Investigation of structures in a highly turbulent mixing layer by using LDA and SPIV

Abstract

There is lack of experimental study on significance or even presence of Brown and Roshko structures in highly turbulent environments. The setup configuration of this experiment is specifically designed to complicate creation and development of these structures by introducing high turbulence intensities. The flow considered is a plane, incompressible, fully developed mixing layer with two airstreams of different turbulence intensities. The Laser Doppler Anemometry (LDA) provides good temporal resolution across the mixing layer to obtain statistical descriptions and the Stereoscopic Particle Image Velocimetry (SPIV) provides good spatial resolution to investigate spatial coherencies. After defining the self-similar region, SPIV measurements are made in this region and different decompositions of Galilean, Reynolds and proper orthogonal decomposition are applied. Characteristics of a mixing layer are well studied and well documented, and the comparisons reveal that the high level of turbulence prevents structures from rolling up in a coherent manner; however, there exists temporary orderly oscillations. This compelling aspect requires appropriate techniques. The Morlet wavelet transform is applied to study non-stationary nature of the flow and a new method similar to the pseudo flow visualization is proposed to ascertain the results. Good qualitative agreements have been observed. The approach proposed is useful to extend the field of view in SPIVs measurement so as to realize larger wave numbers and is of practical significance in flow visualization.