ROUGHNESS ELEMENT SHAPE EFFECTS ON HEAT-TRANSFER AND SKIN FRICTION IN ROUGH-WALL TURBULENT BOUNDARY-LAYERS

Stanton number and skin friction coefficient results are presented for zero pressure gradient incompressible turbulent boundary layer air flows over two rough surfaces composed of 1.27 mm base diameter by 0.635 mm high truncated cones spaced in staggered arrays two (L/d0 = 2) and four diameters (L/d0 = 4) apart, respectively, on otherwise smooth walls. These data are compared with previously reported results obtained in the same test facility under similar flow conditions using equivalently sized and spaced hemisphere roughened surfaces. It is shown that the Stanton number data (which have uncertainties of about 2-4%) exhibit slightly distinguishable differences for the two L/d0 = 4 surfaces and definitive differences for the two L/d0 = 2 surfaces. No dependence of skin friction coefficients on roughness element shape could be concluded considering the uncertainty in the data. Predictions of Stanton number and skin friction coefficient distributions from the finite difference solutions of discrete element equations are presented and compared with the experiments.