Numerical Simulation Study On The Transient Three-dimensional Characteristics Of Car’s Wake Flow

Aerodynamic drag is an important factor that affecting the dynamic performance and economic efficiency of automobile, and the wake flow of a car can have great influence on the aerodynamic drag. A study on the wake of the automobile is of great significance for reducing the aerodynamic drag and developing low-drag cars. The external flow field of automobile is characterized by three-dimension, separation and unsteadiness. The conventional numerical calculation adopted steady simulation method, which was hard to capture all the detailed features of the wake stream. Therefore, it is particularly important to conduct transient numerical simulation on the external flow field of automobile.In this study, the transient numerical simulation method was adopted to research the wake characteristics of M1RA models with different tail shapes, and the application of DES the Detached Eddy Simulation method in the numerical calculation was discussed. It provided reference for improving the shape of cars and control their wake structures. Firstly, the steady external flow field simulation of an Ahmed model is carried out on the software FLUENT, where the three different kinds of turbulence models were used in the steady simulation. The results of simulation were compared with the existing experimental data, and the turbulence model which is better for steady simulation was confirmed. Secondly, the turbulence model were adopted in steady simulation of the MTRA models, the simulation results were compared with the existing experimental data, the rationality of the hybrid grid scheme was verified. Finally, the DES simulation method based on SST k-ε model was adopted in transient simulation of the MIRA models. In this transient simulation the steady calculation results were adopted as the initial conditions. The relevant information of the wake area of the car such as the turbulence energy distribution and velocity distribution were obtained. The following conclusions were made based on the numerical simulation presented here:(1) The turbulent kinetic energy in wake area of squareback car is the highest. And the range of wake vortex and drag coefficient of squareback car are also the maximum. Controling the range of wake vortex can help decreasing the aerodynamic drag of the car;(2) The drag coefficient obtained through transient calculation of the MIRA model with DES simulation method is less and closer to the experimental value than that obtained through steady calculation. Steady simulation may overestimate the turbulence kinetic energy of the wake of models.(3) There are little vortexes randomly distributed in the wake of the car, and the wake structure would vary with the flow of time. Although the car model is geometrically symmetric, the wake structure and transient velocity distribution in the wake area are not completely symmetric.(4) Through the analysis on the transient velocity field of the wake area of the fastback car, it can be seen that unsteady vortices may be produced at the back air window, and the airstream cannot always smoothly pass through the back air window. This means that DES transient simulation method can better capture the wake structure of the car in numerical simulation.