Effect of upstream flow characteristics on the wake topology of a square-back truck

Abstract

The influence of upstream flow characteristics on the bi-stable flow structure in the wake region of a simplified square-back heavy vehicle model at a Reynolds number of 2.7 × 104 was investigated by using the improved delayed detached eddy simulation method. The asymmetric wake structure of this model and its corresponding aerodynamic response were examined, aiming to identify the effect mechanism of three inlet profiles on the asymmetric wake structure of the named ground transportation system (GTS) model in simulations. The accuracy of the numerical method used in this study was validated by comparison with wake structure data, including the flow states, vortex core's location, and aerodynamic drag obtained from previous large eddy simulations and water channel experiments. The numerical results show that different turbulent inlet velocity profiles lead to different wake topologies. When the turbulent velocity profile with a turbulence intensity of 15% generated by TurbSim, a stochastic inflow turbulence tool for generating turbulent velocity inlet on an atmospheric boundary layer profile, is used, the expected bi-stable flow topology is still observed, but it is not shown in the case by means of the turbulence generator incorporated into ANSYS Fluent. Those turbulent inlet velocity profiles contribute to the increase in GTS model's aerodynamic drag forces. Compared to the uniform velocity profile, the TurbSim velocity profile can achieve a drag increase in 7.23%. In addition, this turbulent profile intensifies the flow fluctuations in the wake region and enhances the transient response frequency of the wake region. Thus, when assessing the vehicle aerodynamic performance in open air, especially under crosswinds, the real turbulence velocity profile, e.g., the profile generated by TurbSim in the current study, is recommended to be used for a more accurate prediction in numerical simulations.