Coherent structures in wave boundary layers. Part 2. Solitary motion

2010 ◽  
Vol 646 ◽  
pp. 207-231 ◽  
Author(s):  
B. MUTLU SUMER ◽  
PALLE M. JENSEN ◽  
LONE B. SØRENSEN ◽  
JØRGEN FREDSØE ◽  
PHILIP L.-F. LIU ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Boundary Layers ◽  
Bed Shear Stress ◽  
Laminar Regime ◽  
Short Period ◽  
Flow Features ◽  
Layer Flow ◽  
Wave Boundary ◽  
Flow Experiences

This study continues the investigation of wave boundary layers reported by Carstensen, Sumer & Fredsøe (J. Fluid Mech., 2010, part 1 of this paper). The present paper summarizes the results of an experimental investigation of turbulent solitary wave boundary layers, simulated by solitary motion in an oscillating water tunnel. Two kinds of measurements were made: bed shear stress measurements and velocity measurements. The experiments show that the solitary-motion boundary layer experiences three kinds of flow regimes as the Reynolds number is increased: (i) laminar regime; (ii) laminar regime where the boundary-layer flow experiences a regular array of vortex tubes near the bed over a short period of time during the deceleration stage; and (iii) transitional regime characterized with turbulent spots, revealed by single/multiple, or, sometimes, quite dense spikes in the bed shear stress traces. Supplementary synchronized flow visualization tests confirmed the presence of the previously mentioned flow features. Information related to flow resistance are also given in the paper.

Coherent structures in wave boundary layers. Part 1. Oscillatory motion

2010 ◽  
Vol 646 ◽  
pp. 169-206 ◽  
Author(s):  
STEFAN CARSTENSEN ◽  
B. MUTLU SUMER ◽  
JØRGEN FREDSØE
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Boundary Layers ◽  
Flow Structures ◽  
Bed Shear Stress ◽  
Turbulent Spots ◽  
Onset Of Turbulence ◽  
Stress Signal ◽  
Coherent Flow Structures ◽  
Layer Flow

This work concerns oscillatory boundary layers over smooth beds. It comprises combined visual and quantitative techniques including bed shear stress measurements. The experiments were carried out in an oscillating water tunnel. The experiments reveal two significant coherent flow structures: (i) Vortex tubes, essentially two-dimensional vortices close to the bed extending across the width of the boundary-layer flow, caused by an inflectional-point shear layer instability. The imprint of these vortices in the bed shear stress is a series of small, insignificant kinks and dips. (ii) Turbulent spots, isolated arrowhead-shaped areas close to the bed in an otherwise laminar boundary layer where the flow ‘bursts’ with violent oscillations. The emergence of the turbulent spots marks the onset of turbulence. Turbulent spots cause single or multiple violent spikes in the bed shear stress signal, which has profound implications for sediment transport (in both the laboratory and the field). The experiments also show that similar coherent flow structures exist in the case of combined oscillatory flow and current.

Boundary-layer flow and bed shear stress under a solitary wave: revision

2014 ◽  
Vol 753 ◽  
pp. 554-559 ◽  
Author(s):  
Yong Sung Park ◽  
Joris Verschaeve ◽  
Geir K. Pedersen ◽  
Philip L.-F. Liu
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Solitary Wave ◽  
Boundary Layer Flow ◽  
Bottom Boundary Layer ◽  
Bed Shear Stress ◽  
Bottom Boundary ◽  
Layer Flow

AbstractWe address two shortcomings in the article by Liu, Park & Cowen (J. Fluid Mech., vol. 574, 2007, pp. 449–463), which gave a theoretical and experimental treatise of the bottom boundary-layer under a solitary wave.

Boundary layer flow and bed shear stress under a solitary wave

2007 ◽  
Vol 574 ◽  
pp. 449-463 ◽  
Author(s):  
PHILIP L.-F. LIU ◽  
YONG SUNG PARK ◽  
EDWIN A. COWEN
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Solitary Wave ◽  
Analytical Solutions ◽  
Boundary Layer Flow ◽  
Inertia Force ◽  
Bed Shear Stress ◽  
Viscous Boundary Layer ◽  
Layer Flow ◽  
Viscous Boundary

Liu & Orfila (J. Fluid Mech. vol. 520, 2004, p. 83) derived analytical solutions for viscous boundary layer flows under transient long waves. Their analytical solutions were obtained with the assumption that the nonlinear inertia force was negligible in the momentum equations. In this paper, using Liu & Orfila's solution and the solutions for the nonlinear boundary layer equations, we examine the boundary layer flow characteristics under a solitary wave. It is found that while the horizontal component of the free-stream velocity outside the boundary layer always moves in the direction of wave propagation, the fluid particle velocity near the bottom inside the boundary layer reverses direction as the wave decelerates. Consequently, the bed shear stress also changes sign during the deceleration phase. Laboratory measurements, including the free-surface displacement, particle image velocimetry (PIV) resolved velocity fields of the viscous boundary layer, and the calculated bed shear stress were also collected to check the theoretical results. Excellent agreement is observed.

scholarly journals Experimental study of the turbulent boundary layer in acceleration-skewed oscillatory flow

2011 ◽  
Vol 684 ◽  
pp. 251-283 ◽  
Author(s):  
Dominic A. van der A ◽  
Tom O’Donoghue ◽  
Alan G. Davies ◽  
Jan S. Ribberink
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Stress ◽  
Oscillatory Flow ◽  
Bed Shear Stress ◽  
Flow Conditions ◽  
Oscillatory Boundary Layer ◽  
Layer Flow ◽  
Momentum Integral ◽  
Good Agreement

AbstractExperiments have been conducted in a large oscillatory flow tunnel to investigate the effects of acceleration skewness on oscillatory boundary layer flow over fixed beds. As well as enabling experimental investigation of the effects of acceleration skewness, the new experiments add substantially to the relatively few existing detailed experimental datasets for oscillatory boundary layer flow conditions that correspond to full-scale sea wave conditions. Two types of bed roughness and a range of high-Reynolds-number, $\mathit{Re}\ensuremath{\sim} O(1{0}^{6} )$, oscillatory flow conditions, varying from sinusoidal to highly acceleration-skewed, are considered. Results show the structure of the intra-wave velocity profile, the time-averaged residual flow and boundary layer thickness for varying degrees of acceleration skewness, $\ensuremath{\beta} $. Turbulence intensity measurements from particle image velocimetry (PIV) and laser Doppler anemometry (LDA) show very good agreement. Turbulence intensity and Reynolds stress increase as the flow accelerates after flow reversal, are maximum at around maximum free-stream velocity and decay as the flow decelerates. The intra-wave turbulence depends strongly on $\ensuremath{\beta} $ but period-averaged turbulent quantities are largely independent of $\ensuremath{\beta} $. There is generally good agreement between bed shear stress estimates obtained using the log-law and using the momentum integral equation, and flow acceleration skewness leads to high bed shear stress asymmetry between flow half-cycles. Turbulent Reynolds stress is much less than the shear stress obtained from the momentum integral; analysis of the stress contributors shows that significant phase-averaged vertical velocities exist near the bed throughout the flow cycle, which lead to an additional shear stress, $\ensuremath{-} \rho \tilde {u} \tilde {w} $; near the bed this stress is at least as large as the turbulent Reynolds stress.

Experimental investigation of wave boundary layers with a sudden change in roughness

1993 ◽  
Vol 252 ◽  
pp. 117-145 ◽  
Author(s):  
J. Fredsøe ◽  
B. M. Sumer ◽  
T. S. Laursen ◽  
C. Pedersen
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Sudden Change ◽  
Maximum Velocity ◽  
Velocity Variation ◽  
Bed Shear Stress ◽  
Mean Flow ◽  
Boundary Layer Flows ◽  
Oscillatory Boundary Layer ◽  
Wave Boundary

This study deals with turbulent oscillatory boundary-layer flows over a plane bed with a sudden spatial change in roughness. Two kinds of ‘change in the roughness’ were investigated: in one, the roughness changed from a smooth-wall roughness to a roughness equal to 4.8 mm, and in the other, it changed from a roughness equal to 0.35 mm to the same roughness as in the previous experiment (4.8 mm). The free-stream flow was a purely oscillating flow with sinusoidal velocity variation. Mean flow and turbulence properties were measured. The Reynolds number was 6 × 106 for the major part of the experiments, with a maximum velocity of approximately 2 m/s and the stroke of the motion about 6 m. The response of the boundary layer to the sudden change in roughness was found to occur over a transitional length of the flow. The bed shear stress over this transitional length attains a peak value over the bed section with the larger roughness. It was found that the amplification in the bed shear stress due to this peak could be up to 2.5 times its asymptotic value. Also, it was found that the turbulence is quantitatively different in the two half periods; a much stronger turbulence is experienced in the half period where the flow is towards the less-rough section. The present experiments further showed that a constant streaming occurs near the bed in the neighbourhood of the junction between the two bed sections. This streaming is directed towards the section with the larger roughness.

scholarly journals SOLITARY-WAVE-INDUCED BOUNDARY LAYER FLOWS OVER PERMEABLE BEDS

2020 ◽  
pp. 9
Author(s):  
Nimish Pujara ◽  
Claudio Meza-Valle ◽  
Philip L.-F. Liu
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Solitary Wave ◽  
Boundary Layers ◽  
Accurate Determination ◽  
Bed Shear Stress ◽  
Boundary Layer Flows ◽  
Viscous Effects ◽  
Wave Induced

In the nearshore environment, viscous effects of wave- induced boundary layer flows above sea beds are important in evaluating sediment fluxes and wave damping. These effects need to be included in phase- resolved nearshore models for accurate determination of quantities such as the bed shear stress and the decrease in wave height due to frictional dissipation. In this work, we analyze the boundary layers induced by a solitary wave on permeable sea beds (extending previous work of wave-induced boundary layers on an impermeable bed). We find that the velocity profiles and bed shear stress are sensitive to the hydraulic conductivity of the bed, but not extremely sensitive to the precise boundary condition at the bed-fluid interface.

The Departure from Equilibrium of Turbulent Boundary Layers

1968 ◽  
Vol 19 (1) ◽  
pp. 1-19 ◽  
Author(s):  
H. McDonald
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Kinetic Energy ◽  
Stress Distribution ◽  
Boundary Layers ◽  
Turbulent Boundary Layers ◽  
Two Dimensional ◽  
Pressure Distributions ◽  
Momentum Integral ◽  
State Examination

SummaryRecently two authors, Nash and Goldberg, have suggested, intuitively, that the rate at which the shear stress distribution in an incompressible, two-dimensional, turbulent boundary layer would return to its equilibrium value is directly proportional to the extent of the departure from the equilibrium state. Examination of the behaviour of the integral properties of the boundary layer supports this hypothesis. In the present paper a relationship similar to the suggestion of Nash and Goldberg is derived from the local balance of the kinetic energy of the turbulence. Coupling this simple derived relationship to the boundary layer momentum and moment-of-momentum integral equations results in quite accurate predictions of the behaviour of non-equilibrium turbulent boundary layers in arbitrary adverse (given) pressure distributions.

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