Measurement of coherent fine scale eddies in turbulent mixing layer by DPIV

Abstract

This chapter reviews the existence of coherent fine scale eddies in turbulence that are experimentally confirmed in turbulent mixing layer using digital particle image velocimetry (DPIV). DPIV measurements are conducted with the spatial resolution, which is the same with that of direct numerical simulations (DNS) of turbulence. From the particles images, the velocity vectors are obtained with the help of cross correlation method, where spurious vectors and noises are essentially removed by applying a spatial low-pass filter, which is verified by a DPIV simulation based on DNS of particle-laden homogeneous isotropic turbulence. It is noted that the diameter and maximum azimuthal velocity of coherent fine scale eddies measured are in agreement with the previous DNS results obtained in various turbulent fields within the measurement accuracy. The elliptic features of coherent fine scale eddies on the plane perpendicular to the vorticity vector are also confirmed by the experiment.