especially in the critical regime. Their tests were carried out on smooth cable surfaces with elliptical and semi-elliptical cross-sections.

The present work aims to investigate the impact of increasing the circularity defect of the cable sheath on the aerodynamic behaviour of stay cables by means of bifurcation diagrams and Proper Orthogonal Decomposition (POD). The paper also presents the comparison between the pressure patterns of smooth and helical fillet cable sheaths at the critical flow regime. Finally, the work investigates the impact of large deformations of the sheaths on the effect of the helical fillet.

EXPERIMENTS

Setup

The 4m x 2m wind tunnel of the Scientific and Technical Centre for Building (CSTB) was used for full-scale measurements of surface pressures on cable models. Wind speed was varied from 0 to 28 m/s, corresponding to a Reynolds number range between 1.03 x 10 5 and 4.04 x 10 5 . Static tests were performed on High-Density Polyethylene (HDPE) smooth and helical fillet cable covers normal or inclined to the flow with a mean diameter D=250 mm. The results of the experiment on the smooth cables are reported in [START_REF] Benidir | Impact of roughness and circularity defect on bridge cables stability[END_REF]. A 3D surface characterization device was used to measure the surface roughness. The average roughness values (Ra) of the covers are 1.29m and 1.43m, respectively for smooth sheaths and for sheaths with helical fillet surfaces. The turbulence intensity of the wind tunnel was measured by a Cobra probe and was found to be less than 1.1% at the location of the model. The model length was L = 2 m, representing an aspect ratio of L/D = 8 % and a wind tunnel blockage factor of D/W= 6.2 %, where W = 4 m is the width of the wind tunnel. In addition, the instrumented part of the models was located far from the walls, in the middle of the working section. The helical fillets were 1 mm high and 3 mm wide and were inclined by 45° to the longitudinal axis of the cylinders. The lengthwise spacing of the helical fillet was 550 mm. Figure 1-a shows the helical fillet HDPE cover in the wind tunnel's test section.

Each sheath was equipped with 288 pressure taps distributed around 9 rings (figures 1-a and figure 1b) in the central part of the tube. The distance between the bottom ring (ring number 1) and the top ring (ring number 9) was 1 m. The distribution of the rings was not equidistant along the sheath; the largest separation between two successive rings was 20 cm while the smallest was 5 cm. Thus, the concentration of the pressure taps was higher close to the mid-section of the tube. Each pressure tap was connected via a short tube to a pressure sensor with a measurement range of 2500Pa. These sensors were collected in groups of 32 in a pressure scanner.

A laser displacement sensor with a resolution of 60 μm was used to measure the original circularity defects of the tubes. Each tube was rotated at a steady rate around its axis and the laser sensor was placed normal to the surface at a suitable distance. A second set of tests was carried out in order to determine the effect of the tubes' cross-sectional shape. A controlled deformation was applied on the smooth and helical fillet cable sheaths with the same diameter. The cross-sectional shape distortion procedure consisted of an application of a decompression force inside the cylinder to strain the section in one direction. The mechanism was composed of two metallic reinforcement bars placed inside each cylinder along its axis (figure 1-c). A distortion force was applied by means of five threaded shafts equally spaced along the length of the reinforcement bars. The resulting circularity defect of the two cylinders was controlled with a caliper. The mechanism was used for generating ovalization defects up to 7mm with respect to the original diameter.

Measurement protocol

The objective of the experiments was to study the impact of surface distortion for different wind directions. Thus, for the smooth and helical fillet cable sheaths respectively,16 and 9 wind incidences (i.e. angles of rotation around the longitudinal axis) were tested in the wind tunnel. The step change of the rotation angle was 22.5° during all the tests. Flamand and Boujard (2009), followed by Matteoni and Georgakis (2012) and finally [START_REF] Benidir | Impact of circularity defect on helical fillets HDPE bridge stay covers: Analysis of bi-stability at critical Reynolds number by bifurcation diagrams[END_REF] confirmed the importance of wind incidence in the critical regime when the model is a HDPE sheath with natural defects. The angle of incidence is denoted by θ. For instance, when θ = 0° or θ = 22.5°, it indicates that the wind blows towards taps 1 and 3 respectively. By referring to the DIN8074 standard, the natural circularity defect of cable protection used for cable-stayed bridges is assumed to be less than 1 % of the diameter. In this experiment, the natural defect is artificially increased up to 4.4% of the diameter.

CIRCULARITY DEFECT MEASUREMENT

The results of the circularity defect measurements for both kinds of cable surfaces are plotted in figure 2 and figure 3. Figure 2-a shows the cross-sectional shape of the smooth sheath at ring number 5 for four different distortion forces. Figure 2-b plots the cross-sectional shape at all the rings for the highest distortion force. Similarly, figures 3-a to 3-d plot increasing imposed circularity defects for the sheath with helical fillet. For both types of sheath the deformation ranges from natural (labelled 'Natural defect' in figure 2-a) to 4.4% of the diameter (labelled Dmax=257). The distortion criterion is taken to be the amplitude of deformation along the major axis of the oval shape, called the axis of deformation. Three amplitudes of deformation have been achieved, equal to 253mm, 255mm and 257mm, as presented in figure 2-a and figure 3. As the length of the major axis increases, that of the minor axis decreases. For instance, when the major axis reaches Dmax=257mm the length of the minor axis is close to 245 mm. Figure3: Natural and imposed circularity defect on sheath with helical fillet.

 =0°2  =0°3  =0°4  =0°5  =0°6  =0°7  =0°8  =0°9  =0°1 0  =0°1 1  =0°1 2  =0°1 3  =0°1 4  =0°1 5  =0°1 6  =0°1 7  =0°1 8  =0°1 9  =0°2 0  =0°2 1  =0°2 2  =0°2 3  =0°2 4  =0°2 5  =0°2 6  =0°2 7  =0°2 8  =0°2 9  =0°3 0  =0°3 1  =0°3 2  =0°1

RESULTS

The impact of the shape distortion of smooth and non-smooth surfaces on the aerodynamic behaviour of cable sheaths is discussed below. The interpretation of the data is detailed in two parts. The first part is dedicated to the variation of the mean aerodynamic load coefficients and the second part analyses the instantaneous drag and lift forces inferred from the pressure tap time records.

Mean force coefficient

Mean aerodynamic load coefficients are obtained by integrating the instantaneous pressure distribution at each ring. Lift and drag coefficients are defined by where FD and FL represent the drag and lift forces respectively, ρ the air density, U the airspeed and S the reference surface. The reference surface (S) is obtained by multiplying the original diameter D (250 mm) by the reference length (L = 1 m). L corresponds also to the length of the instrumented area.

𝑪 𝑫 = 𝑭 𝑫 𝟏 𝟐 𝝆𝑼 𝟐 𝑺 (1)
The forces FD and FL are obtained from

𝑭 𝑫 = ∑ 𝑷 𝒊 𝟑𝟐 𝒊=𝟏 𝒙 𝒊 (3) 𝑭 𝑳 = ∑ 𝑷 𝒊 𝟑𝟐 𝒊=𝟏 𝒚 𝒊 (4)
where xi and yi are the characteristic areas of the ith pressure tap projected in 𝑥 ⃗(drag direction) and 𝑦 ⃗⃗⃗⃗(lift direction) respectively and Pi is the instantaneous pressure recorded at the ith tap.

Sheath with helical fillet

Helical fillets are installed on bridge cable sheaths in order to avoid rain-and-wind-induced vibration [START_REF] Flamand | Rain wind induced vibration of cables[END_REF]. The fillet tries to force the water rivulet to follow a modified direction and aims to de-correlate the fluctuations of the flow along the length of the sheath. The impact of natural circularity defect on cable sheath aerodynamics can be studied by comparing two different wind incidences at an angular distance of 180° to each other. Figure 4 plots the mean lift coefficient as a function of Reynolds number at ring number 5. The two wind incidences are θ = 22.5° and θ = 202.5°, meaning that the wind blows directly towards taps 2 and 19 respectively. The cross-sectional shape seen by the oncoming flow is also depicted in each figure. The location of the helical fillet is identical for these two wind incidences, but the cable cross-sections are different due to the natural circularity defect. The mean lift coefficient calculated for these two cases shows a variation in the sign and the intensity of the lift force. In particular, the start of boundary layer transition occurs on opposite sides and the absolute value of the lift force measured for θ = 22.5° is 48 % greater than that obtained for θ = 202.5°. It is worthwhile to note that the TrBL1 regime is established at different Reynolds numbers. These results show that the original circularity defect influences considerably the pressure pattern around the covers at the critical regime, even in the presence of the helical fillet.

The impact of large shape modification on the mean aerodynamic coefficients acting on the sheath is shown in figure 5. The variation of the mean drag and lift coefficients with Reynolds number at ring number 3 are given in figure 5-a and 5-b respectively. The wind is blowing directly towards pressure tap3 (θ = 22.5°) and the cover shape is gradually distorted. The results around the original shape are also plotted. The original shape means that the ovalization setup is not yet installed inside the cable stay. It can be observed that the redistribution of the macroscopic defect around the cable induces a slight increase of the mean drag force at the sub-critical regime for this orientation. The graph shows also that the occurrence of the drag crisis, which indicates the presence of the TrBL1 regime, is shifted to higher Reynolds numbers as the distortion is increased. Additionally, the drop in drag is steeper for the undistorted section than for the distorted ones. In the super-critical regime, the drag coefficient again increases with distortion. For example, at a Reynolds number of 3.36 x 10 5 the drag coefficient is CD = 0.44 for the undistorted sheath; for the same Reynolds number but for Dmax =257mm, the drag coefficient jumps to CD = 0.52, which constitutes an increase of 25%.

In the critical regime, the drag crisis is accompanied by a sudden jump in the mean lift coefficient as shown in figure 5-b. For the original cross-section, this jump appears at Re = 2.25x10 5 , which corresponds to the first drop of the drag. The beginning of the second transition is associated with the second drop in the drag force, noted as the transition from theTrBL1 to the TrBL2 regime. When this transition takes place, the lift forces decrease gradually, as can be observed for a Reynolds number higher than 2. The results presented in this section show that both the original circularity defect and imposed shape distortion of the sheath with helical fillet can adversely impact its aerodynamic behavior. The Reynolds number range of the critical regime becomes wider, which means that non-zero mean lift is generated at a wider range of wind velocities. In addition, for a specific cable orientation, the mean drag coefficient increases as the circularity defect grows.

Smooth surface

The impact of natural circularity defect on smooth cable aerodynamics has been previously reported by [START_REF] Benidir | Impact of roughness and circularity defect on bridge cables stability[END_REF]. This section discusses the impact of artificially increasing the macroscopic defect on cable aerodynamics. Figure 6 shows the variation of the mean lift coefficient as a function of Reynolds number. For the initial (without deformation) circularity defect, the first transition from the TrBL0 to the TrBL1 regime measured by most of the pressure rings along the cable occurs on the same side of the sheath, as illustrated in figure 6-a. However, the maximum of the lift force acting on each ring differs from one location to another. For instance, the discrepancy between the maximum lift coefficient at rings number 2 and 3 is around 50 %. Furthermore, the establishment of the single bubble regime occurs at the same wind velocity, which suggests the presence of a time and spatial correlation between the pressures captured around the rings. It should also be noted that the maximum lift occurs at Re=3x10 5 for the undeformed sheath; this Reynolds number decreases gradually to 2.5x10 5 as the imposed deformation increases.

The same conclusion can be drawn for the sheath with Dmax = 253 mm (graph 6-b), for which the transition occurs on the same side of the model. This result can be justified by the fact that the first step of imposed circularity defect was in the same order of magnitude as the initial circularity defect. Nevertheless, the lift forces generated from the rings at the critical regime are lower than in the case of the initial shape. For the second and third steps of shape distortion, a sign change occurs on the mean lift coefficient. An important area of the cable surface is concerned, as illustrated in figure 6-c and6-d. For Dmax = 255 mm, the pressure pattern of five rings has been modified, which means that the transition from the TrBL0 to the TrBL1 regime takes place on the other side of the model compared to the case of the initial shape or Dmax = 253 mm. This change was confirmed for the largest deformation, where the same area of the model is prone to the same inversion of the local lift force. It is important to note that the wind blows towards tap 3 for all tests, i.e. θ=22.5°. As a conclusion, cable shape distortion can considerably affect the pressure pattern around the smooth circular cylinder especially in the critical regime, resulting in major changes in the mean lift coefficient.

Bi-stability

In the transitional Reynolds number range, the flow around the sheaths was often bi-stable; over a long time duration the flow could jump between different boundary layer transition regimes, resulting in abrupt jumps in the value of the lift. The mean lift coefficient plots on figures 4-6 do not contain any information on bi-stability, as the plotted data are time averages. Conversely, figures 7-10 plot the instantaneous lift coefficient acting on the rings of the sheath with helical fillet, for increasing imposed deformation. The Reynolds numbers corresponding to each figure are respectively Re = 2.35 × 10 5 , 2.42 × 10 5 , 2.51 × 10 5 and 2.51 × 10 5 for the non-deformed sheath, for Dmax = 253 mm, for Dmax = 255 mm and for Dmax = 257 mm. In all cases the wind incidence is θ = 22.5°. The duration of each signal is 60 s; during this time, the lift on several of the rings jumps more than once between two values. One of the values is close to zero, i.e. the corresponding flow is nearly symmetric, while the other value is either positive or negative, i.e. the corresponding flow is asymmetric. The jumps are not always synchronized on all the rings and there are some rings where no jumps occur. Consequently, in this Reynolds number range, the flow can locally switch between different transition regimes (TrBL0 to TrBL1). where Di,j is the within-to-between clusters distance for the i th and j th clusters, determined from : 𝑑 𝑖,𝑗 [START_REF] Nikitas | Critical Reynolds number and galloping instabilities : experiments on circular cylinders[END_REF] where 𝑑 ̅ 𝑖 and 𝑑 ̅ 𝑗 are respectively the average distance between each point and the centroid of the i th and j th centroid of clusters and di,j is the Euclidean distance between the centroids of the i th and j th clusters. More details about the k-means algorithm and cluster separation criterion can be found in MacQueen (1967). Plotting all the different identified clusters at each tested Reynolds number results in a bifurcation diagram. θ = 90° and 135° and in rings 2-7. However, as the deformation is increased and the cross-sectional shape is distorted, the bi-stability percentage at these wind incidences is seriously reduced. The bistability is gradually shifted towards incidences in the ranges 0°-22.5° and 180°-202.5°. Furthermore, the maximum bi-stability percentage is increased up to 90 %, which means that bi-stable phenomena can be observed in nearly the entire Reynolds number range between 1.7 x 10 5 and 3 x 10 5 for some of the rings. The rings most affected by bi-stability are rings 2-7 for all values of the imposed deformation; only the undeformed sheath undergoes bi-stability at ring 8. Furthermore, the length of the sheath affected significantly by bi-stability increases with the imposed deformation. For Dmax = 257 mm and θ=0° this length exceeds 30 cm in total.

Pressure distribution around the sheath with helical fillet

The unsteady pressure distribution around the sheath with helical fillet is analyzed using Proper Orthogonal Decomposition (POD). This approach was already applied to a smooth sheath in [START_REF] Benidir | Impact of roughness and circularity defect on bridge cables stability[END_REF]; more details about it can be found in The data recorded from the pressure taps along the tube can be arranged into a matrix P (N, M), where N and M represent respectively the number of pressure taps and the number of time instances,

P = [ P 1,1 … … … P 1,M … … … … … … … … … … … … … … … … … … P N,1 … … … P N,M ] (5)
The pressure P(x, t) can be written as

𝑃(𝑥, 𝑡) = ∑  𝑖 (𝑥)𝑞 𝑖 𝑁 𝑖=1 (𝑡) (6)
 𝑖 (𝑥) are the spatial POD mode shapes and 𝑞 𝑖 (𝑡) are the generalized coordinates, which are functions of time. The POD procedure makes use of the eigenvalue decomposition

𝐶 𝐴 =  𝐴 ( 7
)
where C is the covariance matrix of P, while A and λ are respectively the matrices of eigenvectors and eigenvalues of C. The energy of the POD modes was ranked using the Cumulative Percentage Variance (CPV) criterion. The formulation of this criterion is given by

𝐶𝑃𝑉(𝐼) = 100 ∑  𝑗 𝐼 𝑗=1 ∑  𝑗 𝑁 𝑗=1 % ( 8
)
where I is the number of retained modes. This criterion is applied to select the most energetic modes from the eigenvalues matrix. The associated generalized coordinates are calculated from Mode 1 in all cases represents the mean flow, as the generalized coordinate corresponding to this mode is constant at all times. It can be seen that this flow is nearly but not completely symmetric for all imposed deformations and even for the undeformed sheath. This asymmetry of the mean flow is due to the presence of the helical fillet and was observed for all wind incidences and Reynolds numbers.

𝑞 = 𝜑 𝑇 𝑃 (9)
Asymmetry due to turbulent transition in the boundary layers is represented by modes 2 and 3. At time instances when the corresponding generalized coordinates are equal to zero, the flow is equal to the mean flow and there is steady asymmetry due to the helical fillet. At times when the generalized coordinates of modes 2 and 3 are non-zero, the flow undergoes transition, becomes temporarily asymmetric and generates lift. Mode 2 always features an area of high pressure and an area of low pressure on the right side, along with a small area of high or low pressure around ring 8 on the left side. Mode 3 features a single area of high/low pressure on the right side, except for Dmax=257mm where there are two distinct areas of high pressure on the right. Every time the generalized coordinates of modes 2 and 3 jump in value, the lift changes magnitude and/or direction. Such changes constitute significant impulses applied on the sheath that will cause structural displacement in flexible cables. Sometimes, the changes in the flowfield are synchronized over the entire instrumented length of the sheath, causing a uniform lift to the left or to the right. Consider the case Dmax=255mm plotted in figure 15. At times when the generalized coordinates of both modes 2 and 3 are positive, there is a uniform high-pressure area over most of the right side of the sheath (rings 3-5), which cancels the low pressure caused by mode 1. A net lift towards the left will be applied on the majority of the sheath, causing a significant impulse when the jump occurs. Nevertheless, figures 13-16 show that, for the sheath with a helical fillet, synchronized changes in the flow field that cause a net lift towards the same direction over the entire length of the sheath are rare.

In most time instances, the generalized coordinates of modes 2 and 3 have incompatible signs or one of them is zero. In such cases, the lift pushes half of the sheath towards the left and the other half towards the right (or vice-versa). This means that, when jumps occur, the resulting impulse acting on the sheath is weaker and the associated structural response is expected to be less pronounced. For the case D=257mm in particular (figure 16), the generalized coordinate of mode 3 is zero almost exclusively when that of mode 2 is not equal to zero. Therefore, there are no time instances when modes 2 and 3 cause a uniform low or high-pressure bubble over the entire length of the sheath. The energy of mode 1 is lower than in the other deformation cases and that of mode 2 is higher, which means that bi-stability is more important. Yet, jumps in the values of the generalized coordinates are less frequent and result in non-uniform changes in pressure along the length of the sheath. Hence, the effect of bi-stability is expected to be less pronounced on the structural response of the cable.

CONCLUSION

This paper reports on the effect of increasing the circularity defect on the sheaths of cables for cablestayed bridges. For smooth sheaths, imposing deformation on their cross-section shifts the critical regime to lower Reynolds numbers. For sheaths with helical fillets, the critical Reynolds number range becomes wider. Important bi-stable phenomena are observed for both types of sheath in the critical regime, causing significant jumps in instantaneous lift.

Imposed deformation has a mixed effect on the aerodynamic stability of the flow around the sheath with helical fillet. On the one hand it increases the Reynolds number range of the critical regime and makes bi-stable phenomena more pronounced. On the other hand, the helical fillet de-correlates the flow, resulting in regions of opposite pressure on the same side of the sheath, even at the highest imposed deformation. This means that jumps in lift are rarely uniform over the entire length of the sheath and any resulting impulses will not have a coherent effect on the structural response of the enclosed cable. This result could be taken to mean that the introduction of a helical fillet has a positive effect on dry galloping stability, as well as on rain-and-wind-induced vibration. Nevertheless, it should be kept in mind that the present experiments were carried out on sheaths of length 2m and flow correlation phenomena might be different for much longer sheaths.

Figure 1 :

 1 Figure 1: (a) HDPE cover with helical fillets, diameter D = 250mm; (b) 288 pressure taps (9 rings of 32 pressure taps); (c) mechanism designed for tuning the ovalization of HDPE tube.

Figure 2 :

 2 Figure 2: (a) Deformations of smooth cable surfaces, (b) Circularity defect variation along the length of the cable for Dmax=257 mm.

4 x 10 5 .Figure 4 : 5 :

 545 The increase and the reorganization of the circularity defect around the cover leads to a wider Reynolds number range in which boundary layer transition occurs. For the original section, the critical regime ranges from Re = 2.25x10 5 to Re = 2.88 x 10 5 while, for Dmax = 257 mm, it stretches from Re = 2.16 x10 5 to Re = 3.04 x105 . Nevertheless, the maximum lift values caused by the different cross-sectional shape deformations at this cable orientation are quite similar. (a) θ=22.5° (b) θ=202.5° Mean lift coefficient vs Reynolds number, original circularity defect, ring 5. (a) Drag coefficient (b) Lift coefficient Figure Mean drag and lift coefficient vs. Reynolds number. θ=22.5°, Ring 3.

Figure 6 :

 6 Figure 6 : Mean lift coefficient vs. Reynolds number, θ=22.5°.

Figure 7 :

 7 Figure 7: Instantaneous lift coefficient, helical fillet cable sheath, Re = 2.35x10 5 , θ=22.5°.

Figure 8 :Figure 9 :Figure10:

 89 Figure 8: Instantaneous lift coefficient, helical fillet cable sheath, Dmax = 253mm, Re = 2.42x10 5 , θ=22.5°.

Figure 11 :

 11 Figure 11: Bifurcation diagram for helical fillet cable surface, Ring 3.

Figures 11 -Figure 12 :

 1112 Figures 11-a to 11-d plot the bifurcation diagrams of the instantaneous lift coefficient around ring 3 for the sheath with a helical fillet at wind incidence θ=22.5° and for four different imposed deformations. Bi-stability occurs in a wider Reynolds number range as the circularity defect increases. For the sheath with natural circularity defect and with Dmax = 253 mm, bi-stability occurs in the Re = 2.5 x 10 5 -3 x

Figure 12

 12 Figure 12 presents contour plots of the bi-stability percentage, plotted against wind incidence (horizontal axis) and ring number (vertical axis). The wind incidence ranged from 0° to 202.5°. The four subplots concern different imposed deformations for the sheath with a helical fillet. For the undeformed sheath (figure12-a) the highest bi-stability percentages occur at wind incidences between

  [START_REF] Andrianne | Flow Visualization and Proper Orthogonal Decomposition of Aeroelastic Phenomena[END_REF] and[START_REF] Dowell | Eigen mode analysis in unsteady aerodynamics: Reduced order models[END_REF]. POD is a procedure for extracting an orthogonal basis for modal decomposition from an ensemble of signals. It relies on statistical processing of the signals to extract some coherence or organization. It is a very powerful tool in the sense that the projection on the subspace used to model the signals will contain the most energetic modes.

Figures 13 -Figure 13 :Figure 14 :

 131314  plot the results of the POD analysis for the sheath with a helical fillet at θ=22.5° and for the different imposed deformations. The Reynolds number is specified below the figures. For each figure, subplot (a) plots CPV(I), subplot (b) plots the first three generalized coordinates and subplots (c)-(e) the first three POD mode shapes. Figures13-a, 14-a, 15-a and 16-a show that the first three modes contain more than 97 % of the energy of the signals for all cases.

Figure 15 :

 15 Figure 15 : Proper orthogonal decomposition of the pressure pattern, helical fillet Dmax = 255mm, Re = 2.51x10 5

( a )

 a Energy of the mode (b) Generalized coordinates (c) Mode shape, λ1 (d) Mode shape, λ2 (e) Mode shape, λ3

Figure 16 :

 16 Figure 16 : Proper orthogonal decomposition of the pressure pattern, helical fillet Dmax = 257mm, Re = 2.51x10 5

281-297. University of California Press, University of California, USA, June 21-July18, 1965 and December 27, 1965-January 7, 1966, pp. 281-297.


Tran Nguyen

Pirovano Alain
email: alain.pirovano@enac.fr

Larrieu Nicolas
email: nicolas.larrieu@enac.fr

Managing aircraft mobility in a context of the ATN/IPS network

Keywords: ATN/IPS, Mobility, LISP, PMIPv6, OMNeT++

For the sake of Air Traffic Management modernization, civil aviation organizations are currently developing IPS for Aeronautical Safety Services in the new ATN/IPS infrastructure. This includes to define new airborne and ground-based communication systems capable of managing both air traffic services (ATS) and aeronautical operational communications (AOC) safety services. One of the main challenges in this new ATN/IPS network is the IPv6 mobility problem. This paper proposes a solution which takes both advantages of ground based Locator/Identifier Separation Protocol and Proxy Mobile IPv6 to manage all the aircraft mobility scenarios. A dedicated OMNeT++ simulation model is also provided and shows the performances of our solution.

I. INTRODUCTION

In the future Air Traffic Management (ATM) concept, in order to increase safety, airspace capacity, flight and cost efficiency, applications will need to exchange more and more data with the aircraft. Aeronautical Telecommunication Network over IP (ATN/IPS) will be the new standard of the future ATM infrastructure. It will allow a more efficient use of datalink communications, which will become in the next decades a major mean of communication. This Future Communication Infrastructure (FCI) aims at providing a resilient and secure way to transfer flight critical data and voice communications over digital links. To meet this requirement, new functionalities in terms of mobility, security and Quality of Services (QoS) must be defined due to the particularities of the aeronautical environment.

This paper focuses on the problem of network mobility in the FCI. The International Civil Aviation Organization (ICAO) is currently working on this aspect but has not defined yet the solutions. Proposals have been made such as Ground-Locator/Identifier Separation Protocol (G-LISP) [START_REF] Haindl | Ground based lisp for multilink operation in atn/ips communication infrastructure[END_REF], Proxy Mobile IPv6 (PMIPv6) [START_REF] Mcparland | Multihoming in the atn/ips[END_REF] and Asymmetric Extended Route Optimization (AERO) [START_REF] Templin | Asymmetric Extended Route Optimization (AERO)[END_REF]. All these solutions resolve only partial mobility issues. PMIPv6 and AERO are intra-domain mobility solutions, whereas Ground-LISP defines a solution to cope with the inter-domain mobility issue. The mobility solution also has to take into consideration multilink properties, which are required by the operating concept defined in SESAR 15.2.4.

The aim of this paper is to deal with both mobility scenarios, which have been identified by T.Whyman in the ICAO mobility working sub-group as use case scenarios. Particularly, coupling G-LISP with PMIPv6 may be an adequate solution as it will allow to solve intra-domain and inter-domain aircraft mobility. This paper will first present the architecture of the ATN/IPS network and all the entities that play a role in the mobility solution. After a short description of PMIPv6 and LISP, the paper describes how these protocols can be combine to handle the aircraft network mobility. A dedicated simulation model we have developed under the OMNeT++ software is provided and a performance evaluation and analysis of our proposal is performed.

II. ATN/IPS OVERVIEW

The Future Communication Infrastructure (FCI) designed for ATN/IPS will be composed of different subnetworks:

• An aircraft which is considered as a mobile subnetwork hosting airborne end systems (A-E). It can be seen either as a mobile node or a mobile network at the IP level.

It includes an airborne router (A-R) responsible for the link selection and the management of the inter-technology handoffs, and also airborne radios (AR) providing the communication over air/ground links. Ground ATN/IPS Internetwork is the core network of ATN/IPS. It maintains a connection between all the different subnetworks. The FCI will be the interconnection of all these subnetworks and hence securing an IP end-to-end communication between all the aeronautical stakeholders. The FCI is represented in the Figure 1, in which we illustrate the fact that there should be more than one CSP providing one type of link technology.

Given ATN/IPS network topology, there are 2 mobility problems to solve: intra-domain mobility, where the aircraft stays in the same domain, and inter-domain mobility where the aircraft changes its network access provider. These mobility scenarios can be either horizontal or vertical, meaning that they do not depend on whether the aircraft keeps using the same technology access or changes to another one. Indeed, a CSP may provide more than one access networks to the aircraft via either a terrestrial or a satellite based access technology, so even if the aircraft has to change between communication means, it may remain in the same administrative domain (CSP domain). The main challenge in these mobility scenarios is to provide a seamless handoff to the application layer. This is realized by keeping the same address for the aircraft, so that it does not have to break any application sessions while changing its network access point. To cope with the heterogeneity of the FCI and handle these mobility scenarios efficiently, subdividing the problem into a local mobility scenario and a global mobility scenario will allow to define a scalable and optimized solution. This solution has to take into account the specificities of the aeronautical environment, thats to say, it must minimize the message exchange on the radio link.

III. MOBILITY SOLUTION A. PMIPv6: an intra-domain mobility solution

In the first time, we focus on the intra-domain mobility scenario. Instead of relying on a routing protocol (for instance OSPF is proposed in [START_REF] Haindl | Ground based lisp for multilink operation in atn/ips communication infrastructure[END_REF]) which has not been developed for such a scenario, using a dedicated mobility protocol is more advantageous. As mentioned in [START_REF] Tran | IP Mobility in Aeronautical Communications[END_REF], Proxy Mobile IPv6 (PMIPv6) is the first IPv6 network mobility protocol to propose such a solution. It provides indeed a unique address to the aircraft when it enters in the domain. Such feature is not provided by other routing protocols, which only constructs a route to reach the aircraft. It is also compatible with multilink scenarios, making him a good candidate to solve the intradomain handoff.

PMIPv6 has gained much attention in the network & telecommunication area recently and has been standardized by IETF in RFC 5213 [START_REF] Chowdhury | Proxy Mobile IPv6[END_REF]. It is a network-based mobility management protocol, inspired from the Mobile IPv6 protocol. It allows the node mobility management inside an administrative domain, also called a proxy domain, and thus hiding the intradomain node mobility to the rest of the network.

PMIPv6 defines two new entities: a local anchor point (LMA) and a proxy router (MAG). The LMA has the role of the Home Agent inside the domain, so it keeps an entry for each MN inside the proxy domain. This entry contains the Home Network prefix allocated for this MN and its proxy Care-of-Address which is the address of the router the MN is connected to. While the MAG handles the mobility signaling for any mobiles nodes entering the proxy domain. The handoff between two MAGs belonging to the same proxy domain is described in Figure 2. The previous MAG (pMAG) sends a Binding Deregistration to the LMA so it can erase the pMAG address in the cache. When the nMAG detects a new mobile node by receiving a RS message, it starts the PMIP procedure for the MN by sending a Binding Update message towards the LMA. The latter modifies the proxy Care-of-Address, i.e the MAG address, in the corresponding entry and replies with a Binding Acknowledgement, containing the home network prefix (HNP) allocated for the MN. The MAG will then send a RA to the MN containing the HNP. The MN will thus receive the same prefix and will still use the corresponding address to communicate. The extension defined in rfc 7864 allows the use of multiple MAGs at the same time to forward packets to the MN.

Without involving the MN in the mobility signalling exchange, PMIPv6 succeeds to manage its mobility. Moreover, the main advantage of this protocol is to allow a fast handoff between different communication means belonging to the same administrative domain. For instance, suppose that a CSP proposes more than one communication means to the aircraft in some regions (i.e LDACs, and future L2 system), a vertical handoff between these 2 communication means will only generate mobility signaling inside the CSP domain.

B. LISP: an inter-domain mobility solution

Whereas PMIPv6 is well suited for intra-domain mobility, it cannot help managing mobile node inter-domain mobility. One solution is to use MIPv6 which is standardized in RFC 3775 [START_REF] Perkins | Mobility Support in IPv6[END_REF] and is described for ATN/IPS internetwork in [START_REF] Bauer | A thorough investigation of mobile ipv6 for the aeronautical environment[END_REF] and [START_REF] Mcparland | Multihoming in the atn/ips[END_REF]. The main drawback of MIPv6 is that it requires a modification in the mobile node's IP stack to announce its Care-of-Address to the Home Agent, and thus implying additional signaling exchange in the radio link. Hence, a network-based solution is recommended such as LISP (or Ground LISP), which introduces a new paradigm to solve inter-domain mobility and multihoming.

LISP [START_REF] Farinacci | The Locator/ID Separation Protocol (LISP)[END_REF] defines a network based mobility solution allowing handoffs between different CSP domains and radio technologies. It splits the role of the IP address into 2 namespaces: Routing Locator (RLOC) and End-system Identifier (EID). RLOC are globally routable addresses used to localize the end-system and route the packet in the network, while EID are addresses dedicated to identify an end host. Doing so ensures a more flexible and scalable routing solution into the network. All the routing functionalities are moved to the ground with the Routing Locator Space (RLOC), and thus it does not require to make any modifications in the end-system, minimizing the implementation cost on-board. LISP routers, namely ingress Tunnel Router (iTR) and egress Tunnel Router (eTR), together with a mapping system maintain a mapping cache between the RLOC and the EID address, and use the encapsulation/decapsulation method to forward packets to the end-system. A detailed explanation of LISP for ATN/IPS can be found in [START_REF] Haindl | Ground based lisp for multilink operation in atn/ips communication infrastructure[END_REF], describing the different steps of the protocol. The interdomain handoff between 2 different CSPs in LISP is described in Figure 3. xTR refers to a LISP router implementing both iTR and eTR mechanisms. Once a MN leaves a CSP domain, its LISP eTR will inform the mapping system that the EID corresponding to the MN is no more reachable via its RLOC. When detecting a new MN in its domain, its LISP Router sends a MAP Register message towards the mapping system to create a new EID-to-RLOC mapping for the MN.

C. A new global mobility solution

We previously described how the intra-domain and interdomain mobility should be resolved. Hence, we propose a solution integrating both protocols to manage the aircraft global mobility. The combination of these two approaches benefits from advantages of both sides, hence renders our solution more optimal. Figure 4 shows how this solution can be deployed in the FCI.

• Mapping System (MS/MR): it is an entity implemented in the ATN/IPS ground network. It is composed of a Map Server (MS) and a Map Resolver (MR). The MS is connected to the eTRs from which it receives LISP map regristration messages and stores the EID-to-RLOC mapping into the database. Furthermore, it must advertises any new EID-to-RLOC mappings to iTRs that have already asked a mapping for this EID, in order to give them an up-to-date mapping information.

The MR responds to LISP map request messages sent by iTRs, whenever an EID-to-RLOC mapping needs to be performed. LISP protocol specifies 2 modes to forward EID-to-RLOC mapping, the direct and indirect mode, as described in [START_REF] Caiazza | Simulating lisp-based multilink communications in aeronautical networks[END_REF]. Here, we choose to use the direct mode, where the MR directly replies to the iTR requesting the mapping, as it includes less signaling traffic.

• PMIP domain: it corresponds to a CSP domain, including a network access for each link technology type provided. However, a CSP providing a satellite link access may not be concerned. Indeed, the mobility inside this type of subnetwork may be solved with a L2 solution.

• AC-R: It takes the role of the MAG inside the PMIP domain. At least one AC-R per network access is recommended. It is responsible for handling the aircraft authentication in the ATN/IPS network and the aircraft mobility signaling. Whenever an aircraft detects a new network access and wants to establish a new connection, the AC-R will start the authentication procedure on behalf of the by querying the AAA server. After a correct authentication, the AC-R will register the aircraft in the LMA.

• A/G-R: In our proposal, this router must integrate the role of both the LMA and the LISP router through eTR/iTR mechanism, and thus allowing the aircraft to be connected to the rest of the FCI. First, it will assign the aircraft home network prefix(HNP) based on ICAO IPS addressing space with 24 bits corresponding to the aircrafts ICAO ID. Then it will use this HNP as the aircraft EID and register it with its RLOC address (i.e its routable address belonging to the ATN/IPS domain) by sending a LISP MAP REGISTER message to the Mapping System (MS/MR). In this way, ground end-systems willing to communicate with the aircrafts HNP will send, through their G/G router, a map request to the mapping resolver to get the corresponding RLOC address. Reversely, when an aircraft initiates the communication to a ground end-system, the A/G-R acts as an iTR, meaning that it has to send a map request to get the RLOC address attached to the ground end-system. After receiving a non empty LISP map reply, the A/G-R will store the mapping in its local cache and then create a LISP tunnel to the G/G-R having the corresponding RLOC address and send aircraft packets through this tunnel. In this way, for further incoming packets to forward, it just needs to use the corresponding tunnel. • Ground/Ground-Router (G/G-R): It is a LISP router acting as an xTR, to interconnect the Applicative Service Provider to the ATN/IPS core network. It is in charge of registering all the ground end-systems into the mapping system, and like the A/G-R, performing EID-to-RLOC mapping.

D. Mobility scenario

To highlight the mechanisms previously described, we consider the flight scenario in Figure 5 where the aircraft performs an inter-domain handoff from CSP1 to CSP2 keeping the same technology access. This type of handoff may happen when the first CSP does not cover the whole area with this technology access, thus requiring a need for an handoff to another CSP providing this type of access. Then, the aircraft remains in the area covered by CSP2 and realizes an intra-domain handoff, changing its network point of access.

1) Inter-domain handoff: Figure 6 shows an aircraft realizing an inter-domain handoff. When the aircraft detects a degradation in its communication, it sends a link disc L2 message to the current AC-R2. The latter informs the A/G-R1 that the aircraft leaves the access network with a Binding Deregistration message. The A/G-R1 must then send a LISP map register to the mapping system to erase the EID-to-RLOC mapping corresponding to its routing locator address. To prevent eTRs from keeping using the old mapping, the mapping system has to inform the concerned iTRs of this change. This could be realized with a publish/subscribe method in which the iTRs subscribe to the mapping systems update messages.

In parallel, the aircraft establishes the radio communication with the CSP2 access network, and sends a Router Solicitation packet to discover the access router on the link. After receiving the RS, the AC-R3 starts the PMIP registration by sending a proxy Binding Registration message to the A/G-R2 (authentication with a AAA server is omitted here). To get the aircraft HNP, the A/G-R2 requests this information to the other A/G-Rs. Once it receives the reply, it sends back a Binding Acknowledgement message with the corresponding HNP to the AC-R3. In the end, the AC-R3 sends a Router Advertisement message to the aircraft advertising the same HNP from which it has already configured an IPv6 address. Hence, the aircraft does not have to configure a new IPv6 address and run the related duplicate address detection procedure.

In the same time, the A/G-R has to advertise the mapping system of a new EID-to-RLOC mapping with a LISP map register message, which in turn broadcasts this information to its subscribers, i.e the iTR that have a mapping with the corresponding aircraft EID.

2) Intra-domain handoff: The aircraft has entered the CSP2 domain and is attached to AC-R3. Figure 7 describes the messages exchange when the aircraft has to achieve an intradomain handoff in the CSP2 domain from AC-R3 to AC-R4. With the use of PMIPv6 protocol, the signaling remains in the CSP2 domain and has no impact on the ATN/IPS ground internetwork, as the aircraft location has already been announced when it has previously entered the domain. Only the AC-Rs and the A/G-R are concerned by the mobility signaling. As described in III-A, the A/G-R2 updates its

IV. PERFORMANCE ANALYSIS AND EVALUATION

Performance evaluation is based on the network infrastructure presented in Figure 5. Our solution is compared with the combination of MIPv6 with PMIPv6 to manage the aircraft global mobility, which is the first proposed solution to handle the global mobility with PMIPv6. Both solutions are implemented in the OMNeT++ software, based on the INET framework. The network infrastructure for the MIPv6/PMIPv6 solution is identical to our solution except that the Mapping system is replaced by the aircraft's Home Agent. In this scenario, the Home Agent attributes a home address to the aircraft, which is based on the ICAO address. When the aircraft is connected to a CSP, it is given a home network prefix, so that it can build a IPv6 Care-of-Address based on this prefix. This paper focuses on the performances of the different handoffs at the network layer an aircraft has to perform during its trajectory. Hence, AC-Rs are equipped with a wireless network interface card to simulate air/ground datalinks and to communicate directly with the aircraft. However, to handle certain aspects of the layer 2 mechanism, a L2 manager was developed [START_REF] Tran | IP Mobility in Aeronautical Communications[END_REF] and allows us to trigger a handoff at the layer 2 (when the signal is under a certain threshold for instance).

For both scenarios, we consider an aircraft moving at a cruise speed, performing successively an inter-domain handoff and an intra-domain handoff as described in the last part III-D. Meanwhile, it receives uplink trafic coming from the ASP endsystem. We implement a representative ATS and AOC traffic during an EN-ROUTE phase, proposed in [START_REF] Ribeiro | A framework for dimensioning vdl-2 air-ground networks[END_REF], based on the UDP module of INET. Figure 8 and Figure 9 show the uplink traffic end-to-end delay for both scenarios. They demonstrate that all the packets are indeed routed to the aircraft under an acceptable delay, even after the successive handoffs, happening at t = 220s and t = 945s. Moreover, under these circumstances and hypotheses, our proposal with LISP is 10% more effective than the MIPv6 protocol in terms of average end-to-end delay. However, to further extend this result, other link technology types must be considered as we just model the case of an aircraft using VdLm2 subnetwork access.

We then evaluate both solutions in terms of handoff latency. The handoff delay for an intra-domain handoff depends only on the PMIPv6 protocol for both scenarios, so there is no difference between the two solutions. It corresponds to the delay from the moment when the aircraft loses its connection with the previous access router until the moment when the A/G-R updates its binding cache entry to insert the new aircraft's proxy Care-of-Address (pCoA). We define the interdomain handoff delay as the delay from the L2 disconnect signal emitted by the aircraft until the time it takes to the G/G-R to update its EID-to-RLOC mapping for the LISP scenario, and until the time it takes to the Home Agent to get the new aircraft's Care-of-Address for the second scenario. This handoff delay is based on 3 factors: 1) New link detection: it corresponds to the time T1 until the aircraft get a new available link and emits a RS message through this link. This time is mainly due to the air/ground datalink protocols (like LDACS) and is not dependent on the network mobility solutions. In our simulation, we do not implement such protocols. Nevertheless, we have a L2 switching mechanism which simulates the same behavior and allows the on-board radio to be "connected" to a unique A/G datalink subnetwork.

2) Local update location: For LISP, it corresponds to the time period T2 from the new link detection until the xTR router gets the new location of the aircraft, i.e when it receives the Binding Registration from an AC-R. For MIPv6, it corresponds to the time period T2 until the aircraft gets its new IPv6 address from the new home network prefix it receives. It includes the duplicate address detection period the aircraft has to perform in order to validate the new IPv6 address.

3) Cache update: it is the time period T3 until the information of the new location of the aircraft reaches the router responsible to route the packet towards the aircraft. This router is the G/G-R for the LISP protocol, whereas it corresponds to the Home Agent for the MIPv6 protocol.

Figure 10 and 11 show the complete inter-domain handoff procedure for both cases. As said earlier, the handoff delay depends on the link technology type. To provide realistic results, we model the delay induced by the MAC sublayer before sending the packets. This delay is representative of a delay perceived by an aircraft in a VHF Datalink Mode 2 (VDLm2) cell (one of the link technology types in ATN/IPS). This study is performed with the VDLm2 protocol developed under OMNeT++ at the ENAC laboratory. We show that the time in queue for downlink and uplink packets follow an exponential law with λ depending on the number of aircraft in the cell. The following results are obtained by varying the cell capacity from 20 to 200 aircraft.

The time T1 is identical for both case scenario as it does not depend on any network layer mechanism. For the LISP solution, we do not take into account the time period until the aircraft receives the HNP inside the solicited RA message as it will receive the same HNP. The time T2 is greater for the MIPv6 scenario because of the DAD procedure which takes approximately 2 seconds, 1 second to join the multicast group, and 1 second to be sure that no other nodes on the link is using the same address. Concerning the time period T3, it is much shorter with our solution as the mobility signaling remains in the network, unlike the MIPv6 scenario where the signaling involves the aircraft. Overall, with our solution, it takes 3,11s at maximum to perform the inter-domain handoff meanwhile with the MIPv6 protocol, the inter-domain handoff delay can reach 6,59s. This result proves that using LISP can help to reduce the handoff delay by 2.

V. CONCLUSION

Coupling LISP with PMIPv6 is a promising solution. It is a full network-based mobility solution keeping on-board complexity as low as possible. Compared to a solution involving MIPv6 protocol for inter-domain mobility where the aircraft is in charge of announcing its location to the Home Agent, it minimizes the mobility signaling exchange with the aircraft, thus saving radio resources. We provide an OMNeT++ simulation model of both LISP and MIPv6 protocols combined with PMIPv6 and we evaluate the performances of successive handoffs. Firstly, the application data end-to-end delay proves that our solution can efficiently manage the different handoff case scenarios, and performs slightly better than the MIPv6 based solution. Results also show that the LISP based solution performs inter-domain handoff faster than the MIPv6 based solution.

To further extend our study, we will implement a modelisation of other link technology types foreseen for ATN/IPS (like LDACS and SATCOM) and verify whether we obtain the same results as shown in this paper. Also, the multilink capability, which is identified as a key aspect to provide high availability and quality in the FCI, of this protocol will be investigated later on.

Fig. 1 .

 1 Fig. 1. The Future Communication Infrastructure

Fig. 3 .

 3 Fig. 3. LISP Handoff Management

Fig. 4 .

 4 Fig. 4. Implementation of a global mobility solution in the FCI

Fig. 6 .

 6 Fig. 6. Inter-domain handoff mechanism

Fig. 8 .

 8 Fig. 8. Traffic application End-to-End delay for LISP scenario

Fig. 10 .

 10 Fig. 10. Inter-domain handoff delay with LISP