Vliv sekundárního proudění na mezní vrstvu v kanále

Abstract

Boundary layer in developing channel flow of air is experimentally studied by using the Stereo Particle Image Velocimetry (PIV) technique. The measurement is performed at fixed distance 400 mm from the channel inlet and the Reynolds number (based on the channel length, i.e. the distance from the boundary layer origin) is controlled via the imposed velocity. Re ranges from 8·104 to 8·105. The displacement boundary layer thickness δ varies from 1.7 to 2.5 mm while the momentum one θ from 0.9 to 1.3 mm. It is found, that the critical Reynolds number of transition to turbulence of the boundary layer is lowered by the vicinity of the other perpendicular wall of the square channel; more accurately - it is accelerated by the larger-scale secondary flow, which results into turbulence at slightly lower Reynolds numbers. The laminar-turbulent transition is first apparent on the profiles of the turbulent kinetic energy, later on the velocity profiles. The mechanism might be probably such, that the turbulent flow structures generated in the secondary flow in the corner via Richardson energy transfer mechanism migrate into the laminar boundary layer. While the large- scale structures cannot feed from the limited-size boundary layer, the smaller ones can strengthen there