Flow and dipole source evaluation of a generic SUV

Paper i proceeding, 2007

Accurately predicting both average flow quantities and acoustic sources at the front side window of today's ground vehicles is still a considerable challenge to automotive companies world-wide. One of the most important aspects for obtaining trustworthy results, but also the most tedious one and therefore perhaps overlooked, is the control and outcome of the mesh generation process. Generating unstructured volume meshes suitable for Large Eddy Simulations with high level representation of geometrical details is both a time consuming and an extremely computer demanding activity. This work investigates two different mesh generation processes with the main aim to evaluate their outcome with respect to the prediction of the two dominating dipole sources in a temporal form of the Curie's equation. Only a handful of papers exists with high level representation of the vehicle geometry and the aim of predicting the fluctuating exterior noise sources. To the author's knowledge no studies have been conducted in which both these source terms are evaluated quantitatively against measurements. The current paper investigates the degree to which the amplitude of these two source terms can be predicted by using the traditional law-of-the-wall and hex-dominant meshes with isotropic resolution boxes for a detailed ground vehicle geometry. For this purpose the unstructured segregated commercial FLUENT Finite Volume Method code is used. The flow field is treated as incompressible, and the Smagorinsky-Lilly model is used to compute the sub-grid stresses. Mean flow quantities are measured with a 14-hole probe for 14 rakes downstream of the side mirror. Dynamic pressure sensors are distributed at 16 different positions over the side window to capture the fluctuating pressure signals. All measurements in this work were conducted at Ford's acoustic wind tunnel in Cologne. All simulations accurately predict the velocity magnitude closest to the side window and downstream of the mirror head recirculation zoner. Some variations in the size and shape of this recirculation zone are found between the different meshes, most probably caused by differences in the detachment of the mirror head boundary layer. The Strouhal number of the shortest simulation was computed from the fundamental frequency of the mirror lift force component. The computed Strouhal number agrees well with the corresponding results from similar objects and gives an indication of an acceptable simulation time. Dynamic pressure sensors at 16 different locations at the vehicle side window were also used to capture the levels of the two dipole source terms. These results are compared against the three simulations. With the exception of three positions, at least one of the three simulations accurately captures the levels of both source terms up to about 1000Hz. The three positions with less agreement as compared to measurements were found to be in regions sensitive to small changes in the local flow direction.