Phase Averaged Transverse Vorticity Measurements in an Excited, Two-Dimensional Mixing Layer

Phase averaged transverse vorticity time series have been obtained in a weakly excited, single-stream mixing layer. The transverse vorticity time series were obtained with an jc-array at each of the 414 points in the measurement grid. These data showed the regions of vortex formation, saturation and decay. The spatial locations of the distinctive vortex motions were found to be in good agreement with previous studies: excitation and the relatively thicker boundary layer (approximately 6.5 cm) in the present study was apparently responsible for the larger isolated vortical motions. Using the vorticity field documentation, the spatial distribution and the temporal evolution of the primary vortex motions in the mixing layer were examined. The spatial distribution revealed deep depressions of the vorticity contours on the high-speed side of the mixing layer. The primary growth of the vortical structure was observed to take place on the low-speed side. In addition, observations revealed (over 15% of the available excitation cycle) that the core of the vortical structures appeared to move at substantially different speeds. This detailed documentation of the vorticity field also produced information showing the tearing and fusing of vortical contours in the phase averaged plots. Background Information I N a natural (unexcited) free shear layer, vorticity from the boundary layer is continuously shed from a backward facing step. This vortical fluid tends to roll-up (or fold) and to form discrete vortices. The roll-up of the vortical fluid is a result of the natural instability process. For the free shear layer this instability is of the Kelvin-Helmholtz type. For convenience, these naturally occurring discrete vorticies are termed "unit vorticies" in the present discussion. When periodic excitation is applied (at the separation lip) to a free shear layer, it causes a two-dimensiona l undulation of the separating boundary layer. This undulation is followed by the agglomeration of the shed vorticity into a large, isolated, vortical structure downstream from the separation lip.1'2 An efficient agglomeration may be accomplished by exciting the natural shear layer with a low-frequency, periodic disturbance.3"5 In addition to agglomeration and nonlinear increases in width, excitation has been found to organize and generate larger vortical structures than would exist in a naturally occurring shear layer. This organization is a result of the vortical structure being 'Hocked" in space with respect to a particular phases (or time) of the excitation mechanism.3'6 Using temperature as a passive contaminant, Fiedler and Korschelt2 measured the transverse space correlation of the temperature fluctuations in a two-dimensional jet. These results reveal a dramatic increase in the space correlation and, hence, in the two-dimensionality of the excited case vs the natural shear layer. These large vortical motions (or structures) are often referred to as coherent structures. Hussain7 defines a coherent structure as "a connected, large-scale, turbulent fluid mass with phase-correlated vorticity over its spatial extent. That is, underlying the three-dimensional random vorticity fluctua