EFFECT OF FREE-STREAM TURBULENCE ON FLAT-PLATE HEAT-FLUX SIGNALS - SPECTRA AND EDDY TRANSPORT VELOCITIES

An experimental study of the eddy structure in a flat-plate turbulent boundary layer with significant levels of free-stream turbulence is pr esented. This is relevant to the enhancement of turbomachinery heal tr ansfer by turbulence and should lead to more realistic CFD modeling. P revious measurements showed that Nusselt numbers may be increased by l ip to 35 percent, and that this increase depended on turbulence integr al length scale as well as intensity. The new results described here p rovide an insight into the mechanism responsible. Thin film gages and hot wires were used to take simultaneous high-frequency measurements o f fluctuating heat transfer rates to the flat plate and the fluctuatin g flow velocity in the free stream and boundary layer. Spectra and cor relation analysis shows that the turbulent eddy structure of the bound ary layer is dominated by the free-stream turbulence at intensities of 3 percent and above. Eddies in the boundary layer mimicked those in t he free stream and convected at the free-stream velocity U, rather tha n the similar to 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 enhancemen t of existing boundary layer turbulence.