Influence Of Bionic Fence Structure On Noise Around A Circular Cylinder

The expanding transportation network brings convenience to human society,but also brings serious noise pollution problems.With the rapid development of high-speed train and aircraft industry,as an important source of noise,the noise control of aircraft landing gear and train pantograph has attracted great attention.The research shows that although the shapes of landing gear and pantograph are quite different,they can be approximately simplified as the sound generation problem of the basic component cylinder.The research on the noise generation mechanism and noise reduction methods of cylinder is conducive to the development of noise reduction technology of landing gear and pantograph.With the development of bionic noise reduction technology,the owl wing structure with the ability of silent flight has attracted people's attention.This paper imitates the owl wing"canopy"structure,and extends and develops it,designs a fence structure which can be used to control the flow noise around a cylinder,and explores the noise reduction effect of the fence structure on the cylinder surface by combining experiment and numerical simulation,The mechanism of noise reduction is analyzed in detail from the aspects of wake flow state and pressure fluctuation characteristics.The main research contents are as follows:(1)Firstly,the actual working conditions of the landing gear and pantograph are investigated.Considering the wind tunnel test conditions,such as wind speed and wind tunnel size,the diameter of the reference cylinder D=30mm and the test wind speed(20m/s?60m/s)are determined.The corresponding Reynolds number range is 4.1×10~4?1.23×10~5.Then,the fence structure is parameterized,and the fence structure is determined by the following parameters:fence diameter A,fence spacing S,fence thickness B.Setting the fence thickness B=3mm unchanged,changing the fence diameter a and fence spacing S,four different fence diameters(48mm,42mm,36mm,33mm)and four different fence spacing(20mm,15mm,10mm,6mm)are considered.(2)In the 0.55M×0.4m acoustic wind tunnel,the acoustic measurement of the reference cylinder and the bionic fence structure cylinder is carried out.The nine channel far-field arc array microphone is used to measure the acoustic field.The results of sound field experiment show that:1)the diameter of fence has great influence on noise reduction and directivity.If the fence diameter is too large,it will not reduce the noise,but will increase the peak noise and the total noise pressure level.In the appropriate diameter range,such as A/D=1.1-1.2,the smaller the diameter,the better the noise reduction effect.Under the same incoming flow condition,A33S10 model can reduce the peak noise by 31d B,while A36S10 model can only reduce the noise by 8d B.2)For example,A33S10 model can reduce the peak noise by about9d B when the incoming flow speed is 20m/s,and it can completely suppress the peak noise when the incoming flow speed is 30m/s.The amount of noise reduction is up to 29d B.At higher speed,the amount of noise reduction is up to 31d B.3)In the range of parameters tested in this experiment,the change of fence spacing has little effect on the noise reduction effect.4)The fence structure has good noise reduction effect on high frequency noise(in the frequency range of 10000-25600Hz).(3)The sound field experiment results show that the noise reduction effect of A36S10and A33S10 models is better,and A33S10 model has the best noise reduction effect,and A33S10 model is not sensitive to the incoming flow conditions,which can obtain robust noise reduction effect under all test conditions.Therefore,particle image velocimetry(PIV)is used to measure the wake velocity field of A33S10 bionic cylinder fence structure model with significant noise reduction effect.The experimental results show that the fence structure can significantly change the wake characteristics of the cylinder,reduce the wake velocity and decrease the vortex shedding of the cylinder,which can significantly inhibit the typical cylinder flow around Carmen vortex street.(4)In order to further reveal the noise reduction mechanism of the fence structure on the cylinder surface,the flow and noise distribution of the reference cylinder,A36S10 and A33S10 models were numerically studied by using large eddy simulation(LES)method when the incoming flow velocity was 60m/s.Firstly,the correctness of the numerical method is verified based on the experimental results.Then the lift and drag coefficients of the three models are studied.Finally,the noise reduction mechanism of the cylindrical surface fence structure is deeply analyzed.The results show that:1)The results show that the surface fence structure can greatly reduce the fluctuation of lift and drag coefficient and the mean value of drag coefficient,and the A33S10 model has the best aerodynamic performance.The model can reduce the RMS of lift coefficient fluctuation by 61.39%,the RMS of drag coefficient fluctuation by 23.25%and the mean value of drag coefficient by 38.69%.2)The fence structure breaks the continuity of shedding vortices in the spanwise direction of the flow around the cylinder,and the continuous large vortices are broken into small vortices,which further destroys the typical Karman vortex street effect,thus effectively reducing the aerodynamic noise of the cylinder.3)The fence structure can significantly reduce the pressure fluctuation amplitude on the cylinder surface,which is conducive to reducing the noise radiation and the total noise pressure level of the cylinder.In conclusion,the fence structure imitating owl wings can effectively improve the aerodynamic performance of the flow around the cylinder,and significantly reduce the aerodynamic noise of the cylinder.Moreover,the smaller the size of the fence can obtain obvious noise reduction benefits,and has little impact on the structural strength of the components,so it has a certain application prospect in engineering noise control.