Visualization of streamwise and crossflow instabilities on inclined circular cylinders

Experimental observations of streamwise and crossflow streaks on circular cylinders are presented over a range of cylinder inclinations. The spanwise wavelength, λ0, of an array of streamwise vortices on a normal circular cylinder was addressed by Kestin and Wood. That theory was for the limiting case of zero sweep and the results are here confirmed experimentally. These observations of streamwise vortices on unswept cylinders confirm the earlier predictions, providing a firm basis for referencing new measurements of vortical behaviour. The work on unswept cylinders provides an excellent benchmark for sweep effects. The principal published collection of results for medium to high sweep angles, Λ, is that of Poll, covering sweep angles between 55° and 71°. Surface flow visualization has been undertaken by the authors giving lateral spacing and streak orientation for sweep angles from zero to 61o. Results for lateral spacing are consistent with those of Poll, covering the range between the zero sweep results of Kestin and Wood and the medium to high sweep cases addressed by Poll.