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

Based on experimental data acquired with particle image velocimetry, we examine turbulent boundary layers that are subjected to an abrupt change in wall roughness the streamwise direction. Three different sandpapers (P24, P36 and P60) together a smooth used form number of surface transition cases, including both R $\rightarrow$ S (where upstream is rough second either or smoother compared surface) downstream surface; surfaces rough). This enables us investigate effect strength ( $M = \ln [y_{02}/y_{01}]$ , where $y_{01}$ $y_{02}$ lengths surfaces, respectively) growth internal layer (IBL) corresponding flow structure. The results show for each group (i.e. R), thickness IBLs proportional transition, thicker cases their counterparts similar $|M|$ when normalised by initial $\delta _0$ ). also rates could be represented power law, consistent previous studies. However, despite wide range scatter literature power-law exponent, average value $0.75$ (varies between $0.71$ $0.8$ no clear trend) obtained present study considering all cases. pre-factor fit, other hand, found related transition. In addition variations velocity defect diagnostic plots IBLs, sweep ejection events appear differ significantly (depending type step change). Two-point spatial correlations, moreover, structure more elongated wall-normal directions, as accelerates over cases). For reverse R, decelerates rougher surface), however, correlation coefficients shrink size directions.