Abstract
This experimental investigation deals with the influence of free-stream turbulence (FST) produced by an active grid on the skin friction of a zero-pressure-gradient turbulent boundary layer. Wall shear stress is obtained by oil-film interferometry. In addition, hot-wire anemometry was performed to obtain wall-normal profiles of streamwise velocity. This enables the skin friction to be deduced from the mean profile. Both methods show remarkable agreement for every test case. Although skin friction is shown to increase with FST, the trend with Reynolds number is found to be similar to cases without FST. Furthermore, once the change in the friction velocity is accounted for, the self-similarity of the logarithmic region and below (i.e. law of the wall) appears to hold for all FST cases investigated.
Highlights
Free-stream turbulence (FST) exists above most of turbulent boundary layers (TBLs) encountered in natural and industrial environments (Sharp et al 2009)
Thole and Bogard (1996) performed extensive research on FST levels up to 20% and presented boundary layer statistics that confirmed the validity of the logarithmic law in the mean profiles of the boundary layer for high turbulence levels by comparing direct measurements of total shear stress with values obtained using a Clauser fit to the log region
Stefes and Fernholz (2004) compared skin-friction data obtained from oil-film interferometry (OFI), wall hot wire, and Preston tube at relatively high Reynolds numbers and FST levels up to 13%, showing that all skin-friction data points lied within an error band of approximately 6% on Cf
Summary
Free-stream turbulence (FST) exists above most of turbulent boundary layers (TBLs) encountered in natural and industrial environments (Sharp et al 2009). Rodríguez-López et al (2015) proposed to leave these constants free to adopt the value that best fits the data This method does not require to prescribe the extent of the logarithmic layer (which can vary under FST conditions, see Dogan et al 2016) and allows a certain uncertainty in the wall-probe initial position. OFI technique is used in this study to obtain direct measure of the wall shear stress This technique is based exclusively on the thinning rate of a thin oil film and the forces acting on the film as flow passes over it. Image-based technique, here, is one of the several variations from the original form proposed by Tanner and Blows (1976)
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