arXiv subjects

Bérengère Podvin

Publications and source records attributed to Bérengère Podvin.

8 recordsLinked to original sources

arXiv2026

Tracking stall cell dynamics at high Reynolds numbers

The spanwise organization of the flow over a thick airfoil is investigated using surface pressure measurements for a range of angles of attack around maximum lift and high Reynolds numbers (1 Million). Locally strong pressure fluctuations, which are not detected in the global lift coefficient, are shown to be associated with the presence of a stall cell. The stall cell width is of the order of the chord length and increases linearly with the angle of attack, with a weak dependence on the Reynolds number. Its dynamics at Reynolds numbers larger than 1 Million is dominated by a coherent motion in the spanwise direction with a characteristic velocity of order tenth of the freestream velocity. The motion can be decomposed into a large-scale, low-frequency sweep with a Strouhal number equal to 0.001 combined with faster, smaller-scale oscillations. The coherence of the stall cell makes it possible to track global dynamics from local measurements.

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arXiv2025

Sparse surface pressure-based reconstruction of the flow around a thick airfoil over a range of angles of attack

We present an efficient neural-based approach to estimate the instantaneous flow field around an airfoil from limited surface pressure measurements. The model, denoted SNN-POD, relies on two independent shallow neural networks to predict the instantaneous flow over a wide range of angles of attack [10{\textdegree},20{\textdegree}]. At all angles the global model correctly recovers the average characteristics of the flow from single-time sensor data, thus allowing combination with local, angle-dependent models. The method is applied to 2D URANS simulations of a thick airfoil at a Reynolds number of Re=4.5e6. The training set consists of snapshots obtained from a coarse sampling (1-2{\textdegree}) of the angle of attack range. A variance-based criterion is used to determine the number and positions of sensors. Tests are carried out for unseen snapshots at angles of attack within the set (sampled angles) as well as outside the set (interpolated angles). The maximum MSE error of attack for sampled and interpolated angles is respectively 2.9% and 6.6%. This makes it possible to develop adaptive strategies to improve the estimation if necessary.

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arXiv2024

Study of the wall pressure variations on the stall inception of a thick cambered profile at high Reynolds number

We present an experimental study of the aerodynamic forces on a thick and cambered airfoil at a high Reynolds number 3.6 x 10^6, which is of direct relevance for wind turbine design. Unlike thin airfoils at low chord-based Reynolds numbers, no consistent description currently exists for the stall process on such airfoils. We consider two chord-wise rows of instantaneous wall pressure measurements, taken simultaneously at two spanwise locations over a range of angles of attack. We show that around maximum lift conditions, a strong asymmetry is observed in the statistics of the normal force on each chord. In this range of angles of attacks, the pressure fluctuations are largest in the adverse pressure gradient region, and the fluctuation peak along the chord is systematically located directly upstream of the mean steady separation point, indicating intermittent flow separation. Moreover, the fluctuations are characterized by bi-stability in both space and time: for each spanwise location, large excursions of the local wall pressure between two different levels can be observed in time (jumps), and these excursions are highly anti-correlated between the two spanwise locations (spatial switches). The characteristic time scale for the switches is found to be well correlated with the amplitudes of the fluctuations. Application of Proper Orthogonal Decomposition (POD) analysis to each row of sensors confirms that the flow separation is an inherently local, three-dimensional and unsteady process that occurs in a continuous manner when the angle of attack increases. The correlation between the dominant POD mode amplitudes is found to be a good indicator of bi-stability. Most of the fluctuations can be captured with the two most energetic POD modes. This suggests that force fluctuations near the maximal lift could be modelled by a low-order approach, for monitoring and control purposes.

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arXiv2024

On the analysis of Rayleigh-Bénard convection using Latent Dirichlet Allocation

We apply a probabilistic clustering method, Latent Dirichlet Allocation (LDA), to characterize the largescale dynamics of Rayleigh-Bénard convection. The method, introduced in Frihat et al. 2021, is applied to a collection of snapshots in the vertical mid-planes of a cubic cell for Rayleigh numbers in the range [106, 108]. For the convective heat flux, temperature and kinetic energy, the decomposition identifies latent factors, called motifs, which consist of connex regions of fluid. Each snapshot is modelled with a sparse combination of motifs, the coefficients of which are called the weights. The spatial extent of the motifs varies across the cell and with the Rayleigh number. We show that the method is able to provide a compact representation of the heat flux and displays good generative properties. At all Rayleigh numbers the dominant heat flux motifs consist of elongated structures located mostly within the vertical boundary layer, at a quarter of the cavity height. Their weights depend on the orientation of the large-scale circulation (LSC). A simple model relating the conditionally averaged weight of the motifs to the relative strength of the corner rolls and of the large-scale circulation, is found to predict well the average LSC reorientation rate. Application of LDA to the temperature fluctuations shows that temperature motifs are well correlated with heat flux motifs in space as well as in time, and to some lesser extent with kinetic energy motifs. The abrupt decrease of the reorientation rate observed at 108 is associated with a strong concentration of plumes impinging layers onto the corners of the cell, which decrease the temperature difference within the corner structures. It is also associated with a reinforcement of the longitudinal wind through formation and entrainment of new plumes.

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arXiv2023

Dissipation-based proper orthogonal decomposition of turbulent Rayleigh-Bénard convection flow

We present a formulation of proper orthogonal decomposition (POD) producing a velocity-temperature basis optimized with respect to an $H^1$ dissipation norm. This decomposition is applied, along with a conventional POD optimized with respect to an $L^2$ energy norm, to a data set generated from a direct numerical simulation of Rayleigh-Bénard convection in a cubic cell ($\mathrm{Ra}=10^7$, $\mathrm{Pr}=0.707$). The data set is enriched using symmetries of the cell, and we formally link symmetrization to degeneracies and to the separation of the POD bases into subspaces with distinct symmetries. We compare the two decompositions, demonstrating that each of the 20 lowest dissipation modes is analogous to one of the 20 lowest energy modes. Reordering of modes between the decompositions is limited, although a corner mode known to be crucial for reorientations of the large-scale circulation is promoted in the dissipation decomposition, indicating suitability of the dissipation decomposition for capturing dynamically important structures. Dissipation modes are shown to exhibit enhanced activity in boundary layers. Reconstructing kinetic and thermal energy, viscous and thermal dissipation, and convective heat flux, we show that the dissipation decomposition improves overall convergence of each quantity in the boundary layer. Asymptotic convergence rates are nearly constant among the quantities reconstructed globally using the dissipation decomposition, indicating that a range of dynamically relevant scales are efficiently captured. We discuss the implications of the findings for using the dissipation decomposition in modeling, and argue that the $H^1$ norm allows for a better modal representation of the flow dynamics.

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arXiv2021

On the relationship between the base pressure and the velocity in the near-wake of an Ahmed body

We investigate the near-wake flow of an Ahmed body which is characterized by switches between two asymmetric states that are mirrors of each other in the spanwise direction. The work focuses on the relationship between the base pressure distribution and the near-wake velocity field. Using direct numerical simulation obtained at a Reynolds number of 10000 based on incoming velocity and body height as well as Bonnavion and Cadot's experiment (Bonnavion et al. 2018), we perform Proper Orthogonal Decomposition (POD) of the base pressure field. The signature of the switches is given by the amplitude of the most energetic, antisymmetric POD mode. However switches are also characterized by a global base suction decrease, as well as deformations in both vertical and lateral directions, which all correspond to large-scale symmetric modes. Most of the base suction reduction is due to the two most energetic symmetric modes. Using the linear stochastic estimation technique of Podvin et al. 2018, we show that the large scales of the near-wake velocity field can be recovered to some extent from the base pressure modes. Conversely, it is found that the dominant pressure modes and the base suction fluctuation can be well estimated from the POD velocity modes of the near-wake.

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arXiv2020

Coherent structure identification in turbulent channel flow using Latent Dirichlet Allocation

Identification of coherent structures is an essential step to describe and model turbulence generation mechanisms in wall-bounded flows. To this end we present a clustering method based on Latent Dirichlet Allocation (LDA), a generative probabilistic model for collections of discrete data. The method is applied for structure identification to the instantaneous Reynolds stress in turbulent channel flow at moderate Reynolds number R τ = 590. LDA computes a robust flow description in terms of a hierarchy of vertically connected structure fragments, the characteristics of which scale with the wall distance, in agreement with the wall-attached eddy hypothesis of Townsend (1961). We show that the model can be used for field reconstruction with a complexity that can be compared with that of Proper Orthogonal Decomposition (POD). It can also be used to generate synthetic fields, the statistics of which mimic those of the original database. These findings highlight the potential of LDA for data analysis, compression and generation.

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arXiv2019

Spatio-temporal Proper Orthogonal Decomposition of turbulent channel flow

An extension of Proper Orthogonal Decomposition is applied to the wall layer of a turbulent channel flow (Re τ = 590), so that empirical eigenfunctions are defined in both space and time. Due to the statistical symmetries of the flow, the igenfunctions are associated with individual wavenumbers and frequencies. Self-similarity of the dominant eigenfunctions, consistent with wall-attached structures transferring energy into the core region, is established. The most energetic modes are characterized by a fundamental time scale in the range 200-300 viscous wall units. The full spatio-temporal decomposition provides a natural measure of the convection velocity of structures, with a characteristic value of 12 u τ in the wall layer. Finally, we show that the energy budget can be split into specific contributions for each mode, which provides a closed-form expression for nonlinear effects.

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