On the conditions at the boundary of a fluid in turbulent motion

Abstract

The experiments here described form part of a general research into the phenomena of skin friction of solid surfaces due to the flow over them of fluids whose motion, not in the immediate vicinity of the surface, is eddying or turbulent. Considerable information has been obtained in recent years as to the magnitude of the frictional forces brought into existence in this condition of flow, and the manner of variation of these forces with the relative mean speed of surface and fluid, the roughness of the surface, and the physical characteristics of the fluid is fairly well known. Practically nothing, however, is known about the mechanism by which the resistance to flow is transmitted to the bounding surfaces. For speeds below the critical when the general motion of the fluid throughout is streamline in character, it is generally accepted that the layer of fluid in contact with the boundary is at rest relative to it, as any slipping of finite amount would be detected in a variation from the Poiseuille law of the relationship between the diameter of a pipe and the time of efflux of a given volume of fluid. At speeds above the critical, observations near the walls have shown that the mean velocity falls rapidly as the solid bounding surface is approached, and it has been suggested that at the walls there may exist a thin layer in which the flow is laminar in character, in which case, if there is no slipping, the frictional resistance would be determined from the slope of the velocity curve in the surface layer and the coefficient of viscosity of the fluid. During the last few years several attempts have been made at the National Physical Laboratory to obtain evidence as to the truth of this assumption. The method adopted has been to set up a condition of turbulent fluid motion over a surface of which the frictional resistance to the flow could be accurately determined, and to measure by means of a very fine Pitot tube the velocity of the fluid at a point as near the wall as possible.