Transmission of thin light beams through turbulent mixing layers

Abstract

This research investigates the effects of spanwise and streamwise coherent structures in a turbulent mixing layer on the deflection of a thin light beam which is transmitting transversely through the mixing layer from the high-speed side to the low speed side. Both equal and unequal density mixing layers of varying pressures and velocities are studied, using a lateral effect detector to dynamically track the motion of a He-Ne laser beam. Beam deflections in the streamwise direction are found to be associated mainly with the spanwise coherent structures; at low Reynolds Numbers the beam deflection is directly related to the part of a spanwise structure through which the beam passes. Maximum deflections are associated with the trailing edge of the spanwise coherent structures. Spanwise deflections are caused mainly by the streamwise coherent structures and as such exhibit large variations across the span of the flow. With the development of the streamwise structures, spanwise deflections are found to exceed streamwise deflections. Mixing transition, as scaled using the momentum thickness of the high-speed side, is found to cause a peak in therms fluctuations of both the streamwise and spanwise deflections.