Effect of externally generated turbulence on wave boundary layer

Jørgen Fredsøe, B. Mutlu Sumer, A. Kozakiewicz, L. Chua, Rolf Deigaard

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel with a smooth bottom. Turbulence was generated ´externally´ as the flow in the oscillator was passed through a series of grids, that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity measurements were carried out. For the velocity measurements, mean and turbulence properties were measured in both the streamwise direction and in the direction perpendicular to the bed. A supplementary measurement for the undisturbed (without grids) case was also carried out, for comparison with the grid results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore the friction coefficient. Other features related to the bed shear stress, such as transition, the friction factor and phase lead are discussed. The range of the Reynolds number studied is 10.000 - 2.000.000
Original languageEnglish
JournalCoastal Engineering
Volume49
Issue number3
Pages (from-to)155-183
ISSN0378-3839
DOIs
Publication statusPublished - Sep 2003

Keywords

  • external turbulence
  • grid-generated turbulence
  • spilling breaker
  • wave boundary layers

Cite this

Fredsøe, Jørgen ; Sumer, B. Mutlu ; Kozakiewicz, A. ; Chua, L. ; Deigaard, Rolf. / Effect of externally generated turbulence on wave boundary layer. In: Coastal Engineering. 2003 ; Vol. 49, No. 3. pp. 155-183.
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Effect of externally generated turbulence on wave boundary layer. / Fredsøe, Jørgen; Sumer, B. Mutlu; Kozakiewicz, A.; Chua, L.; Deigaard, Rolf.

In: Coastal Engineering, Vol. 49, No. 3, 09.2003, p. 155-183.

Research output: Contribution to journalJournal articleResearchpeer-review

TY - JOUR

T1 - Effect of externally generated turbulence on wave boundary layer

AU - Fredsøe, Jørgen

AU - Sumer, B. Mutlu

AU - Kozakiewicz, A.

AU - Chua, L.

AU - Deigaard, Rolf

PY - 2003/9

Y1 - 2003/9

N2 - This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel with a smooth bottom. Turbulence was generated ´externally´ as the flow in the oscillator was passed through a series of grids, that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity measurements were carried out. For the velocity measurements, mean and turbulence properties were measured in both the streamwise direction and in the direction perpendicular to the bed. A supplementary measurement for the undisturbed (without grids) case was also carried out, for comparison with the grid results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore the friction coefficient. Other features related to the bed shear stress, such as transition, the friction factor and phase lead are discussed. The range of the Reynolds number studied is 10.000 - 2.000.000

AB - This experimental study deals with the effect of externally generated turbulence on the oscillatory boundary layer to simulate the turbulence in the wave boundary layer under broken waves in the swash zone. The subject has been investigated experimentally in a U-shaped, oscillating water tunnel with a smooth bottom. Turbulence was generated ´externally´ as the flow in the oscillator was passed through a series of grids, that extended from the cover of the water tunnel to about mid-depth. Two different types of grid porosities were used. Direct measurements of the bed shear stress and velocity measurements were carried out. For the velocity measurements, mean and turbulence properties were measured in both the streamwise direction and in the direction perpendicular to the bed. A supplementary measurement for the undisturbed (without grids) case was also carried out, for comparison with the grid results. The mean and turbulence quantities in the outer flow region are increased substantially with the introduction of the grids. It is shown that the externally generated turbulence is able to penetrate the bed boundary layer, resulting in an increase in the bed shear stress, and therefore the friction coefficient. Other features related to the bed shear stress, such as transition, the friction factor and phase lead are discussed. The range of the Reynolds number studied is 10.000 - 2.000.000

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KW - grid-generated turbulence

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DO - 10.1016/S0378-3839(03)00032-2

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