NACA-W-87

Abstract

Transition of the boundary layer from the laminar to the turbulent regime was investigated on the concave side of a plate with a radius of curvature of 3.5 feet. The critical Reynolds number was found to be considerably lower than on a flat plate and on the concave side of a plate with a 20-feet radius of curvature previously investigated. It was furthermore found that in agreement with the theoretical results of G&#214;rtler, R_&#952; root&#160;^&#952;/_r here termed the "G&#214;rtler parameter," is the proper critical parameter governing boundary layer instability due to concave curvature. The critical parameter at transition was found to have a value of 9.0. <br>The influence of pressure gradient and of an increased free-stream turbulence level on the position of the transition point on the concave side of the plate of 2.5-foot radius of curvature has been studied. Small variations of the pressure gradient did not alter the value of the critical G&#214;rtler parameter. This result is compared with similar measurements on the convex side of a plate of 20-feet radius of curvature. Increased tunnel turbulence lowered the value of the critical parameters 9.0 at a turbulence level of 0.06 percent to 6.0 at a turbulence level of 0.3 percent. <br>The investigation confirms the previous result that the mechanism of the breakdown of the laminar boundary layer is essentially different on convex and concave boundaries. <br>A discussion of the practical applicability of transition measurements is given, and the difference between critical Reynolds number corresponding to laminar instability and critical Reynolds number corresponding to transition is pointed out. A definition of transition Reynolds number, based on the apparent shearing stress caused by the laminar oscillations, is given. In the case of flow along a flat plate, values of transition Reynolds number are calculated approximately for different intensities of the initial disturbance.