Generation of crossflow vortices in a three-dimensional flat plate flow
The ability to generate a crossflow vortex pattern on a swept flat plate, that is typical of swept-wing flows, is presented. A swept flat plate with an elliptic leading edge is mounted vertically in the VPI&SU Stability Wind Tunnel opposite to a swept wall-bump; along with floor and ceiling fairings that duplicate the inviscid streamlines. The resulting pressure gradient over the plate produces a crossflow vortex structure.
Detailed three-dimensional measurements, made within the boundary layer using hot-wire anemometry, are supplemented with different flow-visualization techniques. Freestream measurements are carried out to find the variation of the velocity vector along the model and comparisons are made with the theory showing good agreement. Spanwise measurements are conducted within the boundary layer and show a steady vortex structure. Boundary-layer profiles are taken using both a straight-wire and slant-wire probe in order to obtain the variation of the velocity vector in the region. The components parallel and perpendicular to the freestream velocity vector are extracted, and the perpendicular component is called the crossflow profile. These profiles are compared to the theory. Shape factors and crossflow Reynolds numbers are obtained and analysed.
Two types of flow-visualization techniques are used in the tests. One is the "smoke-wire" technique, where streaklines inside the boundary layer are visualized. This method is not successful in visualizing any vortex structure in the boundary layer. Therefore, the second method was employed: a sublimation technique using trichlorethane and naphthalene is used to visualize the variation of the surface shear stress. A uniform pattern of streaks aligned approximately in the potential flow direction is observed. This pattern has a wavelength on the order of one centimeter which agrees quite well with the theoretical model.