Jie-Min Zhan

Density stratification influences on generation of different modes internal solitary waves

An ideal tide-topography interaction model is utilized for studying the influence of density stratification (pycnocline depth d, thickness δ, and the density difference Δρa across the pycnocline) on nonlinear disintegration of the first (mode-1) and second (mode-2) baroclinic mode internal tides into internal solitary waves (ISWs). The solution methods include weakly nonlinear analysis and fully nonlinear simulation. It is found that as d increases, even though the energy flux into mode-1 internal tides is always larger than that into mode-2 ones at generation, mode-2 ISWs emerge and mode-1 ISWs are suppressed. As δ increases, the total energy conversion and the fluxes into both mode-1 and mode-2 tides all increase first and then decrease. During propagation, a thick pycnocline is actually not favorable for the emergence of mode-2 ISWs, and the simulated well-developed mode-2 ISWs for a pycnocline of intermediate thickness are due to the tide generation process. As Δρa increases, the total conversion and the fluxes into both mode-1 and mode-2 tides all increase almost linearly. Even though the flux into mode-1 tides is always larger than that into mode-2 ones at generation, mode-1 tides cannot disintegrate but mode-2 ISWs develop very well. During propagation, Δρa has no influence on the generation of ISWs. The present work systematically investigates the influence of density stratification on formation of ISWs by considering both internal tide generation and propagation processes.

Numerical Simulation of In-Line Response of a Vertical Cylinder in Regular Waves

ABSTRACT Han, Y.; Zhan, J-M.; Su, W.; Li, Y.S., and Zhou, Q. 2015. Numerical simulation of in-line response of a vertical cylinder in regular waves. In-line response of a flexibly mounted, vertical cylinder in a series of regular waves was studied with numerical simulation. A dynamic mesh scheme and a laminar flow model were adopted. The experimental and simulated time histories of five wave gages and the response of the cylinder were compared. At low-incident wave frequencies, although the ratios of the diameters of the cylinders to the wavelength were small, the oscillating cylinder had little influence on the flow field. The numerical responses of the cylinder were in very good agreement with the experimental data. As the frequency of the incoming wave increased, the diffraction effect caused by the cylinder became significant, and the simulated responses of the cylinder were slightly weaker than those of experiment. When the incident wave frequency approached the natural frequency of the cylinder, the cylinder oscillated in resonance, and the flow field was strongly influenced. The wave height behind the cylinder was reduced. At high-incident wave frequencies, the Reynolds number increased, and the numerical response of the cylinder became stronger than those of experiment. In conclusion, the numerical results of the cylinder response agreed well with the results of the experimental model. To improve the predictive accuracy of the numerical model, further study, such as with the use of a turbulence model, should be carried out in the future.

NUMERICAL SIMULATION OF FLOW THROUGH CIRCULAR ARRAY OF CYLINDERS USING MULTI-BODY AND POROUS MODELS

Two two-dimensional (2D) models for flow through a circular patch of circular cylinders in a channel, the multi-body model and porous model, are established using the scale adaptive simulation (SAS) turbulent model. It is verified that the multi-body model could predict well the average velocity field, turbulent structures and vortex in all cases. The porous model could only predict well the flow pattern and the large-scale vortex motion and fails to predict the small-scale motion. For cases of high solid volume fraction or low cylinder Reynolds number, the differences in prediction between the two models are small even in the region inside the patch. For the case of low solid volume fraction, the performance of the porous model is acceptable only in the region away from the patch. If predictions are required only for large-scale motion away from the patch, the more computational efficient porous model is a good modeling tool.

High-order finite volume WENO schemes for Boussinesq modelling of nearshore wave processes

ABSTRACT This paper presents the formulation and validation of a shock‐capturing hybrid method using the WENO (weighted essential non‐oscillatory)‐HLL (Harten–Lax–Van Leer) scheme for the solution of the two-dimensional extended Boussinesq-type equations. The governing equations are reformulated in a conservative form, using high-order finite volume WENO schemes with HLL Riemann solver to handle the flux terms. The efficient third-order Runge–Kutta time-stepping scheme is used to advance the numerical solution through time. Wave breaking is predicted by applying a wave breaking criterion to automatically degenerate the Boussinesq equations into nonlinear shallow water equations. In order to test whether the present numerical model could accurately reproduce the processes of transformation, breaking, and run-up of nonlinear dispersive waves under different wave conditions in the nearshore region, five test cases have been simulated. The numerical results show that the present shock-capturing model could simulate the nearshore wave processes very well.

Parametric Investigation of Breaking Solitary Wave Over Fringing Reef Based on Shock-Capturing Boussinesq Model

In this paper, a hybrid finite-volume-finite-difference scheme for the solution of the one-dimensional form of the Nwogus extended Boussinesq-type equations was developed to investigate the breaking of a solitary wave propagating over a fringing reef system. The adoption of a high-order finite volume WENO schemes with HLL Riemann solver for advection terms as well as the third-order Runge—Kutta time-stepping scheme makes the present scheme shock-capturing. From the validation case, it is shown that the present shock-capturing model could simulate the shoreline movement and wave breaking as well as bore propagation very well. In order to comprehensively understand the hydrodynamic characteristics when a solitary wave travels over a fringing reef system, a parametric study has been carried out to analyze the interaction process under various conditions. Effects of different physical parameters on wave breaking axe analyzed and the reflection, transmission and dissipation (RTD) coefficients are calculated based on the integration of kinetic and potential energies in different zones.

Hydrodynamic analysis of human swimming based on VOF method

Abstract A 3-D numerical model, based on the Navier-Strokes equations and the RNG k-ε turbulence closure, for studying hydrodynamic drag on a swimmer with wave-making resistance taken into account is established. The volume of fluid method is employed to capture the undulation of the free surface. The simulation strategy is evaluated by comparison of the computed results with experimental data. The computed results are in good agreement with data from mannequin towing experiments. The effects of the swimmer’s head position and gliding depth on the drag force at different velocities are then investigated. It is found that keeping the head aligned with the body is the optimal posture in streamlined gliding. Also wave-making resistance is significant within 0.3 m depth from the free surface.

A Self‐adaption Wave Absorbing Method with Porous Media Model in Numerical Wave Tanks

A two dimensional numerical wave tank (NWT) based on the Navier‐Stokes equations for viscous, incompressible fluid and VOF method is established. The momentum source method is employed to generate the wave. A self‐adaption wave absorbing method based on the porous media model, which can vary as the local flow field, is proposed. The results shows that the self‐adaption wave absorbing method can dissipate the wave energy effectively.