The radial flow of a thin liquid film. V - Influence of wall roughness on laminar-turbulent transition

This paper describes the influence of wall roughness on the transition from laminar to turbulent flow occurring in the radial flow of a thin liquid film. The properties of the liquid surface were visually observed using the following three types of distributed roughnesses: (1) saw-toothed roughness and (2) trapezoidal shaped roughness, both of which were regularly constructed in a concentric configuration, and (3) irregularly distributed sand roughness. It was found that the point of transition shifted upstream as the height of wall roughness increased under otherwise identical conditions, and that the transition occurred in each of the three regions, namely, the stagnant region, the region in which the laminar boundary layer grows, and the region in which the whole flow is the laminar boundary layer. Experimental results showed that the critical Reynolds number decreased as the height of wall roughness increased.