Abstract
The pressure losses for four kinds of PTFE(Poly-Tetra-Fluoro-Ethylene) pile surfaces and a hybrid riblet surface were measured in a rectangular water channel flow using the water and a glycerol-water mixture. The data were compared with those of smooth surfaces. The drag reduction could be obtained for all kinds of PTFE pile surfaces and the hybrid riblet surface; the maximum reduction ratio was estimated to be 6% for PTFE1 (the shorter and the finest fiber) and PTFE4 (the shorter and the thickest fiber). The maximum reduction ratio obtained for the hybrid riblet surface was 7%. It was found that for PTFE pile surfaces, a drag increase due to the effect of surface roughness appeared when the Reynolds number exceeds about 8000. The photograph of x-y and y-z cross-sections of PTFE pile surfaces taken by using a microscope revealed that there was somewhat space between the base fabric and pile fibers. For the hybrid riblet surface, drag-reducing effects due to both the larger and the smaller ribs were observed.
