The Experimental Study On The Characteristics Of Turbulent Boundary Layer Based On The Non-uniform Interrogation Window

Particle image velocity（PIV）measurement and the cross-correlation algorithm are widely used in turbulent boundary layer measurements.With the in-depth study of turbulent boundary layer,more comprehensive and accurate measurement of turbulent field information becomes crucial,compared with the globally uniform interrogation window（IW）,a more flexible interrogate method can be adapted to the non-uniformity of the turbulent boundary layer.In this paper,based on the average flow characteristics of the turbulent field,a non-uniform IW model different from the conventional IW is proposed.This model also considers the image deformation algorithm and the Gaussian sub-pixel peak fitting method.Through the relative uncertainty of the dimensionless average velocity,the IW size is related to the local flow field characteristics,thereby forming a non-uniform IW layout suitable for the turbulent flow field.Using non-uniform IW to calculate DNS channel flow data,the error of the instantaneous stream-wise velocity,the stream-wise average velocity,the root mean square of the fluctuation velocity,and the energy spectrum are analyzed.The results show that the non-uniform IW method improves the accuracy and robustness of the calculation results.Applying non-uniform IW to the PIV data of flat turbulent boundary layer with Re_θ of 355,440,538,the non-uniform IW distribution with IW normal size and wall position is linear relationship and exponential relationship with velocity gradient.Compared with uniform IW results,the turbulence statistics of non-uniform IW are in better agreement with DNS,the relative error is smaller,and the relative error reduction of average velocity,turbulence,Reynolds stress,and flatness factor is within 30%.The average span-wise vorticity,skew factor,turbulent kinetic energy generation and dissipation terms show strong IW size sensitivity in the buffer layer,the relative error reduction of the first two is 40%and 80%,and the relative error of the latter two is controlled below 75%.The results of statistics reflect the advantages of non-uniform IW,and can obtain more accurate statistics of the flow field near the wall.Using the λ_cicriterion to identify the distribution of spanwise vortices,a method for calculating the average flow direction of Q events using the quadrant split method is proposed,and through the velocity structure function,energy spectrum analysis,and eigen orthogonal decomposition of the turbulence of the coherent structure Kinetic energy was studied.The results show that the non-uniform IW method detects a more complete hairpin vortex structure in the y⁺<60 region,and obtains the correct trend that the directional vortex strength decreases with increasing y⁺.Reynolds shear stress contributed by the Q event,the stream-wise average scale and the turbulent kinetic energy of the corresponding scale all have an intersection point around y⁺=15,and the energy of the Q-scale turbulence structure is affected by the combined effect of the Reynolds shear stress and the flow direction scale.The-5/3 spectrum shifts to the low wavenumber region as the wall position decreases,in addition,the non-uniform IW can improve the energy extraction ability of energetic structures,which is reflected in the tendency of the higher energy corresponding to the low and medium wave numbers of the buffer layer and the turbulent kinetic energy corresponding to the modes to be concentrated in lower order modes.