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ESTIMATING THE FRICTION VELOCITY IN A TURBULENT PLANE WALL JET OVER A TRANSITIONALLY ROUGH SURFACE

Noorallah Rostamy
Department of Mechanical Engineering, University of Saskatchewan 57 Campus Drive, Saskatoon, Saskatchewan, Canada S7N 5A9

Donald J. Bergstrom
Department of Mechanical Engineering, University of Saskatchewan, Saskatoon, SK, S7N 5A9, Canada

David Sumner
Department of Mechanical Engineering University of Saskatchewan 57 Campus Drive, Saskatoon, Saskatchewan, S7N 5A2, Canada

James D. Bugg
Department of Mechanical Engineering, University of Saskatchewan, 57 Campus Drive, Saskatoon, Saskatchewan, S7N 5A9, Canada

Résumé

In this paper, a power law velocity profile is used to estimate the friction velocity in a plane turbulent wall jet on a rough surface based on experimental data collected by laser Doppler anemometry (LDA). The Reynolds number based on the slot height and exit velocity of the jet was approximately Re = 7500. A 36-grit sheet was used as the rough surface, creating a transitionally rough flow regime (44 < ks+ < 70). Power law velocity profiles were fitted using both inner and outer scales. The power-law coefficients were obtained by applying the expressions proposed by Seo and Castillo (2004) and Brzek et al. (2009) for rough boundary layer flows. According to the present results, the power-law velocity profiles are in good agreement with the conventional logarithmic profile in the overlap region. The power law coefficients can also be used to evaluate a theoretical expression for the skin friction coefficient. The skin friction coefficient for the rough surface turns out to be substantially greater than that for a wall jet on a smooth surface.