| It is well known from previous work that when a two dimensional vortex interacts with a no-slip boundary flow along the axis of the vortex core, axial core flow is generated. In this work, Molecular Tagging Velocimetry (MTV) is used to provide quantitative measurements of the structure of a developing vortex array.; A NACA-0012 airfoil is pitched about the quarter chord point at varying amplitudes and frequencies to produce a semi-infinite vortex array with vortices of varying characteristics (i.e. circulation, peak vorticity, core size, etc.). Two-component MTV is used to detail the initial formation of the vortex array for the first chord length downstream of the airfoil trailing edge. MTV results combined with flow visualization images indicate that the unsteady Kutta condition is not valid for high reduced frequencies in this flow field. Mean drag results show that the mean force switches from drag to thrust as the reduced frequency is increased.; Cutting walls are placed into the flow field at half a chord downstream of the trailing edge to initiate axial flow within the vortex cores. As expected the results show that axial flow is generated within the vortex core and that fluid moves away from the cutting walls.; Reverse spanwise flow (i.e., flow opposite to the direction of the axial core flow) is observed outside of the vortex cores. The reverse flow indicates a large scale recirculation is set-up to return fluid towards the wall, satisfying the conservation of mass. The vortex array structure changes in character as it convects downstream. The peak vorticity level is reduced and the core radius increases. The vorticity profile changes from Gaussian to non-Gaussian. At some span locations and reduced frequencies “hollow core” vortices are observed. The vortices eventually loose coherence and the spanwise flow, initially limited to the vortex core regions (i.e. the axial core flow), is found throughout the flow field. This break-up of the vortical structures begins near the wall and propagates towards the center of the tunnel. (Abstract shortened by UMI.)... |
