Validation d'un banc d'essais reproduisant les rafales de vent sur véhicule terrestre. Caractérisation de l'écoulement par TR-PIV

Les phénomènes aérodynamiques instationnaires apparaissant autour de véhicules terrestres lors de rafales latérales, demeurent encore mal connus de par les difficultés, tant expérimentales que numériques, à reproduire fidèlement ces situations en laboratoire. L’ISAE a développé un banc d'essai, inspiré des travaux de Ryan et Dominy (Ryan et Dominy [2000]), dans lequel le déferlement de la rafale sur la maquette automobile est reproduit par un jet latéral mobile longitudinalement. L’écoulement généré par ce dispositif est validé PIV résolue en temps dans ce présent document. L’angle de dérapage obtenu lors d’une simulation de rafale présente une évolution temporelle proche du créneau avec un plateau compris voisin de 25°

Etude de l'évolution instationnaire de l'écoulement entourant un véhicule soumis brusquement à un vent latéral

Les techniques de PIV résolue en temps et de PIV stéréoscopique sont appliquées autour de corps automobiles soumis à un coup de vent latéral. Ainsi, l’évolution instationnaire des efforts aérodynamiques peut être interprétée en lien avec la topologie de l’écoulement. On montre en particulier que la réponse instationnaire du véhicule est dominée par le transitoire de la partie arrière. Le développement de la structure tourbillonnaire d’axe longitudinal apparaissant du côté sous le vent du véhicule soumis à un dérapage apparaît jouer un rôle important dans la réponse instationnaire de efforts aérodynamiques

Aerodynamic performances of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models. Additional tests were conducted with geometry modifications as spoilers and underbody diffusers

Forces and flow structures evolution on a car body in a sudden crosswind

A vehicle driver is commonly exposed to strong side air flows, for example when passing through a wind gust. The aerodynamic efforts generated in these situations may induce undesired lateral deviations, which can lead to dramatic effects, if the driver is surprised. In order to simulate a sudden yaw angle change on a moving vehicle, a double wind tunnel facility, adapted from the one of Ryan, Dominy, 2000. Wake Surveys Behind a Passenger Car Subjected to a Transient Cross-wind Gust. SAE Technical Paper No. 2000-01-0874 is developed. Two Windsor car body models, differing from their rear geometry, are analysed. The transient evolution of the side force and yaw moment aerodynamic coefficients are interpreted in connection with the unsteady development of the flow, based on TR-PIV and stereoscopic PIV measurements. Our analysis shows that the region which is most sensitive to crosswind is located at the rear part of the leeward flank. However, changes in the rear geometry (from squareback to fastback body) only affect the established lateral coefficients values while transient duration and the force overshoots appear not to be significantly modified. Furthermore, the circulation of the most energetic leeward vortex appears to be correlated with the lateral coefficients transient evolutions

WAKE STRUCTURE AND DRAG OF VEHICLES WITH ROUNDED REAR EDGES

The wake structure at the rear of road vehicle is known to be of prime importance in aerodynamics performances [3]: about 30% of the total pressure drag derives from the rear end of the vehicle. While production vehicle present significant curvature at the rear end, most of fundamental aerodynamic analyses were carried out around simplified car models presenting sharp edges at the rear. Since recently, very few papers addressed the question of rear edges curvature in aerodynamics performances. Thacker et al. Showed that rounding the edge between the end of the roof and the rear slant suppressed the separation over the rear window and resulted in 10% drag reduction. Fuller et al.Studied the effect on spatial stability and intensity of the pillar vortex when rounding the side rear pillars. For both of these works, the analysis was focused on the flow behaviour over the rear window: the impact of the rear end rounding on the near wake topology was not discussed. The current study aims to understand how the use of rounded pillars with respect to sharp edges modifies the flow field (over the body surface and in the near wake) and hence the global drag. Moreover, an “academic” and an “industrial” model will be characterized to discuss the applicability of simplified models to simulate properly the sensitivity of pillars rounding

Aerodynamic analysis of transitional wings encountering high amplitude streamwise gust

We are interested here in the effect of high amplitude streamwise gusts felt by fixed rigid wings at pre-stall angles of incidence in low Reynolds numbers ($Re\approx 50 \times 10^3 - 200 \times 10^3$) flow conditions. An experimental rig has been developed to reproduce a cyclic variation of the freestream axial velocity representative of the perturbations micro-aerial vehicles can encounter in a urban environment. The temporal evolution of the aerodynamic forces (lift, drag and pitching moment) recorded during the gust show significant deviations from the quasi-static predictions that unsteady but inviscid methods are unable to capture. These deviations appear to be related to the dynamic of both the laminar separation bubble and the laminar separation without trailing edge reattachment, also known as the ``short" and ``long bubble" respectively

Propulsive Performance for an Oscillating Airfoil Applied to Mini Air Vehicles

In the present work, the optimal control to maximize the energy harvesting through a sinusoidal vertical gust profile is investigated through 2D URANS simulations and wind tunnel tests of NACA 0015 wing. The control is defined by a harmonic pitching motion of the wing, with the main objective to determine the optimal control parameters represented by the optimal pitch amplitude and phase shift that maximize the energy harvesting efficiency. The computational fluid dynamics (CFD) based on the k-omega -SST turbulence model is implemented to find the optimal control parameters for a simultaneously heaving and pitching 2D wing. For the experimental investigation, a wind tunnel model is manufactured and used to perform the wind tunnel tests to prove the energy harvesting concept and validate the obtained CFD results. Since it wasn't feasible to generate sinusoidal vertical gust in the wind tunnel, the gust effect is modeled by a sinusoidal heaving motion of the wing. A robotic arm is used to perform the simultaneous heaving and pitching motions of the wing. The numerical results showed the significant effect of the control activation to increase the energy harvesting where an optimal efficiency of 67 % is achieved at a gust amplitude of 0,5 m/s and frequency of 0,4 Hz. It was also found that an increase in the amplitude of the sinusoidal gust profile brings significant increment in the amount of energy harvested. Wind tunnel tests proved the concept of energy harvesting and exhibit the same trends of efficiency variation with pitch amplitude as that obtained through the numerical simulations. The obtained results showed that the energy harvesting flight technique is very promising regarding the improvement of the performance of mini-UAVs

Influence of afterbody rounding on the near wake of fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with different afterbody rounding. Rounding the side pillars generated moderate changes in drag and important lift reductions. The minor effect on the drag force was found to result from opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to afterbody rounding

Drag and crosswind sensitivity of rounded fastback vehicle

Experimental and numerical analyzes were performed to investigate the aerodynamic performances of a realistic vehicle with a different afterbody rounding. This afterbody rounding resulted in a reduction to drag and lift at a yaw angle of zero, while the crosswind performances were degraded. Rounding the side pillars generated moderate changes to the drag and also caused important lift reductions. A minor effect on the drag force was found to result from the opposite drag effects on the slanted and vertical surfaces. The vorticity distribution in the near wake was also analyzed to understand the flow field modifications due to the afterbody rounding. Crosswind sensitivity was investigated to complete the analysis of the aerodynamic performances of the rounded edges models

Aeroelastic implications of active winglet concept aimed to improve civil transport aircraft performances

Reduction of aircraft environmental footprint has become over years a key objective for the industry. Particularly, for decades winglets have been proven to eficiently reduce drag and fuel consumption. However, the design of those wingtip extensions mainly relies on an aerodynamic shape optimisation for a given cruise condition resulting in suboptimal behaviour for the rest of the flight. Active winglet concept proposes to optimise the winglet cant angle along the flight to compensate the loss of eficiency inherent to fixed designs. The variation of winglet deflection impacts the lift distribution with repercussion on wing deformation that must be investigated. Besides, the presence of moving masses at the tip of the wing also has influence on dynamic response and particularly on flutter onset. This work proposes to evaluate those impacts through an aeroelastic analysis of both static and dynamic implications of active winglets combined with an aerodynamic performances optimisation. The XRF1, an Airbus provided industrial standard multi- disciplinary research test case representing a typical configuration for wide body long- range aircraft, is used as the baseline aircraft. Coupled CFD/CSM computations are performed to assess the evolution of wing shape with respect to winglets deflections and the consequences on mission performance optimisation. While a parametric flutter analysis is carried-out to highlight the dependence of critical flutter speed on winglet cant angle