The structure and development of streamwise vortex arrays embedded in a turbulent boundary layer
Abstract
An investigation of the structure and development of streamwise vortices embedded in a turbulent boundary layer was conducted in the test facility CW-22 at NASA Lewis Research Center. The vortices were generated by a single spanwise row of rectangular vortex generator blades. A single embedded vortex was examined, as well as arrays of embedded counter-rotating vortices produced by equally spaced vortex generators. Measurements of the secondary velocity field in the crossplane provided the basis for characterization by four descriptors. The center of each vortex core was located at the spanwise and normal position of peak streamwise vorticity. Vortex concentration was characterized by the magnitude of the peak streamwise vorticity, and the vortex strength by its circulation. Measurements of the secondary velocity field were conducted at two crossplane locations to examine the streamwise development of the vortex arrays. Large initial spacings of the vortex generators produced pairs of strong vortices which tended to move away from the wall region while smaller spacings produced tight arrays of weak vortices close to the wall. The crossplane structure of embedded vortices is observed to be very similar to that exhibited by the two dimensional Oseen vortex with matching descriptors. Quantitative comparisons are established. A model of vortex interaction and development is constructed using the experimental results. The model is based on the structure of the Oseen vortex. Vortex trajectories are successfully modelled by including the convective effects of neighbors, and images to represent the wall. The streamwise decay of circulation is successfully modelled for the single vortex, and for large initial spacings, by accounting for the effects of wall friction. An additional mechanism associated with the turbulent stress field in the near vicinity of the vortex cores is postulated to explain the large losses in the circulation obtained for the smaller initial spacings. The streamwise decay of vortex circulation at the smaller spacings is successfully modelled by summing wall friction losses and 'proximity' losses. These proximity losses are found to be proportional to the gradient in streamwise vorticity occurring between an embedded vortex and its adjacent counter-rotating neighbors.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- July 1991
- Bibcode:
- 1991PhDT........55W
- Keywords:
-
- Flow Characteristics;
- Turbulent Boundary Layer;
- Velocity Distribution;
- Vortices;
- Friction;
- Turbulence;
- Vortex Generators;
- Wind Tunnel Tests;
- Fluid Mechanics and Heat Transfer