Jean-Pierre P Kahn

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Bruno Etain

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Frank Bellivier

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Thierry Bougerol

Philippe Courtet

Caroline Dubertret

Sébastien Gard

Chantal Henry

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Disorders (FACE-BD Marion Leboyer

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Reconsideration of the factorial structure of the Barratt Impulsiveness Scale (BIS-11): Assessment of impulsivity in a large population of euthymic bipolar patients

niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

The BIS-11 is the most widely internationally used instrument to assess impulsivity in clinical populations. Since the seminal work of Ernest Barratt and his first version of the Barratt Impulsiveness Scale [START_REF] Barratt | Anxiety and impulsiveness related to psychomotor efficiency[END_REF], the concept of impulsivity has evolved from an unidimensional to a multidimensional model, through successive revisions of the Barratt Impulsiveness Scale (BIS) until the last and 11 th version of the BIS, published in 1995 [START_REF] Patton | Factor structure of the Barratt impulsiveness scale[END_REF].

The BIS-11 was designed from ratings obtained from a heterogeneous population of 733 persons (412 undergraduate students, 248 psychiatric in-patients and 73 male prison inmates).

Using two successive "Principal Component Analyses" (PCA), the authors isolated three dimensions of the scale, which they called "motor impulsiveness", "non-planning impulsiveness" and "attentional impulsiveness". In 2007, Spinnela presented a shortened 15 items version of the BIS-11, without questioning the three factors structure of the scale [START_REF] Spinella | Normative data and a short form of the barratt impulsiveness scale[END_REF]. Unfortunately this three factors structure could not be replicated in several subsequent studies [START_REF] Haden | A Comparison of Factor Structures of the Barratt Impulsiveness Scale in a Mentally Ill Forensic Inpatient Sample[END_REF][START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF][START_REF] Steinberg | New tricks for an old measure: The development of the Barratt Impulsiveness Scale-Brief (BIS-Brief)[END_REF]. [START_REF] Haden | A Comparison of Factor Structures of the Barratt Impulsiveness Scale in a Mentally Ill Forensic Inpatient Sample[END_REF], in a sample of 327 mentally ill forensic in-patients found only two factors, which they named "motor impulsivity" and "nonplanning impulsivity". [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF], using the BIS-11 in a community sample (N = 691), also concluded that only a model with two factors could accurately account for their results. They called these two factors "behavioral impulsivity" and "cognitive impulsivity". According to Reise et al., behavioral impulsivity depends on factors present at the time of action, while cognitive impulsivity 2 JPK. Barratt/IS-12/V2 14/03/2019 depends on the awareness of factors present before the action and on the consequences of the action.

Suicidality has been strongly associated with both bipolar disorder (BD) and impulsivity [START_REF] Brezo | Personality traits as correlates of suicidal ideation, suicide attempts, and suicide completions: a systematic review[END_REF][START_REF] Liu | A behavioral and cognitive neuroscience perspective on impulsivity, suicide, and non-suicidal self-injury: Meta-analysis and recommendations for future research[END_REF][START_REF] Lynam | Testing the relations between impulsivity-related traits, suicidality, and nonsuicidal self-injury: A test of the incremental validity of the UPPS model[END_REF] . Psychological autopsy studies have shown that around 25% of suicides occur in patients suffering from bipolar disorder [START_REF] Isometsä | Suicide in Bipolar I Disorder in Finland: Psychological Autopsy Findings from the National Suicide Prevention Project in Finland[END_REF]. Twenty five to 60 % of BD patients will attempt suicide at least once during lifetime [START_REF] Goodwin | Manic-depressive illness: bipolar disorders and recurrent depression[END_REF][START_REF] Novick | Suicide attempts in bipolar I and bipolar II disorder: a review and meta-analysis of the evidence[END_REF] and, according to recent estimations, around 6 to 7 % of BD patients will die by suicide [START_REF] Schaffer | Epidemiology, neurobiology and pharmacological interventions related to suicide deaths and suicide attempts in bipolar disorder: part I of a report of the International Society for Bipolar Disorders Task Force on Suicide in Bipolar Disorder[END_REF].

On the other hand, patients with BD have been shown to present a higher level of trait impulsivity than controls, as measured by the BIS-11 total score, even when euthymic [START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF]Moëller et al., 2001;[START_REF] Swann | Measurement of inter-episode impulsivity in bipolar disorder[END_REF]. In bipolar disorder, our group has shown that impulsivity was higher in patients with a history of mixed states, rapid cycling and substance misuses [START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF] ; impulsivity was also higher in subjects with a predominant depressive polarity, a longer duration of illness, a history of psychotic mood episodes and a history of suicide attempts [START_REF] Ekinci | Relationship of trait impulsivity with clinical presentation in euthymic bipolar disorder patients[END_REF]. When assessed by a behavioral task, the Continuous Performance Test (CPT), impulsivity is associated with suicidal attempts as well as with the severity of suicidal attempts in BD patients [START_REF] Swann | Increased impulsivity associated with severity of suicide attempt history in patients with bipolar disorder[END_REF].

The relationships between suicidality and impulsivity have been studied using different psychometric instruments. Most studies using the "Urgency, (lack of) Premeditation, (lack of) Perseverance, Sensation seeking Scale" (UPPS, [START_REF] Whiteside | The five factor model and impulsivity: Using a structural model of personality to understand impulsivity[END_REF] to assess impulsivity consistently demonstrate its links with suicidality [START_REF] Ammerman | Suicidal and violent behavior: The role of anger, emotion dysregulation, and impulsivity[END_REF][START_REF] Klonsky | Rethinking impulsivity in suicide. Suicide Life[END_REF][START_REF] Lynam | Testing the relations between impulsivity-related traits, suicidality, and nonsuicidal self-injury: A test of the incremental validity of the UPPS model[END_REF] ; in contrast, when assessing impulsivity with the BIS, results are inconsistent [START_REF] Doihara | Trait impulsivity in suicide attempters: Preliminary study: Impulsivity in suicide attempters[END_REF][START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF][START_REF] Ferraz | Hostility and childhood sexual abuse as predictors of suicidal behaviour in Borderline Personality Disorder[END_REF] JPK. Barratt/IS-12/V2 14/03/2019 [START_REF] Olié | Clinical and neuropsychological characteristics of euthymic bipolar patients having a history of severe suicide attempt[END_REF][START_REF] Swann | Increased impulsivity associated with severity of suicide attempt history in patients with bipolar disorder[END_REF][START_REF] Wu | Multidimensional assessments of impulsivity in subjects with history of suicidal attempts[END_REF]. These results seem to demonstrate the instable predictive value of the BIS-11 total score. Therefore, we wanted to address the question of the use of subscores of the BIS to improve its predictive value. To this day, no research has shown, in a large population of euthymic patients with bipolar disorder, if a two factors structure of the BIS-11 could have a better predictive value than the classical three factors structure. This is the aim of the present article.

AIMS OF THE STUDY

The aims of this study are to check if 1: a two factors structure of the BIS-11, could be assessed in a reliable and reproducible manner in bipolar disorder patients; 2: if this bidimensional version has a better predictive value than the classical three factor structure and 3: if a specific factorial structure is associated with clinical characteristics of bipolar disorder, such as a history of suicide attempts, mixed states, rapid cycling and substance misuse. These associations could demonstrate the criterion validity of the BIS bi-dimensional version.

MATERIALS AND METHODS

Clinical samples

Five hundred eighty euthymic bipolar patients have been included, among which 305 women (age: m = 41.3, SD= 12.7), 240 men (age: m = 41.6, SD= 13.3) and 35 subjects (6% of the sample) for which gender was not recorded (age: m = 40.9, SD= 11.7). The patients were recruited through the "FondaMental Advanced Centers of Expertise for Bipolar Disorder" network (FACE-BD). The aims and organization of this network have been described in detail elsewhere [START_REF] Henry | A French network of bipolar expert centres: A model to close the gap between evidence-based medicine and routine practice[END_REF]. JPK. Barratt/IS-12/V2 14/03/2019

The protocol of evaluation was approved by the French ethical committee (CPP-Ile de France IX, January 18th, 2010) and has had an authorization of the "Commission Nationale de l'Informatique et des Libertés" (DR-2011-069). Patients have been systematically informed that their data could be used within the frame of scientific studies but, since the measurements were part of immediate care, a written consent was not necessary.

Inclusion criteria were : to be 16 years of age or over, being diagnosed with bipolar disorder (type I, II or NOS) as described in the Diagnostic and Statistical Manual of Mental Disorders, 4th ed. Text Revised (DSM-IV-TR, American Psychiatric Association, 2000). Patients had to be euthymic and their scores had to be inferior to 8 at the Montgomery and Asberg Depression Rating Scale (MADRS, [START_REF] Montgomery | A new depression scale designed to be sensitive to change[END_REF] and inferior to 6 at the Young Mania Rating Scale (YMRS, [START_REF] Young | A rating scale for mania: reliability, validity and sensitivity[END_REF]. Other bipolar disorder characteristics (rapid cycling, history of mixed states, etc.) and comorbidity (substance use disorders) were also diagnosed according to DSM-IV-TR.

Data collection

Based on the Structured Clinical Interview for DSM-IV (SCID: First, Spitzer, Gibbon, & Williams, 2012), a large standardized clinical evaluation was performed to confirm bipolar disorder diagnosis, as well as psychiatric comorbidities and history of mood disorder (i.e.: age at onset, number and polarity of past episodes). Moreover, sociodemographic factors were collected (age, gender, education). A standardized clinical interview was used to assess manic symptoms with the YMRS and depressive symptoms with the MADRS. Evaluations were performed by senior clinical psychiatrists and psychologists.

Barratt Impulsiveness Scale

The French version of the Barratt Impulsiveness Scale, version 10 (BIS-10) was used, since it is the latest validated version in French to this day [START_REF] Bayle | Factor analysis of french translation of the Barratt impulsivity scale (BIS-10)[END_REF][START_REF] Patton | Factor structure of the Barratt impulsiveness scale[END_REF]. JPK. Barratt/IS-12/V2 14/03/2019

This 10 th version comprises 34 items but, in Patton's analysis (1995), four of the items of the BIS-10 did not meet their criteria to be included in the final version: BIS-11. Therefore, these four items of the BIS-10 were not included in our analysis. The BIS-11 is a thirty items selfreport questionnaire which includes nineteen items indicating impulsivity, which are answered on a 4-point scale ("rarely or never" = 1; "occasionally"= 2; "often"= 3; "almost always" = 4) and 11 items indicating non-impulsivity, which are scored in a reverse order ("rarely or never" = 4; …). The BIS-11 is considered to evaluate three major factors: attentional (8 items), motor (11 items) and non-planning impulsivity (11 items). High scores indicate high levels of the different dimensions of impulsivity.

Statistical analysis

Scores and internal consistency coefficients of the BIS-11

The internal consistency of the total score and the three subscores described by [START_REF] Patton | Factor structure of the Barratt impulsiveness scale[END_REF] were computed. An alpha reliability coefficient ≥ .70 was considered as satisfactory, according to [START_REF] Nunnally | Psychometric theory[END_REF].

Fit of the original three factor model:

A Confirmatory Factor Analysis (CFA) of the BIS-11 was computed to check if the original three factor structure of the BIS-11 fitted in our sample. The CFA model was fitted with package Lavaan (Rosseel, 2012) for R statistical software, with the diagonal weighted least squares (WLSMV) estimation method [START_REF] Beauducel | On the Performance of Maximum Likelihood Versus Means and Variance Adjusted Weighted Least Squares Estimation in CFA 37-41[END_REF][START_REF] Brown | Confirmatory Factor Analysis for Applied Research[END_REF]. The variance of the first indicator of each latent variable was set to 1, to ensure model identification.

As an indication of a fit of a good model, the following indicators were used across this study, based on the Satorra-Bentler χ² : the Comparative Fit Index (CFI: cut-off >.90), the Tucker-Levis-Index (TLI: cut-off >.90), Root Mean Square Error of Approximation (RMSEA: < .08 for a fair fitting model or <.05 for a well-fitting model) and the Weighted Root Mean Square Residual (WRMR, cut-off < .90;[START_REF] Mcdonald | Test theory: A unified treatment[END_REF][START_REF] Yu | Evaluating cutoff criteria of model fit indices for latent variable models with binary and continuous outcomes[END_REF]. The same statistical criteria (CFI, TLI, RMSEA and WRMR) were used to study the fit of all the other CFA models. In the first sample, a series of analysis were performed to select the items which could constitute a better factorial model. First, a hierarchical cluster analysis of the items (with package Psych, Revelle, 2014) was performed to study the clusterization of the items. The items used in the following Exploratory Factorial Analysis (EFA) were selected by studying the coefficients of reliability of the different clusters (coefficients alpha and beta >.70).

Thereafter, an EFA was used to find the best factorial solution: the items with loadings above 0.32 on a unique factor after an oblimin rotation were selected [START_REF] Tabachnick | Multivariate statistics[END_REF]. Afterwards, a CFA was performed to check if the newly defined bi-dimensional structure fitted according to the statistical criteria (CFI, TLI, RMSEA and WRMR).

The last CFA was then repeated in the second sub-sample (see supplementary material for detailed description of the statistical analysis), in order to test if the scale constructed step by step in the first sub-sample could be validated. JPK. Barratt/IS-12/V2 14/03/2019

Predictive validity of the new scale for specific clinical variables

In order to test the predictive validity of the new scale, we studied its association with the specific clinical variables, previously studied in [START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF]: residual mood symptoms, past clinical episodes, suicidal behavior, substance use disorder.

The correlation between residual symptomatology and the subscores of our new scale were computed. For sub-depression, subjects with a MADRS < 8 were included and for hypomania subjects with a YMRS <6 were included.

A succession of linear regressions was computed with successively, as dependent variables the two scores at the new scale ("cognitive impulsivity": F1 and "behavioral impulsivity": F2). The following independent variables were used successively as quantitative predictors: number of past mixed, depressive and manic episodes. The suicide attempts were predicted with a hurdle regression model with the five subscores of the two scales (BIS-11 and our new scale) as predictors, to detect the most important predictor(s) (Frees, 2009).

For the two scale's sub-scores, the means obtained by the following clinical groups were compared: rapid cycling vs. non rapid cycling, alcohol use disorder (dependence and abuse) vs. no alcohol use disorder, cannabis use disorder (dependence and abuse) vs. no cannabis use disorder. MANOVA (with the two subscores as dependent variables) followed by one-way ANOVA and Tukey's HSD tests were used to identify exact sources of variations. All differences were considered as significant at a p < .05 level.

Finally, to compare our results with the ratings obtained by [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF], Pearson correlations between our two subscores and Reise et al.'s two factor scores were computed.

Our variables comprised few missing data (less than 5 %); therefore the listwise deletion method was chosen. JPK. Barratt/IS-12/V2 14/03/2019

The software R 3.2 (R Development Core Team, 2015) was used for all the analyses.

RESULTS

Socio-demographic and clinical variables

The main clinical characteristics of the 580 patients are presented in Table 1.The mean age of patients at interview was 41.4 years (± 12.9, range 16-86). The median age at onset of illness was 18 years (±10.7, range 7 -66). The proportion of suicide attempters was 36.2 % among the total population, 42.0 % among women and 25.5 % among men. The mean number of past suicide attempts for suicide attempters differed significantly according to gender (t = 2.74; p < .01): it was higher for women: 2.58 (± 2.87, range 1-20), than for men: 1.80 (± .98, range 1-5).

(Insert Table 1 about here)

Scores and internal consistency of the BIS-11

On the whole sample (N=580), the mean total score of the BIS-11 was 64.4 (± 10.2, range 42-99, median = 63). The alpha reliability coefficient of the total scale and the original three modifications (freeing of covariance between residuals of the variables), an acceptable fit was not obtained.

Selection of the items constituting reliable dimensions of impulsivity from the BIS-11 in the first half of the sample

Step 1: Hierarchical clustering of items Some clusters of items were identified in the first half of our sample. A large cluster of items (n=18) with good psychometric reliability (α = 0.83; β = 0.79) was identified. These items belong to two clusters which aggregate in one higher level cluster (cf. figure S1 in supplementary material).

Step 2: Exploratory Factorial Analysis In order to determine the loading on each factor of these 18 items, an EFA was performed with two factors, as suggested by the results of the previous analysis. The results are presented as factorial results (loadings after rotation, cf. Table 4). The items number 2, 5, 7, 10 and 13 were excluded (loadings < 0.32).

Therefore, we obtained a structure with two dimensions: the first was called "cognitive impulsivity" and the second "behavioral impulsivity" (see discussion for explanations).

(Insert Table 2 about here)

Step 3: Fit in CFA of the newly defined structure in the first half of the sample This model was then tested in CFA on the first half of the sample, in order to find the best fitting model. Item 26 was removed, as it loads on the dimension "behavioral impulsivity" (loading = 0.433), but is also highly correlated with the score of "cognitive impulsivity" (r = .43), thus lacking specificity. A new analysis of the data without item 26 was performed JPK. Barratt/IS-12/V2 14/03/2019

To summarize, our results confirm that impulsivity could be reliably explored with a scale comprising 12 items from the BIS-11, aggregating in two correlated but distinct dimensions.

We named this new scale "IS-12" (for: 12 items Impulsivity Scale). We kept Reise et al.'s denomination and named its two dimensions "cognitive impulsivity" (F1) and "behavioral impulsivity" (F2).

Construct validity and clinical utility of the 12 items impulsivity bidimensional scale

To obtain the score on the two factors, the scores at each item of each dimension were summed and a total score was obtained for the two dimensions: "cognitive impulsivity" (F1) and "behavioral impulsivity" (F2).

Correlations with residual symptoms:

The scores of these two dimensions (F1 and F2) from the IS-12 correlated significantly with residual depressive symptoms (MADRS, F1: r = .21; p < .001; F2: r= .11; p < .001), but only "behavioral impulsivity" was correlated with residual hypomanic symptoms (YMRS, r = .11; p < .01).

Relationship with presence or absence of rapid cycling and past clinical episodes:

Neither rapid cycling nor past episodes were associated with any of the scores at our 12 items impulsivity scale (all p > .10).

Relationship with suicidal attempts:

A hurdle regression model with "cognitive impulsivity", "behavioral impulsivity" (both from IS-12) and the classical three subscores of the BIS-11 as predictors of the number of past suicide attempts showed that "behavioral impulsivity" only was linked to the history of suicide attempts and with the number of suicide attempts, whereas other variables were not significantly associated with suicidal attempts (p < .05).

Substances use disorders:

The diagnostic of alcohol dependence disorder was associated with greater mean scores on cognitive impulsivity and on behavioral impulsivity subscales from IS-12 than in the group without alcohol use disorder (p < .05); other differences were not significant (see table S2).

The diagnostic of cannabis dependence disorder was associated with greater scores on the cognitive impulsivity and behavioral impulsivity subscales from IS-12 than in the non cannabis use disorder group and the cannabis abuse disorder group; whereas the patients with abuse disorder were not significantly different from patients without cannabis use disorder.

(Insert Table 3 about here)

DISCUSSION

The Barratt Impulsiveness Scale (BIS, 11 th version) is the most widely used scale to assess impulsivity, but its total score is used more frequently than its three sub-scores, according to the original authors [START_REF] Stanford | Fifty years of the Barratt Impulsiveness Scale: An update and review[END_REF]. They advise to use the sub-scores, since impulsivity is clearly a multidimensional concept. But, many researchers did not succeed to confirm the original three second order structure of the scale and rather found a bidimensional structure [START_REF] Haden | A Comparison of Factor Structures of the Barratt Impulsiveness Scale in a Mentally Ill Forensic Inpatient Sample[END_REF][START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF]. Even the total score, sometimes, failed to be associated with suicidal behavior [START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF][START_REF] Olié | Clinical and neuropsychological characteristics of euthymic bipolar patients having a history of severe suicide attempt[END_REF][START_REF] Swann | Increased impulsivity associated with severity of suicide attempt history in patients with bipolar disorder[END_REF]. If the three dimensional structure is not reliable, then this strongly impacts the usefulness of these sub-scores. The present study confirms previous studies and the lack of replicability of the initially proposed three dimensional structure of the BIS, 11 th version. It introduces a new impulsivity JPK. Barratt/IS-12/V2 14/03/2019 scale with only 12 items (named IS-12), aggregating in two dimensions, which we named "cognitive impulsivity" (5 items) and "behavioral impulsivity" (7 items). They correlated highly with the rating of [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF], who proposed these denominations. These two dimensions of impulsivity present a strong content proximity to those regularly described in previous studies [START_REF] Haden | A Comparison of Factor Structures of the Barratt Impulsiveness Scale in a Mentally Ill Forensic Inpatient Sample[END_REF][START_REF] Ireland | Impulsivity among adult prisoners: A confirmatory factor analysis study of the Barratt Impulsivity Scale[END_REF][START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF].

Cognitive impulsivity seems to involve the anticipation of the result of actions and therefore be closer to the concept of "non-planed impulsivity" found in the previous analysis of the BIS-11 (Haden and Shiva, 2009). This cognitive impulsivity dimension comprises the same items as dimension 1 of [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF] except item 7, which did not load significantly in dimension 1 in our study. Behavioral impulsivity seems to be dependent of an excessive sensitivity to present stimuli and is therefore close to "motor impulsivity", as found by [START_REF] Haden | A Comparison of Factor Structures of the Barratt Impulsiveness Scale in a Mentally Ill Forensic Inpatient Sample[END_REF]. Compared to [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF], the "behavioral impulsivity" dimension shares only 4 items among the 7 proposed by Reise et al., but the dimension "behavioral impulsivty" in Reise et al. and in our study correlated highly. Interestingly, the same two dimensions appeared in Reise's and our studies, although using different statistical methodologies and different populations (non-clinical in [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF] and clinical in our study). Our psychometrical methodology allowed us to select 12 items among the original thirty, which reliably assess two distinct dimensions of impulsivity. This study highlights, once more, the multidimensional aspect of impulsivity and suggests that a reliable assessment of sub-dimensions of impulsivity is possible and useful. The remaining 18 items of the BIS-11 were discarded from the new scale, since they did not segregate in a coherent separate dimension, nor segregate distinctively between the two dimensions F1 and F2.

The free covariances added between residuals of items 22 ("I buy things on impulse") and 25 ("I spend or charged more than I earned") could be explained since they explore the same facet of impulsivity. We suppressed item 26 of the BIS-11, because it loaded simultaneously on both dimensions of the new scale. Furthermore, its suppression allowed a better fit of the model and a better distinction between the two dimensions (cognitive impulsivity and behavioral impulsivity).

The IS-12 showed a correlation between subclinical depressive symptoms and both cognitive and behavioral impulsivity dimensions, but only on the behavioral impulsivity dimension for hypomanic symptoms. Therefore, even if subjects were euthymic according to stringent criteria (MADRS< 8 and YMRS < 6), the IS-12 allowed us to detect subtle relationships between impulsivity and subclinical symptoms. This result underscores the sensitivity of the new scale and the importance of residual mood symptomatology in the persistence of a strong impulsivity, during inter-episodic periods.

The association between rapid cycling and impulsivity did not appear, even though this result was found in a previous publication of our group [START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF]), but in a different sample.

The presence and number of past suicide attempts only correlated with "behavioral impulsivity". Suicidal behavior, in a sub-group of bipolar patients, could be an impulsive behavior, since behavioral impulsivity has a strong relationship with the immediate psychological condition of the patient. This phenomenon could contribute to explain why suicide is frequent in patients with bipolar disorder. These patients often present two particularly dangerous features when associated: an acute awareness of psychological pain during depression, associated with a general hyper-emotionality [START_REF] Chan | Coping with amplified emotionality among people with bipolar disorder: A longitudinal study[END_REF][START_REF] Henry | Inhibition/activation in bipolar disorder: validation of the Multidimensional Assessment of Thymic States scale (MAThyS)[END_REF] and a strong tendency to act impulsively [START_REF] Etain | Clinical features associated with trait-impulsiveness in euthymic bipolar disorder patients[END_REF][START_REF] Olié | Clinical and neuropsychological characteristics of euthymic bipolar patients having a history of severe suicide attempt[END_REF].

These two characteristics (general emotional dysregulation and impulsivity) have been found associated with suicidal behavior in a general population of 2295 students among which were 108 past suicidal attempters [START_REF] Ammerman | Suicidal and violent behavior: The role of anger, emotion dysregulation, and impulsivity[END_REF]. Our results confirm the relevance of our new scale, since the original three subscores of the BIS-11 were not associated to JPK. Barratt/IS-12/V2 14/03/2019 suicidality in the hurdle regression models. The results of this study suggest the potential utility of the IS-12 to predict suicidality in patients with bipolar disorder. Future studies will be necessary to test the IS-12 in various clinical populations, where impulsivity is frequent.

LIMITATIONS

From a metrological perspective, a weakness of the IS-12 is the formulation used in the assessment of its both dimensions. In the behavioral impulsivity dimension, the items all have the same rating system (the higher the score, the higher impulsivity), while items of the cognitive dimension are all reverse-rated. But we demonstrated that behavioral and cognitive impulsivity related differently to suicidality.

Another limit is intrinsic to our study: our population comprised only euthymic bipolar disorders patients. It would be of clinical interest to test the IS-12 in healthy controls and other clinical populations.

STRENGTHS

A strength of this study is the size of the cohort (N total = 580), which allowed us to split the sample in two sufficiently large sub-samples. The minimal size of a sample to use factor analysis is around three hundred, according to [START_REF] Rouquette | Sample size requirements for the internal validation of psychiatric scales[END_REF], which corresponds approximately to the size of each sub-sample (N=290).

This large sample allowed us to use up to date statistical methodology and therefore to use specific methods which take into account, in all phases of the study, the ordinal scale of the responses to items from the BIS-11 (Likert scale; see supplementary material).

Another strength of the study comes from the use of strict criteria to define bipolar diagnosis, using semi-structured clinical interviews (SCID-I; DSM-IV-TR criteria) performed by JPK. Barratt/IS-12/V2 14/03/2019 experienced clinicians and the strict definition of euthymic states in patients (MADRS < 8 and YMRS < 6). Therefore, the sample was highly homogenous.

CONCLUSIONS

This study did not support a three factors structure of the BIS-11, as published by [START_REF] Patton | Factor structure of the Barratt impulsiveness scale[END_REF]. In line with the contributions of [START_REF] Haden | A Comparison of Factor Structures of the Barratt Impulsiveness Scale in a Mentally Ill Forensic Inpatient Sample[END_REF] and [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF], our results support a structure with two dimensions that we name "cognitive impulsivity" and "behavioral impulsivity". We present a new twelve items impulsivity scale (IS-12), with the same criterion validity for substance use disorder and better criterion validity for suicidal behavior than the 30 items Barratt Impulsiveness Scale. We demonstrated that the behavioral impulsivity subscale only is related to the presence and number of previous suicidal attempts, thus emphasizing the usefulness of the distinction between cognitive and behavioral impulsivity. Therefore, we recommend that, besides the total score, both sub-scores be analyzed separately in future impulsivity research. Indeed, impulsivity is highly correlated to suicidal behavior and patients with bipolar disorder are at high risk for suicide. Therefore, a short scale reliably assessing impulsivity, could be a useful instrument to assess risk factors of suicidality. Longitudinal studies are required to investigate further the predictive validity of the IS-12 in other healthy and clinical populations.

Bibliography:

American Psychiatric Association, 2000. Diagnostic and statistical manual of mental disorders: DSM-IV-TR. American Psychiatric Association. I plan for job security 0.213 0.039 Eighteen items, among 30 of the original BIS-11, were retained after a hierarchical cluster analysis. Among those, 13 had loadings superior to 0.32 (in bold) When two groups present no statistically significant difference, they share the same letter (i.e. "a" or "b"), whereas when two groups present statistically significant difference at a p < .05 level, they are associated with different letters ("a" versus "b").

  of the best factorial structure of the BIS-11: construction of a new scale Since, in our analysis, these indices showed a really poor fit of the original three-dimensional factorial model of the BIS-11, the sample was randomly split in two sub-samples of 290 subjects. On the first sub-sample, we searched step-by-step for a better factorial structure of the BIS. The second half of the sample allowed us to check directly if the newly identified scale's structure could be replicated, since in the first sub-sample the psychometric characteristics of the scale could have been due to try-and-error processes.

  subscales of the BIS-11 were the following: total score: α = 0.802, 95%CI [0.776; 0.824]; motor impulsivity: α = 0.551, 95% CI [0.484; 0.615]; attentional impulsivity: α = 0.713, 95% CI [0.675; 0.753]; non-planning impulsivity: α = 0.642, 95% CI [0.590; 0.683]. As observed, only attentional impulsivity and the total score obtained an alpha superior to 0.70. 4.3. Fit of the BIS-11 original factorial structure The original three dimensional structure of the BIS-11 did not fit in the CFA model: CFI=0.675; TLI=0.648; RMSEA = 0.145; 95% CI [0.141-0.149], WRMR= 2.745. These results indicated a strong misfit of the original model in our sample. The modification indices were computed in an attempt to maximize the fit of the model, but, even after seven steps of JPK. Barratt/IS-12/V2 14/03/2019

Table 1 :

 1 Clinical characteristics of the total cohort Presence during at least one major mood episode ** Lifetime history *** Lifetime history for patients with at least one suicide attempt

	Variables	% or Means (SD) N = 580
	Gender (Men / Women / missing)	41.4% / 52.6% / 6.0%
	BD subtype (I/II/NOS)	55.7% /33.2% /11.1%
	Age at onset (years)	20.68 (10.65)
	Age at interview (years)	41.39 (12.87)
	Duration of ilness (years)	22.49 (12.32)
	Number of hospitalizations	2.9 (2.9)
	Psychotic features (yes / missing)*	28.4% / 30.2%
	Suicide attempt (Total/ Women / Men) **	36% / 42.0% / 25.5%
	Suicide attempts***	2.34 (2.38)
	Number of Depressive Episodes	4.18 (3.27)
	Number of Manic Episodes	1.43 (2.02)
	Number of Hypomanic Episodes	6.03 (5.52)
	Number of Mixed Episodes	0.33 (1.06)
	Alcool abuse / dependant	9.8% / 5.0%
	Cannabis abuse / dependant	7.1% / 4.8%
	*

Table 2 :

 2 Analysis of the two dimensional structure of the

		Item
		number	Item' content	F1	F2
	Items with main	Item1	I plan tasks carefully	0.481	-0.08
	loading on	Item 8	I am self controlled	0.445	0.028
	F1 > 0.32	Item 9	I concentrate easily	0.55	0.048
	"cognitive	Item 12	I am a careful thinker	0.761	-0.021
	impulsivity"	Item 20	I am a steady thinker	0.659	0.046
		Item 6	I have "racing" thoughts	-0.14	0.367
		Item 14	I say things without thinking	-0.011	0.478
	Items with main	Item 17	I act "on impulse"	0.054	0.74
	loading on	Item 19	I act on the spur of the moment	-0.073	0.586
	F2 > 0.32	Item 22	I buy things on impulse	-0.034	0.614
	"behavioral impulsivity"	Item 24 Item 25	I change hobbies I spend or charge more than I earn	0.024 0.053	0.347 0.344
		Item 26	I often have extraneous thoughts when thinking	0.158	0.433
		Item 2	I do things without thinking	0.071	0.179
	Items with loading < 0.32 on both dimensions	Item 5 Item 7 Item 10	I don't "pay attention" I plan trips well ahead of time I save regularly	0.236 0.236 0.211	0.16 -0.046 0.187
		Item 13

Table 3 :

 3 Dimensions of impulsivity in bipolar patients with and without substance use disorder, as evaluated with the 12 items Impulsivity Scale

	Alcohol	No Alcohol use disorder	Abuse	Dependence
	IS12 Cognitive impulsivity	10.89 (±2.94) a	11.84 (±3.32) b	12.79 (±2.57) b
	IS12 Behavioral impulsivity	13.79 (±3.51) a	14.47 (±3.88) a,b	15.41 (±4.38) b
	Cannabis	No cannabis use disorder	Abuse	Dependence
	IS12 Cognitive impulsivity	11.03 (±2.94) a	11.15 (±2.82) a	13.64 (±3.13) b
	IS12 Behavioral impulsivity	13.82 (±3.54) a	14.00 (±4.10) a	16.07 (±3.73) b

IS-12: our 12 items revised Impulsivity Scale Results are expressed as means ± standard deviation.

JPK. Barratt/IS-12/V2 14/03/2019

Therefore, the structure was replicated in the second half of our sample.

A model without covariance between "cognitive impulsivity" and "behavioral impulsivity" (orthogonal model) was computed and a badly fitting model was obtained according to our criteria; therefore, the covariance between the two factors is necessary to the good fit of our model.

The rating of [START_REF] Reise | The Barratt Impulsiveness Scale-11: Reassessment of its structure in a community sample[END_REF] was used to compute correlations between their dimensions and our dimensions. The scores at the "cognitive impulsivity" following our rating and at the same dimension following Reise's rating were highly significantly correlated (r = .96; p < .0001) and the scores at the "behavioral impulsivity" according to our rating and at the same dimension following Reise's rating were equally highly significantly correlated (r = .85; p < .0001).


Maria Ikhennicheu
email: maria.ikhennicheu@ifremer.fr

Grégory Germain
email: gregory.germain@ifremer.fr

Philippe Druault
email: philippe.druault@sorbonne-universite.fr

Benoît Gaurier

Experimental study of coherent flow structures past a wall-mounted square cylinder

Keywords: Turbulence, Experimental trials, Wall-mounted cylinder, PIV, LDV, POD, Lamb-Oseen

In high flow velocity areas like those suitable for tidal applications, turbulence intensity is high and flow variations may have a major impact on tidal turbines behaviour. Large boils that can be observed at the sea surface are emitted from the sea floor and may interact with the tidal turbine. These boils have then to be characterized. The Reynolds number, based on the rugosity height and the mean flow velocity, is rather high in this context: R e = 2.5 × 10 7 . For that purpose, experiments are carried out in a flume tank with R e as high as achievable in Froude similitude (in the tank: Re = 2.5 × 10 5 and F r = 0.23) in order to study coherent flow structures emitted behind seabed obstacles. The obstacle is here a canonical square wall-mounted cylinder chosen to be representative of specific in-situ bathymetric variations. Using PIV and LDV measurements, the flow past the cylinder is investigated. Using a POD filter, large coherent structures are identified and their trajectories are analysed. By means of a Lamb-Oseen profile approximation, properties of these structures are determined. The formation mechanism of such structures is discussed in this paper and their behaviour is characterized. It is assumed that vortices periodically shed from the obstacle interact and generate Hairpin structures.

Introduction

Industrial projects have started to emerge in tidal energy harvesting.

There is a strong tidal potential in Europe and over 25% of the European potential is located in French waters [START_REF] Edf | Énergies marines hydrolienne et houlomotrice[END_REF]]. The area of interest is the Alderney Race that presents currents up to U insitu = 5m/s. In-situ, strong velocity variations exist [START_REF] Myers | Simulated electrical power potential harnessed by marine current turbine arrays in the alderney race[END_REF]]. Such variations are causing a high turbulence rate in the water column. Some turbulent events can sometimes rise up to the surface and erupt to create a boil. Turbulence can have a major impact on the tidal turbines behaviour, on their production [START_REF] Medina | Turbulence analysis and multiscale correlations between synchronized flow velocity and marine turbine power production[END_REF]] and on the structural fatigue [START_REF] Mycek | Experimental study of the turbulence intensity effects on marine current turbines behaviour. Part I: One single turbine[END_REF]]. Before trying to reproduce complex bathymetric structures, we chose to introduce the topic by studying elementary obstacles representative of real life condition: i.e. with an aspect ratio (A R = Width/Height) of the magnitude of the mean bathymetry variations encountered between France and Alderney. Surveys performed in this area give access to the bathymetry (see Fig. 1) showing an average depth variation of H insitu = 5m. Some bathymetry variations have been represented using elementary elements like wall-mounted cubes and an inclined floor representing smooth depth variations [Ikhennicheu et al. (2018a)]. The wake past a wall-mounted cube has been described and results show that the addition of an inclined floor downstream of the cube impulses turbulent structures higher than the simple floor translation. However, the floor inclination does not have an impact on the nature of turbulent structures emitted from the obstacle wake. The aspect ratio impact on the wake development has been investigated in [START_REF] Ikhennicheu | Experimental study of the wake past cubic wall-mounted elements to predict flow variations for tidal turbines[END_REF]], showing that for an aspect ratio 7 times supe-rior, the recirculation length triples.

The matter of interest of this work is the development of the wake and the analysis of turbulent structures past large A R obstacles. The case of A R = ∞ is a classical example of a 2D obstacle in the literature. Among the firsts to ever characterize the flow past a 2D block (=rib), [START_REF] Castro | Relaxing wakes behind surface-mounted obstacles in rough wall boundary layers[END_REF]] characterized its wake in a flow at R e = 1.5 × 10 4 . More recently, [START_REF] Liu | Unsteady separated and reattaching turbulent flow over a two-dimensional square rib[END_REF]] carried out experiments on a rib in a turbulent flow for R e = 1.3×10 4 . They give a detailed explanation of the intermittency aspect of the reattachment of the recirculation region. They also found that at x/H = 15.75 in the obstacle wake, the streamwise velocity is not fully recovered, illustrating the wake extension past larger A R obstacles. Indeed, at this location behind a 3D wall-mounted cube [START_REF] Hearst | Effect of turbulence on the wake of a wall-mounted cube[END_REF]], the wake of the obstacle is fully dissipated. Finally, the experimental works of [START_REF] Wang | Turbulent flow in a ribbed channel: Flow structures in the vicinity of a rib[END_REF]] offer a topology of the fluid past a rib at R e = 2.2×10 4 . Detailed investigation on the turbulent organisation past a vertical cylinder can be found in the studies of [START_REF] Sattari | On the vortex dynamics in the wake of a finite surface-mounted square cylinder[END_REF]] although different mechanism are expected for a horizontal cylinder. These studies on 2D ribs are completed by numerical works such as the one of [START_REF] Panigrahi | Multi-modal forcing of the separated shear flow past a rib[END_REF]] who simulated a turbulent flow at R e = 1.4 × 10 4 on 2D square ribs. For obstacles with high A R , the two ends of the horseshoe vortex are farther apart and have smaller influence on the middle region of the wake, unlike for small A R obstacles where they interact and cause the 3D behaviour of the wake. Intermediate A R were experimentally studied by [START_REF] Martinuzzi | The flow around surface-mounted, prismatic obstacles placed in a fully developed channel flow[END_REF]]. They found that for A R ≥ 6, the recirculation region downstream of the cube switches from a 3D aspect to a nominally 2D aspect. The main differences between 2D and 3D square obstacles are extensively described by [START_REF] Schofield | Turbulent shear flow over surface mounted obstacles[END_REF]].

Few of the studies exposed in this bibliographic review present cases with R e > 10 5 . It is generally assumed that, for R e > 4 × 10 4 , the flow achieves a certain Reynolds number independency as found by [START_REF] Castro | The flow around a surface-mounted cube in uniform and turbulent streams[END_REF]]. However, that conclusion has been questioned in [START_REF] Lim | Bluff bodies in deep turbulent boundary layers: Reynolds-number issus[END_REF]] where it is explained that for vortex-dominated flows, mean flow is likely to be Re-dependant. The Reynolds number, based on H insitu and U insitu is substantially high: R e,insitu = H insitu U insitu /ν = 2.5 × 10 7 , with ν the kinematic viscosity of water. Hence, there is a necessity to extend the existing database for marine energy application through high Reynolds number experiments.

Few studies are lead on the far field of wide wall-mounted obstacles in a high Reynolds number flow and especially with a focus on the turbulent structures that are emitted from the wake. Large turbulent events can have a major impact on turbines [START_REF] Mycek | Experimental study of the turbulence intensity effects on marine current turbines behaviour. Part I: One single turbine[END_REF]], hence the need to investigate them. In many cases in the literature, coherent turbulent structures can be observed past an obstacle or in a turbulent boundary layer. One of the most extensive analysis of structures emitted from turbulent boundary layers is the work of [START_REF] Adrian | Vortex organization in the outer region of the turbulent boundary layer[END_REF]]. They detailed the creation, detection and transport of hairpin structures in the flow. For wide square plates, the numerical study of [START_REF] Diabil | Coherent structures and flow topology of transitional separated-reattached flow over two and three dimensional geometrical shapes[END_REF]] for R e = 6.5 × 10 3 shows that Kelvin-Helmholtz vortices are shed from the upstream face of the obstacle. Periodically shed vortices create corotating pairs that lead to vortex pairing as described by the annual review of [START_REF] Leweke | Dynamics and instabilities of vortex pairs[END_REF]]. Their analysis uses a Lamb-Oseen model for vortex profile fitting as will be done here. [START_REF] Müller | On the vortex formation in the mixing layer behind dunes[END_REF]] demonstrate that these vortices interact or merge to create hairpin structures. These hairpins are sometimes called Kolk vortices in other studies [START_REF] Best | Kinematics, topology and significance of dune-related macroturbulence: some observations from the laboratory and field[END_REF]]. Such structures are very energetic and can rise up to the surface to form a large boil [Best (2005a)]. [START_REF] Volino | Turbulence structure in a boundary layer with two-dimensional roughness[END_REF]] studied coherent structures created over a rough bottom. They show that their size varies with the upstream flow conditions however the rising angle is conserved. [START_REF] Sabatino | Tomographic PIV measurements of a regenerating hairpin vortex[END_REF]] proposed tomographic PIV measurements of hairpin vortex developing in laminar boundary layers. They illustrate how hairpin vortices regenerate with the creation of secondary hairpin vortices. This kind of turbulent structures shall be characterized in order to predict their trajectory and frequency as well as the incoming velocity profile on tidal turbines.

In this paper, coherent structures developing from the wake of a wallmounted cylinder representative of seabed element is studied. The experimental set up is first presented and test cases are detailed. Then, the characterization of the wake past the cylinder is carried out from experimental results in and out of the symmetry plane. Vortices are studied and a method to detect their centre and track their position is proposed. A Lamb-Oseen vortex approximation is proposed for the determination of the vortex properties. A temporal analysis is added to the turbulent structures analysis. Finally, a discussion on the turbulent organization formed in the obstacle wake is proposed.

Experimental set-up

The tests are carried out in the wave and current circulating flume tank of Ifremer located in Boulogne-sur-Mer (France) presented in Fig. 2(a). The test section is: 18m long × 4m wide × 2m high. In this work, the three instantaneous velocity components are denoted (U, V, W ) along the (X, Y, Z) directions respectively (Fig. 2(b)). Each instantaneous velocity component is separated into a mean value and a fluctuating part according to the Reynolds decomposition: U = U + u , where an overbar indicates the time average. The incoming flow (U ∞ , V ∞ , W ∞ ) is assumed to be steady and constant. By means of a grid and a honeycomb (that acts as a flow straightener) placed at the inlet of the working section (see Fig. 2(b)), a turbulent intensity of I = 1.5% is achieved for a flow velocity of 1m/s. Turbulence intensity I in the incoming flow is defined as follows: The wall-mounted cylinder is chosen to be representative of real-life conditions. As there is no obstacle upstream of the cylinder: the upstream flow is a simple boundary layer developing over the tank floor. Hence the experimental set-up represents a bump considerably higher than its neighbours. To consider turbulent events interaction with the free surface, experiments shall be achieved in Froude similitude (see Table 1) where F r = U∞ √ gD , with g the gravity and D the tank depth. Furthermore, Reynolds number must be as high as achievable to be closer to real conditions. The obstacle of interest in this study is a wall-mounted cylinder of aspect ratio A R = Width/Height = 6 represented in Fig. 3. This element represents a key bathymetric element in the area of interest at a 1:20 scale (see Table 1). In the following study, non dimensional lengths are used for all parameters indexed by * : x * = x/H, y * = y/H, z * = z/H and δ * = δ/H, with H the cylinder height (H = 0.25 m). At the obstacle position, the bound-ary layer height δ * is calculated as δ * 95 = z * (U = 0.95U ∞ ). It yields δ * = 1.3. To characterize the flow, two Laser Velocimetry techniques are used: LDV (Laser Doppler Velocimetry) and PIV (Particle Image Velocimetry) (see Fig. 3). Beforehand, the tank is seeded with 10 µm diameter silver coated glass micro-particles. For the PIV measurements, a Nd-yag Laser Gemini-Like is used: power is 200 mJ per probe and wavelength is 532 mm. The laser is synchronized with a Camera FlowSens EO-2M 1600pix × 1200pix that makes double images with a time step of 1600 µs. A particle is detected on 3 to 5 pixels. Cross-correlation peak intensity is between 0.3 and 0.8 and peak detectability (ratio of the tallest correlation peak and the second tallest correlation peak [START_REF] Westerweel | Particle image velocimetry[END_REF]) is 8 in average. PIV acquisitions are made for 150s, hence 2250 double images are taken with a 15H z acquisition frequency. The data are post processed with the software Dynamic Studio. The displacement of particles is calculated using a Cross-Correlation on 32pix × 32pix interrogation windows with 50% overlap [START_REF] Meinhart | A parallel digital processor system for particle image velocimetry[END_REF]]. Outliers are replaced with the Universal Outlier Detection [START_REF] Westerweel | Universal outlier detection for PIV data[END_REF]]. Depending on the plane, from 1% to 13% of the total vectors number are substituted. Singlepass method is used as, in our case, multi-pass adaptive PIV leads to errors generating artefacts rather than improving resolution. PIV measurements are carried out at various transverse positions: y * = 0, y * = 1 and y * = 2 (denoted y0, y1 and y2 respectively) summarized in Table 2 and represented in Fig. 4(b). Fig. 4(a) shows the 13 measurement planes carried out for each transverse position. In the tank, using a wide angle lens for a large surface detection, only vertical PIV measurement planes can be achieved. Hence, only 2D measurements of (U, W ) will be performed in this study. The experimental set-up allows measurements for z * ∈ [0.5; 7]. Fig. 5 illustrates the post-processing steps applied in the plane y0: from the raw image to the Universal Outlier Detection (UOD). The Cross-Correlation (CC) results are obtained with a mask applied on the cylinder. The LDV measurements are made using a 2D DANTEC FiberFLOW system. The probe is positioned horizontally for (U, V ) measurements at various streamwise positions along the Z axis. With LDV measurements, the acquisition frequency is not constant. It depends on the number of particles passing through the measurement volume. Then, a re-sampling is done in the post processing. Based on previous works performed in the tank [START_REF] Medina | Correlation between synchronised power and flow measurements, a way to characterize turbulent effects on a marine current turbine[END_REF]], the re-sampling is done using the mean sample rate of the measurements set considered. At a specific streamwise position, f e varies from 70 to 270 H z depending on the turbulent agitation.

I = 100 1 3 (u 2 + v 2 + w 2 ) U ∞ 2 + V ∞ 2 + W ∞ 2 (1)
Various sources of experimental errors can be identified. Positions fixed using the human eyes are considered to have a maximum error of 0.5 cm and 0.5 • ; it is the case for the obstacle and the lasers positioning. Particles motion follows the Basset-Boussinesq-Oseen equation; for silver-covered glass particle, the response of particle is of 0.14 mm/s. The laser sheet induces a magnification that causes an error of 0.3% [START_REF] Westerweel | Particle image velocimetry[END_REF]]. Measurement standard deviation goes up to 0.1%. Calibration is achieved by the LDV manufacturer. Additionally, vibration of the LDV probe due to vortex induced vibration can induce an error up to ±0.5 cm. PIV calibration is precise at 1 mm. The pixel size is added to the errors for PIV measurements. Hence, the errors on the results are of 2% and 2.6% for LDV and PIV respectively. A comparison is made in Fig. 6 between PIV and LDV measurements in the symmetry plane showing a good coherency.

Characterization of the wake past the wall-mounted cylinder

Average velocity maps of U and W in plans y0, y1 and y2 are presented in Fig. 7. In these figures, the classical behaviour of the flow past a wallmounted element can be observed: the flow separates at the leading edges of the cylinder into the outer steady region and the recirculation area downstream of the cylinder. A shear layer develops in-between and then the flow reattaches. The recirculation length downstream of the obstacle is evaluated for the 3 planes: l y * 0 = 6, l y * 1 = 5.5 and l y * 2 = 4.5. As expected, the apparition of 3D effects mainly impacts planes for y > y1: the l * reduction between y0 and y1 is 8% and l * reduction increases to 18% between y1 and y2. In [START_REF] Martinuzzi | The flow around surface-mounted, prismatic obstacles placed in a fully developed channel flow[END_REF]], at a similar R e and A R , a recirculation length of l * = 5.7 has been measured in the symmetry plane, which is close to our results. Maps of the average vertical velocity W out of the symmetry plane indicate that the flow passing over the obstacle is similar in the 3 planes for x * < 2. However, downstream of the cylinder, the flow is considerably redirected towards the floor for position y2 with a large and intense W < 0 zone. In y0, the flow shows an area of W > 0 beginning at x * = 6 from the floor up to z * ∼ 1. The wake is impulsed towards the surface.

Fig. 8(a) and 8(b) show that, in y0, the transverse component V is relatively unaffected by the cylinder wake. The velocity fluctuations at z * = 2, indicates a large shear stress area for u and w that does not affect v . At z * = 2 for instance, values are U/U ∞ = 0.64 ± 0.2, V /U ∞ = 0.003 ± 0.04 and W/U ∞ = -0.05 ± 0.16.

In order to better understand the development of the wake past the cylinder, average Reynolds shear stress τ uw = u w maps are plotted in Fig. 9. Some areas suffer from the poor lightning and data in these area are not fully converged. However, it appears that the shear layer reduces faster for y1 compared to y0 although the global orientation is similar. In field y2, the shear layer length reduces by half and is reoriented towards the floor. Here the shear layer length is estimated as the length corresponding to the position at which |τ uw /U 2 | < 0.02. This effect is also visible when comparing W maps in Fig. 7. In x * = 7, the shear layer starts disappearing for y2 but persists for the other two planes.

Turbulent structures dynamics

During the experiments performed for this study, large boils are observed at the tank free surface as illustrated in Fig. 10 (taken 8m past the obstacle). Such events originate in the shear layer past the cylinder and rise up to the surface, located at 2 m from the bottom. These events are highly energetic with a ∼ 1 m diameter. It is the same kind of vortices that can be observed at the sea surface in the Alderney Race. These events evolving in the water column and carrying high velocity variations might have a strong impact on tidal turbines. Their investigation is detailed in this section in order to be able to quantify their effect on the flow impacting a potential turbine. In this part, POD (Proper Orthogonal Decomposition) is used as a filter on PIV measurement results in order to highlight the highest energetic structures and leave aside the non-coherent turbulent part. Then, based on the extracted energetic flow structures, a centre detection algorithm is implemented on POD filtered data in order to characterize the dynamic of turbulent events present in the flow. Structures properties are also investigated using a Lamb-Oseen vortex approximation. POD is an efficient and optimal mathematical tool to extract energetic coherent structures embedded in a turbulent flow. It was first introduced by [START_REF] Lumley | The structure of inhomogeneous turbulent flows[END_REF]]. The snapshot version of POD [START_REF] Sirovich | Turbulence and the dynamics of coherent structures. Part I: Coherent structures[END_REF]] is used in the present work. Details of the implementation of POD technique applied to PIV database can be found in [START_REF] Druault | Use of proper orthogonal decomposition for time interpolation from PIV data. Application to the cycle-to-cycle variation analysis of in-cylinder engine flows[END_REF][START_REF] Druault | POD investigation of the unsteady turbulent boudary layer developing over porous moving flexible fishing net structure[END_REF]]. Following previous works [Ikhennicheu et al. (2018a)], a vectorial POD is performed in this study. In order to determine the number of POD modes (N P OD ) used to filter the large turbulent structures, the effect of POD filter on the fluctuating velocity field is investigated in Fig. 11 during the passing of a vortex in plane B (see Fig. 4). This specific vortex is selected because its centre position is in the middle of the plane and because there is no other vortex close by. In the raw field without filter, a vortex appears but its centre is difficult to properly position. With N P OD ≥ 20, the POD filter is efficient although some turbulent noise is still present and that could hinter the centre detection. With N P OD = 10 rotating velocity is weaker since it is less energetic. Hence the required POD modes number lies between 10 and 20 which corresponds to between 69% and 76% of the total kinetic energy level. Arbitrary, it is chosen to take, in the following, 70% of the total kinetic energy for each measurement plane. For plane B, it corresponds to 13 modes.

Vortex properties

A centre detection algorithm, inspired by the method proposed by [START_REF] Graftieaux | Combining PIV, POD and vortex identification algorithms for the study of unsteady turbulent swirling flows[END_REF]] and adapted for PIV measurements by [START_REF] Druault | Use of proper orthogonal decomposition for time interpolation from PIV data. Application to the cycle-to-cycle variation analysis of in-cylinder engine flows[END_REF]] is here considered. In each point M, the tangential fluctuating velocity V θ is evaluated for the 2 × 8 = 16 neighbours as represented in Fig. 12. If every V θ around M has the same sign, a vortex centre is detected. A sensibility analysis allowed to fix 16 vortices to achieve a good centre detection. To have trajectories long enough for statistic analysis, this study is restricted to detected on at least 7 temporally consecutive detected points, corresponding to a minimal detection time of 7 × ∆ t = 0.47 s. The centre detected with the algorithm is indicated with a red dot in Fig. 13. The full trajectory obtained is represented in Fig. 14. Following previous studies [START_REF] Hamed | Vortical structures in the near wake of tabs with various geometries[END_REF]; [START_REF] Leweke | Dynamics and instabilities of vortex pairs[END_REF]] the head of vortices (supposed to be a hairpin vortex) detected in 2D PIV planes can be modelled thanks to a simplified Lamb-Oseen vortex model following [START_REF] Moriconi | Minimalist turbulent boundary layer model[END_REF]]. Such vortex is defined by equation 2.

V θ (r) = Γ 2πr [1 -exp(- r 2 r c 2 )] (2)
with Γ the circulation and r the radius, r c is the vortex core size (see Fig. 15(b)). Dimensionless values are used: r * c = r c /H and Γ * = Γ/(HU ∞ ). Only vortices that are in the middle of the plane can be properly fitted since a sufficient quantity of data points is necessary. A vortex passing in the middle of plane B is chosen. Its tangential velocity V θ is plotted with r * = r/H ∈ [-2; 2] and θ = 0 • (i.e. z * constant) as illustrated in Fig. 15(a). It is then fitted by a Lamb-Oseen profile in Fig. 15(b). The Lamb-Oseen model is designed for circular symmetrical vortices. Due to a ratio between the size of the vortex and the height of the measurement plane close to 1, it is not possible to perform a Lamb-Oseen vortex approximation for θ = 0 • to θ = 90 • for all vortices to make sure the approximation is valid in every directions. It is only possible for some vortices that are located at mid-height of the plane. Fig. 16 shows two cases of vortices for which the approximation is performed for θ = 0 • and θ = 90 • . Most of the vortices are symmetrical, similar to the one on Fig. 16(a). However, some vortices present an elliptical form which results in the non-identical profiles presented on Fig. 16(b). More precisions on the specific case of elliptical vortices will be given later in the paper.

Analysis of the turbulent structures

The algorithm is applied to every PIV measurement planes. All the trajectories are gathered in Fig. 17 are generated in the beginning of the shear layer and are then emitted in the flow. Close to the obstacle (first PIV planes: x * < 4), it is expected that trajectories detected are Kelvin-Helmholtz vortices. Higher in the water column and further in the flow, detected vortices are expected to resemble hairpin vortices. Trajectories are also presented in plane y1 in Fig. 17(b). Results are similar to those obtained for y0, while in y2, no vortices are detected. It is assumed that the same wide 3D structure is detected on both 2D planes (y0 and y1) as sections of a larger 3D structure, even if vortices detected in one plane are not linked with vortices detected in the contiguous plane. The 3D shape of the vortices is then contained in a 2H × 3H × 2H volume (x,y,z directions). The planes A, B and C contain the most vortices. They are also the most representative of the vortex behaviour since they are located just above the shear layer where turbulent structures are emitted. Fig. 18 illustrates the trajectories detected for planes A, B and C. On plane A, trajectories are impulsed from the shear layer and oriented towards the surface with an angle of ∼ 9 • . All trajectories are localized in the low part of the plane. Then, for planes B and C, trajectories appear more horizontal and more scattered. They are being dispersed and carried out by the flow. It is worth noticing that some trajectories in plane B (in bold in Fig. 18(b)) are particularly oriented towards the surface. It is assumed that these are the trajectories of vortices that might erupt at the surface.

A linear interpolation is achieved on every trajectory in order to study the rising angle γ of each vortex. In average, the mean rising angle γ is found between 6 • to 10 • . It is substantially higher than the average rising angle of the shear layer, which is around 4 • (see Fig. 9(a)). There is a strong dispersion of γ due to the large range of trajectory angles, however the average rising angle reduces when x * increases. The discrepancy of rising angle results in figure 17 might be caused by 3D effects in the wake or by the unsteady interaction of emitted vortices with one another as explained by [START_REF] Müller | On the vortex formation in the mixing layer behind dunes[END_REF]]. The boil represented in Fig. 10 corresponds to a rarely occurred event with an angle of 14 • which is in the higher range of vortex trajectories. This indicates that some trajectories are particularly impulsed towards the surface compared to others. The low rising angle trajectories cannot be observed in the present set up since they would erupt downstream of the working section end (where the boil picture represented in Fig. 10 is taken).

The approximation of the vortex fluctuating tangential velocity with a Lamb-Oseen profile is applied to vortices located in the middle of the measurement plane in order to have enough data points for profile fitting. The superposition of fitted profiles for plane C are plotted in Fig. 19. This shows that the vortex profiles are generally symmetrical about their vertical axis. The radius (r c * ) and the circulation (Γ * ) obtained with the Lamb-Oseen vortex approximation are presented in Table 3 for plane y0. The number of vortices that present enough data points for profile fitting is indicated compared to the total number of trajectories detected in the measurement plane. Standard deviation is around 25% on r c * but higher on Γ * . Nevertheless, global behaviour can be drawn from these results. |Γ * | decreases when x * increases and r c does not variate significantly. Vortices that are emitted from the shear layer weaken due to dissipation effect. In average, vortices have a diameter of 0.30 m (∼ H) in the symmetry plane in the sense of the Lamb-Oseen vortex approximation. However, the vortices fluctuations themselves are present as long as V θ > 0 and in that perspective, the diameter of the vortex velocity perturbations is of the order of 1 m. The Lamb-Oseen modelling is also applied to y1 vortices and averaged results are presented in table 3. Results are within the same range as in plane y0 especially when accounting for the standard deviations. The same global behaviour is observed: Γ reduces when x * increases and the radius r c * does not variate significantly. Nevertheless, both metrics are inferior to those in the symmetry plane. It means that weak 3D effects exist although they cannot be more thoroughly investigated with the available database. Structure are then larger and more energetic in the symmetry plane. It appears to be consistent with hairpin vortices centred around the symmetry plane. Otherwise, some of the trajectories are more oriented towards the surface, with a higher rising angle. On Fig. 18(b), trajectories with the highest rising angle are highlighted. It appears that these trajectories present a common behaviour: while rising up, the vortex shifts from a circular to an elliptical form. Circulation around it becomes more intense. The vortex dynamic of the yellow trajectory in bold in Fig. 18(b) is illustrated in Fig. 21. For this example, the rising velocity is v vtx = 0.8, more than 2 times the horizontal case, with a rising angle of 16.2 • , three times superior. For the nine selected events rising up, averaged results give a radius of 0.64 ± 0.08 and a circulation of -1.7 ± 0.3. In the previous section, Lamb-Oseen vortex approximation gave in average r c * = 0.68 ± 0.2 and Γ * = -1.4 ± 0.4 for all trajectories in plane B. These results confirm the previous observations. Indeed, as the vortex becomes elliptical its horizontal radius r c reduces. Hence, rising angle vortices are more energetic and their shape changes as they extend towards the surface rising up in the water column.

Temporal analysis

Using LDV measurements, a spectral analysis is performed in the symmetry plane y0. Acquisitions last 6 minutes and frequency varies. for x * = 11, the energetic content is high since those points are located in the shear layer. Furthermore, a sharp peak is detected for S t 0.062. Second, there is a transitional part in-between where spectrum do not show any peak. And third, above z * = 3.4 for x * = 7 and above z * = 3.6 for x * = 11, the energetic content is lower and the peak is detected for a higher frequency of S t 0.072. The higher peak S t 0.072 is close to the literature for dunes cases in [START_REF] Omidyeganeh | Large-eddy simulation of twodimensional dunes in a steady, unidirectional flow[END_REF]], it corresponds to the vortex shedding. Periodic events indicated by the lower peak S t 0.062, happening underneath the shed vortices in the shear layer do not impact the higher altitudes of the water column. Therefore, every event occurring in the top part of the water column must originate from vortices shed from the shear layer. The same results was found by [START_REF] Omidyeganeh | Large-eddy simulation of twodimensional dunes in a steady, unidirectional flow[END_REF]] that carried out LES simulation on dunes. Fig. 22(c) shows the presence of a peak in the far field, close to the surface. That measurement point is the only one available for x * > 11. The peak is around S t = 0.072 confirming that the events detected above the shear layer are still present at the same frequency close to the surface and they will presumably erupt at the free surface. The peak intensity is lower than the peak close to the obstacle indicating a lower energetic content. In Fig. 23, a map of instantaneous u w (obtained from PIV measurements) for z * versus t is plotted for a chosen streamwise position at x * = 11. A vortex passing is detected with a high u w > 0 peak followed by u w < 0 with amplitude superior to 0.02. At mid-height (z * = 3), periodic vortex passing is observed. It matches the frequency found in Fig. 22 for vortex shedding. The constant shedding of vortices is visible until z * = 3.5 which is the boundary of the shear layer. Some events are more energetic than others and are still persistent above z * = 4, at t = 40 s for instance. These highly turbulent events that rise higher are more energetic than others. On Fig. 23 and 24, two zones are highlighted. In red, a zone that presents highly turbulent events rising higher than the average turbulent events, they are still visible on the signals at z * = 4. In green, a zone where vortices do not show higher than z * = 3.5. Instantaneous u w signals are plotted in Fig. 24 at the same streamwise position: x * = 11 and for two altitudes: z * = 3 and z * = 4 for the 150 s of the PIV acquisition. This figure illustrates what is explained in the previous section: at z * = 3, vortex shedding induces a periodic fluctuating velocity variation. It occurs at the frequency detected in the spectrum in Fig. 22(b). At a higher position (z * = 4), although the frequency is still visible on the spectrum, it does not show on the signals with the used scale. Some intense events are detected. These are the very energetic events that rise higher than the shear layer and they will be assimilated to hairpin structures in the next section.

Discussion on detected vortices

Looking at previous results, there seems to be two kinds of structures coexisting in the cylinder wake. The presence of the obstacle in a flow promotes the creation of a shear layer and, as in a mixing layer, Kelvin-Helmholtz instabilities are generated from the velocity difference between the outer flow and the recirculation region underneath. These vortices are detected on the edges of the shear layer, close to the obstacle, which is consistent with Kelvin-Helmholtz instabilities. The shedding frequency is detected in the spectral analysis. Then, as demonstrated in [START_REF] Müller | On the vortex formation in the mixing layer behind dunes[END_REF]; [START_REF] Leweke | Dynamics and instabilities of vortex pairs[END_REF]], these events can merge, pair-up or interact to form hairpin structures. It is indeed illustrated by the works of [START_REF] Diabil | Coherent structures and flow topology of transitional separated-reattached flow over two and three dimensional geometrical shapes[END_REF]] and by [START_REF] Chaurasia | Three-dimensional instabilities in the boudary-layer flow over a long rectangular plate[END_REF]], both working on the flow past large plates. Hairpin structures are then more persistent and energetic with a higher rising angle and they are ejected towards the free surface. Structures transverse extension is around 2H = 0.5 m and the resulting boils are around 1 m wide: it implies that hairpin structures generate boil wider than their width when erupting at the surface [Best (2005a)]. Temporal analysis validates the assumption that these structures are hairpin boilgenerating vortices created from Kelvin-Helmholtz vortices interactions in the separated shear layer as detailed by [START_REF] Müller | On the vortex formation in the mixing layer behind dunes[END_REF]]. This assumption opposes to hairpin vortices generated underneath the shear layer from near-wall bursts or reattachment flapping conjectures [START_REF] Kiya | Structures of large-scale vortices and unsteady reverse flow in the reattaching zone of a turbulent separation bubble[END_REF]; [START_REF] Kadota | Three-dimensional structure of space-time correlation on coherent vortices generated behind dune[END_REF]]. Such explanations are consistent with vortices detected in the PIV measurement planes being 2D sections of a larger 3D event. Furthermore, these events are only visible past large A R obstacles. Indeed, in previous works [Ikhennicheu et al. (2018a)], it is shown that in the case of low A R obstacles such as a wall-mounted cube, Kelvin-Helmholtz vortices do not form structures able to rise up to the free surface. It is also explained by [START_REF] Diabil | Coherent structures and flow topology of transitional separated-reattached flow over two and three dimensional geometrical shapes[END_REF]] in their numerical study on large and low A R plates.

Conclusion and perspectives

In this study, the flow past a wide wall-mounted square cylinder of aspect ratio A R = 6 is studied. From PIV measurements, the wake past the obstacle is characterized. Key-elements of the wake past the obstacle are identified. Recirculation length in the symmetry plane is l * = 6, it then reduces by 8% in y1 and 18% in y2. Average Reynolds shear stress maps illustrate the shear layer behaviour, its position is very similar between y0 and y1 whereas in y2 it is redirected towards the floor. The flow behaviour is similar between y0 and y1 and shows significant differences for y2 with the apparition of side effects. At the free surface of the tank, 1 m wide boils can be observed. These events originate in the cylinder wake and rise up to the surface. In order to understand these structures behaviour, PIV measurements are filtered using Proper Orthogonal Decomposition. Based on existing methods, a centre detection algorithm is applied. It allows to elucidate vortices trajectories. No vortex is large or energetic enough to be detected in field y2. Authors conclude that the transverse extend of the vortex lies between y0 and y1, hence from 2H to 4H (0.5 m to 1 m). In average, rising angle of vortices is ∼ 8 • , which is higher than the shear layer rising angle of ∼ 4 • . Velocity profiles are fitted with a Lamb-Oseen profiles showing that the vortices radius stays similar with x * : r * c = 0.69 in y0. However circulation reduces with x * . It is an effect of vortices dissipation. It is then showed that some vortices are impulsed towards the surface. They show a stronger circulation, a larger rising velocity and their shape shifts from a circular to an elliptical form. Finally, a temporal analysis validates that some events rise higher in the water column than others. In the discussion, authors suggest that two phenomena coexist. Kelvin-Helmholtz instabilities develop downstream of the obstacle, in the shear layer, close to the obstacle. Then, these events merge, pair up or interact to form hairpin structures ejected towards the free surface to form large boils. Such boils, that exist at sea can have a major impact on turbines and must be considered in calculations for loads on tidal turbines. The problem investigated here is also under development from a numerical perspective [START_REF] Mercier | Towards the modelling of turbulence at tidal stream power sites with the lattice boltzmann method[END_REF]]. After validation, numerical analysis will offer a 3D understanding of the turbulent phenomenon occurring in the cylinder wake.

Figure 1 :

 1 Figure 1: Bathymetry of the English Channel with a zoom of the Alderney Race area [SHOM (2015)].

Figure 2 :

 2 Figure 2: Ifremer Flume tank description and schematic view of the experimental set-up.

Figure 3 :

 3 Figure 3: Cylinder during PIV measurements.

Figure 4 :Figure 5 :

 45 Figure 4: PIV measurement planes locations in the wake of a wall-mounted cylinder.Origin is taken at the middle of the bottom face of the cylinder.

∞

Figure 6 :

 6 Figure 6: LDV measurements (dotted lines) and PIV measurement (full lines) U /U∞ profiles comparison in y * = 0.

Figure 7 :Figure 8 :

 78 Figure 7: U /U∞ (left) and W /U∞ (right) maps in plane y0 (top), y1 (middle) and y2 (bottom). White parts indicate the different PIV measurement planes separation.

Figure 9 :

 9 Figure 9: τuw/U 2 ∞ maps in plane y0 (left), y1 (middle) and y2 (right).

Figure 10 :

 10 Figure 10: Picture of a boil of diameter ∼ 1m at the tank surface at ∼ 8m of a wallmounted cylinder.

  Figure 11: Effect of the POD modes number on the fluctuating velocity u and w field in the plane B (y0).

M

  Schematic representation of vortex centre detection using tangential fluctuating 12: Figure velocities V θ around a centre located in M for the 16 = 2 × 8 neighbours in red and blue.

Figure 13 :

 13 Figure 13: Vortex centre in plane B (y0) from t = 26s to t = 27.5s.

Figure 14 :

 14 Figure 14: Trajectory of the vortex detected in Fig.13.

  (a) Velocity map with vortex centre detected (red dot). Red dotted line indicates the altitude for V θ profile. (b) POD filtered V θ profile (blue) fitted with a Lamb-Oseen profile (green). r * c (red) is the vortex radius.

Figure 15 :

 15 Figure 15: Lamb-Oseen vortex approximation of a vortex passing in plane B (y0), at t = 58.4s.

  (a) for the plane y0. The top of the shear layer is determined from Fig.9where τ uw > -0.02. This figure shows that vortices

Figure 16 :

 16 Figure 16: Tangential velocity V θ * profiles versus the radius r * of Lamb-Oseen vortex approximation of vortices passing in plane B (y0), at t = 100s (a) and t = 2.7s (b).

Figure 17 :

 17 Figure 17: Vortices trajectories in y0 (top) and y1 (bottom) (black line: top of the shear layer).

  V θ * varies between [-0.21; 0.25]. Velocity profiles are slightly non-symmetrical about their horizontal axis. |V θ * | is larger upstream of the vortex centre, where its value is positive: it is connected to the rising vortices trajectory towards the free surface.

Figure 19 :

 19 Figure 19: Tangential velocity V θ * profiles versus the radius r * fitted with a Lamb-Oseen vortex for plane C in y0 and centred at the inflection point (derivative maximum abscissa set to 0).

  ± 0.1 Γ * (y0) -1.6 ± 0.6 -1.4 ± 0.4 -1.3 ± 0.4 Fitted/Total (y * = 1Most of the vortices whose trajectories are described in Fig.18 have a low rising angle trajectory (γ = 5.3 • ) with circular form as shown in Fig.20. The rising velocity v vtx = || v z ||./U is evaluated here at: v vtx = 0.3.

Figure 20 :

 20 Figure 20: Velocity map of a vortex trajectory with horizontal behaviour between t = 81.3s and t = 82.4s in plane B (y0).

Figure 21 :

 21 Figure 21: Velocity map of a vortex trajectory rising up between t = 54.6s and t = 55.1s in plane B (y0).

  For each streamwise position x * , the lowest average acquisition frequency for each altitude is taken: f e (x * = 7) = 84 Hz and f e (x * = 11) = 75 Hz. Using f e , LDV signals are re-sampled. Signals are then cut into blocks of 1024 points and a Fast Fourier Transform is applied on every bloc. Power Spectrum Densities are then plotted versus the Strouhal number: S t = f H/U ∞ in Fig.22 (a) and (b) in the near field. Altitudes are chosen to represent the flow above, under and at the shear layer position. Three different flow behaviours are detected. First: below z * = 3 in x * = 7 and below z * = 3.6

Figure 22 :

 22 Figure 22: PSD(u ) versus St = f H/U∞ in y0 at x * = 7 (left), x * = 11 (middle) and x * = 19 (right).

Figure 23 :

 23 Figure 23: u w in a z * versus t map in y0 at x * = 11, from PIV measurements.

Figure 24 :

 24 Figure 24: Instantaneous u w signals for (x * = 11, y * = 0, z * = 3) (top) and (x * = 11, y * = 0, z * = 4) (bottom).

Table 1 :

 1 in-situ and experimental conditions (1:20 scale).

		Scale U ∞	Rugosity	Depth D	R e =	F r =
			[m/s] height H [m]	[m]	HU∞ ν	U∞ √ gD
	Alderney Race	1	5	5	40	2.5 × 10 7 0,25
	Flume tank	1/20	1	0.25	2	2.5 × 10 5 0.23

Table 3 :

 3 Lamb-Oseen vortex modelling results.

Acknowledgment

This work benefits from a French State grant managed by the National Research Agency under the Investments for the Future program bearing the reference ANR-10-IEED-0006-11. The authors also acknowledge the financial support of Ifremer and the Hauts de France Regional Council for this PhD study. We are most grateful to Thomas Bacchetti, Inès Belarbi and Jean-Valery Facq for their assistance and precious advices.

Highlights:

Experimental study of coherent flow structures past a wall-mounted square cylinder

Ocean Engineering Editor

Boulogne-sur-mer, 11 th June 2017.

Highlights for paper entitled: Experimental study of coherent flow structures past a wallmounted square cylinder:

-Tests are carried out on a wall-mounted cylinder representative of seabed elements, in Froude similitude with high Reynolds number.

-PIV measurements are performed in vertical measurement planes and spatial analyses are performed.

-POD analysis and center detection allow to study the vortices behaviour.