UPSTREAM TURBULENCE EFFECTS IN THE SPATIO-TEMPORAL CHARACTERISTICS OF A MODEL A-PILLAR VORTEX

Conical vortices generated over surfaces having a swept angle with the incidence wind are found in a large class of practical applications. The study presented in this paper is related to the aerodynamic and aero-acoustic of passenger vehicles. More particularly, we focus on the Apillar vortex, arising on the corner edge, between the windshield and the front side window. The incoming flow fields encountered by passenger cars is highly turbulent due to the natural wind or other road traffic. The goal of the paper is then twofold: (i) educe the spatial and temporal characteristics of the A-pillar conical vortex, (ii) investigate the influence of a turbulent stream on these characteristics. For that purpose, a database of simultaneous stereoscopic HSPIV and fluctuating wall pressure measurements was compiled. A spectral analysis of the fluctuating pressure under the vortex is used to analyze the link between the unsteady aerodynamics and the wall pressure field. It has been observed a significant amplification of the meandering of the structure highlighting the high receptivity of this structure to perturbations generated by external turbulence. Introduction Conical vortices generated over surfaces having a swept angle with the incidence wind are found in a particularly large class of practical applications as aerodynamics (delta-wings), civil engineering and transport engineering. The study presented in this paper has a strong link with the aerodynamic and aero-acoustic of passenger vehicles. More particularly, we focus on the A-pillar vortex, arising on the corner edge, between the windshield and the front side window. Previous studies (Alam et al.,2003) have shown that the fluctuating pressures in the A-pillar region of a passenger car are the primary source of ”in cabin” aerodynamic noise. Indeed, the wall stresses induced by the A-pillar vortex are sufficient to make the front side window vibrate and generate noise disturbances inside the car (Levy and al., 2013). The particularity of these A-pillar structures is their strong interaction with the lateral wall of the car. This differs, for example, from delta-wings leading edge vortices at high angle of attack. A mirror image can be used to qualitatively understand the mean downstream evolution of these structures of concentrated vorticity that shift to the roof when traveling toward the back of the car (Hucho 1998). Most of the work related to these typical structures has been experimental and led in wind tunnel in flows with low levels of free stream turbulence (FST). However, incoming flow fields encountered by passenger car are highly turbulent due to the natural wind or other road traffic. This study is then more focused on the effects of freestream turbulence on this conical vortex structure. Effects of FST on bluff body aerodynamic have been studied for a long time. Bearman and Morel (1983) described three basic mechanisms by which FST and the mean flow over bluff bodies interact: accelerated transition to turbulence in shear layers, enhanced mixing and entrainment and distortion of FST itself by the mean flow. In this way, this work is a continuation of the work already carried out in the laboratory by Hoarau et al. (2008) which led experiments coupling wall pressure measurements with Doppler Laser Anemometry in a uniform and laminar stream. The analysis of fluctuating pressure spectra have shown two principal contributions which may correspond to a global meander-