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The Limitations of “Ra” to Describe Surface Roughness

Goodhand, Martin N., Walton, Karl, Blunt, Liam, Lung, Hang W., Miller, Robert J. and Marsden, Reg (2015) The Limitations of “Ra” to Describe Surface Roughness. In: ASME Proceedings | Transitional Flows (With Turbomachinery) : ASME Turbo Expo 2015: Turbine Technical Conference and Exposition. ASME, V05CT16A003. ISBN 978-0-7918-5673-4

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Current criteria used to determine whether rough surfaces affect skin friction typically rely on a single amplitude parameter to characterize the roughness. The most commonly used criteria relate the centreline averaged roughness, Ra, to an equivalent sandgrain roughness size, ks. This paper shows that such criteria are oversimplified and that Ra/ks is dependent on the roughness topography, namely the roughness slope defined as the roughness amplitude normalized by the distance between roughness peaks, Ra/λ.

To demonstrate the relationship, wake traverses were undertaken downstream of an aerofoil with various polished surfaces. The admissible roughness Reynolds number (ρ1u1Ra/μ1) at which the drag rose above the smooth blade case, was determined. The results were used to demonstrate a 400% variation in Ra/ks over the roughness topographies tested.

The relationship found held for all cases tested, except those where the roughness first initiated premature transition at the leading edge. In these cases, where the roughness was more typical of eroded aerofoils, the drag was found to rise earlier.

Item Type: Book Chapter
Additional Information: ASME Turbo Expo 2015: Turbine Technical Conference and Exposition, Quebec, June 15–19, 2015 Paper No. GT2015-43329, pp. V05CT16A003; 8 pages
Subjects: T Technology > TJ Mechanical engineering and machinery
Schools: School of Computing and Engineering > Centre for Precision Technologies > EPSRC Centre for Innovative Manufacturing in Advanced Metrology
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Depositing User: Karl Walton
Date Deposited: 27 Jan 2016 08:51
Last Modified: 28 Aug 2021 11:58


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