An Instrument for the Direct Measurement of Skin Friction in High Speed Flow

Abstract

In recent years the development of high speed aircraft and missiles has shown the importance of the effects of compressibility and heat transfer on boundary layer flow and hence on drag. The general problem is a highly complex one and involves processes like turbulence and shock waves in a real fluid. Such phenomena pose formidable theoretical difficulties for an analytical solution of the drag problem (for theoretical work on the subject see Refs. 1 and 2). in the case of shock free laminar flow the mechanism of the resistance experienced by a given body is fairly well understood and the difficulties are mainly of a mathematical nature. The case of turbulent flow, however, involves conceptual difficulties in addition to mathematical ones. Under such circumstances the role of experiments is a vital one. They provide evaluation of existing theory and contribute to a general understanding of the phenomenon of drag at high speeds. The quantity of most significance, in many cases, for practical reasons and theoretical analysis, is the viscous shearing stress on the surface of a body moving through a fluid. For reasons of simplicity the case of flow past a thin flat surface is usually singled out for close examination. In experimental terms this involves detailed boundary layer studies on a thin flat plate in a wind tunnel. Now boundary layers in high speed flow are extremely thin. A laminar boundary layer at atmospheric stagnation conditions and at a Mach number ≐ 1.4 and Reynolds number ≐ 10^6, on a 10 cm. long flat plate, is of the order of 1/2 mm thick and a turbulent boundary layer under similar conditions is about 1 mm thick. Hence accurate determination of skin friction by the measurement of velocity profiles is very difficult and the accuracy questionable. The development of an instrument for direct measurements of the local friction force is reported here.

Files

Additional details