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Abstract
Understanding high freestream turbulence effects on the transport of heat and momentum is needed for better predictive schemes. This dissertation presents the effects of very high freestream turbulence levels, between Tu = 10% and 20%, on surface heat transfer and skin friction in terms of the turbulent transport of heat and momentum.
Both the velocity and thermal fields are quantified for a boundary layer influenced by high freestream turbulence and are compared to a boundary layer with negligible freestream turbulence. In addition to quantifying the mean and rms values of temperature and velocity, the turbulent heat flux profiles are quantified through simultaneous temperature/velocity measurements. Spectra and length scales for both temperature and velocity are also quantified in the freestream and throughout the boundary layer.
The results of this study indicate that scaling the heat transfer enhancement is best accomplished using St$\sp\prime$ which uses the maximum rms streamwise velocity as the velocity scale in the Stanton number. Similarly, scaling the skin friction enhancement is best accomplished using Cf$\sp\prime$ which also uses the maximum rms streamwise velocity as the velocity in the skin friction coefficient. In the case of St$\sp\prime$, a constant value occurs for all turbulence levels and was found to be independent of turbulent length scales and momentum Reynolds numbers. Cf$\sp\prime$ remains constant for turbulence levels less than 15%, but then decreases as the turbulence levels increase. When using the other scaling parameters that depend on turbulence level and length scale, more consistent trends were found for the integral length scale rather than the dissipation length scale.
The present data indicate that a highly turbulent freestream does not enhance the skin friction as much as the heat transfer. However, the turbulent transport of momentum does scale with the surface shear stress and the turbulent transport of heat does scale with the surface heat flux. Thus, these turbulent transport mechanisms are similarly affected by the high freestream turbulence. The reduced shear stress enhancement is attributed to large-scale, inactive motions from the highly turbulent freestream which penetrate into the turbulent boundary layer, displace the active motions, and reduce the average shear stress. These actions result in reduced correlation of the turbulent fluctuations which counter-balances the enhancement effect of increased turbulence levels so that there is no net increase in wall shear. However, these inactive motions have less effect on the normal heat flux. Therefore, heat transfer continues to be enhanced as turbulence levels increase.
