Experimental Studies on Laminar Sub-layer in Turbulent Boundary Layer involving Separation

Abstract

Observations were made on the mean and fluctuational velocities of the flow near a wall in turbulent boundary layer with non-zero pressure gradient. The edge of the sub-layer was defined as the maximum-point of the u -distribution curve. Important results obtained are as follows (suffixes 1 and 0 denoting here the values at the edge of the sub-layer and just outside the boundary layer): (i) In the field with constant pressure the curves of \bar U /\bar U 1 versus y /δ 1 are similar to those in the field with a fixed value of \bar U 1 δ 1 /ν equal to about 165, indicating that the well-known relation between \bar U / U τ and y U τ /ν holds near the wall, U τ being the frictional velocity; (ii) The curve of \bar U /\bar U 1 varies in such a manner that ∂ 2 (\bar U /\bar U 1 )/∂( y /δ 1 ) 2 increases with d P 0 / d x near the wall; (iii) The laminar sub-layer persists until the boundary layer separates and the separation of the sub-layer seems to be equivalent to turbulent separation phenomenon.