Analysis And Evaluation Of Aerodynamic Noise Of A Convertible Based On Flow Field Characteristics

In recent years,as the level of national economy incresed,the modern automobile industry is developing rapidly,people’s demand for automobiles is also increasing,and the quality requirements of automobiles are also increasing.In addition to the most basic transport function of car,personality,science and technology are also required.The opentop car body structure of the convertible car can not only meet people’s individual needs,but also enable people to experience a good outdoor sense while pursuing a sense of science and technology.It is a way of travel closer to nature.However,the ’open’ characteristics of the convertible are bound to affect the driving experience.Therefore,this paper takes a convertible as the research object,and performs steady-state and transient simulation calculations through the computational fluid dynamics CFD platform to analyze the development of the flow field of cockpit.A-weighted sound pressure level and the objective evaluation parameters of sound quality are introduced to analyze the noise feeled by passengers,and then the local modeling is optimized based on the flow field results,which provides a reference for the subsequent development of the convertible.The research contents are as follows :Firstly,the CAS model of the convertible car is built and pre-processed to simplify the detail characteristics that have little effect on the cabin flow field and aerodynamic noise.The convertible car model is meshed,and then the stable / transient flow field and aerodynamic noise numerical simulation parameters are set based on the CFD fluid simulation analysis platform.The final mesh scheme is confirmed by mesh independence verification,and the accuracy of the simulation method is verified by the standard MIRA model.Secondly,through the visualization of steady-state and transient flow fields,the characteristics of flow field changes under different speeds and different yaw angles are analyzed,and the causes of aerodynamic noise are explored through turbulent kinetic energy,vortex generation and development,and velocity field.Then,the broadband noise source model is used to predict the distribution and level of the aeroacoustic noise source.At the same time,monitoring points are set near the left and right passengers’ ears to monitor the pressure pulsation.The pressure curve is converted into an A-weighted sound pressure level spectrum curve by FFT,and then the total sound pressure level of the monitoring points is calculated to quantitatively analyze the human ear noise,and the psychoacoustic objective evaluation parameters are introduced to analyze the human ear noise characteristics in multiple dimensions.Finally,based on the flow field analysis,the local shape of the car body which affects the aerodynamic noise and the cabin flow field is optimized,and the influence of a single variable on the aerodynamic noise and the cabin flow field is studied.The following conclusions are obtained through simulation analysis :(1)There is a reverse backflow near the back of the convertible,which collides with the separated airflow of the front windshield and causes the high-speed vortex to develop towards the cockpit.The airflow from the rear of the seat into the cabin will form two opposite vortices centered on the left and right passenger positions,and form multiple relatively small vortices near the human head and seat.(2)The negative pressure in the cabin decreases with the increase of velocity,and the flow separation point of the front windshield gradually moves forward with the increase of velocity.Under different yaw angle conditions,the airflow involved in the cockpit from the windward side will directly hit the head and face of the occupant on the leeward side,so that the vortex around the two occupants without yaw angle gradually moves to the leeward side with the increase of yaw angle and forms multiple local small vortices on the leeward side.(3)The noise source of the convertible car is mainly dipole and quadrupole sound source,the distribution range is almost the same,all over the whole body surface,but the dipole sound energy is much larger than the quadrupole sound energy;the spectrum curve shows that the noise characteristic is broadband noise source.The A-weighted sound pressure level of the outer ear of the two passengers near the outside of the cockpit is about 5 d BA higher than that of the inner ear of the passenger near the middle of the cockpit.For every 20 km/h increase in vehicle speed,the A-weighted sound pressure level increases by about 5 d BA.(4)From the sound quality evaluation parameters,loudness,sharpness and roughness,it can be seen that the aerodynamic noise of the convertible car belongs to the ’ rough ’ sound with larger loudness and smaller sharpness;when the speed reaches 100 km/h,the loudness value begins to be greater than 50 sone,which has reached the range of damage to human hearing.(5)When exploring the influence of a single variable on the cabin flow field and aerodynamic noise during structural reshape,it is found that raising the rear extension height of the front windshield,extending the length of the front windshield,increasing the tilt angle of the front windshield,reducing the windward area of the rearview mirror handle,and reducing the back height can improve cockpit flow field reduce the aerodynamic noise of the passengers’ ear.The purpose of this research is to help the research on aerodynamics of convertible cars,and to provide reference for the development of subsequent car companies and academic research related to convertible cars,which has certain academic value and guiding significance.