Turbulent boundary layer analysis over a rough wall with increasing development length

Numerous studies have examined turbulent boundary layers (TBLs) over streamwise homogeneous roughness leading to a comprehensive understanding of their internal structure, particularly in the fully rough regime. In addition, the study of the TBL response to step changes (smooth to rough or rough to smooth) provided new insight into their expected behaviour. In both cases, the length of the roughness fetch over which the TBL develops was never actively scrutinized, leaving the open question of where the threshold between them is. This study aims to determine the minimum streamwise roughness length needed for the TBL to stop feeling the smooth-to-rough transition effects and start displaying homogeneously rough behaviour. This is achieved by using a floating drag balance positioned ≈ 7 m downstream of the wind tunnel's test section inlet to measure the TBL response to 0-to-40 ms⁻¹ inlet-velocity sweeps for 22 different roughness lengths upstream of the balance (up to ≈ 40δ). Here, the roughness consists of stripes of P24 sandpaper sequentially taped, one at a time, over the test section floor of the University of Southampton Boundary Layer Wind Tunnel (BLWT) with dimensions 9x1.2x1 m and an expected δ of about 0.15 m. Finally, the TBL response is compared with previous studies of both smooth-to-rough transitioning TBLs and homogeneously rough ones based on the equivalent sand-grain roughness parameter (Ks).