Numerical study of airflow around vehicle A-pillar region and windnoise generation prediction

American Journal of Applied Sciences, Feb, 2009 by M.H. Shojaefard, K. Goudarzi, H. Fotouhi

INTRODUCTION

As engine, tire, and other noise reduced and as driving speeds increased, aerodynamic noise sources on ground vehicles were becoming relatively more important. They often dominate at cruise speeds over 100 km [h.sub.-1]. Reduction of aerodynamic noise has a significant effect to occupants comfort on a long way trip. Consequently, the interior noise in a vehicle is important to its sales, particularly for luxury cars.

It is well known that the pressure fluctuations on the front side window surface of a road vehicle are a major sound source for both the external and interior wind noises (1).

The Previous research studies revealed that the unsteady pressure on the front side window surface is due to the existence of three-dimensional vortex separated from the A-pillar region (1), (2). These researches also suggested that the size and magnitude of the A-pillar flow separation mainly depend on the local A-pillar and windshield geometry and vehicle yaw angles (3), (4), (5). Consequently, in order to reduce wind noise, understanding the mechanics of airflow behaviour around the A-pillar region is very important.

In this study, a series of three-dimensional Navire-Stockes simulations were carried out for the vortical flow about basic car models with different A-pillar/windshield geometry and at different cruising speed for different yaw angles; the models geometries were similar to Alam (6). The airflow behaviour behind slanted-sharp edged model and semi-small ellipsoidal model were simulated using CFD and qualitatively analyzed. Pathlines analysis, contours of mean pressure coefficient and contours of turbulent kinetic energy helped to find out the characteristics of the airflow around the vehicle A-pillar. It is noted that the airflow simulation around the A-pillar region was investigated in this research.

In order to investigate windnoise around vehicle A-pillar, the turbulence quantities, which were acquired by CFD analysis, were used to estimate local sound source strength by using Boundary Layer Noise Source Model which is an acoustic source strength broadband noise model.

VEHICLE GEOMETRY MODELS

Geometry configuration used in these simulations were obtained from Alam (6). In his wind tunnel tests, 40% scale idealized model vehicles with different A-pillar and windshield radii were made. All models were simple without the complication of engine compartment, fore-body, side mirrors, wheels and wheel arches. In this study, two of them were selected which they are made with 60[degrees] flat windshield inclination angles but with different A-pillar and windshield curvature; a slanted sharp-edged model, a small semi-ellipsoidal model. The full length of the model was 1.963 m, the width was 0.748 m and the height was 0.588 m (Fig. 1).

[FIGURE 1 OMITTED]

GOVERNING EQUATION

Turbulent flow over the vehile A-pillar is three-dimensional, steady and incompressible; the continuity and momentum equations (Navier-Stokes equations) with a turbulence model were used to solve the flow.

[[[partial derivative]u]/[[partial derivative]x]] + [[[partial derivative]v]/[[partial derivative]y]] + [[[partial derivative]w]/[[partial derivative]z]] = 0 (1)

u[[[partial derivative]u]/[[partial derivative]x]] + v[[[partial derivative]u]/[[partial derivative]y]] + w[[[partial derivative]u]/[[partial derivative]z]] = - [1/[rho]][[[partial derivative]p]/[[partial derivative]x]] + [1/[rho]]([[partial derivative][[tau].sub.xy]]/[[partial derivative]y] + [[partial derivative][[tau].sub.xz]]/[[partial derivative]z]) + [B.sub.xspace]space (2)

[[[partial derivative]u]/[[partial derivative]x]] + v[[[partial derivative]u]/[[partial derivative]y]] + w[[[partial derivative]u]/[[partial derivative]z]] = - [1/[rho]][[[partial derivative]p]/[[partial derivative]x]] + [1/[rho]]([[partial derivative][[tau].sub.xy]]/[[partial derivative]y] + [[partial derivative][[tau].sub.xz]]/[[partial derivative]z]) + [B.sub.x] (3)

u[[[partial derivative]w]/[[partial derivative]x]] + v[[[partial derivative]w]/[[partial derivative]y]] + w[[[partial derivative]w]/[[partial derivative]z]] = - [1/[rho]][[[partial derivative]p]/[[partial derivative]z]] + [1/[rho]]([[partial derivative].sub.[tau]xz]/[[partial derivative]x] + [[partial derivative].sub.[tau]yz]/[[partial derivative]y]) + [B.sub.z] (4)

Where u is x-component of velocity vector, v is y-component of velocity and w is z-component of velocity vector. [rho] is density of air, P is static pressure, [tau] is shear stress and [B.sub.x],[B.sub.y],[B.sub.z] are body forces (7).

TURBULENCE MODEL

In this research, the turbulence model used for simulations was realizable (K-[epsilon]) model. The term realizable means that the model satisfies certain mathematical constraints on the normal stresses, consistent with the physics of turbulent flows. It provides superior performance for flows involving rotation, boundary layers under strong adverse pressure gradients, separation, and recirculation, so realizable (K-[epsilon]) model is a proper model for visualization of airflow around vehicle A-pillar region. The modeled transport equations for k and e in the realizable (K-[epsilon]) model are