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

Neutrally- and Stably-Stratified Boundary Layers Adjustments to a Step Change in Surface Roughness

“…Consistently, n ≈ 0.8 was also reported for cases of S → R or R 1 → R 2 (where R 1 stands for a rough upstream surface smoother than the rough downstream surface R 2 ) by Andreopoulos and Wood (1982), Efros and Krogstad (2011) and Gul and Ganapathisubramani (2022) in the laboratory where the downstream surfaces are constructed using sandpaper or recessed elements, and by Rouhi et al (2019) where the downstream rough surface is a sine function. Much smaller values of the exponent (0.21 < n < 0.5) have, however, been observed in other S → R or R 1 → R 2 scenarios when the roughness elements were raised.…”

Statistical properties of neutrally and stably stratified boundary layers in response to an abrupt change in surface roughness

“…Two different methodologies were employed to determine the friction velocity as it responds to the change in roughness. The first methodology, based on the model in Elliott (1958), has been widely used to determine the response of the wall shear stress to a roughness change (Gul & Ganapathisubramani 2022;Li et al 2022). This model assumes that the flow within the IBL reaches equilibrium promptly, hence there is a constant roughness length of the downstream surface z 02 , while the mean streamwise velocity assumes the form of a piecewise function…”

“…The power exponent n varies within the range of [0.2, 0.8] in the existing literature, and it is dependent on the roughness arrangement and sensitive to the estimation procedure. The approaches to identify the IBL can be categorised into two branches according to the variables of interest, which are the mean streamwise velocity (Elliott 1958;Antonia & Luxton 1971a;Cheng & Castro 2002;Gul & Ganapathisubramani 2022) and normal stress (Efros & Krogstad 2011;Li et al 2021). Thus, recent work applied distinctive identification procedures to compare the growth curves of IBLs (see Rouhi, Chung & Hutchins (2019), Bou-Zeid, Meneveau & Parlange (2004), Sessa, Xie & Herring (2018) and Li et al (2021) amongst others).…”

“…The power exponent varies within the range of in the existing literature, and it is dependent on the roughness arrangement and sensitive to the estimation procedure. The approaches to identify the IBL can be categorised into two branches according to the variables of interest, which are the mean streamwise velocity (Elliott 1958; Antonia & Luxton 1971 a ; Cheng & Castro 2002; Gul & Ganapathisubramani 2022) and normal stress (Efros & Krogstad 2011; Li et al. 2021).…”

Large Eddy Simulation Study of Atmospheric Boundary Layer Flow over an Abrupt Rough-to-Smooth Surface Roughness Transition

“…Several experiments have been reported recently (Hanson and Ganapathisubramani 2016;Li et al 2019Li et al , 2021Gul and Ganapathisubramani 2022) that are mostly in the engineering domain. These studies have focused on the one-and two-dimensional turbulent spectra and on the integral and smaller length scales in the flow field behind an abrupt roughness transition.…”