Velocity and Temperature Fluctuation Measurements in a Turbulent Boundary Layer Downstream of a Stepwise Discontinuity in Wall Temperature

Abstract

Abstract Results are presented of an experimental investigation of the coexisting thermal and velocity fluctuation fields in a turbulent boundary layer downstream of a small stepwise discontinuity in wall temperature. The statistical behaviors of several relevant fluctuating quantities have been obtained at a typical cross section of the boundary layer in the region where the thermal boundary layer has not yet reached the free stream. The instantaneous surface of demarcation between heated and unheated fluid was found to be sharp and distinct at all points, resulting in intermittent temperature fluctuation signals well within the fully turbulent region of the momentum boundary layer. From the measurements at one section, the balances of turbulent kinetic energy and of turbulent temperature fluctuations across the boundary layer have been evaluated. In addition, the distribution of the local turbulent Prandtl number (ratio of the turbulent diffusivities of momentum and heat) through the boundary layer has been obtained; it is not constant and varies between limits of 0.8 and 1.2.