Computer Simulation Helps Engineers Address Side Window Buffeting
By: Sandeep Sovani, Ph.D.
CFD Engineer
Fluent Inc.
Ann Arbor MI
Computer simulation is providing automotive engineers with a powerful new tool to reduce wind buffeting, the noise and pulsating forces that are experienced when driving with side windows open. It's ironic that wind buffeting has become a significant factor in the overall passenger experience in recent years precisely because engineers have been so successful in reducing noise from other sources. Until now, the difficulty in addressing this issue is that engineers have to build and test a particular design in order to measure how it performs from a wind buffeting standpoint, and these measurements provide little or no information to help them improve the design. Recently however, automotive engineers have begun accurately simulating wind buffeting using commercial computational fluid dynamics (CFD) software. Accurate simulation makes it possible to quickly evaluate the impact of various design changes without the time and cost involved in building a prototype. Simulation also makes it possible to view important performance parameters such as pressure and flow velocity at any point in the vehicle and surrounding airstream, providing an intuitive understanding that leads to better designs. In simulations of a number of different designs under different operating conditions, results were found to closely match experimental measurements. DaimlerChrysler plans to begin using these methods both to help improve specific designs and to develop general guidelines for reducing wind buffeting.
Understanding wind buffeting
Wind buffeting is caused by an unstable shear layer that is established at the upstream edge of the window opening. Disturbances are shed from this location and travel along the side of the vehicle. When they reach the rear edge of the window opening, a pressure wave is generated that propagates both inside and outside the passenger compartment. Outside the vehicle, this wave propagates both forward and backward along the side of the car. When the forward traveling wave reaches the front edge of the opening, it triggers another disturbance that moves back to the rear edge. This process is repeated many times every second and causes the shear layer to develop a characteristic buffeting frequency, which depends on the speed of the automobile and the geometry of the opening. Often the frequency is below the range that can be heard by human ears but it still can be felt by passengers as a pulsating wind force that can be very fatiguing. The results of wind buffeting are easy to measure with microphones but the complicated pressure waves that are its cause are very difficult to measure. The result, in the past, is that engineers have had to wait until relatively late in the design process when prototypes become available to measure this phenomena. These measurements typically give them little or no information on what areas of the design are affecting wind buffeting and what could be done to reduce it. The only option is to modify and test the prototypes to see whether individual changes have any effect. This process is so costly and time-consuming that it is difficult to identify changes that will improve the design.
In recent years, dramatic reductions in structure-borne and airborne noise in the passenger compartment have made wind buffeting much more noticeable to passengers than it was in the past. At the same time, the performance of simulation tools has dramatically advanced, including the availability of robust CFD solvers with accurate turbulence models and computers with the speed and memory capacity to perform the complex computations required to simulate this phenomenon. A CFD simulation provides fluid velocity, pressure, temperature, and other variables, as appropriate, throughout the solution domain for problems with complex geometries and boundary conditions. As part of the analysis, an engineer may change the geometry of the system or the boundary conditions, and observe the effect of the changes on fluid flow patterns or distributions of other variables. CFD also can provide detailed parametric studies that can significantly reduce the amount of experimentation necessary to develop new or modified equipment.
DaimlerChrysler's drive to simulate wind buffeting
For these reasons, DaimlerChrysler has a major effort underway to accurately simulate wind buffeting. Engineers have selected FLUENT CFD software from Fluent Incorporated, Lebanon, New Hampshire, because results in simulating other areas of the vehicle have provided strong confidence in the ability of the FLUENT solver to closely match physical testing results. Fluent also offers a wide range of proven turbulence models, which are relevant because of the important role played by turbulence in wind buffeting. In one recent study [Ref. 1], the modeling process began by importing a surface model of the outer shape of the vehicle and a CAD model of the vehicle interior into the ICEM CFD pre-processor. The simulation domain was defined to include the entire passenger cabin, which is connected to the external flow domain through the open window. Dummies representing the passengers were included in the model to correctly represent the volume of the passenger compartment. The vehicle surface was modeled to a significant degree of detail to capture flow development from the vehicle front end to the window opening. Several levels of local mesh refinement were used. Two refinement levels were applied outside the vehicle to capture the wake behind the vehicle. One refinement level was applied inside the passenger compartment to capture wave propagation inside the cabin. The finest refinement level was applied at the area of the opening to capture the shear layer. Models were developed for the case where the left side front window was open and all other windows were closed and for the case where the left side rear window was fully open and all other windows were closed.
DaimlerChrysler engineers modeled the turbulent flow using the RNG k-e and LES turbulence models, both of which have been shown in the past to provide good results for a range of turbulent conditions. Interior surfaces of the vehicle were assumed to be solid walls instead of soft surfaces such as carpeting or fabric. Actual car surfaces are less reflective and more absorbent than solid walls, giving the model a tendency to overpredict the wind buffeting phenomena. A steady state solution was first obtained for each case. The continuity and momentum equations were converged until the residuals dropped more than three orders of magnitude. These results were used as initial conditions for a subsequent series of transient simulations that were used to capture the pressure fluctuations in the vicinity of the open windows. Monitors were set up at the driver's and co-driver's ear locations and static pressure was recorded at these locations at every time step. The initial transients died down and the pressure traces reached a dynamically steady solution in roughly 300 to 500 time steps. Subsequently, pressure traces were recorded for time periods between 1.0 and 2.0 seconds - long to obtain a sound pressure spectrum. The pressure signals were then converted to the sound frequency spectrum by taking a discrete Fourier transform using a Hanning window filter. The sound pressure level (SPL) was finally converted to dB units.
Simulation provides superior diagnostic information
The CFD predictions were validated by comparing them to experimental measurements conducted in a wind tunnel. The CFD simulations not only accurately predicted buffeting frequency and sound pressure level but their predictions also accurately matched SPL and frequency variation trends observed in physical experiments. A key advantage of simulation is that CFD provides far more diagnostic information than can be obtained with physical testing. For example, to investigate air pressure around the critical front window area, engineers generated charts that displayed air pressure on the opening with filled contours while line contours were used to display air pressure on a plane parallel to the opening. A vertical stripe of low pressure followed by a high pressure stripe was observed in the opening plane. The stripes indicated that a vertical vortex occurs behind the A-pillar (the structural member between the windscreen and the front window). Animation of the transient solution clearly showed vortex movement with the local flow that impinges on the B-pillar (the structural member between the front and rear windows). The wave generated at the B-pillar was also shown as it propagated towards the passenger compartment. Being able to visualize the formation of the pressure waves and obtained detailed measurements of velocity and pressure at any point in simulations such as these has the potential to help engineers understand exactly how wind buffeting is occurring in a particular design and iterate quickly to an optimized solution.
These correlations provided engineers with the confidence to use CFD results to investigate a much greater number of possible design alternatives than would be possible with physical testing. The simulations showed that both the RNG k-e and LES turbulence models provide accurate results. The simulations correlated well with experimental measurements in showing that as the vehicle speed is reduced from 60 mph to 50 mph, the buffeting SPL and frequency go down 1 dB and 1 Hz respectively for the front window open case and by 5 dB and 1 Hz for the rear window open case. The frequency spectra and sound pressure level were very similar for all locations studied within the vehicle. An exhauster that connects the passenger compartment to the outside atmosphere was also evaluated and shown to have an insignificant impact on buffeting. Simulations with the left front window wide open and the right rear window open 1 inch were also performed. These showed that buffeting was substantially reduced. Simulations with a modified side mirror design that eliminates the neck reduced buffeting by 13 dB, which correlated well with experimental measurements. DaimlerChrysler engineers are making use of these results by simulating other vehicles, evaluating the influence of more parameters, and evaluating different modeling techniques. (The buffeting simulation methodologies were also recently applied to predicting noise levels for an SUV [Ref-2].) As simulation is more fully integrated into the design process, this approach should make it possible to substantially reduce wind buffeting.
SOURCE: Structured Information
2. An C.-F., Alaie S.M., Sovani S.D., Scislowicz M.S., Singh K., "Side Window Buffeting Characteristics of an SUV," SAE Paper No. 2004-01-0230 (2004).
References
1. Hendriana D., Sovani S.D., Scheimann M.K., "On Simulating Passenger Car Window Buffeting," SAE Paper No. 2003-01-1316 (2003).