On transition from laminar to turbulent flow in the boundary layer

Information on transition from laminar to turbulent flow in boundary layers on stream line bodies of revolution in a water stream is obtained from visual an d photographic observation of the movements of small particles and filament bands. The cases considered are transition caused by isotropic turbulence in the free stream , by a falling velocity in a stream substantially free from turbulence, and by flow disturbances from surface wires. A non-dimensional number, specified in terms of the intensity and scale of turbulence, the boundary-layer thickness, and the mean velocity just outside the layer, is found to be representative of the conditions of flow in the boundary layer of a body in a turbulent stream: and values of this number at transition obtained from observations made for a body of revolution have the same order of magnitude as those calculated for a flat plate from measurements taken by Hall and Hislop. Transition in a region of falling velocity in a steady stream arises from a separation of the laminar boundary layer from the surface; and after separation, the flow in the layer becomes turbulent and then rejoins the surface. The observed position of transition is near the position of separation given by a solution of the momentum equation for laminar flow and for the observed distribution of velocity just outside the layer. Flow disturbances from a surface wire placed in a laminar boundary layer cause fully developed turbulent flow to be established at the wire when u2d/v> 400, where d is the wire diameter, u2 is the velocity in the laminar layer at a distance d from the surface, and v is the kinematic viscosity. The measured maximum value of the downstream component of turbulent velocity in the stream behind a grid is about 3.3 times the root-mean-square value.