Research On Automotive Aerodynamic Lift And Its Effects On Straight-Line Driving Ability

With the rapid growth of high-grade highway traffic mileage, the average speed of automotive is greatly improved. The body weight is significantly reduced for the tendency of making light automotive which is caused by energy crisis and rising oil prices. For these reasons, the automotive aerodynamic characteristics, especially aerodynamic lift, have increasing influence on the automotive performance. With the continuous improvement of living standards, consumers put more emphasis on vehicle safety, comfort and so on, thus higher requirements for the quality of the automotive is put forward. As one of the most important evaluating indicator of aerodynamic performance, aerodynamic lift not only directly affects the safety of the automotive, but also indirectly affect the automotive’s fuel economy. The phenomenon that the driver feels the automotive shimmy dangerously at high speed arouses the concern of the relevant researcher on aerodynamic lift.So far, the experimental and computational methods about drag coefficient have been developed maturely, and experimental results agree well with computational results. As to the lift coefficient, experimental results are always different from computational results. Relative to the drag coefficient, the lift coefficient is more vulnerable to the uncertainties of the model installation, wind tunnel structure and so on, thus the repeatability of the lift coefficient is bad, and the lift coefficients are quite different for the same model which is measured in different wind tunnels. The lift of automotive is mainly from the pressure difference between its upper surface and underbody. For the uneven surface of chassis, the flow field around the bottom is complex when the automotive is driving close to the ground, thus the computational results are vulnerable to the influence of the grid, turbulence model and so on, and are often different from the experimental results. In addition, different from drag coefficient which is the smaller the better, the lift coefficient optimization objectives need to be analized specificly according to the actual situation, for a racing car a large amount of negative lift is required to help it cornering at high-speed, while for a heavy duty truck,.the negative lift is want to be reduced to decrease the frictional risistance between the tires and the floor, so as to achieve low fuel consumption. In summary, start the experimental and computational study on aerodynamic lift, and find out a reasonable way to evaluate the aerodynamic lift characteristics of different automotive not only of great theoretical significance, but also to conduct the project development.According to the problems mentioned above, the objective of this paper is to find out a method which can obtain high accuracy aerodynamic lift coefficient by a large number of experiments and simulations. On this basis, the influence of aerodynamic lift on vehicle straight-line driving ability is analyzed by combining with the multi-body dynamics; meanwhile the styling parameters which are sensitive to aerodynamic lift are identified. Finally, series schemes which are useful to improve vehicle straight-line driving ability are put forward. The main research contents are as follows:1. The influence of model installation and the thickness of the boundary layer on the results of lift measurement are discussed by conducting a large amount of experiments in HD-2wind tunnel of Hunan University. Based on this, the relationship between drag coefficient and lift coefficient is discussed by conducting wind tunnel tests with9different vehicle models. Finally, the laws that the lift coefficients change with the ground clearances, pitch angles, yaw angles are also studied by wind tunnel tests, its mechanism was analyzed by combining with surface pressure tests and PIV tests.2. How to improve the numerical accuracy and efficiency is discussed by simulations, the works were done as follows:①Comparing the results of different grids, different wall functions, different turbulence models, different pressure-coupled schemes and different pressure discretization schemes;②Discussing the influence of velocity gradient adaptation and Y+adaptation on improving the grids and the results accuracy;③Based on Ahmed model, the empirical parameters in the Realizable k-ε model were optimized by DOE.3. Based on a large number of simulations on model and the wind tunnel tests about some MPV, the sensitivity of the aerodynamic lift coefficient to the styling parameters was analyzed.4. Created the dynamic model of some automotive, and discussed the influence of aerodynamic lift on the straight-line driving ability of this automotive.5The aerodynamic characteristics of some automotive which is from an engineering project is analized, and its straight-line driving ability under aerodynamic force is also discussed. Based on this, Styling of this automotive is optimized according to relative optimization rules, and its straight-line driving ability is improved.