Evaluation of the Effect of Discrete Fluid Injection on Turbulent Boundary Layer Skin Friction in the Presence of Wall Roughness.

摘要

The purpose of the present experimental study was to determine if discrete mass injection into a turbulent boundary layer over a rough wall would effectively smooth the surface and reduce the skin friction drag. Accordingly, a test program was planned and a series of tests were conducted in the Free Surface Water Tunnel at the California Institute of Technology. In addition to a laser doppler velocimeter system which was used for measuring boundary layer velocity profiles the skin friction could be inferred) surface mounted instruments consisting of heated-film gages, Presgon tubes, and Stanton tubes were used to measure the local skin friction. In general these sensors indicated local reductions in skin friction over the full range of injection rates and slot gaps sizes and at the highest flow rates resulted in reductions of about 15 percent near the injection slot. A brief tradeoff study was performed and indicated that this concept was not feasible for drag reduction of submersibles since the power required to operate a fluid injection system was about an order of magnitude greater than the power saving achieved through drag reduction with the same system. (Author)