Characteristics of turbulent boundary layers over smooth surfaces with spanwise heterogeneities

Abstract

An experimental investigation of a turbulent boundary-layer flow over a heterogeneous surface is carried out to examine the mean flow and turbulence characteristics, and to document the variation of skin friction that might affect the applicability of traditional scaling and similarity laws. The heterogeneity is imposed along the spanwise direction and consists of streamwise-aligned smooth raised strips whose spanwise spacing $S$ is comparable to the boundary-layer thickness ($S/\unicode[STIX]{x1D6FF}=O(1)$). Single-point velocity measurements alongside direct skin-friction measurements are used to examine the validity of Townsend’s similarity hypothesis. The skin-friction coefficients reveal that the drag of the heterogeneous surface increased up to 35 % compared to a smooth wall, while velocity measurements reveal the existence of a log layer but with a zero-plane displacement and a roughness function that vary across the spanwise direction. Lack of collapse in the outer region of the mean velocity and variance profiles is attributed to the secondary flows induced by the heterogeneous surfaces. Additionally, the lack of similarity also extends to the spectra across all scales in the near-wall region with a gradual collapse at small wavelengths for increasing $S$. This suggests that the effect of surface heterogeneity is not necessarily felt at the smaller scales other than to reorganise their presence through turbulent transport.

Turbulent channel flow over heterogeneous roughness at oblique angles

“…There is some disagreement in the predictions, although it is stressed that the Bons et al (2001) topography represents a case of complex, multiscale roughness with a predominant spanwise heterogeneity, which would thus challenge the predictive potential of any model. Unfortunately, comparison against the datasets from Ganapathisubramni and company (Vanderwel & Ganapathisubramani 2015;Medjnoun et al 2018) was not possible, since they consider continuous streamwise-aligned rows of LEGO elements, for which λ f = ∞. To further diversify the study, results from a complementary LES investigation have been included in figure 5(e) (Zhu et al 2016).…”

“…A range of such cases are shown in figure 1, where panel (a) shows a canonical spanwise-heterogeneous case, for which the flow streamwise direction is aligned parallel to the roughness heterogeneity. Spanwise heterogeneities are responsible for Reynolds-averaged flow heterogeneities (Barros & Christensen 2014;Vanderwel & Ganapathisubramani 2015;Medjnoun, Vanderwel & Ganapathisubramani 2018), which are known to be a realization of Prandtl's secondary flow of the second kind (Anderson et al 2015a) Oblique flow-surface heterogeneity 886 A15-3 (discussion to follow). The stripes of elements in figure 1(a-e) could notionally be represented by z 0,h , while the surrounding white space represents the 'less rough' region and could be represented by the lower roughness length, z 0,l .…”

Controlling secondary flow in Taylor–Couette turbulence through spanwise-varying roughness

“…Since the secondary flows are driven by a spatial gradient of the RSS, they find that the mean secondary flows are Prandtl's secondary flow of the second kind (Bradshaw 1987). Medjnoun et al (2018) observed a breakdown of outer layer similarity in the local profiles of the mean flow, turbulent intensity, and the energy spectra, evidently induced by the presence of the secondary vortices. Finally, Chung et al (2018) studied the influence of the spacing of idealized (i.e.…”

On the decay of dispersive motions in the outer region of rough-wall boundary layers

“…Therefore, beyond this roughness sub-layer, the outer layer of the flow is usually independent of local details of surface roughness, resulting in a mean flow that is nearly homogeneous in wall-parallel directions, with flow statistics that mainly depend on the wall-normal direction (Castro, 2007). However, for rough surfaces with spatial heterogeneities where dominant spanwise length scales of the roughness distribution are on the order of the outer length scale of the flow, large secondary motions are excited by the roughness arrangement, and can penetrate into the outer layer (Nezu & Nakagawa, 1984;Wang & Cheng, 2005;Barros & Christensen, 2014;Anderson et al, 2015;Vanderwel & Ganapathisubramani, 2015;Kevin et al, 2017;Medjnoun et al, 2018;Hwang & Lee, 2018). Therefore, dispersive stress can be significant across the entire turbulent layer.…”