Large-scale secondary flows can sometimes appear in turbulent boundary layers formed over rough surfaces creating low-and high-momentum pathways along the surface (Barros & Christensen 2014). We experimentally investigate the dependence of these secondary flows on surface/flow conditions by measuring the flows over streamwise strips of roughness with systematically-varied spanwise spacing. We find that the large-scale secondary flows are accentuated when the spacing of the roughness elements is roughly proportional to the boundary layer thickness, and do not appear for cases with finer spacing. Cases with coarser spacings also generate δ-scale secondary flows with tertiary flows in the spaces in between. These results show that the ratio of the spanwise length scale of roughness heterogeneity to the boundary layer thickness is a critical parameter for the occurrence of these secondary motions in turbulent boundary layers over rough walls.

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.

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Secondary flows can develop in turbulent boundary layers that grow over surfaces with spanwise inhomogeneities. In this article, we demonstrate the formation of secondary flows in both experimental and numerical tests and dissect the instantaneous structure and topology of these secondary motions. We show that the formation of secondary flows is not very sensitive to the Reynolds number range investigated, and direct numerical simulations and experiments produce similar results in the mean flow as well as the dispersive and turbulent stress distributions. The numerical methods capture time-resolved features of the instantaneous flow and provide insight into the near-wall flow structures, that were previously obscured in the experimental measurements. Proper orthogonal decomposition was shown to capture the essence of the secondary flows in relatively few modes and to be useful as a filter to analyse the instantaneous flow patterns. The secondary flows are found to create extended regions of high Reynolds stress away from the wall that comprise predominantly sweeps similar to what one would expect to see near the wall and which are comparable in magnitude to the near-wall stress. Analysis of the instantaneous flow patterns reveals that the secondary flows are the result of a non-homogeneous distribution of mid-size vortices.

This paper presents the results of a detailed experimental investigation into the effectiveness of sinusoidal leading edge serrations on aerofoils for the reduction of the noise generated by the interaction with turbulent flow. A detailed parametric study is performed to investigate the sensitivity of the noise reductions to the serration amplitude and wavelength. The study is primarily performed on flat plates in an idealized turbulent flow, which we demonstrate captures the same behaviour as when identical serrations are introduced onto 3D aerofoils. The influence on the noise reduction of the turbulence integral length-scale is also studied. An optimum serration wavelength is identified whereby maximum noise reductions are obtained, corresponding to when the transverse integral length-scale is roughly one-forth the serration wavelength. This paper proves that, at the optimum serration wavelength, adjacent valley sources are excited incoherently. One of the most important findings of this paper is that, at the optimum serration wavelength, the sound power radiation from the serrated aerofoil varies inversely proportional to the Strouhal number St h = f h U , where f , h and U are frequency, serration amplitude and flow speed, respectively. A simple model is proposed to explain this behaviour. Noise reductions are observed to generally increase with increasing frequency until the frequency at which aerofoil self-noise dominates the interaction noise. Leading edge serrations are also shown to reduce trailing edge self-noise. The mechanism for this phenomenon is explored through PIV measurements. Finally, the lift and drag of the serrated aerofoil are obtained through direct measurement and compared against the straight edge baseline aerofoil. It is shown that aerodynamic performance is not substantially degraded by the introduction of the leading edge serrations on the aerofoil.