An URANS Simulation of the Kelvin-Helmholtz Aerodynamic Effect over the Ahmed Body

Abstract

The reduction of energy consumption of cars is always a significant issue in automotive design. Turbulent flow around a car body is very difficult to simulate accurately due to the complexity of the flow conditions around the body, such as complex flow separation and laminar to turbulent flow transition. In particular, flow over the Ahmed body with a rear angle of 25o is considered a challenging problem for the RANS approach with two-equation turbulence models. In this study, we aim to analyze the Kelvin-Helmholtz instability associated with this flow with a URANS approach. Methodology for utilizing the URANS method is fully discussed. The predicted velocity profiles and drag coefficient are com-pared with experimental results. Three turbulence models, such as the k-ε, k-ω and SST models, are assessed and validated with experimental data. The aim of the study is to evaluate the performance of these models for the study of the Kel-vin-Helmholtz instability over the Ahmed body and for car bodies generally us-ing experimental data for their validations. It is found that the URANS approach with the turbulence models with proper numerical treatment can perform as well as or even better than the LES. And the SST model shows the best performance compared with other turbulence models.

An Arbitrary Hybrid Turbulence Modeling Approach for Efficient and Accurate Automotive Aerodynamic Analysis and Design Optimization

“…As a result, Both LES and DES have very high computation costs, which hinders the practical application of DES and LES for high Reynolds number flows around Ahmed Body [13] for ADO. It is also noteworthy that Igali et al [5] and Sadykov et al [8] showed that the RANS and URANS models could generate time-averaged results that are comparable to those of LES. This is a challenge to the established assumption that the LES/DES is always superior to the more efficient RANS/URANS approaches if the detailed resolution of the turbulent eddies is not required in engineering design.…”

“…There are many published studies in the literature that present the results of simulations aimed to provide insight into the flow around the Ahmed body. For example, two recent studies showed the wake structures of the vehicle in detail by Igali et al [5] and Sadykov et al [8], respectively, with the former using the RANS approach and the latter the URANS one. Their usefulness is in the comparison of results obtained by the RANS and URANS approaches with different turbulence models, where Igali et al [5] achieved 2.474% of error in comparison with experimentally measured drag coefficient using the RANS approach, while Sadykov et al [8] obtained 1.68% error using the URANS method.…”

“…For example, two recent studies showed the wake structures of the vehicle in detail by Igali et al [5] and Sadykov et al [8], respectively, with the former using the RANS approach and the latter the URANS one. Their usefulness is in the comparison of results obtained by the RANS and URANS approaches with different turbulence models, where Igali et al [5] achieved 2.474% of error in comparison with experimentally measured drag coefficient using the RANS approach, while Sadykov et al [8] obtained 1.68% error using the URANS method. Moreover, each of the papers has a detailed flow visualization of the wake region at the back of the vehicle model.…”

“…As a result, Both LES and DES have very high computation costs, which hinders the practical application of DES and LES for high Reynolds number flows around Ahmed Body [13] for ADO. It is also noteworthy that Igali et al [5] and Sadykov et al [8] showed that the RANS and URANS As a result, it can capture the turbulent eddies near and at the rear of the Ahmed Body, which is crucial for properly capturing the Kelvin-Helmholtz instability of shear layers there. However, as the VLES model is a relatively new approach, there are no crucial studies that have been performed to evaluate its performance and efficiency for its potential application in Aerodynamic Design Optimization (ADO) for automotive vehicles.…”

“…The URANS model The URANS model used is a revised RANS model that describes unsteady turbulent flows with the largest scale above the integral one. Léonard et al [25] and Sadykov et al [8] claimed that the URANS method was a good model that could provide a qualitative understanding of the key physics in transient turbulent flows.…”