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Technical Paper

The BMW AVZ Wind Tunnel Center

The new BMW Aerodynamisches Versuchszentrum (AVZ) wind tunnel center includes a full-scale wind tunnel, "The BMW Windkanal" and an aerodynamic laboratory "The BMW AEROLAB." The AVZ facility incorporates numerous new technology features that provide design engineers with new tools for aerodynamic optimization of vehicles. The AVZ features a single-belt rolling road in the AEROLAB and a five-belt rolling road in the Windkanal for underbody aerodynamic simulation. Each of these rolling road types has distinct advantages, and BMW will leverage the advantages of each system. The AEROLAB features two overhead traverses that can be configured to study vehicle drafting, and both static and dynamic passing maneuvers. To accurately simulate "on-road" aerodynamic forces, a novel collector/flow stabilizer was developed that produces a very flat axial static pressure distribution. The flat static pressure distribution represents a significant improvement relative to other open jet wind tunnels.
Technical Paper

The New BMW Climatic Testing Complex - The Energy and Environment Test Centre

The Energy and Environment Test Centre (EVZ) is a complex comprising three large climatic wind tunnels, two smaller test chambers, nine soak rooms and support infrastructure. The capabilities of the wind tunnels and chambers are varied, and as a whole give BMW the ability to test at practically all conditions experienced by their vehicles, worldwide. The three wind tunnels have been designed for differing test capabilities, but share the same air circuit design, which has been optimized for energy consumption yet is compact for its large, 8.4 m₂, nozzle cross-section. The wind tunnel test section was designed to meet demanding aerodynamic specifications, including a limit on the axial static pressure gradient and low frequency static pressure fluctuations - design parameters previously reserved for larger aerodynamic or aero-acoustic wind tunnels. The aerodynamic design was achieved, in-part, by use of computational fluid dynamics and a purpose-built model wind tunnel.
Journal Article

Integrated Numerical and Experimental Approach to Determine the Cooling Air Mass Flow in Different Vehicle Development Stages

This paper presents an integrated numerical and experimental approach to take best possible advantage of the common development tools at hand (1D, CFD and wind tunnel) to determine the cooling air mass flow at the different vehicle development stages. 1D tools can be used early in development when neither 3D data nor wind tunnel models with detailed underhood flow are available. A problem that has to be resolved is the dependency on input data. In particular, the pressure coefficients on the outer surface (i.e. at the air inlet and outlet region) and the pressure loss data of single components are of great importance since the amount of cooling air flow is directly linked to these variables. The pressure coefficients at the air inlet and outlet are not only a function of vehicle configuration but also of driving velocity and fan operation. Both, static and total pressure coefficient, yield different advantages and disadvantages and can therefore both be used as boundary conditions.
Technical Paper

Experimental Study on the Influence of Model Motion on the Aerodynamic Performance of a Race Car

While race cars run in a highly dynamic environment, aerodynamic testing through state of the art wind tunnel tests, as well as CFD analyses, are mostly performed under static or stationary conditions. Therefore, other than track data, only very limited data are available on time resolved aerodynamic forces and pressures for a moving car. To investigate these effects a new model manipulator was developed which allows substantial pitch and heave movements up to 20Hz. Wind tunnel tests with a former LeMans type race car model have shown that the difference between a steady state and a true dynamic analysis is significant.
Technical Paper

Experimental Investigations and Computations of Unsteady Flow Past a Real Car Using a Robust Elliptic Relaxation Closure with a Universal Wall Treatment

In the present work we investigated experimentally and computationally the unsteady flow around a BMW car model including wheels*. This simulation yields mean flow and turbulence fields, enabling the study aerodynamic coefficients (drag and lift coefficients, three-dimensional/spatial wall-pressure distribution) as well as some unsteady flow phenomena in the car wake (analysis of the vortex shedding frequency). Comparisons with experimental findings are presented. The computational approach used is based on solving the complete transient Reynolds-Averaged Navier-Stokes (TRANS) equations. Special attention is devoted to turbulence modelling and the near-wall treatment of turbulence. The flow calculations were performed using a robust, eddy-viscosity-based ζ - ƒ turbulence model in the framework of the elliptic relaxation concept and in conjunction with the universal wall treatment, combining integration up to the wall and wall functions.
Technical Paper

Moving Belt with Distributed Suction in the Porsche Model Wind Tunnel

The Porsche 1:4-scale model wind tunnel was upgraded with a moving belt in combination with basic and distributed boundary layer suction devices. The belt is placed between the rotating wheels of the model with the external underfloor balance in operation. Special attention was given to providing a sufficient length of the belt with regard to road simulation for the aerodynamic optimization of race car models with their specific wake behavior. The boundary layer suction systems control the regions beyond the belt. Furthermore the balance was equipped with a pitch angle adjustment system in order to simulate the spring deflections of a moving vehicle. Results from a series of measurements taken from one race car model and one passenger car model are presented in order to investigate the influence of the boundary layer-, wheel rotation- and pitch adjustment-controls on the characteristic aerodynamic figures of the models.
Technical Paper

The Porsche Wind Tunnel Floor-Boundary-Layer Control - A Comparison with Road Data and Results from Moving Belt

The design of the PORSCHE wind tunnels - two facilities, one in full- and the other in quarter-scale - was determined by the demand for simulating both passenger car models and racing vehicles. One peculiarity, the very low ride height of the latter requires a reduction of the oncoming boundary layer that develops along the test- section floor. The number of difficult practical engineering problems in using and operating full-scale moving belts (*Bearman et al. [14]), led to the development of two suction systems using porous plates in the test section floor. These have been installed in the full-scale and in the 1:4 - scale windtunnels. For verification or optimization of the originally estimated suction rates required to meet realistic road conditions, a number of experiments on the road and in a moving-belt facility were conducted and the results compared to values from the suction facilities.
Technical Paper

Influence of Plenum Dimensions on Drag Measurements in 3/4-Open-Jet Automotive Wind Tunnels

The size of the room surrounding the wind tunnel test section, the so called wind tunnel plenum, is always seen as an important parameter of the wind tunnel building, but has rarely been the subject of systematic investigation regarding minimal requirements to meet quality objectives for aerodynamic testing. Experimental investigations of this object were made in a quarter-scale wind tunnel (nozzle area 1.4m2). The plenum dimensions were changeable by combinations of different side wall and ceiling positions. The results have shown, that the plenum can have a significant effect on the flow around the vehicle and therefore on the measured forces. Drag coefficient is under prediced if the wind tunnel plenum is too small. Recommendations are provided for the geometric dimensions of a wind tunnel plenum. The data obtained are a valuable tool for the layout of wind tunnel design concepts and for the evaluation of interference free wind tunnel simulation.
Technical Paper

Developing a Theory for Active Grille Shutter Aerodynamics—Part 1: Base Theory

The aim is to develop a theory to describe the aerodynamic behavior of active grille shutters (AGS). The theory correlates the cooling air mass flow and drag of a vehicle with the angle and number of air flaps on the AGS. The relatively simple mathematical formulation of this theory provides an insight into the aerodynamic behavior and characteristic curve shape of AGS. It illustrates how the number of air flaps changes and influences the shape of the AGS characteristic curve. The theory is validated by experiments using wind tunnel measurements on real vehicles with AGS. The comparisons show good agreement between theory and experiment.
Technical Paper

A Two-Measurement Correction for the Effects of a Pressure Gradient on Automotive, Open-Jet, Wind Tunnel Measurements

This paper provides a method that corrects errors induced by the empty-tunnel pressure distribution in the aerodynamic forces and moments measured on an automobile in a wind tunnel. The errors are a result of wake distortion caused by the gradient in pressure over the wake. The method is applicable to open-jet and closed-wall wind tunnels. However, the primary focus is on the open tunnel because its short test-section length commonly results in this wake interference. The work is a continuation of a previous paper [4] that treated drag only at zero yaw angle. The current paper extends the correction to the remaining forces, moments and model surface pressures at all yaw angles. It is shown that the use of a second measurement in the wind tunnel, made with a perturbed pressure distribution, provides sufficient information for an accurate correction. The perturbation in pressure distribution can be achieved by extending flaps into the collector flow.
Technical Paper

Measurement of Reference Dynamic Pressure in Open-Jet Automotive Wind Tunnels

In automotive open-jet wind tunnels reference velocity is usually measured in terms of a static pressure difference between two different cross-sectional areas of the tunnel. Most commonly used are two sections within the nozzle (Method 1: ΔP-Nozzle). Sometimes, the reference velocity is deduced from the static pressure difference between settling chamber and plenum (Method 2: ΔP-Plenum). Investigations in three full-scale open-jet automotive wind tunnels have clearly shown that determination of reference dynamic pressure according to ΔP-Plenum is physically incorrect. Basically, all aerodynamic coefficients, including drag coefficient, obtained by this method are too low. For test objects like cars and vans it was found that the error ΔcD depends on the test object's drag blockage in an open-jet wind tunnel.