scholarly journals Mean flow structure and velocity–bed shear stress maxima phase difference in smooth wall, transitionally turbulent oscillatory boundary layers: direct numerical simulations

2021 ◽  
Vol 928 ◽  
Author(s):  
Dimitrios K. Fytanidis ◽  
Marcelo H. García ◽  
Paul F. Fischer
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Numerical Simulations ◽  
Phase Difference ◽  
Free Stream ◽  
Direct Numerical Simulations ◽  
Stream Velocity ◽  
Phase Lag ◽  
Bed Shear Stress ◽  
Oscillatory Boundary Layer

Direct numerical simulations of oscillatory boundary-layer flows in the transitional regime were performed to explain discrepancies in the literature regarding the phase difference ${\rm \Delta} \phi$ between the bed-shear stress and free-stream velocity maxima. Recent experimental observations in smooth bed oscillatory boundary-layer (OBL) flows, showed a significant change in the widely used ${\rm \Delta} \phi$ diagram (Mier et al., J. Fluid Mech., vol. 922, 2021, A29). However, the limitations of the point-wise measurement technique did not allow us to associate this finding with the turbulent kinetic energy budget and to detect the approach to a ‘near-equilibrium’ condition, defined in a narrow sense herein. Direct numerical simulation results suggest that a phase lag occurs as the result of a delayed and incomplete transition of OBL flows to a stage that mimics the fully turbulent regime. Data from the literature were also used to support the presence of the phase lag and propose a new ${\rm \Delta} \phi$ diagram. Simulations performed for ${\textit {Re}}_{\delta }=671$ confirmed the sensitivity in the development of self-sustained turbulence on the background disturbances ( $\textit{Re}_{\delta}=U_{o}\delta/\nu$ , where $\delta=[2\nu/\omega]^{1/2}$ is the Stokes' length, $U_{o}$ is the maximum free stream velocity of the oscillation, $\nu$ is the kinematic viscosity and $\omega=2{\rm \pi}/T$ is the angular velocity based on the period of the oscillation T). Variations of the mean velocity slope and intersect values for oscillatory flows are also explained in terms of the proximity to near-equilibrium conditions. Relaminarization and transition effects can significantly delay the development of OBL flows, resulting in an incomplete transition. The shape and defect factors are examined as diagnostic parameters for conditions that allow the formation of a logarithmic profile with the universal von Kármán constant and intersect. These findings are of relevance for environmental fluid mechanics and coastal morphodynamics/engineering applications.

scholarly journals Prediction of Heat Transfer From Laminar Boundary Layers, With Emphasis on Large Free-Stream Velocity Gradients and Highly Cooled Walls

1966 ◽  
Vol 88 (3) ◽  
pp. 249-256 ◽  
Author(s):  
L. H. Back ◽  
A. B. Witte
Keyword(s):  
Heat Transfer ◽  
Boundary Layer ◽  
Shear Stress ◽  
Velocity Gradient ◽  
Free Stream ◽  
Variable Density ◽  
Similarity Solutions ◽  
Free Stream Velocity ◽  
Stream Velocity ◽  
Gradient Parameter

Laminar boundary-layer heat transfer and shear-stress predictions from existing similarity solutions are extended in an approximate way to perfect gas flows with a large free-stream velocity gradient parameter β and variable density-viscosity product ρμ across the boundary layer resulting from a highly cooled wall. The dimensionless enthalpy gradient at the wall gw′, to which the heat flux is related, is found not to vary appreciably with β. Thus the application of similarity solutions on a local basis to predict heat transfer from accelerated flows to an arbitrary surface may be a reasonable approximation involving a minimum amount of calculation time. Unlike gw′, the dimensionless velocity gradient at the wall fw″, to which the shear stress is related, is strongly dependent on β.

scholarly journals A Numerical Study of Sheet Flow Driven by Skewed-Asymmetric Shoaling Waves Using SedWaveFoam

2021 ◽  
Vol 9 (9) ◽  
pp. 936
Author(s):  
Yeulwoo Kim ◽  
Ryan S. Mieras ◽  
Dylan Anderson ◽  
Timu Gallien
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Sediment Transport ◽  
Transport Rate ◽  
Phase Lag ◽  
Bed Shear Stress ◽  
Sheet Flow ◽  
Sediment Transport Rate ◽  
Wave Conditions ◽  
The Impact

SedWaveFoam, an OpenFOAM-based two-phase model that concurrently resolves the free surface wave field, and the bottom boundary layer is used to investigate sediment transport throughout the entire water column. The numerical model was validated with large-scale wave flume data for sheet flow driven by shoaling skewed-asymmetric waves with two different grain sizes. Newly obtained model results were combined with previous nonbreaking and near-breaking wave cases to develop parameterization methods for time-dependent bed shear stress and sediment transport rate under various sediment sizes and wave conditions. Gonzalez-Rodriguez and Madsen (GRM07) and quasi-steady approaches were compared for intra-wave bed shear stress. The results show that in strongly asymmetric flows, considering the separated boundary layer development processes at each half wave-cycle (i.e., GRM07) is essential to accurately estimating bed shear stress and highlights the impact of phase-lag effects on sediment transport rates. The quasi-steady approach underpredicts (∼60%) sediment transport rates, especially for fine grains under large velocity asymmetry. A modified phase-lag parameter, incorporating velocity asymmetry, sediment stirring, and settling processes is proposed to extend the Meyer-Peter and Mueller type power law formula. The extended formula accurately estimated the enhanced net onshore sediment transport rate observed under skewed-asymmetric wave conditions.

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.

scholarly journals BED SHEAR STRESS IN UNSTEADY FLOW

2011 ◽  
Vol 1 (32) ◽  
pp. 8 ◽  
Author(s):  
Paul Andrew Guard ◽  
Peter Nielsen ◽  
Tom E Baldock
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Pressure Gradient ◽  
Water Surface ◽  
Free Stream ◽  
Shear Stresses ◽  
Bed Shear Stress ◽  
Horizontal Pressure Gradient ◽  
Friction Factors ◽  
Horizontal Pressure

Standard engineering methods of estimating bed shear stress using friction factors can fail spectacularly in unsteady hydrodynamic conditions. This paper demonstrates this fact using direct measurements of bed shear stresses under irregular waves using a shear plate apparatus. The measurements are explained in terms of the influence of the horizontal pressure gradient and the shear stresses acting on the surface of the plate. The horizontal fluid velocity at the edge of the boundary layer and the water surface elevation and slope were also measured. The paper demonstrates that the water surface measurements can be used to obtain accurate estimates of the forces on the bed, by employing Fourier analysis techniques or an innovative convolution integral method. The experimental results indicate that an offshore bed shear stress may be recorded while the free stream velocity remains onshore at all times. This demonstrates the failure of the standard engineering friction factor method in this scenario, since negative friction factors would be required. Important questions are raised regarding the appropriate definition for the bed shear stress when the vertical gradient of the shear stress is large. It is shown that it is problematic to define a single value for a “bed” shear stress in the presence of a strong horizontal pressure gradient. It is also argued that the natural driver for any model used to predict bed shear stress is the pressure gradient (or its proxy the free stream acceleration), rather than the velocity. This allows for accurate calculation of both acceleration effects (more rapid acceleration leads to a thinner boundary layer and higher shear stress) and also the direct action of the horizontal pressure gradient.

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.

Conditional statistics and flow structures in turbulent boundary layers buffeted by free-stream disturbances

2019 ◽  
Vol 866 ◽  
pp. 526-566 ◽  
Author(s):  
Jiho You ◽  
Tamer A. Zaki
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Kinetic Energy ◽  
Boundary Layers ◽  
Large Scale ◽  
Free Stream ◽  
Turbulent Boundary Layers ◽  
Direct Numerical Simulations ◽  
Free Stream Turbulence ◽  
Conditional Statistics

Direct numerical simulations are performed to study zero-pressure-gradient turbulent boundary layers beneath quiescent and vortical free streams. The inflow boundary layer is computed in a precursor simulation of laminar-to-turbulence transition, and the free-stream vortical forcing is obtained from direct numerical simulations of homogeneous isotropic turbulence. A level-set approach is employed in order to objectively distinguish the boundary-layer and free-stream fluids, and to accurately evaluate their respective contributions to flow statistics. When free-stream turbulence is present, the skin friction coefficient is elevated relative to its value in the canonical boundary-layer configuration. An explanation is provided in terms of an increase in the power input into production of boundary-layer turbulence kinetic energy. This increase takes place deeper than the extent of penetration of the external perturbations towards the wall, and also despite the free-stream perturbations being void of any Reynolds shear stress. Conditional statistics demonstrate that the free-stream turbulence has two effects on the boundary layer: one direct and the other indirect. The low-frequency components of the free-stream turbulence penetrate the logarithmic layer. The associated wall-normal Reynolds stress acts against the mean shear to enhance the shear stress, which in turn enhances turbulence production. This effect directly enlarges the scale and enhances the energy of outer large-scale motions in the boundary layer. The second, indirect effect is the influence of these newly formed large-scale structures. They modulate the near-wall shear stress and, as a result, increase the turbulence kinetic energy production in the buffer layer, which is deeper than the extent of penetration of free-stream turbulence towards the wall.

scholarly journals Interface-resolved direct numerical simulations of sediment transport in a turbulent oscillatory boundary layer

2020 ◽  
Vol 885 ◽  
Author(s):  
Marco Mazzuoli ◽  
Paolo Blondeaux ◽  
Giovanna Vittori ◽  
Markus Uhlmann ◽  
Julian Simeonov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sediment Transport ◽  
Numerical Simulations ◽  
Direct Numerical Simulations ◽  
Oscillatory Boundary Layer ◽  
Image Position

scholarly journals VELOCITY DEFECT LAW IN THE WAVE BOTTOM BOUNDARY LAYER

2011 ◽  
Vol 1 (32) ◽  
pp. 30
Author(s):  
Tiago Abreu ◽  
H. Michallet ◽  
P. A. Silva ◽  
F. Sancho ◽  
P. Nielsen
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Vertical Profile ◽  
Bottom Boundary Layer ◽  
Bed Shear Stress ◽  
Simple Method ◽  
Velocity Defect ◽  
Bottom Boundary ◽  
Velocity Magnitude ◽  
Oscillatory Boundary Layer

This work presents a simple method based on the defect law (Nielsen,1992) to reproduce the velocity vertical profile within the wave bottom boundary layer. The model results agree fairly well with the ADVP measurements and show that the defect law reproduces typical features of the oscillatory boundary layer: the velocity magnitude first increases with distance from the bed, with an overshoot at approximately 3 cm above the bed. There is a phase shift in the velocity that is maximum at about 1cm above the bed. Processing of the bed shear stress as well as velocities estimates within the sheet flow layer is under progress. (e.g. Ruessink et al., submitted)

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