Experimental investigation of secondary flows in turbulent boundary layers

Abstract

Wall-bounded flows developing over spanwise heterogeneous surfaces exhibit timeaveraged streamwise vortices when the spanwise characteristic length scale of the surface (S) is comparable to the dominant length scale of flow (δ) (Barros and Christensen, 2014). In this work, we experimentally examine a turbulent boundary layer developing over such surfaces, with particular attention to the large-scale secondary motions and their impact of the primary flow. We specifically examine three pivotal questions. (i) Can inner- and outer-layer similarity be possible in the presence of secondary flows? (ii) Are there other relevant length scales that carry the heterogeneity information? (iii) What are the effects of a heterogeneous to homogeneous step-change on the turbulent boundary layer secondary flows? A series of measurement campaigns were conducted over various surface conditions. The heterogeneity consists of smooth streamwise-aligned ridges of different spacings, shapes and widths and a step-change all with S/δ ≈O(1). Oil-film interferometry, hot-wire anemometry along with stereoscopic-PIV are used to determine the flow properties. The skin-friction coefficients revealed that the drag of a heterogeneous surface can increase up to 35% compared to a smooth-wall. Lack of collapse in the outer region is shown for the mean, variance and also extends to the spectra across all scales in the near-wall region with a gradual collapse at small wavelengths for increasing S. Swirling strength maps revealed remarkable changes in the secondary motions for different ridge shapes. For wide ridges, tertiary structures can form appearing to compete with the secondary ones. An imbalance in favour of the former structures occurs over a certain width, causing a swap in the locations of the low- and high-momentum pathways. Furthermore, the results indicate that the spanwise spacing alone is insufficient to characterise the surface heterogeneity. A new parameter (ξ) based on the ratio of perimeters over and below the mean surface height is shown to adequately capture the variations in the flow properties. Finally when subjected to a step-change, the upstream secondary motions are shown to persist downstream. The velocity profiles in defect form and diagnostic plot showed that self-similarity is maintained hinting that the flow has built a pseudo-state of equilibrium in the presence of these secondary motions. These observations indicate that secondary flows posses a mechanism through which they are capable to self-sustain for long distances. Consequently, this means that both the growth rates of the internal layer as well as the turbulent boundary layer will depend on the evolution process of these large-scale features.

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Identifiers

ORCID for Takfarinas Medjnoun: orcid.org/0000-0002-8699-1305

ORCID for Bharathram Ganapathisubramani: orcid.org/0000-0001-9817-0486

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