On Transition from Laminar to Turbulent Flow in the Boundary Layer
Proceedings of the Royal Society A
Plate 5] Information on transition from laminar to turbulent flow in boundary layers on streamline bodies of revolution in a water stream is obtained from visual and 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
... 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 measure ments 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. 14-2 on July 20, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from 202 A. Fage and J . H. P reston * The work was carried out in the Aerodynamics Department of the National Physical Laboratory, and it is presented on the recommendation of the Aeronautical Research Committee and by permission of the Director of the Laboratory. on July 20, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from on July 20, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from * The name fluid-motion microscope now introduced is more appropriate than the name ultramicroscope used formerly.