1DV bottom boundary layer modeling under combined wave and current: Turbulent separation and phase lag effects (original) (raw)
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An analytical model of capped turbulent oscillatory bottom boundary layers
Journal of Geophysical Research, 2010
1] An analytical model of capped turbulent oscillatory bottom boundary layers (BBLs) is proposed using eddy viscosity of a quadratic form. The common definition of friction velocity based on maximum bottom shear stress is found unsatisfactory for BBLs under rotating flows, and a possible extension based on turbulent kinetic energy balance is proposed. The model solutions show that the flow may slip at the top of the boundary layer due to capping by the water surface or stratification, reducing the bottom shear stress, and that the Earth's rotation induces current and bottom shear stress components perpendicular to the interior flow with a phase lag (or lead). Comparisons with field and numerical experiments indicate that the model predicts the essential characteristics of the velocity profiles, although the agreement is rather qualitative due to assumptions of quadratic eddy viscosity with time-independent friction velocity and a well-mixed boundary layer. On the other hand, the predicted linear friction coefficients, phase lead, and veering angle at the bottom agreed with available data with an error of 3%-10%, 5°-10°, and 5°-10°, respectively. As an application of the model, the friction coefficients are used to calculate e-folding decay distances of progressive internal waves with a semidiurnal frequency.
Numerical Modeling of Turbulent Bottom Boundary Layer over Rough Bed under Irregular Waves
IPTEK The Journal for Technology and Science, 2011
AbstractA numerical model of turbulent bottom boundary layer over rough bed under irregular waves is reviewed. The turbulence model is based upon Shear Stress Transport (SST) k- model. The non-linear governing equations of the boundary layer for each turbulence models were solved by using a Crank-Nicolson type implicit finite-difference scheme. Typical the main velocity distribution, turbulence kinetic energy and time series of the bottom shear stress are presented. These results are shown to be in generally good agreement with experimental result. The roughness effects in the properties of turbulent bottom boundary layer for irregular waves are also presented with several values of the roughness parameter (a m /k s ) from a m /k s =5 to a m /k s =3122. The roughness effect tends to decrease the main velocity distribution and to increase the turbulent kinetic energy in the inner boundary layer, whereas in the outer boundary layer, the roughness alters the mean velocity distribution and the kinetic energy turbulent is relatively unaffected. The effect of bed roughness on the bottom shear stress under irregular waves is found that the higher roughness elements increase the magnitude of bottom shear stress along wave cycle. And further, the bottom shear stress under irregular waves is examined with the existing calculation method and the newly proposed method.
TURBULENCE MODELING OF A WAVE BOUNDARY LAYER ON A ROUGH BOTTOM
Coastal Engineering 2008 - Proceedings of the 31st International Conference, 2009
The original k-co model by Wilcox (WL) and two versions of blended k-w/k-S model, namely; Baseline (BSL) model and Shear Stress Transport (SST) model were applied to the wave boundary layers on a rough bottom. The model results were compared with the available experimental data. The three models show good agreement with the experimental data for velocity, turbulent kinetic energy and Reynolds stress. The SST model is superior in predicting shear velocity in one of the experimental cases. However, a detailed comparison revealed that SST model underestimates the friction factor for lower values of particle excursion length to roughness ratio, whereas, WL and BSL models showed good agreement with the experimental data. The results of this study would be useful for practicing engineers and researchers in choosing an appropriate model for calculating bottom shear stress.
Turbulent marine bottom boundary layer by V t2-F turbulence model
MARINE VI : proceedings of the VI International Conference on Computational Methods in Marine Engineering, 2015
In this work, we propose to implement the v 2 − f turbulence model rarely used in the marine environment to study the marine bottom boundary layer (MBBL). This model will complete the series of the turbulence models already implemented in the operational model 1DV-MoSeTT (1D Vertical Model of Sediment Transport and Turbulence) developed for the MBBL dynamics analysis. To show the performance of v 2 − f turbulence model first, we give a comparison between this model and q 2 − q 2 model. This comparison is based in various laboratory data proposed in the literature and widely used by the scientific community. Second, and in comparison with in-situ suspended sediment transport measurements, we examine the impact of the v 2 − f and the q 2 − q 2 turbulence models on the quantification of flux sediment at the bottom and on the estimation of the vertical profile of the suspended particle matter (SPM).
Three versions of the low Reynolds number k-ε model and three versions of the k-ω model (one original model and two two-layer versions) are tested against the DNS data of one-dimensional sinusoidal and flat-crested oscillatory boundary layers, and experimental data of cnoidal wave boundary layer. A detailed comparison has been made for cross-stream velocities, turbulent kinetic energy (T.K.E.), ratio of Reynolds stress and turbulent kinetic energy and wall shear stress. It is observed that the newer versions of the k-ε model can predict the velocity and turbulent kinetic energy in a better way. The k-ω model and two-layer models underestimate the peak value of turbulent kinetic energy, which may be explained by the Reynolds stress to T.K.E ratio in the logarithmic zone. The bottom shear stress peak is predicted by the k-ω model in an excellent manner. The results of the present study may be useful for the practicing engineers and researchers for choosing appropriate turbulence models in a certain field condition.
Journal of Fluid Mechanics, 2020
The flow within an oscillatory boundary layer, which approximates the flow generated by propagating sea waves of small amplitude close to the bottom, is simulated numerically by integrating Navier-Stokes and continuity equations. The bottom is made up of spherical particles, free to move, which mimic sediment grains. The approach allows to fully-resolve the flow around the particles and to evaluate the forces and torques that the fluid exerts on their surface. Then, the dynamics of sediments is explicitly computed by means of Newton-Euler equations. For the smallest value of the flow Reynolds number presently simulated, the flow regime turns out to fall in the intermittently turbulent regime such that turbulence appears when the free stream velocity is close to its largest values but the flow recovers a laminar like behaviour during the remaining phases of the cycle. For the largest value of the Reynolds number turbulence is significant almost during the whole flow cycle. The evaluation of the sediment transport rate allows to estimate the reliability of the empirical predictors commonly used to estimate the amount of sediments transported by the sea waves. For large values of the Shields parameter, the sediment flow rate during the accelerating phases does not differ from that observed during the decelerating phases. However, for relatively small values of the Shields parameter, the amount of moving particles depends not only on the bottom shear stress but also on flow acceleration. Moreover, the numerical results provide information on the role that turbulent eddies have on sediment dynamics.
A Numerical Study of Wave-Current Interaction in the Bottom Boundary Layer
Coastal Engineering Proceedings
In the present work, a numerical wave-current flume has been developed, based on a standard k-ε model. The numerical flume was 12.86m in length, with a numerical beach at one end of the flume. The Volume of Fluid (VOF) method was used to capture the free surface in the flume. The velocity profile obtained at the test section from the numerical simulation has then been compared with experimental data and good agreement found. Periodic velocities in the bottom boundary layer have been obtained which agree well with the experimental data. The model provides an insight to the changes in bed shear stress time histories that characterise wave current interaction.
Modelling horizontal velocities within the wave bottom boundary layer
Proceedings of the IASTED International Conference on Modelling and Simulation, 2012
As waves travel and shoal towards a beach, their surface elevation becomes peaky (sharp crests) and asymmetric relative to the vertical, differing from the sinusoidal profile of linear waves. Below the surface, the passage of the progressive waves induces fluid velocities, showing similar (time) asymmetries. These nonlinearities are inextricably linked to sediment transport, but the processes involved are not well understood. This work analyses the data collected during a recent experimental project under skewed oscillatory flows. It validates a simple method based on the defect law to reproduce the horizontal velocities within the wave bottom boundary layer. Results indicate a good agreement between the measured and modeled velocities using this methodology.
Coastal Engineering, 2008
A large number of studies have been done dealing with sinusoidal wave boundary layers in the past. However, ocean waves often have a strong asymmetric shape especially in shallow water, and net of sediment movement occurs. It is envisaged that bottom shear stress and sediment transport behaviors influenced by the effect of asymmetry are different from those in sinusoidal waves. Characteristics of the turbulent boundary layer under breaking waves (saw-tooth) are investigated and described through both laboratory and numerical experiments. A new calculation method for bottom shear stress based on velocity and acceleration terms, theoretical phase difference, ϕ and the acceleration coefficient, a c expressing the wave skew-ness effect for saw-tooth waves is proposed. The acceleration coefficient was determined empirically from both experimental and baseline k-ω model results. The new calculation has shown better agreement with the experimental data along a wave cycle for all saw-tooth wave cases compared by other existing methods. It was further applied into sediment transport rate calculation induced by skew waves. Sediment transport rate was formulated by using the existing sheet flow sediment transport rate data under skew waves by . Moreover, the characteristics of the net sediment transport were also examined and a good agreement between the proposed method and experimental data has been found.