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The general objective of this research is the analysis of the instability of passenger vehicles associated with transient crosswind gusts. Experimental and numerical tests are performed on the squareback Willy test model, which is realistic compared to a van-type vehicle. Experiments were carried out in a semi-open test section at the Conservatoire National des Arts et Métiers (CNAM) and computations were performed at the Ecole Centrale de Nantes (ECN). The ISIS-CFD flow solver, developed by the CFD Department of the Fluid Mechanics Laboratory of ECN used the incompressible unsteady Reynolds-averaged Navier-Stokes equations. The experimental results obtained at a Reynolds number of 0.9 106 were compared with numerical data for steady flow. Agreement between experimental and computed aerodynamic forces, wall pressures, and tomographies of total pressure is fairly good.

This research focuses on the analysis of the instability of passenger vehicles associated with transient crosswind gusts. A new vehicle model, created to analyze the behavior of unsteady wakes on bluff bodies, is proposed. The tests presented here were performed on a squareback model animated by an oscillating yaw angle in a steady wind. The initial unsteady results were obtained for a yaw angle of 10°, a Reynolds number close to 106, and a maximum value of the Strouhal number of 0.07. The analysis of unsteady total-pressure tomographies and unsteady particle-image velocimetry (PIV) shows the presence of phase shifting and hysteresis that evolve as a function of the Strouhal number. This work was performed in the framework of a collaboration between the Conservatoire National des Arts et Métiers, (CNAM), the Ecole Nationale Supérieure de l’Aéronautique et de l’Espace (SUPAERO) and the European automotive constructor Peugeot Citroën SA.

An analytical approach based on an integral balance of momentum is used to define and analyze the parameters which contribute to cooling air drag of road vehicles. The results obtained serve to propose a number of recommendations aimed at design and test departments, and to clarify the influence of a geometric or architectural under-bonnet modification on cooling air drag.

Increasing design emphasis on factors such as styling, fuel reduction and soundproofing raises a number of additional problems concerning under-bonnet aerodynamics and heat exchange. Because experimental work on successive prototypes entails heavy penalties in terms of development lead-time, it is becoming more and more important to integrate simulation from the pilot study stage, as a way to minimize the number of prototypes. Fortunately, early integration of under-bonnet air-flow modelling is becoming an increasingly viable proposition, thanks to the spectacular increase in computer processing power, which stimulates the development of more efficient meshing software and facilitates the generalized implementation of CAD techniques throughout the design processes. Modelling thus emerges as a new investigatory method that enhances the design office's capabilities by enabling it to adopt a sharper design focus right from the pilot project stage.

In this paper we discuss a method for determining heat transfer behaviour during tests in aerodynamic wind tunnels. The method involves computing the heat power evacuated by the heat exchanger as a function of the cooling airflow drag. Using dimensional analysis techniques, we define a Thermal Efficiency Coefficient (TEC). Systematic tests on three geometrically different types of vehicles show that there is a universal law governing the variation in TEC with cooling airflow drag. By associating the curve for this universal law with the characteristic curve for the heat exchanger alone, we can compute a TEC value for a measured cooling airflow drag value, and thus determine the vehicle's engine cooling performance during aerodynamic tests.

In parallel to and with the development of numerical methods for calculating flow around road vehicles, experimental techniques are improving in order to obtain physical information at the same level of precision as those given by computation. In this way, RENAULT has developed in 1984 a numerical tomography technique which permits an analysis of vortical flow in wakes of road vehicles. Nevertheless, interpretations of the maps obtained by this method are sometimes difficult for complex aerodynamic shapes, mainly concerning the source of free share layers. By coupling this last method with oil-flow visualizations, and a simultaneous reading, it is possible to overcome this problem and to obtain a more complete understanding of flow structures around a car. Results of this technique are presented for some typical models, and discussed.