A comparison of RANS-based turbulence modeling and PIV experiments for flow over a simplified road vehicle

Abstract

Purpose: This study compares a simplified car model's numerical and experimental result with different slant angles by PIV and CFD (Figure A) Theory and Methods: The freestream velocity on the numerical simulations and PIV measurements was 0.2 m/s, and the Reynolds number defined by the characteristic length of the Ahmed body was 4.18x10(4). The realizable k-is an element of, RNG k-is an element of, and SST k-is an element of turbulence models were used. The PIV was used to visualize the flow and to obtain the instantaneous and time-averaged velocity measurements. Results: The effect of the back tilt angle theta=15 degrees, 25 degrees, and 35 degrees on the flow characteristics downstream of the wake area of the Ahmed body was studied experimentally and compared with the CFD results. It can be seen that the flow separation point changes with the increasing slant angle. It is evident from SST k-is an element of, and PIV results that the flow separation from Ahmed body surface occurs at the lower side of the slant surface at theta=15 degrees, whereas at theta=25. and theta=35 degrees the separation point shifts upwards and occurs at the upper edge of slant surface. For the results of the realizable k-e and the RNG k-e models, the separation point shifts upwards at the upper edge at theta=35.. The SST k-is an element of model predicts the flow separation region in agreement with the experimental results for the cases of theta=15. and theta=35 degrees. Conclusion: The flow separation from Ahmed body surface occurs at the lower side of the slant surface at theta=15 degrees, whereas at theta=25 degrees and theta=35 degrees, the separation point shifts upwards and occurs at the upper edge of the slant surface. Therefore, it is also concluded that the most proper slant angle for aerodynamic and hydrodynamic application flow structure angle is 25 degrees as it has the lowest wake region according to experimental results. Moreover, the pressure drag is around 90% of the total drag and mainly increases with the increasing slant angles.