Riblet-generated flow mechanisms that lead to local breaking of Reynolds analogy

Rouhi, A. ORCID: 0000-0002-7837-418X, Endrikat, S., Modesti, D., Sandberg, R., Oda, T., Tanimoto, K., Hutchins, N. and Chung, D., 2022. Riblet-generated flow mechanisms that lead to local breaking of Reynolds analogy. Journal of Fluid Mechanics, 951: A45. ISSN 0022-1120

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Abstract

We investigate the Reynolds analogy over riblets, namely the analogy between the fractional increase in Stanton number Ch and the fractional increase in the skin-friction coefficient Cf, relative to a smooth surface. We investigate the direct numerical simulation data of Endrikat et al. (Flow Turbul. Combust., vol. 107, 2021, pp. 1–29). The riblet groove shapes are isosceles triangles with tip angles α=30°,60°,90°, a trapezoid, a rectangle and a right triangle. The viscous-scaled riblet spacing varies between s+≈10 to 60. The global Reynolds analogy is primarily influenced by Kelvin–Helmholtz rollers and secondary flows. Kelvin–Helmholtz rollers locally break the Reynolds analogy favourably, i.e. cause a locally larger fractional increase in Ch than in Cf. These rollers induce negative wall shear stress patches which have no analogue in wall heat fluxes. Secondary flows at the riblets’ crests are associated with local unfavourable breaking of the Reynolds analogy, i.e. locally larger fractional increase in Cf than in Ch. Only the triangular riblets with α=30° trigger strong Kelvin–Helmholtz rollers without appreciable secondary flows. This riblet shape globally preserves the Reynolds analogy from s+=21 to 33. However, the other riblet shapes have weak or non-existent Kelvin–Helmholtz rollers, yet persistent secondary flows. These riblet shapes behave similarly to rough surfaces. They unfavourably break the global Reynolds analogy, and do so to a greater extent as s+ increases.

Item Type:	Journal article
Publication Title:	Journal of Fluid Mechanics
Creators:	Rouhi, A., Endrikat, S., Modesti, D., Sandberg, R., Oda, T., Tanimoto, K., Hutchins, N. and Chung, D.
Publisher:	Cambridge University Press
Date:	14 November 2022
Volume:	951
ISSN:	0022-1120
Identifiers:	Number Type 10.1017/jfm.2022.880 DOI 1618966 Other
Divisions:	Schools > School of Science and Technology
Record created by:	Jonathan Gallacher
Date Added:	16 Nov 2022 11:53
Last Modified:	16 Nov 2022 11:53
URI:	https://irep.ntu.ac.uk/id/eprint/47417

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