Research On Flow Noise Of Engine Exhaust System With Simulation And Experiment

Engine exhaust system main role is to reduce the pollutant content in the exhaust gas and reduce engine exhaust noise.In the design of the exhaust system,in addition to requiring lower levels of exhaust noise,noise is also required to have sound quality such as acceleration or comfort.When the engine is running at a high speed,there will be a large airflow noise in the exhaust system,which makes people feel extremely uncomfortable,so it is necessary to control the airflow noise.In this paper,a threedimensional flow field air noise simulation model including the hot and cold ends of the exhaust system is established for a car exhaust system.The sound power distribution law of airflow noise in the exhaust system and the frequency spectrum characteristics of the sound pressure level at the measurement point were obtained.Based on this,the influence of the structure of the hot and cold ends on airflow noise was studied.The results of this study have important engineering application value for guiding the design of the exhaust system.The main research content of this paper is as follows:Firstly,a three-dimensional flow field noise simulation model for an exhaust system including a hot end and a cold end was established.Starting with geometric preprocessing,meshing and physical model selection,using structural meshing software to mesh the exhaust system flow field.Based on the obtained grid model,a threedimensional airflow field noise model of the exhaust system is established,and the engine quasi-dimensional work process simulation model is built to calculate the boundary conditions of the exhaust system.The effect of boundary layer meshes on the number of model meshes and simulation results was studied.It is found that as the number of boundary layers increases,the number of grids grows linearly;And when the k-ε turbulence model with high Reynolds number is used for calculation,the y+ value of the first boundary layer grid is greater than 30 and the closer to 30 is,the more accurate the pressure loss calculation result is.Secondly,simulation analysis and experimental research on airflow noise in exhaust system.Based on the broadband noise source model and the FW-H noise model,the sound power distribution law and the sound pressure spectrum characteristics at the measurement point of the entire exhaust system are calculated.At the same time,the flow field distribution characteristics analysis shows that When the structure of a gradually expanding pipeline,a sudden change in cross-section,and an elbow structure appear in an exhaust system,airflow noise is generated,and the greater the turbulent kinetic energy at the noise source,the greater the flow noise generated.Comparing the sound pressure level spectrum of the sound receiving point which is at the tail pipe with GT-POWER tailpipe noise simulation results and experimental tailpipe noise test results,the maximum difference between the simulation results and the test results in this paper is about 10 dB,and the curve consistency is better which can prove the simulation method is effective.Finally,study on the effect of different structures on airflow noise in exhaust systems.Based on the established airflow noise simulation model of the exhaust system and combined with the distribution characteristics of the flow field,the noise emission mechanism of the airflow noise in the manifold catalytic converter,the secondary catalytic converter and the expansion chamber silencer unit was obtained.The analysis and comparison of the sound power distribution law and the sound pressure spectrum characteristics of the measured points show that the airflow noise produced by the week cloth manifold catalytic converter is smaller than the in-line airflow noise;the airflow noise decreases with the increase of the catalyst expansion angle;Expanding the volume of the chamber increases the airflow noise;the airflow noise decreases with the length of the expansion chamber inserted into the tube.