Study of disturbed boundary layers using a hot-wire anemometer

One of the most important problems in fluid mechanics which has attracted the interest of investigators for many years is the transition from laminar to turbulent flow. This problem is of greatest importance in practice for flows over aerofoils and in closed channels, where the effects of transition are marked. Little is known, however, about the fundamental physical processes which cause a laminar flow to break down to turbulence. The particular case of boundary-layer flow has received considerable attention and has been treated with some measure of success. Linearized theories have been useful in predicting the nature of the mechanism of instability of a laminar flow; but the non-linear problem, namely, the transition from an unstable laminar to a fully developed turbulent flow, presents formidable mathematical difficulties, so that few theoretical results have been obtained. The development of air and water tunnels together with improved measuring techniques in fluid flow has led to attempts by engineers and experimental physicists to relate the unstable laminar to the turbulent flow regime. The aim of the work reported in this thesis was to gain further insight into the mechanism of transition in the boundary layer. The present experimental work was confined to the simplest example, the flow of an incompressible fluid along a smooth flat plate at zero angle of incidence with zero pressure gradient in the flow direction. The intention was to develop a hot -wire anemometer system to study the downstream development of disturbances introduced under controlled conditions into the boundary layer. As the work progressed emphasis was placed on one particular aspect of the non- linear development of the disturbances, the generation of higher harmonics of the fundamental wave