Rw. Moss et Mlg. Oldfield, EFFECT OF FREE-STREAM TURBULENCE ON FLAT-PLATE HEAT-FLUX SIGNALS - SPECTRA AND EDDY TRANSPORT VELOCITIES, Journal of turbomachinery, 118(3), 1996, pp. 461-467
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.