Aerodynamic and heat transfer effects of distributed hemispherical roughness elements inducing step changes in a turbulent boundary layer

Abstract

This work details an experimental investigation aiming at characterizing aerodynamic and heat transfer effects induced by roughness elements on a fully-rough zero-pressure-gradient turbulent boundary layer (TBL). The studied rough surfaces were composed of distributed hemispherical elements in staggered arrangement over part of an otherwise flat plate, inducing step changes in roughness height and wall temperature. While the arrangement and the geometry of the roughness elements were fixed (constant density, constant plan and frontal solidities), the effect of varying the ratio of the boundary layer thickness δ to the roughness height k was investigated with a particular emphasis on low values of δ∕k. In a first part of the study, mean friction coefficients and equivalent sand heights were estimated for four configurations. Quasi-wall-similarity was observed on velocity statistics, two-point correlations and spectra measured by PIV and hot-wire anemometry, suggesting that TBL large-scale structures were not significantly influenced by such large roughness elements. In a second part, heat transfer effects were investigated using mean temperature profiles and transient surface temperature measurements, highlighting discrepancies between the two approaches. Relying on the first one, a parametrization of the non-dimensional wall temperature step function was obtained for this configuration of roughness elements. Based on this result, a semi-empirical relation for the Stanton number associated with such roughness geometries and accounting for a step change in surface temperature was derived.