A comparison of the viscosity balance and Preston tube methods of skin friction measurement

Abstract

In the viscosity balance method of skin friction measurement an oil film is placed on the surface over which gas is flowing, and moves under the influence of skin friction. The oil thickness variation with time at a fixed point is measured by interferometry. The paper gives a comparison of this method with the well known Preston tube method in conditions (thick, turbulent boundary layer) favourable to the latter. The results show no significant difference between the methods. The Preston tube method is simpler and probably more accurate when conditions are suitable for its use. The advantage of the viscosity balance method is that it is independent of conditions in the boundary layer. It can be used whether the boundary layer is thick or thin, laminar, transitional or turbulent, two- or three-dimensional, on flat or curved surfaces, in compressible or incompressible flow, with or without pressure gradients.