The effect of spanwise wavelength of surface heterogeneity on turbulent secondary flows

Abstract

We examine the behaviour of turbulent boundary layers over surfaces composed of spanwise-alternating smooth and rough strips, where the width of the strips S varies such that 0.32⩽S/δ¯¯⩽6.81, where δ¯¯ is the boundary-layer thickness averaged over one spanwise wavelength of the heterogeneity. The experiments are configured to examine the influences of spanwise variation in wall shear stress over a large S/δ¯¯ range. Hot-wire anemometry and particle image velocimetry (PIV) reveal that the half-wavelength S/δ¯¯ governs the diameter and strength of the resulting mean secondary flows and hence the observed isovels of the mean streamwise velocity. Three possible cases are observed: limiting cases (either S/δ¯¯≪1 or S/δ¯¯≫1), where the secondary flows are confined near the wall or near the roughness change, and intermediate cases (S/δ¯¯≈1), where the secondary flows are space filling and at their strongest. These secondary flows, however, exhibit a time-dependent behaviour which might be masked by time averaging. Further analysis of the energy spectrogram and fluctuating flow fields obtained from PIV show that the secondary flows meander in a similar manner to that of large-scale structures occurring naturally in turbulence over smooth walls. The meandering of the secondary flows is a function of S/δ¯¯ and is most prominent when S/δ¯¯≈1.

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ORCID for Dea Daniella Wangsawijaya: orcid.org/0000-0002-7072-4245

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