Study On The Aerodynamic Characteristics Of Fastback Passenger Car

This paper is studied on the external flow field of a fastback car with numerical simulation methods. The results are analyzed qualitatively and quantitatively. This project is based on a fast MIRA model and the distributing of the flow field around this car model is shown with numerical simulation methods. Based on the parameter we have known, a 3-D model is built. The mesh generation and flow analysis have been done in commercial software, STAR-CD. And the 3-D flow field is simulated. The velocity and pressure distributing of some section are shown. The aerodynamic force coefficients are obtained. The simulation results are compared with the test results and the aerodynamic characteristic of this fastback car is studied. The concrete contents are following: 1) Based on the fastback MIRA model's basic dimension, the 3-D model is built, and so the external flow field is simulated. First, the wake isn't refined, then the wake is refined in some region, at last the whole wake is refined. With the three meshes, the results are compared. 2) As for the refined wake, the velocity field and pressure field are shown and its drag coefficient are compared and analyzed with the test value. And its wake survey is analyzed in detail. 3) Based on the flow around the car model, the flow complexion is shown. And so its wake's influence on the aerodynamic characteristic is studied. 1. The three numerical simulation results'analysis 1) Based on the three numerical simulation results, the results of refined wake is better. The trailing eddy and the cut eddy of the fastback car are shown clearly, which shows that the most information of the flow field are obtained. But the unrefined meshes'results has got the greater error and these eddies aren't clear. The results of the whole wake refined are better than the results of some region of wake refined. This has shown the modality of the refined meshes has influence on the results and the results the refined meshes are better in qualitative analysis. 2) As for quantitative analysis of the three results, the drag coefficient of the results of some region of wake refined is more closely to the test value whose error is the lest. And the error of unrefined meshes is greater than the others. But the error of the whole wake meshes refined is no better than the results of some region of wake refined and the precision has decreased by contraries. From this point of view, the pertinent mesh-refinement work would not only improve the precision but also cut down the excess time which has been spent on large scale mesh-refinement. 2. The analysis on the aerodynamic characteristics of the fast back car 1) It is clear of the pressure distribution on the front part of the automobile, the bonnet, the front windscreen and the roof. The coming flow from ahead is slowed down due to the obstruction of the head. And as a result the positive pressure area is formed and airflow is divided into two parts. One goes upwards passing the bonnet, the front windscreen, the roof; the other part goes downwards beneath the car to the rear. 2) The airflow above separates in its passing way and flows so fast as to form a huge attraction peak around the upper corner. Descending onto the bonnet at high speed, the airflow formed a relatively large negative pressure on the front part of the bonnet and obstructed by the front windscreen, there also exists a concave corner which turns to a stagnation area with the positive pressure. 3) The airflow passes through a corner to the upper side of windscreen, and brings a negative pressure on the front top of the car. As the top of car is in the shape of arc, the airflow flows fast but doesn't change very much. There is no separated flow appears. The airflow changes its direction but doesn't separate at the rear windscreen. 4) In the wake, the high-speed airflow from the top and the side converge and brings portrait eddies. At the same time, the airflow from the beneath and the side converge and cause complex backsets. At the beginning, the two backsets are separated and circle in different clockwise and the upper has the higher energy. But in very short time, the two backsets influence each other and a pair of trailing vortex comes into being. With the increase of distance from the rear of the car, the...