Patrick Gilliéron

Separated Flows Around the Rear Window of a Simplified Car Geometry

A 3D numerical simulation based on the lattice Boltzmann method is carried out on a simplified car geometry (initially proposed by Ahmed, Ramm, and Falting, 1984, SAE Technical Paper series No. 840300) to analyze and establish a method for controlling the near-wake flow topology of a generic blunt body model. The results indicate the existence of a complex flow topology consisting of transverse and longitudinal vortices emanating from flow separations that occur on the top and the lateral edges of the slanted rear window, respectively. The topology of each structure is detailed and the numerical results are compared with the experimental results in the literature. The results presented in this paper can then be used to develop and parametrize active control solutions conducive to improving the aerodynamic performances of automobile vehicles.

Passive Control Around the Two-Dimensional Square Back Ahmed Body Using Porous Devices

The control of two-dimensional flows around the square back Ahmed body is achieved by using porous devices added on some parts of the body. The square back Ahmed body is considered either in an open domain or on top of a road. The modeling of the flow in different media is performed by means of the penalization method. A good choice of the location of the porous interfaces yields a significant improvement of the aerodynamic quantities, especially for the square back body.

Contribution to the analysis of transient aerodynamic effects acting on vehicles

The transient phenomena occurring when a vehicle exits a tunnel into a prevailing crosswind or when vehicles overtake or cross are analysed with an approach based on the assumption of pseudo-harmonic variations of the aerodynamic characteristics. The findings provide magnitudes of the frequencies and the Strouhal numbers associated with the transient variation of the side forces. Results are compared with the computational simulations of the overtaking process. The pseudo-harmonic behaviour is validated and the values of the frequencies and Strouhal numbers are verified. The transient aerodynamics is then analysed on 1/5-scale models using a test bench specially developed for simulating the overtaking process.

Aerodynamic Drag Reduction by Synthetic Jet: A 2D Numerical Study Around a Simplified Car

Active synthetic jet control technique is numerically examined in order to obtain an innovative flow control solution capable of significantly reducing vehicle fuel consumption. The potential of the synthetic jet is evaluate d according to a two dimensional analysis on simplified rear body of an automobile vehicle. The aerodynamic drag coefficient (C d) and the topology of the mean flow in the wake are analyzed according to two characteristic parameters, namely the momentum coe fficient and the reduced frequency. Several efficient operating configurations are identified. In the optimum configuration, at which a C d reduction of 28% is obtained, a complete reorganization of the time averaged flow topology and of the unsteady dynami cs of the wake is observed.

Drag reduction by pulsed jets on strongly unstructured wake: towards the square back control

Experimental studies have been performed in a wind tunnel in order to control the flow separation on rear window of a generic vehicle shape (the Ahmed body with a scale of 0.7 and a slope angle of 35 degrees). The rear part of the geometry has been modified by replacing the sharp edge between roof and rear window by smooth curved surfaces. This model is equipped at the end of the roof with a strip of pulsed jets in order to control the flow with a velocity of 30 m.s−1. A 20% drag reduction has been obtained with a pulsed frequency at 500 Hz and a momentum coefficient Cµ = 2.75 10−3. This result confirms the interest in using pulsed jets in order to reduce aerodynamic drag and pollutant emission.

Active procedures to control the flow past the Ahmed body with a 25° rear window

Ahmed body with a 25 degree rear window is used to represent a simpli ed car geometry. Two and three-dimensional simulations are performed to analyse the flow behaviour around such a vehicle. Sucking and blowing jets or slots are added on the body to control the flow. The results presented show that good drag reductions are achieved for a good choice of the active procedure.

Influence of the Slant Angle of 3D Bluff Bodies on Longitudinal Vortex Formation

This paper presents the experimental results and analytical arguments concerning simplified geometries of the rear window and windscreen of automotive vehicles in order to contribute to a better understanding of the swirling structure formation and vortex bursting processes. Static pressure distributions and skin friction line visualizations on both sides of the edge where the swirling structure is generated on the rear window of an Ahmed body are presented for different slant angles. Results show the influence of the slant angle on the swirling structure formation and further show that the vortex bursting process can be promoted by small rear window angles. These results are then extrapolated with the help of analytical demonstrations to the windscreen configuration to demonstrate that large windscreen slopes would have the same disintegration effect.

Contrôle des écoulements appliqué a l'automobile. État de l'art

The control of the flows forms the basis for the work of development in automotive aerodynamics. Apart from the traditional techniques usually used, many experimental techniques often developed for aeronautics can constitute effective means of reduction of consumption, noise and stains. These techniques can also be used to improve the safety and the dynamic stability of the vehicles at the time of the phases of crossing, overtaking, entry or exit of tunnel with cross-wind. This article constitutes a state of the art of the new techniques which enable to control the flow and the position of separations lines. For each solution, the experimental conditions are recalled and the principal results presented.

Characterization of longitudinal vortices in the wake of a simplified car model

computation code based on the Lattice Boltzmann Method (LBM). Air flow is anal yzed in the vicinity of the axis of the longitudinal vortex in terms of vorticity, pressure and velocity. The findings relative to the topology of the longitudinal vortices complement those found in the literature . By analyz ing the longitudinal and azimuth al velocity profiles it is possible to characterize the manner in which the longitudinal vortex develops and identify dissipation methods conducive to reducing the aerodynamic drag intensity on automobile vehicles .

Phase Locked Analysis of a Simplified Car Geometry Wake Flow Control Using Synthetic Jet

This paper presents a numerical unsteady analysis of a SJA impact on a car wake flow. First, for the optimal reduced frequency F+ , the influence of the Cμ on the mean aerodynamic drag reduction 〈Cd 〉 is observed. A spectral analysis of the vortex shedding coming from the upper and the lower part of the car and of the drag coefficient is then presented for different Cμ values. Preliminary results suggest that maximum drag reduction is obtained when most energy in the wake comes from the actuator forcing frequency rather than the natural vortex shedding frequencies of the two contributions. This work is completed by a phase locked analysis of the synthetic jet actuator local effect on the turbulent boundary layer just before the flow separation. For the fixed optimal F+ , different Cμ values are compared. The streamwise velocity profiles seem to show that maximal efficiency of the control is obtained when the synthetic jet injected momentum is introduced in the logarithmic sub-layer part of the turbulent boundary layer.Copyright