Effect Of Floor Boundary Layer On Aerodynamic Characteristics Of Vehicle And Its Control In Wind Tunnel

When the car is driving on the actual road,the car is moving relative to the ground and air,but the ground and air are relatively stationary,so there is no boundary layer on the ground.In the wind tunnel experiment of the car model,the air is moving relative to the floor and the car,and the floor and the car model are relatively stationary.The surface of the floor will develop and generate thicker and thicker boundary layer.The existence of the boundary layer will affect the flow field around the car model and its aerodynamic forces.Focusing on the above issues,the research of this paper is mainly divided into following three parts.:1 The development process of boundary layer of the floor surface and its influencing factors are studied,and corresponding conclusions are drawn:(1)In the interval x = 0.75 ~1m,the wall shear stress suddenly changed due to the transition.As the thickness of boundary layer continually increases,its growth rate continually decline.Due to the increasingly thicker boundary layers,a certain static pressure and dynamic pressure gradient are generated in the calculation domain.The laminar boundary layer is dominated by viscous stress,which rapidly decays after maintaining stability within a certain height.The turbulent boundary layer is dominated by viscous stress in the viscous sublayer,the Reynolds stress is dominant in the log law layer and the outer layer,and both exist in the buffer layer.The turbulent boundary layer differs only in the outer layer at different locations on the surface.During the transition process,the Reynolds stress continuously increases and the corresponding normal height decreases continuously when it reaches the maximum value.As for the viscous stress,the plateau near the wall disappeared and also continuously increased.There are intersection points between the velocity profile curve and the normal velocity gradient curve of the laminar and turbulent boundary layers and the abrupt change of the total pressure in the transition interval is detected at the height of intersection points of the normal velocity gradient.(2)The increase in roughness delays the occurrence of transitions,increases the growth rate of the turbulent boundary layer,makes the viscous sublayer of the turbulentboundary layer disappear,and the other layers move downwards.Increased turbulence intensity and wind speed make boundary layer transition more likely to occur,but decrease the growth rate of turbulent boundary layer.The increase of pressure gradient will delay the transition of the boundary layer,which will greatly reduce the growth rate of the boundary layer but will not affect the structure of the turbulence boundary layer.2 The driver model is used to study the influence of the floor boundary layer on the force of the car model and the surrounding flow field.The research results are as follows:(1)The presence of the floor boundary layer reduces the flow velocity at the front and bottom of the vehicle,causing a decrease in pressure at the head and an increase in pressure at the bottom of the vehicle.Affected by the change of the vehicle bottom flow field,the vortex shape in the upper part of the wake does not change but its intensity changes.Meanwhile,the shape and intensity of the lower vortex also change.The change in the wake causes the rear pressure to increase.(2)For a 1:2.5 scale model,the smaller the wind speed,the greater the impact of the boundary layer,but for the 1:4 scale model,the influence of the floor boundary layer is basically not affected by the wind speed.The smaller the scale model at the same wind speed,the greater the influence of the floor boundary layer.The influence of the floor boundary layer in the 1: 1 scale model is basically negligible.The floor boundary layer has different effects on different models,the notchback model has the most impact,and the fastback model has the least impact.3 The control scheme of the floor boundary layer of "lifting the floor + distributed suction" is proposed,the experimental bench is designed and the suction rules and characteristics of the porous suction plate are studied:(1)By designing the front and back edges,support,and platform dimensions of the test bench,it is ensured that the test bench will not have a negligible effect on the flow field around the model and its forces.The surface of the experimental bench uses porous suction plate to eliminate the boundary layer on its surface.(2)The DOE matrix of 120 samples is constructed and calculated using the full factor method.A rule is found that when the ratio of the hole spacing to the hole diameter is greater than 2.5,the thickness of the boundary layer has a negative correlation with the suction pressure,and vice versa.The negative pressure and high pressure are alternately changed onthe center line of each row of suction holes.When the ratio between the hole spacing and the hole diameter is less than 2.5,the low-speed high-pressure area formed above the flat plate of the trailing edge of the hole increases as the suction pressure increases.As a result,the thickness of the boundary layer increases.Based on the DOE matrix,a second-order response surface model was established as a prediction model.It is found that the hole spacing has the greatest effect on the suction effect,and that the interaction between the hole spacing and the hole diameter is the strongest.Based on the DOE matrix,a radial basis neural network model is established,and based on this,an adaptive simulated annealing method is used for optimization to find the optimal solution with the goal of minimizing the thickness of the boundary layer.The optimal solution successfully control the thickness of the boundary layer to less than 1.9mm,and the correctness of the optimal solution is verified.