Effect of convex transverse surface curvature on transition and other properties of incompressible boundary layer.

Abstract

A study of the incompressible boundary layer with transverse surface curvature has been made with particular reference to transition from laminar to turbulent flow. Calculations based on linearised hydrodynamic stability equations show that the basic effect of convex transverse curvature alone is destabilising. However, when the influence of transverse curvature in producing a fuller velocity profile is included, it is found that for curvature Reynolds numbers below about 11,000, all small disturbances are damped out. Wind tunnel measurements of growth rates of artificially created turbulent spots and intermittency in transition were made on a circular cylinder placed axially in the wind stream. Above and below curvature Reynolds number of 11,000, these quantities exhibited properties similar to those on a flat plate without and with favourable pressure gradient respectively. Analysis of measured turbulent boundary layers suggested the development of an axisymmetric law of the wall. A theoretical analysis of the experimentally observed strong asymmetry of the laminar boundary layer suggests the cause to be small transverse pressure fields in the wind tunnel. For these measurements, a constant?current hot?wire amplifier with controlled calling? to?floor ratio was developed. This technique was found to reduce the noise level due to compensation by as much as 70%.