Turbulence modifications in a turbulent boundary layer over a rough wall with spanwise-alternating roughness strips

Abstract

Turbulence modifications over a rough wall with spanwise-varying roughness are investigated at a moderate Reynolds number Reτ ≈ 2000 (or Reθ ≈ 6400), using particle image velocimetry (PIV) and hotwire anemometry. The rough wall is comprised of spanwise-alternating longitudinal sandpaper strips of two different roughness heights. The ratio of high- and low-roughness heights is 8, and the ratio of high- and low-roughness strip width is 0.5. PIV measurements are conducted in a wall-parallel plane located in the logarithmic region, while hotwire measurements are made throughout the entire boundary layer in a cross-stream plane. In a time-average sense, large-scale counter-rotating roll-modes are observed in the cross-stream plane over the rough wall, with downwash and upwash common-flows displayed over the high- and low-roughness strips, respectively. Meanwhile, elevated and reduced streamwise velocities occur over the high- and low-roughness strips, respectively. Significant modifications in the distributions of mean vorticities and Reynolds stresses are observed, exhibiting features of spatial preference. Furthermore, spatial correlations and conditional average analyses are performed to examine the alterations of turbulence structures over the rough wall, revealing that the time-invariant structures observed are resultant from the time-average process of instantaneous turbulent events that occur mostly and preferentially in space.

Effects of heterogeneous surface geometry on secondary flows in turbulent boundary layers

“…To further investigate the influence of the surface heterogeneity on the characteristics of the turbulence structures, the two-point correlation function of the velocity fluctuations is examined. This allows qualitative and quantitative characterisation of the spatial structures at different critical locations such as the high-and low-momentum pathways that characterise this flow (Wu & Christensen 2010;Yang & Anderson 2017;Bai et al 2018;Kevin et al 2019). The two-point correlation coefficient tensor R uiuj in the (y, z)plane is defined as, (y, z) is the standard deviation of u j in the (y, z)-plane.…”

Boundary layer flow interaction with a permeable wall

“…Varano, 27 on the other hand, investigated the rough surface with fetches of hemispherical roughness and found that the turbulent kinetic energy and shear stress production increase downstream of the element due to the delay in the flow separation over the top of the element. Bai et al 30 studied the application of spanwise altering a roughness strip with two different roughness heights and showed that the largescale counter-rotating roll-modes are observed over the rough wall, with reduced and increased streamwise velocities occurring over the low-and high-roughness strips, respectively. The results also indicated that pronounced modifications in the mean vorticities, swirling strength, and Reynolds stresses can be observed over the roughness strip.…”

Secondary currents and very-large-scale motions in open-channel flow over streamwise ridges

“…Nezu & Nakagawa 1993;Wang & Cheng 2006;Vanderwel & Ganapathisubramani 2015;Hwang & Lee 2018;Medjnoun, Vanderwel & Ganapathisubramani 2018;Vanderwel et al 2019;Zampiron et al 2020), alternating streamwise strips of different surface roughness (e.g. Nezu & Nakagawa 1993;Wang & Cheng 2006;Willingham et al 2014;Anderson et al 2015;Bai, Kevin & Monty 2018;Chung, Monty & Hutchins 2018;Wangsawijaya et al 2018) and converging/diverging riblet patterns (e.g. Nugroho, Hutchins & Monty 2013;Kevin et al 2017;Kevin & Hutchins 2019).…”