Experimental observations on turbulent boundary layers subjected to a step-change in surface roughness

Abstract

Based on experimental data acquired with particle image velocimetry, we examineturbulent boundary layers that are subjected to an abrupt change in wall roughness inthe streamwise direction. Three different sandpapers (P24, P36 and P60) together with asmooth wall are used to form a number of different surface transition cases, includingboth R→S (where upstream surface is rough and second surface is either smooth orsmoother compared with the upstream surface) and S→R (where upstream surface issmoother compared with the downstream surface; both surfaces are rough). This enablesus to investigate the effect of the surface transition strength (M = ln[y02/y01], where y01and y02 are the roughness lengths of the upstream and downstream surfaces, respectively)on the growth of the internal boundary layer (IBL) and the corresponding flow structure.The results show that for each surface transition group (i.e. R→S and S→R), thethickness of the IBLs is proportional to the strength of the surface transition, and thatthe IBLs are thicker for the S→R cases compared with their R→S counterparts forsimilar |M|, when normalised by the initial boundary layer thickness (δ0). The results alsoshow that the growth rates of the IBLs could be represented by a power law, consistentwith the previous studies. However, despite a wide range of scatter in the literature forthe power-law exponent, an average value of 0.75 (varies between 0.71 and 0.8 withno clear trend) is obtained in the present study considering all the surface transitioncases. The pre-factor for the power-law fit, on the other hand, is found to be related tothe strength of the surface transition. In addition to the variations in the velocity defectand diagnostic plots with the growth of the IBLs, sweep and ejection events appearto differ significantly (depending on the type of the step change). Two-point spatialcorrelations, moreover, show that the structure is more elongated in the wall-normal andstreamwise directions, as the flow accelerates over the downstream surface (i.e. R→Scases). For the reverse transition cases (i.e. S→R, where the flow decelerates over the

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Identifiers

ORCID for Melika Gul: orcid.org/0000-0001-5843-7895

ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

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