DAEP seminars 2022

Latent Representation of CFD Meshes and Application to Aerodynamics

• Friday, December 9, 2022 - 11:00 a.m. - room 38.051 - by 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 to the problem of the permanence in flight of drones

• Friday, November 18, 2022 - 11:00 a.m. - room 38.051 - by Jean-Marc Moschetta

Robust trajectory planning for light vehicles in varying and uncertain flows

• Friday, October 21, 2022 - 11:00 am - room 38.051 - by Bastien Schnitzler(presentation)

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 Unmanned Aerial Vehicle : Synthesis of robust control laws

• Friday, September 16, 2022 - 11:00 am - room 38.137 - by Romain Jan

We will discuss the different aerological phenomena that can be exploited on aircraft and the different strategies associated with them: exploitation of lift, gradient flight and exploitation of turbulence. The numerical tool chosen for the thesis will be presented (ASWING), with emphasis on the improvements made as well as the validations on the following proven experimental test cases:

1. Pure aerodynamics : prediction of CL and CD on different geometries with wake interaction and pre-stall behavior (10 cases), unsteady prediction of 2D and 3D lift coefficients with in particular the study of amplitude and reduced frequency as well as a variation of angle of attack (2 cases), effect of the interaction of the wake of a propeller with an airfoil with application on the A400M (CFD case) and finally deflection of the wake of a propeller by an airfoil.
2. Pure structure: study of nonlinear static deflections under point load on a set of blades and wings with variable fiber arrangement (7 cases) and modal response on a vibrating pot for all cases
3. Aerostructure : static torsional divergence velocity on 3 distinct cases, control reversal velocity on trapezoidal wing, right wing flutter velocity (3 cases) with the impact of the angle of attack and stall, impact of the position of a concentric mass on the flutter speed of a wing, impact of the precession effects of a propeller on the flutter of a straight wing (2 cases - Whirl Flutter) and study of the oscillations of limit cycles, post flutter behavior (3 cases)

The seminar will end with the presentation of the results of the following works: the possibility of exploiting orographic waves on a Boeing 737, the effect of fiber arrangement on the propulsive effects of a straight wing subjected to gusts and the use of automatic tools for the optimal placement of sensors on a flexible wing.

Characterization of acoustic shock wave propagation in an urban environment

• Friday, May 20, 2022 - 11:00 am - room 38.137 - by Samuel Deleu(presentation)

The ability to accurately determine the location of acoustic sources is of interest to the DGA. However, most of the current localization methods are based on the assumption of linearity (first-order approximation of the Euler equations) of the recorded acoustic signal and thus remain limited to the hypothesis of small pressure fluctuations around the steady state. These methods are nevertheless used on the basis of highly non-linear impulse signals (explosions, supersonic booms or sniper shots). The amplitudes of these signals are large enough that the quadratic terms of the propagation equations are no longer neglected. The result is a distortion of the signal: as the wave propagates, it becomes steeper, resulting in the formation of a discontinuous front. The characteristic “N” shape of these acoustic shock waves is due to the presence of a gradual relaxation following the compression phase. In addition to the non-linear effects of the propagation, the effects of the interactions of these waves with the structures are added, giving rise to more complex patterns of reflections which it is important to take into account. These non-linear signals give rise to localization errors that it is interesting to quantify. The main objective is therefore to identify and quantify the non-linearities of propagation and interactions from recorded signals in order to be able to propose corrections or at least to identify the configurations in which these non-linearities are preponderant on the accuracy of the source localization.

Modification of a turbulent boundary layer by circular cavities

• Friday, May 13, 2022 - 11:00 a.m. - room 38.137 - by 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

• Friday, March 18, 2022 - 11:00 a.m. - room 38.137 - by Thomas Jaroslawski(presentation)

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 an 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 damping of the convective growth of modal instabilities in the LSB.

Aeroacoustic study of low Reynolds number rotors using LES

• Friday, March 11, 2022 - 11:00 a.m. - room 38.137 - by Dhanush Vittal-Shenoy(presentation)

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

• Friday 4 February 2022 - 11:00 - room 38.051 - by Aleksandar Joksimovic