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Turbulent boundary layers over permeable walls: scaling and near wall structure

Turbulent boundary layers over permeable walls: scaling and near wall structure
Turbulent boundary layers over permeable walls: scaling and near wall structure
This paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions
geophysical and geological flows, shear layer turbulence, turbulent boundary layers
0022-1120
141-170
Manes, C.
7d9d5123-4d1b-4760-beff-d82fe0bd0acf
Poggi, D.
f7f62aa4-d650-40bc-8715-3fe0e4acaa4d
Ridolfi, L.
260d7bfe-13ed-48c8-bc9e-7e529d6ed1df
Manes, C.
7d9d5123-4d1b-4760-beff-d82fe0bd0acf
Poggi, D.
f7f62aa4-d650-40bc-8715-3fe0e4acaa4d
Ridolfi, L.
260d7bfe-13ed-48c8-bc9e-7e529d6ed1df

Manes, C., Poggi, D. and Ridolfi, L. (2011) Turbulent boundary layers over permeable walls: scaling and near wall structure. Journal of Fluid Mechanics, 687, 141-170. (doi:10.1017/jfm.2011.329).

Record type: Article

Abstract

This paper presents an experimental study devoted to investigating the effects of permeability on wall turbulence. Velocity measurements were performed by means of laser Doppler anemometry in open channel flows over walls characterized by a wide range of permeability. Previous studies proposed that the von Kármán coefficient associated with mean velocity profiles over permeable walls is significantly lower than the standard values reported for flows over smooth and rough walls. Furthermore, it was observed that turbulent flows over permeable walls do not fully respect the widely accepted paradigm of outer-layer similarity. Our data suggest that both anomalies can be explained as an effect of poor inner–outer scale separation if the depth of shear penetration within the permeable wall is considered as the representative length scale of the inner layer. We observed that with increasing permeability, the near-wall structure progressively evolves towards a more organized state until it reaches the condition of a perturbed mixing layer where the shear instability of the inflectional mean velocity profile dictates the scale of the dominant eddies. In our experiments such shear instability eddies were detected only over the wall with the highest permeability. In contrast attached eddies were present over all the other wall conditions. On the basis of these findings, we argue that the near-wall structure of turbulent flows over permeable walls is regulated by a competing mechanism between attached and shear instability eddies. We also argue that the ratio between the shear penetration depth and the boundary layer thickness quantifies the ratio between such eddy scales and, therefore, can be used as a diagnostic parameter to assess which eddy structure dominates the near-wall region for different wall permeability and flow conditions

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More information

e-pub ahead of print date: 10 October 2011
Published date: November 2011
Keywords: geophysical and geological flows, shear layer turbulence, turbulent boundary layers
Organisations: Energy & Climate Change Group

Identifiers

Local EPrints ID: 204129
URI: http://eprints.soton.ac.uk/id/eprint/204129
ISSN: 0022-1120
PURE UUID: 9493fee2-3fba-4bac-b6f6-601718d53f42

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Date deposited: 24 Nov 2011 15:32
Last modified: 14 Mar 2024 04:30

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Contributors

Author: C. Manes
Author: D. Poggi
Author: L. Ridolfi

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