Prediction Research On The Mutation Phenomenon Of Supersonic Jet Noise

Supersonic jet is widely used in various industrial fields and the noise it generates also has attracted much attention of the researchers.The results of the muffler noise measurement experiment indicate that the supersonic jet noise generated by some nozzles in the discharge process doesn’t decrease as the upstream reservoir pressure decreases,but increases rapidly in a particular pressure condition,which can’t meet the requirement of the experiment.This kind of noise problem can only be found in the sample test step previously and it will cause the waste of resources.The study purpose of this article is to find a way to predict whether the jet noise would mutate in the nozzle design phase,in order to avoid the useless input in the design and manufacture step.Firstly,the numerical simulation of supersonic jet flow fields under various pressure ratios generated by two kinds of nozzles with typical structures is conducted and one kind of nozzle is along with the noise mutation phenomenon,the other is not.After the detailed comparison and analysis of the shock structures and the parameters distribution along the axis in the near filed,it is discovered that the mutation phenomenon is marked by the “L” type axial total pressure or Pitot pressure distribution curve,which is caused by the jet transforming into extremely underexpanded condition and the vanishing of the quasi-periodic shock cell structure.So it can be regarded as a criterion to judge whether the noise sound pressure level will mutate.Then the experiment of measuring the Pitot pressure distribution condition along the jet axis of the two kinds of nozzles is performed.The experimental data and the simulation results agree reasonably well,and it verifies the validity of the numerical calculation method and the above judgement criterion of the noise mutation phenomenon.Finally,the influence of the inner diameter of throttle orifice plate and the length of nozzle after the orifice on the jet flow field and the noise mutation phenomenon is investigated by means of experiment and numerical simulation.The research results indicate that this judgement criterion applies to different nozzles of various geometries,and within a certain range,the upstream pressure corresponding to the noise mutation phenomenon decreases as the inner diameter of the orifice plate or the length of nozzle after the orifice increases.