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The instantaneous structure of secondary flows in turbulent boundary layers

Published online by Cambridge University Press:  16 January 2019

C. Vanderwel*
Affiliation:
Faculty of Engineering and Physical Sciences, University of Southampton, Southampton SO17 1BJ, UK
A. Stroh*
Affiliation:
Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
J. Kriegseis
Affiliation:
Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
B. Frohnapfel
Affiliation:
Institute of Fluid Mechanics, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany
B. Ganapathisubramani
Affiliation:
Faculty of Engineering and Physical Sciences, University of Southampton, Southampton SO17 1BJ, UK
*
Email addresses for correspondence: [email protected], [email protected]
Email addresses for correspondence: [email protected], [email protected]

Abstract

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.

Type
JFM Papers
Copyright
© 2019 Cambridge University Press 

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Vanderwel et al. supplementary movie 1

Video of sequential snapshots of the instantaneous flow field from the experiment recorded at 2 frames per second.

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Video 1.3 MB

Vanderwel et al. supplementary movie 2

Time-resolved video of the instantaneous flow field from the numerical simulation.

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Video 6.3 MB

Vanderwel et al. supplementary movie 3

Complete instantaneous flow field with its POD-reconstructed counterpart (upper row) and the corresponding velocity fluctuations (lower row). In the upper row, the colour code corresponds to the streamwise velocity component and the arrows indicate the in-plane motion, whereas in the bottom row, the color code represents the instantaneous deviation of the streamwise velocity component from (y).

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Video 19.3 MB

Vanderwel et al. supplementary movie 4

Instantaneous structure of the flow visualized using isosurfaces of $\lambda_2$-criterion ($\lambda_2$ = −0.005) and colored by their rotational direction.

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Video 67.8 MB