Secondary flow over surfaces with spanwise heterogeneity

Wangsawijaya, Dea Daniella, de Silva, Charitha, Baidya, Rio, Chung, Daniel, Marusic, Ivan and Hutchins, Nicholas (2018) Secondary flow over surfaces with spanwise heterogeneity. In The 21st Australasian Fluid Mechanics Conference. 4 pp .

Abstract

Surfaces with heterogeneous roughness are known to alter the behaviour of canonical turbulent boundary layers. In the case where roughness varies in the spanwise direction, the turbulent boundary layer is modified by secondary flows in the form of counter-rotating streamwise roll modes. Previous studies on various spanwise heterogeneity models have shown that the behaviour of these secondary flows is determined by the type,spacing, and the width of the roughness elements. To examine this further, we here conduct a series of hot-wire anemometry measurements over surfaces with strips of spanwise heterogeneous roughness, where the ratio of the roughness strip widthΛ and the spanwise-averaged boundary layer thickness δ varies in the range of Λ/δ = 0.32–3.63. Our results show that the secondary flow appears to amplify a particular energetic mode in the outer layer (z/δ ≈ 0.5), as evidenced by a peak in the 1-D energy spectra at this wall-normal location for all test cases. This behaviour is observed to be the strongest at Λ/δ ≈ 1. In thecases where Λ/δ approaches its limit (Λ/δ 1 or Λ/δ 1),the magnitude of this spectral peak diminishes and the size and location of the secondary flow are limited by either the spanwise extent of Λ or the height of δ. Outside the region affected by the secondary flow, the behaviour of surfaces with spanwise heterogeneity appears to approach that of homogeneous flow.

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Contributors

Author: Dea Daniella Wangsawijaya

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