Fluid flow and heat transfer over straight and curved rough surfaces

Measurements of fluid pressure, velocity, including turbulent fluctuations and heat transfer were made over the straight and curved lower surface of a wind tunnel with an upper surface which could be flexed to control the streamwise pressure gradients. Laser–Doppler techniques were employed for flow visualisation and detailed investigation of the flow structure. These were roughened in a structured manner with machined excrescences of pyramidal and trapezoidal shape. Fluid dynamic data are presented for smooth and rough surfaces and heat transfer results from roughnesses of three different heights. Velocity gradients and curvature, less than often found in turbo-machinery, in the ranges examined, had relatively little effect compared to the roughness. The present observations are compared with relevant data in the literature. The elements of the present work showed enhancement of heat transfer at comparable conditions over 70% more than the more commonly reported spherical forms. The observations are also compared with the predictions of a computational fluid dynamic procedure developed in the authors’ laboratory (F.H.A. Tarada, Heat transfer to rough turbine blading. University of Sussex, D.Phil thesis, 1987) with which there is fair to good agreement. Although the code overpredicts the effects of surface curvature and pressure gradients, and the level of heat transfer at the higher velocities, discrepancies are usually within the experimental uncertainties. The comprehensive range of fluid dynamic and heat transfer results are presented by Hubbe-Walker (S.E. Hubbe-Walker, An experimental study of the effects of roughness and curvature on heat transfer in turbulent boundary layers. University of Sussex, D.Phil. thesis, 1996), is intended to provide a database for the development of more recent analytical and CFD procedures.