arXiv subjects

Caroline Braud

Publications and source records attributed to Caroline Braud.

7 recordsLinked to original sources

arXiv2026

Stall cells over an airfoil. Part 1: Three-dimensional flow organisation and vorticity dynamics

This study investigates the three-dimensional organisation and evolution of stall cells in the separated flow region over an airfoil. Using a hybrid RANS/LES approach based on the DDES-SST turbulence model, we characterise the formation and development of these structures, which remain challenging to capture experimentally. Initial validation confirms accurate reproduction of global loads when comparing with both experimental data and RANS simulations. The complex three-dimensional flow organisation is analysed through investigating the vorticity, revealing that spanwise variation of the separation location leads to non-uniform load distribution along the airfoil span. The mid-span experiences premature separation due to flow bifurcation, while flow attraction at $\pm1$ chord length successfully sustains attached flow further along the chord. The separated flow generates a shear layer culminating in a separation vortex tube, which exhibits a Crow-type instability when interacting with the counter-rotating trailing edge vortex tube. This instability induces a wave-like bending of the vortex tubes and shear layer, generating significant vertical vorticity (y-vorticity) that drives spanwise flow. We identify a previously unreported phenomenon where the maxima of spanwise velocity structures exhibit rotation around fixed spanwise axes, with the rotation angle evolving linearly with downstream distance according to $\zeta = 14.5(x/c) - 0.8$. This study provides new insights into the mechanisms underlying stall cell formation and highlights the importance of three-dimensional effects in separated flows, which has implications for aerodynamic load prediction and control strategies.

physics.flu-dyn↗

arXiv2026

Stall cells over an airfoil. Part 2: A vortex-based analytical model for their formation and saturation

Stall cells are spanwise-periodic flow structures that spontaneously form on airfoils operating near stall, fundamentally altering the aerodynamic loading distribution. Despite decades of experimental observations, a complete theoretical framework connecting vortex dynamics to the characteristic flow patterns has remained elusive. In this work, we develop an analytical model for stall cell formation based on the interaction between finite-length, counter-rotating vortex tubes representing the separation vortex and trailing-edge vortex. Linear stability analysis of the coupled vortex system yields the growth rate and wavelength selection of the Crow-type instability responsible for the wave-like bending of the vortex structures. A weakly nonlinear analysis using the method of multiple scales is performed to derive the Stuart--Landau amplitude equation, providing an explicit expression for the saturation amplitude at which nonlinear effects arrest the instability growth and establish quasi-steady cellular structures. The vortex sheet representing the separated shear layer is coupled to the vortex tube dynamics through the Birkhoff--Rott equation, from which we derive the induced vertical vorticity $\Omega_y$ that drives the alternating spanwise velocity characteristic of stall cells. The model predicts quantitatively the spanwise velocity magnitude, vertical vorticity distribution, and vortex sheet deformation. The resulting framework provides a unified, first-principles description connecting the Crow-type instability of counter-rotating vortex tubes to the observed flow topology of stall cells. The model is validated against the DDES simulation data presented in the companion paper, demonstrating strong agreement.

physics.flu-dyn↗

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.

physics.flu-dyn↗

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.

physics.flu-dyn↗

arXiv2024

Aerodynamic Performance and Impact Analysis of a MEMS-Based Non-Invasive Monitoring System for Wind Turbine Blades

Wind power generation plays a crucial role in transitioning away from fossil fuel-dependent energy sources, contributing significantly to the mitigation of climate change. Monitoring and evaluating the aerodynamics of large wind turbine rotors is crucial to enable more wind energy deployment. This is necessary to achieve the European climate goal of a reduction in net greenhouse gas emissions by at least 55% by 2030, compared to 1990 levels. This paper presents a comparison between two measurement systems for evaluating the aerodynamic performance of wind turbine rotor blades on a full-scale wind tunnel test. One system uses an array of ten commercial compact ultra-low power micro-electromechanical systems (MEMS) pressure sensors placed on the blade surface, while the other employs high-accuracy lab-based pressure scanners embedded in the airfoil. The tests are conducted at a Reynolds number of 3.5 x 10^6, which represents typical operating conditions for wind turbines. MEMS sensors are of particular interest, as they can enable real-time monitoring which would be impossible with the ground truth system. This work provides an accurate quantification of the impact of the MEMS system on the blade aerodynamics and its measurement accuracy. Our results indicate that MEMS sensors, with a total sensing power below 1.6 mW, can measure key aerodynamic parameters like Angle of Attack (AoA) and flow separation with a precision of 1{\deg}. Although there are minor differences in measurements due to sensor encapsulation, the MEMS system does not significantly compromise blade aerodynamics, with a maximum shift in the angle of attack for flow separation of only 1{\deg}. These findings indicate that surface and low-power MEMS sensor systems are a promising approach for efficient and sustainable wind turbine monitoring using self-sustaining Internet of Things devices and wireless sensor networks.

eess.SP↗

arXiv2023

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.

physics.flu-dyn↗

arXiv2016

Robust and fast online identification of streamwise vortices properties for closed-loop control purposes

We propose to combine the active vortex generators with the particle image velocimetry (PIV) measurements and post-processing streamwise vortex characterization algorithms into a feedback based closed-loop control system for wind turbine applications. We develop two vortex identification and characterization methods that use PIV realizations for the purpose of a real-time (online or on-the-fly) feedback-based control. Both methods can extract centers and strengths of streamwise vortices generated behind active vortex generators in a turbulent boundary layer flow, and we show how to integrate those in a closed-loop control strategy. For demonstration purposes we use stereoscopic PIV measurements at the wind tunnel facility obtained in the transverse-wall-normal plane behind active vortex generators. A robust algorithm is using the $Q$-criteria and the integration of vorticity of each extracted vortex. Results show that a moving window average of a small number of instantaneous fields is nevertheless needed for increased robustness. The robust method requires the full field PIV computation followed by spatial derivatives calculations. A faster method is developed, which, using only horizontal lines of vertical velocity, has a high potential to significantly cut down the computational effort relative to the robust method. We compare the two methods and discuss their shortcomings and the potential for the real-time, online, closed-loop control of turbulent boundary layers of the wind turbine blades.

physics.flu-dyn↗