Experimental study of the viscous sublayer in turbulent pipe flow

Abstract A flash photolysis method for nondisturbing turbulent flow measurements has been applied to study conditions in the viscous sublayer in a square smooth pipe. Experimental results are obtained that are extremely difficult to obtain by any other known technique. The results show that, adjacent to the wall, there is a layer of very small thickness y+ = 1.6 ± 0.4 in which a linear velocity gradient occurs virtually at all times, but the slope of the gradient changes with time. Beyond y+ = 34.6 essentially turbulent flow exists. In the transition region in between, very disturbed flow conditions prevail. However, the technique used clearly indicates the instantaneous velocity distribution in the flow field bewteen the wall and the fully turbulent region. Statistical distributions of the instantaneous wall shear stress (evaluated from the slope of the velocity gradient), the laminar sublayer thickness, and the velocity at the edge of the laminar sublayer are obtained. It follows that the partial turbulence mechanism whereby the flow at a point is turbulent only a fraction of the time is, at least to an extent, responsible for the reduction of eddy diffusivity by kinematic viscosity close to the wall. Experimental velocity profile obtained in the present investigation is compared with the proposed and experimental velocity profiles of other authors.