Effect of Free-Stream Turbulence on Flat-Plate Heat Flux Signals: Spectra and Eddy Transport Velocities

Abstract

An experimental study of the eddy structure in a flat-plate turbulent boundary layer with significant levels of free-stream turbulence is presented. This is relevant to the enhancement of turbomachinery heat transfer by turbulence and should lead to more realistic CFD modeling. Previous measurements showed that Nusselt numbers may be increased by up to 35 percent, and that this increase depended on turbulence integral length scale as well as intensity. The new results described here provide an insight into the mechanism responsible. Thin film gages and hot wires were used to take simultaneous high-frequency measurements of fluctuating heat transfer rates to the flat plate and the fluctuating flow velocity in the free stream and boundary layer. Spectra and correlation analysis shows that the turbulent eddy structure of the boundary layer is dominated by the free-stream turbulence at intensities of 3 percent and above. Eddies in the boundary layer mimicked those in the free stream and convected at the free-stream velocity U, rather than the ∼0.8U characteristic of boundary layers. The main heat transfer enhancing mechanism is due to the penetration of free-stream turbulent eddies deep into the boundary layer, rather than enhancement of existing boundary layer turbulence.