Research On Vehicle Drag Reduction Based On Plasma Active Flow Control

Plasma flow control technology has gradually become one of the key directions in the field of flow control due to its simple structure,fast response,low power consumption and easy layout.In this paper,the plasma actuator was used to study the wake flow control of automobile.Through wind tunnel test methods such as aerodynamic measurement,surface pressure measurement,and PIV velocity measurement,the control mechanism and law of the linear plasma actuator and the plasma vortex generator on the wake field of Ahmed model were described.At the same time,the parameters of Suzen simulation model were optimized based on the characteristics of ion wind velocity obtained from the static test,which improved the simulation accuracy of ion wind,was successfully applied to the simulation of complex wake flow control,and it could be used as an important reference model in the flow control research in different fields.Firstly,the aerodynamic characteristics of the ion wind were studied in a static environment,and the effect of the structural parameters of the linear plasma actuator and the excitation parameters on the strength of the ion wind was analyzed.The maximum ion wind velocity was used as the main index to determine some parameters such as the width of the upper and lower electrodes,electrode gap,dielectric thickness and excitation frequency of the actuator so as to improve the performance of the actuator and obtain the characteristics of ion wind velocity under different excitation voltages,which provided detailed data reference for establishing the relationship between ion wind intensity and flow control effect as well as parameter correction of simulation model.Secondly,the linear actuator was used to conduct a comprehensive study on the wake flow control of the fastback Ahmed model and the effects of the actuator installation position,excitation voltage,freestream velocity,crosswind environment and combined conditions of multiple actuator on the drag reduction were analyzed.It was found that the linear actuator could inhibit the generation of separation bubble at the tail of the model by injecting momentum at the airflow separation point.The change of tail pressure was affected by two factors: local airflow acceleration and separation bubble suppression above the slanted surface.Although the airflow acceleration at the top of the slanted surface caused the pressure drop in this area,the suppression of separation area led to the overall pressure rise in the tail area,thus the aerodynamic drag was reduced.When the velocity ratio between the maximum ion wind and the free steam reached over 17%,the maximum drag reduction rate could be achieved by the actuator control,which was about-7.9%.In the crosswind environment,the drag reduction effect of the actuator gradually decreased with the increase of yaw angle.Thirdly,the linear plasma actuator combined with curved tail plates was used to conduct a comprehensive study on the wake flow control of the square-back Ahmed model,and the effects of the diameter of the tail plate,the installation angle of the actuator,and the excitation voltage on the drag reduction were analyzed.It was found that the linear plasma actuator could effectively delay the flow separation on the tail plate,shorten the wake length and smooth the wake at the same time,resulting in the increase of the average pressure in the wake,thus reducing the drag.Through comparing of the drag reduction rates at different installation positions,it was found that the maximum drag reduction was obtained at an installation angle of 15 °.The optimal size of the tail plate was not fixed,which depends on the ratio of the active drag reduction rate of the actuator and the passive drag reduction rate of the tail plate to the total drag reduction rate.When the wind speed was low,the active drag reduction rate was dominant.At this time,the optimal tail plate diameter was the same as that when the maximum active drag reduction rate was obtained,which was 40 mm.At high wind speeds,the ionic wind control capacity decreases,the passive drag reduction rate dominated and the optimal tail plate diameter was the same as that when the maximum passive drag reduction rate was obtained,which was 50 mm.At low wind speeds,an excitation voltage of 17 kV could obtain a maximum total drag reduction rate of 9.02%.If the ionic wind intensity continued to increase,it was expected to achieve greater drag reduction effects.Fourthly,the plasma vortex generator(DBD-VG)was applied to the wake flow control study of the fastback Ahmed model and the effects of some factors such as counter distance,backward distance,stream-wise length and installation position of the vortex generator on the drag reduction were analyzed.It was found that the stream-wise vortex induced by DBD-VG could promote the mixing of high-speed airflow above the slanted surface at the end of the model and low-speed airflow near the wall by increasing the turbulence of airflow,thus inhibiting the generation of separation bubbles and reducing drag.The drag reduction decreased with the decrease of the total discharge length of DBD-VG,but its back distance had the largest effect on the drag reduction rate,followed by the counter distance,while the shortening the stream-wise length had the least effect on the drag reduction rate.When installing the DBG-VG,the end of the actuator should be arranged near the separation line,so that the generation and development of the stream-wise vortex was prior to the separation line,and the flow control effect was the best at this time.It was no need for DBD-VG to be precisely arranged at the airflow separation point,so it was more versatile than linear plasma actuator.At low speed,the maximum drag reduction rate of-8.51% was obtained at 13 kV excitation voltage under the control of DBD-VG,which had stronger a flow control ability compared with linear actuator,but at the same time it required more energy consumption.Lastly,using Suzen's numerical simulation model,the Lorentz force obtained from solving the plasma equation was introduced into the body force term of Navier Stokes Equation to realize the coupling solution of the plasma equation and the fluid equation.Based on the static test results of the ionic wind,the Suzen model was modified to make it suitable for different excitation voltage conditions,which provided a simulation parameter for the study of plasma flow control under the different excitation intensity.And the modified body force and charge density distribution conformed to the variation trend of plasma discharge,and the error of the maximum ionic wind speed to the test results was within 5%.The modified Suzen model was successfully applied to the flow control of the vehicle outflow field,which was in good agreement with the wind tunnel test results.At the same time,the results of transient numerical simulation using large eddy simulation showed that the actuator restrained the generation of periodic separated vortex at the tail of the model and the vorticity in the wake flow decreased significantly,thus reducing the energy dissipation and aerodynamic resistance of the model.This paper has promoted the development of plasma flow control technology in the field of automobile drag reduction through wind tunnel tests and numerical simulation studies,and has accumulated important empirical methods and data foundations for the practical application of this technology.