Séminaires DAEP 2022

Latent Representation of CFD Meshes and Application to Aerodynamics

• Vendredi 9 décembre 2022 - 11h00 - salle 38.051 - par Wei Zhen

I will present our work on the latent model for CFD meshes. We aim to develop a mesh representation that eliminates handcrafts and provides full differentiation during object parameterization and manipulation. To this end, we design an auto-decoder with latent space to learn geometric prior and propose a regularization to preserve mesh quality. The talk includes technical details and various applications on 2D airfoils.

Contributions au problème de la permanence en vol des drones

• Vendredi 18 novembre 2022 - 11h00 - salle 38.051 - par Jean-Marc Moschetta

Robust trajectory planning for light vehicles in varying and uncertain flows

• Vendredi 21 octobre 2022 - 11h00 - salle 38.051 - par Bastien Schnitzler (présentation)

The Mermoz challenge at ISAE-SUPAERO consists in making an Unmanned Air Vehicle (UAV) cross the Atlantic between Dakar and Natal autonomously and without en-route CO2 emissions. The large-scale trajectory planning for this UAV (3000km, 40 hours of flight) is challenging because the airspeed is of the same order of magnitude as the wind speed, and furthermore the wind varies in time and is only known probabilitiscally. Time-optimality, energy-optimality or risk-optimality are relevant criteria for this problem and require an adapted modeling of the problem. In this presentation, I will come up with the work done so far on time-optimal navigation in steady and deterministic wind fields after one year of PhD thesis. I will also describe future work on energy-optimality and stochastic wind fields.

Energy harvesting on flexible Unmaned Aerial Vehicule : Synthesis of robust control laws

• Vendredi 16 septembre 2022 - 11h00 - salle 38.137 - par Romain Jan

Nous aborderons les différents phénomènes aérologiques exploitables sur les aéronefs et les différentes stratégies qui leurs sont associées : exploitation d’ascendances, vol de gradient et exploitation de turbulences. L’outil numérique retenu pour la thèse sera présenté (ASWING), en mettant l’accent sur les améliorations apportées ainsi que les validations sur des cas tests expérimentaux éprouvés suivants :

1. Aérodynamique pure : prédiction de CL et CD sur différentes géométries avec interaction de sillage et comportement pré-décrochage (10 cas), prédiction instationnaire des coefficients de portance 2D et 3D avec en particulier l’étude de l’amplitude et fréquence réduite ainsi que d’une variation d’angle d’attaque (2 cas), effet de l’interaction du sillage d’une hélice avec une surface portante avec application sur l’A400M (cas CFD) et enfin déviation du sillage d’une hélice par une surface portante.
2. Structure pure : étude des déflexions statiques non linéaires sous charge ponctuelle sur un ensemble de pâles et ailes à agencement de fibres variable (7 cas) et réponse modale sur pot vibrant pour l’ensemble des cas
3. Aérostructure : vitesse de divergence statique en torsion sur 3 cas distincts, vitesse d’inversion de commande sur aile trapézoïdale, vitesse de mise de flottement d’aile droite (3 cas) avec l’impact de l’angle d’attaque et du décrochage, impact de la position d’une masse concentrique sur la vitesse de mise en flottement d’une aile, impact des effets de précession d’une hélice sur la mise en flottement d’une aile droite (2 cas - Whirl Flutter) et étude des oscillations de cycles limites, comportement post mise en flottement (3 cas).

Le séminaire se terminera sur la présentation des résultats des travaux suivants : la possibilité d’exploitation d’ondes orographiques sur Boeing 737, l’effet de l’agencement des fibres sur les effets propulsifs d’une aile droite soumise à des rafales et l’utilisation des outils de l’automatique pour le placement optimal de capteurs sur une voilure souple.

Caractérisation de la propagation d’ondes de choc acoustiques en milieu urbain

• Vendredi 20 mai 2022 - 11h00 - salle 38.137 - par Samuel Deleu (présentation)

La capacité à déterminer avec précision la localisation de sources acoustiques est d’intérêt pour la DGA. Toutefois, la plupart des méthodes de localisation actuelles reposent sur les hypothèses de linéarité (approximation au premier ordre des équations d’Euler) du signal acoustique enregistré et restent ainsi limitées à l’hypothèse de fluctuations de pression faibles autour de l’état d’équilibre. Ces méthodes sont malgré tout utilisées sur la base de signaux impulsionnels (explosions, booms supersoniques ou de tirs de sniper) fortement non-linéaires. Les amplitudes de ces signaux sont suffisamment importantes pour que les termes quadratiques des équations de propagation ne soient plus négligés. Il en résulte une distorsion du signal : à mesure que l’onde se propage, elle se raidit, engendrant la formation d’un front discontinu. Il est alors question d’ondes de choc acoustiques dont la forme caractéristique en « N » tient de la présence d’une détente graduelle à la suite de la phase de compression. Aux effets non-linéaires de la propagation, s’ajoutent les effets des interactions de ces ondes avec les structures, donnant lieu à des schémas de réflexions plus complexes dont il est important de tenir compte. Ces signaux non-linéaires donnent ainsi lieu à des erreurs de localisation qu’il est intéressant de quantifier. L’objectif principal est donc d’identifier et de quantifier les non-linéarités de propagation et d’interactions à partir de signaux enregistrés pour être en mesure in-fine d’en proposer des corrections ou à minima d’identifier les configurations dans lesquelles ces non-linéarités sont prépondérantes sur la précision de la localisation de source.

Modification of a turbulent boundary layer by circular cavities

• Vendredi 13 mai 2022 - 11h00 - salle 38.137 - par Francesco Scarano

Skin friction drag accounts for more than 50% of the overall drag for an aircraft in cruise condition. Reducing the skin friction drag would then have a strong impact on the CO2 emissions reductions since 1% of drag reduction is converted into a 0.75% in fuel-burn savings. The skin friction drag is mainly generated by the turbulent boundary layer grazing over the surfaces of the aircraft. In a turbulent boundary layer, the coherent structures are responsible for the majority of the turbulent kinetic energy production and transport in the near wall region and the skin friction drag. During the so-called bursting process, the Reynolds shear stress is generated. The Reynolds shear stress is responsible for accelerating the flow near the wall, which leads to an increase of the mean flow in the vicinity of the wall. As a result the mean velocity gradient at the wall increases and subsequently the wall shear stress and the skin friction increase. The control of this near-wall activity would lead to a potential skin friction reduction.

It is shown how well-chosen perforations in a wall flow can locally reduce skin friction drag by modifying the generation of bursts in the boundary layer. For this purpose, a detailed experimental boundary layer investigation of the flow past a perforated plate, complemented with large eddy simulations, is carried out and compared to the smooth case. The experimental techniques employed are hot wire anemometry and Particle Image Velocimetry (PIV). The perforated plate is obtained with an array of flush-mounted circular cavities. These cavities are disposed in a periodic staggered arrangement. For the three tested flow velocities, the momentum thickness based Reynolds number varies from Reθ = 1830 to 3380 and the cavity diameter and spacing in wall units respectively from d+ =130 to 250 and L+ = 587 to 1075, the latter being identical in both spanwise and streamwise directions. The mean velocity profiles evidence a thickening of the viscous sublayer and a decrease of the friction velocity as compared to the smooth wall case. The application of the Variable Interval Time Averaging (VITA) technique highlights an upward shift of the bursts from the wall and an attenuation of the average burst intensity and duration. Spanwise measurements evidence an overall bursts attenuation despite the lack of spanwise uniformity. The three dimensional (3D) mean flow topology arising from the large eddy simulations provides evidences of the qualitative similarities between the current setup and the spanwise wall oscillations.

The modal and non-modal growth of disturbances in a laminar separation bubble subjected to freestream turbulence

• Vendredi 18 mars 2022 - 11h00 - salle 38.137 - par Thomas Jaroslawski (présentation)

Laminar separation bubbles (LSBs) are common features in low Reynolds number flows, and can have considerable performance impacts on applications such as Unmanned Aerial Vehicles (UAVs), with the global fleet of UAVs in urban environments projected to drastically increase in the coming years. Therefore, the impact of freestream perturbations on flows relevant to UAVs is of current interest. Boundary layer measurements using hotwire anemometry are employed to study the flow development of an LSB over the suction side of a NACA0015 aerofoil, at a chord based Reynolds number of 125 000 fixed and at an angle of incidence of 2.3 degrees, in a open circuit wind tunnel subjected to a wide range of turbulence intensity levels. An increase in the level of freestream turbulence intensity, advances the transition position, decreasing the size of the bubble, with its eventual elimination at the highest levels. Local Linear Stability Theory (LST) is shown to accurately model incipient disturbance growth, unstable frequencies and eigenfunctions for configurations subjected to levels of turbulence up to 3%, suggesting modal growth of instabilities, even at elevated levels of turbulence. Additionally, the presence of streaks is highly probable for configurations with freestream turbulence levels greater than 1%, with unfiltered wall-normal disturbance profiles agreeing remarkably well with theoretical optimal perturbation profiles. This observation leads to the conclusion that the co-existence of both modal and non-modal convective growth of instabilities is present in the LSB. Furthermore, increasing the freestream turbulence intensity resulted in the range of unstable frequencies to decrease and the Reynolds number dependence to increase due the inflection point shifting closer towards the wall. This suggests that a viscous, rather than an invicid formulation of the stability equations is appropriate for modeling modal instabilities in the fore portion of the current LSB, especially in the presence of freestream turbulence. It is ascertained that streaks (non-modal instability) modify the mean flow field, resulting in an increased importance of viscosity, which contributes to the the damping of the convective growth of modal instabilities in the LSB.

Aeroacoustic study of low Reynolds number rotors using LES

• Vendredi 11 mars 2022 - 11h00 - salle 38.137 - par Dhanush Vittal-Shenoy (présentation)

Usage of unmanned air vehicles (UAV) and drones can range from simple recreational to military activities. Along with increased interests in air-taxis in recent years, the regulatory laws on noise are deemed to become more and more stringent. With electric motors becoming more silent, various studies have shown that the rotor blades are the main noise sources. In order to mitigate these sources, it is paramount to understand the noise mechanisms in low Reynolds number and low Mach number regimes. Rotors in these regimes face various flow phenomena such as Laminar Separation Bubbles (LSB), Laminar-to-Turbulent transition (LTT) etc.
Depending on the configuration, some of the main components of unsteady loading noise from an isolated rotors are blade-vortex interaction noise (BVI) and blade self-noise. BVI noise occurs when the blade interacts with the wake from the previous blade ; and blade "self-noise" which is the interaction between the blade and the turbulence produced by its own boundary layer. Present study deals with the numerical investigation of flow-acoustic interactions of such a rotor using wall-resolved Large Eddy Simulation (LES).

Two Decades of European Incentives for (Sustainable) Civil Aviation Research, a Review

• Vendredi 4 février 2022 - 11h00 - salle 38.051 - par Aleksandar Joksimovic