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The difficulties of testing a bluff automotive body of sufficient scale to match the on-road vehicle Reynolds number in a closed wall wind tunnel has led to many approaches being taken to adjust the resulting data for the inherent interference effects. But it has been impractical if not impossible to experimentally analyze the effects that are occurring on and around the vehicle when these blockage interferences are taking place. The present study is an extension of earlier work by the author and similarly to that study uses the CFD (computational fluid dynamics) analysis of several bodies of differing configurations to examine the interference phenomena in solid wall wind tunnels and the effects that they have on the pressures, forces and force increments experienced by the vehicle model. This is accomplished by executing a series of CFD configurations with varying sized cross sections from 0.2% to 16% blockage enabling an approximation of free air conditions as a reference.

In the authors’ previous work, a database was generated documenting the effects of variable blockage ratios on the drag and lift of simplified and generic automotive bodies in solid wall wind tunnels. This database displays significant differences in the responses of different vehicle architectures to changes in wind tunnel blockage. What was not examined in this previous work was the effect of wind tunnel blockage on the incremental values of geometry changes to these generic models. This is critical knowledge related to the aerodynamic development process of automotive vehicles in wind tunnels. To complement that work, the present paper examines the effects of changes in solid wall blockage on the incremental force values of geometry changes on the simplified sedan geometry known as the Pilot Fastback, the Pilot Squareback and the Ford GTU pickup.

The difficulties of testing a bluff automotive body of sufficient scale to match the on-road vehicle Reynolds number in a closed wall wind tunnel has led to many approaches being taken to adjust the resulting data for the inherent interference effects. But it has been impractical if not impossible to experimentally analyze the effects that are occurring on and around the vehicle when these blockage interferences are taking place. The present study is an extension of earlier work by the author and similarly to that study uses the CFD (computational fluid dynamics) analysis of several bodies of differing configurations to examine the interference phenomena in solid wall wind tunnels and the effects that they have on the pressures, forces and force increments experienced by the vehicle model. This is accomplished by executing a series of CFD configurations with varying sized cross sections from 0.2% to 13% blockage enabling an approximation of free air conditions as a reference.

Establishing data adjustments that will give an interference free result for bluff bodies in automotive wind tunnels has been pursued for at least the last 45 years. Recently, the Two-Measurement correction method that yields a wake distortion adjustment for open jet wind tunnels has shown promise of being able to adjust for many of the effects of non-ideal static pressure gradients on bluff automotive bodies. Utilization of this adjustment has shown that a consistent drag results when the vehicle is subjected to the various gradients generated in open jet wind tunnels. What has been lacking is whether this consistent result is independent of the other tunnel interference effects. The studies presented here are intended to fill that gap on the performance of the two-measurement technique. The subject CFD studies are designed to eliminate all wind tunnel interference effects except for the variation of the (linear) static pressure gradient.

Window buffeting is a major source of flow induced sound and vibration. This paper will describe window buffeting measurements acquired on a full scale vehicle as well as two different simplified small scale models. The experimental data sets included microphone and phase averaged Particle Image Velocimetry (PIV) measurements both of which show that the flow physics are qualitatively and quantitatively similar in all cases. The implication of this result is that simplified laboratory models of a vehicle are sufficient to study the various aspects of window buffeting in full scale vehicles.

This paper presents applications of the Two-Variable method for the correction of solid-wall boundary interference of both wind tunnel and CFD data for a simplified automobile model at zero yaw angle and to a flat-plate wing over a 90° angle range. The latter model has flowfields that vary from those of a streamlined body at 0° yaw to those of a bluff body at 90° yaw. The Two-Variable method utilizes measurements on the wind tunnel walls to estimate the interference velocity components induced by the solid boundaries. The correction of the forces and moments from these interference velocities are obtained by Hackett's force model. The paper compares this method to a simpler analytical method that is more practical to apply in closed-wall wind tunnels. It is shown that the effect of the wind tunnel walls or CFD domain boundaries can accurately removed by these techniques for model/domain area ratios of up to 0.15.

To develop vehicle exterior aerodynamics, a CFD surface morphing tool was applied to change the vehicle exterior surface. Morphing is applied to the surface mesh which then is used in CFD simulation to measure the aerodynamic parameters. Three kinds of general surface modifications and results are discussed, and some comparisons are given for analysis. In this paper, the methodology of surface mesh morphing is described, and its implementation in CFD for aerodynamic simulation is analyzed. Utilization of the mesh morphing process to optimize vehicle surface for aerodynamic drag will be the subject of a future study.

During the design process for a vehicle, the CAD surface geometry becomes available at an early stage so that numerical assessment of aerodynamic performance may accompany the design of the vehicle's shape. Accurate prediction requires open grille models with detailed underhood and underbody geometry with a high level of detail on the upper body surface, such as moldings, trim and parting lines. These details are also needed for aeroacoustics simulations to compute wall-pressure fluctuations, and for thermal management simulations to compute underhood cooling, surface temperatures and heat exchanger effectiveness. This paper presents the results of a significant effort to capitalize on the investment required to build a detailed virtual model of a pickup truck in order to simultaneously assess performance factors for aerodynamics, aeroacoustics and thermal management.

A process of prediction of side window buffeting at a very early program stage is introduced. The process includes CFD simulations, a full scale aero-acoustic buck and pilot vehicle wind tunnel tests. The results of the three different tools are compared and the conclusions are very encouraging and unique. It proves that this process is very useful for early program buffeting improvement.

This paper provides an overview of the design and commissioning results for the DaimlerChrysler full-scale vehicle Aeroacoustic Wind Tunnel (AAWT) brought online in 2002. This wind tunnel represents the culmination of the plan for aeroacoustic facilities at the DaimlerChrysler Corporation Technical Center (DCTC) in Auburn Hills, Michigan. The competing requirements of excellent flow quality, low background noise, and constructed cost within budget were optimized using Computational Fluid Dynamics, extensive acoustic modeling, and a variety of scale-model experimental results, including dedicated experiments carried out in the 3/8-scale pilot wind tunnel located at DCTC. The paper describes the project history, user requirements, and design philosophy employed in realizing the facility. The AAWT meets all of DaimlerChrylser's performance targets, and was delivered on schedule. The commissioning results presented in this paper show its performance to be among the best in the world.

A wind tunnel investigation was made of a new high-lift system consisting of a leading edge cusp flap combined with split upper and lower trailing edge flaps. The idea behind the system was to generate two strong spanwise vortices that would increase maximum lift and drag simultaneously. Test results were not encouraging. The spanwise vortices were observed, but were not sufficiently strong to generate the anticipated high lift. Several interesting flow phenomena were observed and are described. The purpose of this paper is to present a summary of results obtained from this wind tunnel test program.