AN EXPERIMENTAL INVESTIGATION OF CURVATURE AND FREESTREAM TURBULENCE EFFECTS ON HEAT TRANSFER AND FLUID MECHANICS IN TRANSITIONAL BOUNDARY LAYER FLOWS

An experimental investigation of the effects of convex curvature and free-stream turbulence on momentum and heat transfer in a boundary layer undergoing natural transition was performed. The test section was specially designed for providing a two-dimensional boundary layer flow on a uniformly heated curved surface. The unique feature of the test wall was its flexibility to be bent to various radii of curvature. A qualification test was fund for a case in which only laminar flow is present throughout the test section. Three cases of various radii of curvature; R = (INFIN), 180cm, and 90cm, were next undertaken each with two free-stream turbulence intensity levels, 0.68% and 2%. A coarse grid was inserted upstream of the tunnel contraction section to generate homogeneous turbulence of 2% intensity for the higher free-stream turbulence study. Mean velocity and temperautre profiles and local Stanton number data were taken in a heated flow. Streamwise Reynolds normal stress SQRT.(u'('2)), and Reynolds shear stress u'v' profiles were taken in an isothermal flow.; Mild convex curvature (R = 180cm) is shown to delay transition substantially. Bending the test wall to a smaller radius of curvature (90cm) resulted in no further delay of transition. Increasing the free-stream turbulence intensity results in an earlier transition. Cases with both effects present a pronounced dominance of the free-stream turbulence effect over that of convex curvature. Heat transfer rates and temperature profiles are shown to change more slowly after transition than wall shear stress values and velocity profiles. This can be observed through higher values of turbulent Prandtl number and lower values of the Reynolds-analogy factor, 2St/Cf, in the transitional and early turbulent flows than values known to apply for mature turbulent flows.; The results of this study increase the data base for transition prediction modelling and provide increased understanding of the fundamentals of transition. Incorporating the effects of convex surface curvature and elevated free-stream turbulence levels into prediction models will represent an important advancement in gas turbine blade design.