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The Response of Turbulent Shear Flows to Discontinuous Changes in Surface Roughness.

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Mechanical engineering rept.,

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A large amount of data now exists on the response of turbulent shear layers to discontinuous or step changes in surface roughness. The data cover a range of different types of turbulent shear flows over a range of discontinuities in surface roughness. The various authors have used a wide variety of methods to reduce and present the data and, because of this, a consistent, general analysis and description of turbulent shear flow over a step change in roughness has not as yet been presented. It is the subject of this report. All available data have been reduced in a uniform way. The mean flow responses to a step change in roughness are considered in the context of growth of the new internal layer, of re-establishment of wall similarity and of wall shear distribution. It was found firstly, that Townsends prediction for the growth rate of the new internal layer was very good for layers that conformed to the assumptions of his theory it also gave reasonable results for other layers. Secondly the data showed that deviations from wall similarity immediately after a step can be accounted for by a variation in Karmans constant alone. Finally for layers without pressure gradient, the wall shear stress distribution downstream of a step change in roughness has been shown to have the qualitative trend predicted by Townsends theory but is often in poor quantitative agreement. Author

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  • Fluid Mechanics

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