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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.

One of the remaining challenges in the simulation of the aerodynamics of ground vehicles is the modeling of the airflows around the spinning tires and wheels of the vehicle. As in most advances in the development of simulation capabilities, it is the lack of appropriately detailed and accurate experimental data with which to correlate that holds back the advance of the technology. The flow around the wheels and tires and their interfaces with the vehicle body and the ground is a critical area for the development of automobiles and trucks, not just for aerodynamic forces and moments, and their result on fuel economy and vehicle handling and performance, but also for the airflows and pressures that affect brake cooling, engine cooling airflows, water spray management etc.

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 and add more realistic model and wind tunnel conditions to the evaluations of the performance of the two-measurement technique. The subject CFD studies are designed to greatly reduce all wind tunnel interference effects except for the variation of the non-linear static pressure gradients. A zero gradient condition is generated by simulating a solid wall test section with a blockage ratio of 0.1%.

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.

In an environment of tougher engineering constraints to deliver tomorrow's aerodynamic vehicles, evaluation of aerodynamics early in the design process using digital prototypes and simulation tools has become more crucial for meeting cost and performance targets. Engineering needs have increased the demands on simulation software to provide robust solutions under a range of operating conditions and with detailed geometry representation. In this paper the application of simulation tools to wheel design in on-road operating conditions is explored. Typically, wheel and wheel cover design is investigated using physical tests very late in the development process, and requires costly testing of many sets of wheels in an on-road testing environment (either coast-down testing or a moving-ground wind-tunnel).

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.

A correlation study was performed between static wind tunnel testing and computational fluid dynamics (CFD) for a small hatchback vehicle, with the intent of evaluating a variety of different wheel and tire designs for aerodynamic forces. This was the first step of a broader study to develop a tool for assessing wheel and tire designs with real world (rolling road) conditions. It was discovered that better correlation could be achieved when actual tire scan data was used versus traditional smooth (CAD) tire geometry. This paper details the process involved in achieving the best correlation of the CFD prediction with experimental results, and describes the steps taken to include the most accurate geometry possible, including photogrammetry scans of an actual tire that was tested, and the level of meshing detail utilized to capture the fluid effects of the tire detail.

A correlation of aerodynamic wind tunnels was initiated between Chrysler, Ford and General Motors under the umbrella of the United States Council for Automotive Research (USCAR). The wind tunnels used in this correlation were the open jet tunnel at Chrysler's Aero Acoustic Wind Tunnel (AAWT), the open jet tunnel at the Jacobs Drivability Test Facility (DTF) that Ford uses, and the closed jet tunnel at General Motors Aerodynamics Laboratory (GMAL). Initially, existing non-competitive aerodynamic data was compared to determine the feasibility of facility correlation. Once feasibility was established, a series of standardized tests with six vehicles were conducted at the three wind tunnels. The size and body styles of the six vehicles were selected to cover the spectrum of production vehicles produced by the three companies. All vehicles were tested at EPA loading conditions. Despite the significant differences between the three facilities, the correlation results were very good.

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.

Numerous efforts have been made to adjust the interference effects of solid wall wind tunnels on bluff body forces and moments. Most of these have been based upon a theoretical assumption of the mechanisms that caused the distortion of the wind tunnel data relative to free air conditions; some have been based upon empirical data from wind tunnel testing. But it has been impractical if not impossible to experimentally analyze all of the effects that are actually occurring on and around the vehicle when these blockage interferences are taking place. The present study uses the CFD analysis of two simple bodies, a fastback shape and a squareback shape, to examine the interference phenomena actually experienced in solid wall wind tunnels and the effects that those phenomena have on the pressures, velocities, and forces on the model.

In the development of several outside mirror designs for vehicles, a high frequency noise (whistling) phenomenon was experienced. First impression was that this might be due to another source on the vehicle (such as water management channels) or a cavity noise; however, upon further investigation the source was found to be the mirror housing. This “laminar whistle” is related to the separation of a laminar boundary layer near the trailing edges of the mirror housing. When there is a free stream impingement on the mirror housing, the boundary layer starts out as laminar, but as the boundary layer travels from the impingement point, distance, speed, and roughness combine to trigger the transition turbulent. However, when the transition is not complete, pressure fluctuations can cause rapidly changing flow patterns that sound like a whistle to the observer. Because the laminar boundary layer has very little energy, it does not allow the flow to stay attached on curved surfaces.

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 summarizes the major activities of CFD study on rear window buffeting of production vehicles during the past two years at DaimlerChrysler. The focus of the paper is the attempt to find suitable solutions for buffeting suppression using a developed procedure of CFD simulation with commercial software plus FFT acoustic post-processing. The analysis procedure has been validated using three representative production vehicles and good correlation with wind tunnel tests has been attained which has gained the confidence in solving the buffeting problem. Several attempts have been proposed and tried to find solution for buffeting reduction. Some of them are promising, but feasibility and manufacturability still need discussion. In order to find suitable solution for buffeting reduction, more basic research is necessary, more ideas should be collected, and more joint efforts of CFD and testing are imperative.

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.