Effect of high free-stream turbulence on boundary-layer skin friction and heat transfer

Abstract

The effects of high free-stream, turbulence (FST) on skin friction and heat transfer ha a constant-pressure boundary layer have been measured in a low-speed wind tunnel. To avoid the uncertainties of other experiments using various unconventional devices to produce high FST, a traditional square-mesh, square-bar biplane grid has been used. High eifective FST is obtained by replacing the test-section floor with a moving belt, reducing the velocity difference across the boundary layer for a given free-stream turbulence intensity- Heat transfer h*-1* been measured by a novel quasi-transient technique using liquid crystals. I. Introduction A major reason for studying the effects of high freestream turbulence is its effect on heat transfer to gas turbine blades, which operate in the highly turbulent flow from the combustion chamber- Turbulence intensities can be as high as 30 percent, far larger than obtainable with the square-mesh grids traditionally used for experiments on decaying homogeneous turbulence and on FST effects. Several recent investigators15 have used non-standard high-FST generators, including those intended to simulate combustion-chamber turbulence, and the results have been very confusing: it was uncertain how much of the effect on skin friction and heat transfer was the result of high FST as such, and how much was the result of anisotropy and inhomogeneity of the turbulence or of shear in the mean flow. In the present work, more fully reported elsewhere6, a traditional square-mesh, square-bar biplane grid has been used to provide baseline results: high effective FST is obtained by replacing the floor of a low-speed wind-tunnel test by a moving belt. If the downstream speed of the moving floor is Uw and the externalstream mean speed and rms turbulence intensity are Uoa and u'gz respectively, the effective FST - i.e. the ratio of u'^ to the mean-velocity difference across the boundary layer - is w^A^co—#»)> which can be made Copyright (§) 1997 by Donald M. Bott. Published ! by the American Institute of Aeronautics and Astro