A Numerical Study on Water Waves Generated by A Submerged Moving Body in A Two-Layer Fluid System

This is a numerical study on the time development of surface waves generated by a submerged body moving steadily in a two-layer fluid system, in which a layer of water is underlain by a layer of viscous mud. The fully nonlinear Navier- Stokes equations are solved on FLUENT with the Volume-of-Fluid （VOF） mnltiphase scheme in order to simulate, the free surface waves as wen as the water-mud interface waves as functions of time. The numerical model is validated by mimicking a reported experiment in a one-layer system before it is applied to a two-layer system. It is found that the presence of bottom mud in a water layer can lead to large viscous damping of the surface waves. For the investigation of the problem systematically, the effects of the Froude number and the mud layer thickness, density and viscosity relative to those of water are evaluated and discussed in detail.

EXPERIMENTAL RESEARCH ON INTERNAL WAVES GENERATED BY A MOVING BODY IN A STRATIFIED FLUID

Two-dimensional internal waves generated by a moving flat body in a continuous- ly stratified fluid are investigated by using three kinds of flow-visualization methods:electrolytic precipitation,hydrogen bubbles,and dye streaks.Attentions are paid mainly to the generation and propagation process in the upstream region under low internal Froude number(1/10π<F_r<1/2π).The features of the upstream disturbances as well as their relationship with stratification number K (K=1/F_r) are illustrated.The corresponding theoretical analysis is briefly presented,and by comparison,the experimental and theoretical results agree well.

Numerical Simulation of TWo Different Flexible Bodies Immersed in Moving Flow

The coupled motion of two flexible bodies with different lengths immersed in moving fluid is studied numerically. The flapping frequency, flapping amplitude and average drag coefficient of each body are calculated and the influences of the arranging manner and separation distance are analyzed. In our simulation, when placed in the flow individually, the flexible body with a longer length will flap in period and the shorter one will maintain still straightly in the flow direction. The numerical results show that, two different flexible structures near placed in moving flow would strongly interact. When they are placed side by side, the existence of the stable shorter flexible body will restrain the flapping of the longer one while the existence of the longer flexible body may also induce the shorter one to flap synchronously. When placed in tandem with the shorter flexible body in upstream, the flapping of the longer one in downstream will be obviously enhanced. In the situation for the longer flexible body placed in upstream of the shorter one, the coupled flapping amplitude and average drag coefficients increase and decrease periodically with increasing the arranging space, and peak values appear as a result of the mediate of the tail wakes.

VELOCITY FIELD IN SHIP WAVES ON THE VISCOUS FLUID

From the Navier-Stokes equations, the integral expressions of the free-surface elevation and the velocity field in ship waves of a moving waterborne body are obtained. Next, Lighthill' s two-stage scheme is employed to change the above-mentioned integral expressions to algebraic expressions. Compared with the results obtained when the seawater is idealized to an inviscid fluid, the singularities are dispelled or weakened, and the accuracy of the digit information of ship waves is improved.

A Neo Explanation of the"Mass-Velocity Relation

A reinterpretation of the well-known formula of the 'mass-velocity relation' is exactlyderived from a new viewpoint with new concepts, such as the finiteness of the transmitting velocityof force (TVF), effective action, and the coupled effect of the TVF for two EM fields, etc. Then, atrue meaning hidden in the Lorentz factor is exploited : i.e., when a charged particle is moving at aspeed v under an EM field, the effective action exerted on it by the field varies inversely with thespeed ratio β= v / U, where U is the TVF, which probably is equal to the propagation velocity ofEM field. The actual reduction of the effective action gives a false impression of mass gain.Accordingly, it is a major mistake in orientation to ascribe the (genuine) electrodynamics of movingbodies to any observation, or to any motion of an observer, while disregarding the facts of mutualaction.

A Three Dimensional Gas-Kinetic Scheme with Moving Mesh for Low-Speed Viscous Flow Computations

The paper introduces the gas-kinetic scheme for three-dimensional(3D)flow simulation.First,under a unified coordinate transformation,the 3D gaskinetic BGK equation is transformed into a computational space with arbitrary mesh moving velocity.Second,based on the Chapman-Enskog expansion of the kinetic equation,a local solution of gas distribution function is constructed and used in a finite volume scheme.As a result,a Navier-Stokes flow solver is developed for the low speed flow computation with dynamical mesh movement.Several test cases are used to validate the 3D gas-kinetic method.The first example is a 3D cavity flow with up-moving boundary at Reynolds number 3200,where the periodic solutions are compared with the experimental measurements.Then,the flow evolution inside a rotating 3D cavity is simulated with the moving mesh method,where the solution differences between 2D and 3D simulation are explicitly presented.Finally,the scheme is applied to the falling plate study,where the unsteady plate tumbling motion inside water tank has been studied and compared with the experimental measurements.

A GPU-accelerated two-phase flow model for fluid-solid interaction using the sharp interface immersed boundary method

A two-phase flow model accelerated by graphical processing unit(GPU)is developed to solve fluid-solid interaction(FSI)using the sharp-interface immersed boundary method(IBM).This model solves the incompressible Navier-Stokes equations using the projection-based fractional step method in a fixed staggered Cartesian grid system.A volume of fluid(VOF)method with second-order accuracy is employed to trace the free surface.To represent the intricate surface geometry,the structure is discretized using the unstructured triangle mesh.Additionally,a ray tracing method is employed to classify fluid and solid points.A high-order stable scheme has been introduced to reconstruct the local velocity at interface points.Three FSI problems,including wave evolution around a breakwater,interaction between a periodic wave train and a moving float,and a 3-D moving object interacting with the free surface,were investigated to validate the accuracy and stability of the proposed model.The numerical results are in good agreement with the experimental data.Additionally,we evaluated the computational performance of the proposed GPU-based model.The GPU-based model achieved a 42.29 times speedup compared with the single-core CPU-based model in the three-dimension test.Additionally,the results regarding the time cost of each code section indicate that achieving more significant acceleration is associated with solving the turbulence,advection,and diffusion terms,while solving the pressure Poisson equation(PPE)saves the most time.Furthermore,the impact of grid number on computational efficiency indicates that as Fluid-solid interaction(FSI)immersed boundary method(IBM)graphical processing unit(GPU)two-phase flow moving rigid bodythe number of grids increases,the GPU-based model outperforms the multi-core CPU-based model.