Molecular tagging velocimetry measurements of axial flow in a concentrated vortex core

The characteristics of the axial flow within the core of a concentrated line vortex are investigated using molecular tagging velocimetry (MTV). A well-defined array of isolated vortices of alternating sign is generated in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. The circulation and peak vorticity of the vortices are varied by the choice of oscillation frequency. Interaction of these two-dimensional vortices with the walls of the test section generates an axial flow within the vortex cores. The magnitude of the axial flow and its spatial/temporal characteristics are quantified using the MTV technique. Results show that the peak axial flow speeds can be very high, of the order maximum swirl speed of the vortices. The maximum axial speed ratio (maximum axial speed normalized by maximum swirl speed) is found to vary in the range 0.6–1.0 for the parameters investigated here. Initially, the axial flow is spatially confined in isolated structures corresponding to the core of vortices. As the vortex convects downstream, however, the spatial structure of axial flow changes from isolated regions to a continuous region for the highest reduced frequency investigated here. This change in structure is correlated with a significant decrease in the peak axial flow speed.