Experimental And Numerical Research On Flow And Noise Characteristics Of Impinging Jet

At present,many studies on the acoustic field of impinging jet mainly focus on the noise characteristics and sound source mechanism of high-speed or supersonic impinging jet,but few studies on the acoustic field of low-speed and unsteady-speed impinging jet have been mentioned.Low-speed impinging jet and unsteady-speed impinging jet are widely applied in the field of heat dissipation and cooling.However,most studies focus on their flow and heat transfer characteristics,but neglect the noise problem caused by them.Therefore,an experimental platform for measuring the velocity and noise of impinging jet is designed and built,and a numerical simulation scheme for the acoustic field of impinging jet is established.The noise characteristics and sound source mechanism of low-speed impinging jet and unsteady-speed impinging jet are studied.Firstly,the flow and noise characteristics of jets impinging on flat plate and cones are experimentally studied.The noise characteristics and sound source mechanism of jet impinging on flat plate and cones are analyzed and compared.The flow field and acoustic field of jet impinging on cones are obtained under different impinging velocities,impinging distances and cone angles.The effects of cone angles and impinging distances on the sound source mechanism of jet impinging on cones are investigated.The noise directivity of of jet impinging on cones is obtained under different monitoring points.It provides an experimental basis for optimization of numerical scheme for the acoustic field of jet impinging on the flat plate.Secondly,based on large eddy simulation(LES)and FW-H acoustic analogy,the numerical simulation scheme for noise characteristics of jet on impinging a plate are established.Effects of the size of acoustic integration surface and the sub-grid scale model on the simulation results are studied for the optimization of numerical scheme.In the optimization scheme,the size of acoustic integration surface is 130 mm ×130mm×130mm,and the sub-grid scale model is KET model.The maximum error between the simulation results and the experimental data is 1.3dB and the maximum relative error is 2.8% under different impinging distances,which validates the reliability of the optimization scheme.The research provides a basis for the numerical study on the noise characteristics of the periodic impinging jet on a flat plate.Then,the the noise characteristics of the periodic impinging jet on a flat plate are numerically studied by the above optimization scheme.The periodic waveforms of the impinging velocity are sinusoidal,triangular,sawtooth and rectangular,respectively.The periodic frequency varies from 5Hz to 40 Hz.The results show that compared with the impinging jet with constant velocity,the noise of periodic impinging jet is larger and increases with the increase of the periodic frequency.The noise in the high-frequency band is greatly affected by the periodic waveform and the periodic frequency.Compared with the other three waveforms,the rectangular waveform has a large difference in noise characteristics.The main reason is that the unsteady velocity leads to the unstable fluctuation of the fluid pressure at the nozzle outlet.The greater the change of unsteady speed is,the greater the intensity of unstable fluctuations is.Finally,The symplectic algorithm is applied to the numerical study of noise propagation of impinging jet.The acoustic propagation equation is expressed in Hamilton system.The stability conditions and calculation errors of symplectic algorithm for solving acoustic wave equation are analyzed.The numerical dispersion and computational efficiency of the symplectic algorithm are compared to those of the finite difference algorithm in solving the acoustic wave equation.The results show that the symplectic algorithm can effectively suppress the numerical dispersion and the simulation results are more accurate when the time and space accuracy are the same.When the results in same accuracy are obtained,the CPU calculation time of the symplectic algorithm in coarse grids is only 3.6% of that of the finite difference algorithm in fine grids.The feasibility and superior computational performance of symplectic algorithm in numerical study of aeroacoustic field of impinging jet are preliminarily validated.The noise characteristics and sound source mechanism of the jet impinging on the flat plate and cones are systematically revealed.The numerical scheme of the acoustic field of jet impinging on a plate is optimized from two new perspectives of the acoustic integral surface size and the sub-grid model.The acquisition of noise characteristics and the analysis of sound source mechanism are realized for periodic jet impinging on a flat plate with unsteady impinging velocity.The symplectic algorithm is applied to the calculation of noise propagation of impinging jet to realize the innovation of the numerical calculation method of noise of impinging jet.The above researches provide investigation method and theoretical basis for the prediction,analysis and control of noise of impinging jet in engineering application.