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 centerline 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.


    Access

    Access via TIB

    Check availability in my library

    Order at Subito €


    Export, share and cite



    Title :

    The Limitations of Using “Ra” to Describe Surface Roughness



    Published in:

    Publication date :

    2016




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English



    Classification :

    BKL:    52.30 Strömungskraftmaschinen, Turbomaschinen / 52.30




    Surface Roughness Effects

    Shih, Tom I-P. / Yang, Vigor / Bons, Jeffrey P. | AIAA | 2014


    SURFACE ROUGHNESS (AA)

    SAE Technical Papers | 1963


    SURFACE ROUGHNESS (AA)

    SAE Technical Papers | 1960


    SURFACE ROUGHNESS (AA)

    SAE Technical Papers | 1955