Enhanced heat transfer and shear stress due to high free-stream turbulence

Surface heat transfer and skin friction enhancements, as a result of free-stream turbulence levels between 10 percent < Tu < 20 percent, have been measured and compared in terms of correlations given throughout the literature. The results indicate that for this range of turbulence levels, the skin friction and heat transfer enhancements scale best using parameters that are a function of turbulence level and dissipation length scale. However, as turbulence levels approach Tu = 20 percent, the St’ parameter becomes more applicable and simpler to apply. As indicated by the measured rms velocity profiles, the maximum streamwise rms value in the near-wall region, which is needed for St’, is the same as that measured in the free stream at Tu = 20 percent. Analogous to St’, a new parameter, Cf’, was found to scale the skin friction data. Independent of all the correlations evaluated, the available data show that the heat transfer enhancement is greater than the enhancement of skin friction with increasing turbulence levels. At turbulence levels above Tu = 10 percent, the free-stream turbulence starts to penetrate the boundary layer and inactive motions begin replacing shear-stress producing motions that are associated with the fluid/wall interaction. Although inactive motions do not contribute to the shear stress, these motions are still active in removing heat.