Application And Analysis Of Mode Decomposition In Multi-scale Turbulence Motions

Large Eddy Simulation(LES)is applied to study the noise of leading edge of slat.The first part of this paper is to take advantage of proper orthogonal decomposition(POD)to analyze the distribution of energy around slat free shear layer.Each Eigenvalue of POD method represent the system energy of relative POD mode.We can order the eigenvalues and relative modes by eigenvalue from largest to smallest,then will find POD modes come in pairs.The first third couples occupy 80% of system energy.The fluctuations with same direction appear at opposite direction intervals.High energy modes always have a big scale area of high value of pressure of fluctuation around the reattachment point and the relatively low energy modes have an opposite situation.It is crucial to know the mechanism of low-frequency noise from the leading-edge slat,which can guide an ad-vanced active and passive control of slat noise.POD method can extract the structures by POD eigenvalues.However,each POD mode is composed all frequency components.Dynamic Mode Decomposition method can extract single-frequency modes which can give a significant help while analyze the noise of slat.This paper reveals the mechanism of slat noise using the method of dynamic mode decomposition(DMD).The results indicate that the dipole features of slat noise come from the collision between large scale vortexes in the free shear layer and the lower surface of the slat.There is a close loop feedback mechanism of flow-acoustic coupling existing between large scale vortexes and low-frequency acoustic waves from the reattachment point.Based on this mechanism,a theoretical predictive model is also proposed and verified.Direct numerical simulation(DNS)is applied to study channel turbulence.Fully developed compressible turbulent channel flow(Re=2800,Ma=0.3)is simulated.Empirical Mode Decomposition(EMD)method is applied to extract each streamwise velocity EMD mode.According the theory of EMD method,we can get different scale flow structures.EMD method can effectively separate different scale structures and analyze the scale of each EMD mode quantificationally.