Research On Aerodynamic Characteristics Of High Speed Bus Based On Fluid-structure Interaction

Energy conservation, environmental protection and safety are the main themes of the automotive industry. The high speed bus is the blunt body with a large windward area, which leads to a larger aerodynamic drag force. So it is significant to study the influence of the aerodynamic characteristics on high speed bus. The windshields suffered the front pressure can generate stress concentration even lead to be brokendue to an unreasonable design, and it is dangerous for the driving safety. Therefore, it is significant to study the aerodynamic characteristics and the wind-induced vibration characteristics of the windshields based on the fluid-structure interaction (FSI) for the energy conservation and the driving safety of the high speed bus.In this paper, the methods of theoretical analysis, numerical simulation and experimental study are combined to study the aerodynamics characteristics and the windshield’s wind-induced vibration characteristics of the model car and the high speed bus.First, the aerodynamics characteristics of a homemade high speed bus of steady state are studied. A one-tenth high speed bus model is made. The aerodynamics drag coefficient of the bus model is measured by low speed wind tunnel and the precision balances. The RNG k-ε turbulence model are used to study the aerodynamic characteristics of the high speed bus, and experiment results verified the correctness of numerical simulation. The aerodynamic characteristics of steady state are obtained via flow field analysis. Then influence of seven different structures parameters of windshield of the bus on the aerodynamic characteristics are studied, i.e., the inclined angle of windshield, the arc radius of the front side, the inclined angle of the head underside, the inclined angle of the top rear side, the arc radius of the rear side, the dihedral angle of the rear floor panel, the dihedral angle of the front floor panel, and the influence law of the structure parameters changes on the aerodynamic characteristics are obtained. The results indicate that the dihedral angle of the tail underside influence most on the aerodynamics drag coefficient, which can decrease8.5%.Second, the aerodynamic characteristics and wind-induced vibration characteristics of the model car are studied. A steel model car with a400mm X400mm×2mm thin glass windshield is made. The aerodynamic drag coefficient of the model car and the deformation of the windshield’s center point are measured by a wind tunnel experiment with a precision eddy current displacement sensor. The results of the experiments show that the deformation of the windshield is approximately proportional to the wind speed, in other words, the glass can be regarded as a linear elastic material under certain pressure range. The numerical calculation with the FSI model is carried out to study the aerodynamic characteristics of the model car and the wind-induced vibration characteristics of the windshield at transient state. And experiment results verify the numerical simulation methods. The results indicate that the aerodynamic drag coefficient considering the FSI is higher than that ignoring the FSI. The windshield undergoes a noticeable deformation under the wind load, and the maximum displacement is at the center of the windshield. Both of the deformation curves and the stress curves at different wind speed have the same regular pattern, and both of them increase with the speed. In addition, the deformation is approximately proportional to the wind speed. The maximum stress area locates on the upper, lower and side edge of the glass, and the maximum stress point locates on the upper and lower corner, while the minor main stress area locates in the central region of the glass, and the distribution of the main stress seems like a plum blossom, where the stress concentration occurs easily. Both of the transient maximum displacement and stress caused by variable wind load is much higher than the time average deformation caused by the uniform wind load.Based on the above researches, the transient aerodynamic characteristics of a homemade high speed bus and the transient wind induced vibration characteristics of the PVB windshield is studied finally. The equivalent thickness method is used to deal with the PVB windshield; The FSI model is used to study the aerodynamic characteristics of the bus, and the wind induced vibration characteristics of the PVB windshield under the steady wind load, the fluctuating wind load and different thickness of the windshields.The results under the steady wind load show that the aerodynamic drag coefficient of the bus considering the FSI is slightly higher than that regardless of the FSI under long-term steady wind load, and the increasing degree is related to the deformation. The windshield undergoes a slight deformation in the travelling process, which increases with the speed. The deformation is much higher when a accelerated motion just change to a uniform motion; The deformation descends quickly and fluctuates violently due to the action of the inertia force, then becomes stable quickly, at last the deformation is very approaching to the time average deformation. The transient maximum deformation is much higher than the time average deformation throughout the process. The deformation distribution of the windshield under long-term steady wind load is symmetrical, and the maximum displacement is slightly lower than the center point of the windshield. The transient stress trends of the windshield are similar to the transient deformation trends of the windshield. The transient maximum stress is much higher than the time average deformation throughout the process. The main stress areas of the windshield locate on the upper, lower and side edges, and the maximum stress point locates on the corners.Under three different thicknesses of the windshields, i.e.,4mm+0.76mmPVB+4mm,3.5mm+0.76mmPVB+3.5mm,3mm+0.76mmPVB+3mm, the results show that the influences of the windshield thickness on the transient deformation and stress of the PVB windshield are significant. The deformation increases significantly with the thickness decreases, but the stress changes irregularly.The results under the transient wind load show that the influences of the fluctuating wind’s characteristics on the transient aerodynamics characteristics of the high speed bus are significant. While the amplitude and frequency of sine function pulsating speed load change, the time averaged value of the bus’s aerodynamic drag coefficient varies with the cosine function regulation. It is entirely different to the constant time averaged value under steady wind load. The changing laws of the transient aerodynamic drag coefficient, transient wind induced deformation and the transient stress of the windshield are similar. The transient fluctuation intensity of the three parameters increases obviously with the increase of the amplitude, and the amplitude of time averaged value increases significantly. Analogously, the transient fluctuation intensity of the three parameters increases with the frequency. However, not only the frequency of time averaged value increases homogonously, the amplitude increases significantly.In this paper, effects on the steady aerodynamic characteristics owing to structural parameters are investigated, and the influence rules are obtained. The transient aerodynamic characteristics and the transient wind-induced vibration characteristics of the model car and high speed bus with different thickness of windshields under the steady wind load, the fluctuating wind load are studied based on the FSI, and the relevant laws are obtained. Especially consider the effect of the fluctuating wind conditions on the drag coefficient, wind-induced windshield deformation and stress are far greater than the steady wind conditions. The results of this study can provide a theoretical basis for the aerodynamic shape optimization, the safety design and the lightweight design of the high speed bus, and have important theoretical and engineering application value.