Philippe Devinant

Unsteady analyses of the flow separation on the rea r window of a simplified ground vehicle model.

This paper presents the results of an experimental investigation of the separated flow over the 25 deg. inclined rear slanted surface of an Ahmed body. Experiments performed using hot wire anemometry and PIV concentrate on the symmetry plane of the geometry. Results show an average separated flow as well as a low Strouhal number spectral activity. POD analysis applied on the PIV instantaneous velocity fields show that the lower order mode correspond to a “flapping” behavior of the separation on the rear inclined surface, oscillating between a fully separated and a quasi-fully attached configurations.

Unsteady Experimental Characterization of the Natural Wake of a Squareback Ahmed Model

The increasing demand for such vehicles as station wagons or minivans has focused aerodynamic research for drag reduction on squareback models of which, the one proposed by Ahmed et al. [1] is one of the most used. The presented study aims at finely characterize the natural wake of a 1:1 squareback Ahmed body with experimental measurements, in order to draw possible strategies for drag reduction. Our analysis focused both on the detection of the mean wake topology, by means of 2D PIV as well as parietal pressure measurements, and on the study of the spectral fluctuations of velocity and pressures. The experimental results exhibit a left/right bistable behavior for the wake, highly sensitive to the incoming wind yaw angle, so that, most generally, the mean aerodynamic field shows an asymmetric behavior due to the predominance of one of the two stable positions. The characterization of the mean wake by averaging the fields on the whole acquisition time can then be biased and show an asymmetrical wake. Then, as far as the symmetric average field is of interest, it is relevant to study separately each topology using conditional averaging, and then build a symmetrical field by averaging the two bistable phases. The analysis shows that the well-known symmetrical wake and the corresponding rear pressure field derive from a random rotation of the toric recirculation bubble, which can induce instantaneous pressure losses of about 25%. The bistability does not affect the spectral energy distribution. Our results show a spectral activity mostly at low frequencies. In particular, we found in measurements along the vertical symmetry plane a peak at StH = 0.07, indicating the presence of the “bubble pumping” phenomenon, whereas pressure measurements at vehicle mid-height indicate the existence of vortex shedding.© 2014 ASME

Aerodynamic drag and flow rate through engine compartments of motor vehicles

Engine compartments of motor vehicles are analyzed in terms of aerodynamic drag and cooling airflow by means of a simplified geometry. The geometry is based on an Ahmed body with a blunt rear end, equipped with a porous media modeling heat exchangers and a realistic engine section. Analytical results, wind tunnel exp erimentations and numerical simulations are conducted on this simplified geometry to provide a better understanding of the flows around and inside engine compartments, to connect the flows in teraction to the drag value, and to establish a link between the aerodynamic drag and the flow rate through the heat exchangers. Configurations favorable to low drag values and hig h engine cooling are put in evidence. Nomenclature ( ) z y, x, = coordinate system subscripts: L

Comparison of flow modification induced by plasma and fluidic jet actuators dedicated to circulation control around wind turbine airfoils

In order to reduce the aerodynamic load fluctuations on wind turbine blades by innovative control solutions, strategies of active circulation control acting at the blade airfoil trailing edge are studied, allowing lift increase and decrease. This study presents a comparison of results obtained by performing surface plasma and continuous fluidic jet actuation on a blade airfoil designed with a rounded trailing edge. In the present study, both actuator types are located at the trailing edge. Plasma actuators act uniformly in the spanwise direction, whereas fluidic jets blow through small squared holes distributed along the span, and therefore, provide a three-dimensional action on the flow. Load and velocity field measurements were performed to assess the effectiveness of both actuators and to highlight the flow mechanisms induced by both actuation methods for lift-up configurations. Results are presented for a chord Reynolds number of 2. 105 and for a lift coefficient increase of 0.06.

Experimental Characterization of the Unsteady Flow Over the Rear Slant of an Ahmed Body

This study presents results of an experimental analysis of the unsteady features of the flow around the rear part of an Ahmed body with a rear slant angle of 25°. This analysis focuses on the half elliptic separation bubble that developps on the rear slanted surface and brings new information, improving the understanding of the flow unsteadiness. Flow investigations are carried out using hot wire probe measurements for velocity fluctuations in the plane of symmetry above the rear slanted surface and five unsteady flush mounted pressure taps (Kulite transducers) simultaneously acquiring static pressure fluctuations along the middle line of the slanted surface. Spectral analysis and Proper Orthogonal Decomposition of the output signal show the emergence of a low frequency unsteadiness and high frequency activities which, in accordance with bibliography about separated and reattaching flow configurations, is related to a global flapping of the separated shear layer and a large scale vortices shedding. Characteristic frequencies of both instabilities is given and physical effects of the low frequency unsteadiness is related with the flapping motion of the separated shear layer.Copyright