The Influence of Viscous Effects on the Motion of a Body Floating in Waves

1993 ◽  
Vol 115 (1) ◽  
pp. 40-45 ◽  
Author(s):  
M. J. Downie ◽  
J. M. R. Graham ◽  
X. Zheng
Keyword(s):  
Three Dimensional ◽  
Vortex Method ◽  
Inviscid Flow ◽  
The Body ◽  
Viscous Effects ◽  
Local Flow ◽  
Discrete Vortex ◽  
Outer Flow ◽  
Viscous Forces ◽  
Singularity Distribution

This paper describes a method for calculating the forces experienced by a body floating in waves, including those due to vortex shedding from its surface. The method uses a purely theoretical approach, incorporating viscous forces, for calculating the motions of the body in the frequency domain. It involves the matching of an outer inviscid flow with the local flow in the regions of flow separation on the body, which must be well defined. The outer flow is computed by a three-dimensional singularity distribution technique and the inner flow by the discrete vortex method. The technique has been applied to the prediction of the motion response of barges floating in waves. The results compare favorably with experimental data.

Numerical Simulation of Rivulet Dynamics Associated With Rain-Wind Induced Vibration

2010 ◽  
Author(s):  
Ian J. Taylor ◽  
Andrew C. Robertson
Keyword(s):  
Flow Field ◽  
Thin Liquid Film ◽  
Vortex Method ◽  
The Body ◽  
Wind Loading ◽  
External Loading ◽  
Discrete Vortex ◽  
Unsteady Aerodynamic ◽  
Karman Vortex ◽  
Wind Induced Vibration

On wet and windy days, the inclined cables of cable-stayed bridges can experience large amplitude, potentially damaging oscillations known as Rain-Wind Induced Vibration (RWIV). The phenomenon is believed to be the result of a complicated nonlinear interaction between rivulets of rain water that run down the cables and the wind loading on the cables due to the unsteady aerodynamic flow field. A numerical method has been developed at the University of Strathclyde, to simulate aspects of RWIV, the results of which can be used to assess the importance of the water rivulets on the instability. This combines a Discrete Vortex Method solver to determine the external flow field and unsteady aerodynamic loading and a pseudo-spectral solver based on lubrication theory to model the water on the surface of the body and which is used to determine the evolution and growth of the water rivulets under external loading. These two models are coupled to simulate the interaction between the aerodynamic field and the thin liquid film on a horizontal circular cylinder. The results illustrate the effects of various loading combinations, and importantly demonstrate rivulet formation in the range of angles previous research has indicated that these may suppress the Karman vortex and lead to a galloping instability. These rivulets are found to be of self limiting thickness in all cases.

scholarly journals Investigation of the Unsteady Flow Behaviour on a Wind Turbine Using a BEM and a RANSE Method

2016 ◽  
Vol 2016 ◽  
pp. 1-12
Author(s):  
Israa Alesbe ◽  
Moustafa Abdel-Maksoud ◽  
Sattar Aljabair
Keyword(s):  
Unsteady Flow ◽  
Wind Turbine ◽  
Pressure Distribution ◽  
Three Dimensional ◽  
Panel Method ◽  
Flow Behaviour ◽  
Viscous Effects ◽  
Horizontal Axis Wind Turbine ◽  
Local Flow ◽  
Ranse Solver

Analyses of the unsteady flow behaviour of a 5 MW horizontal-axis wind turbine (HAWT) rotor (Case I) and a rotor with tower (Case II) are carried out using a panel method and a RANSE method. The panel method calculations are obtained by applying the in-house boundary element method (BEM) panMARE code, which is based on the potential flow theory. The BEM is a three-dimensional first-order panel method which can be used for investigating various steady and unsteady flow problems. Viscous flow simulations are carried out by using the RANSE solver ANSYS CFX 14.5. The results of Case I allow for the calculation of the global integral values of the torque and the thrust and include detailed information on the local flow field, such as the pressure distribution on the blade sections and the streamlines. The calculated pressure distribution by the BEM is compared with the corresponding values obtained by the RANSE solver. The tower geometry is considered in the simulation in Case II, so the unsteady forces due to the interaction between the tower and the rotor blades can be calculated. The application of viscous and inviscid flow methods to predict the forces on the HAWT allows for the evaluation of the viscous effects on the calculated HAWT flows.

scholarly journals Bilge-Keel Influence on Free Decay of Roll Motion of a Realistic Hull1

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Yichen Jiang ◽  
Ronald W. Yeung
Keyword(s):  
Free Surface ◽  
Fluid Motion ◽  
Three Dimensional ◽  
Vortex Method ◽  
Roll Motion ◽  
Two Dimensional ◽  
Desktop Computer ◽  
Roll Damping ◽  
Discrete Vortex ◽  
Bilge Keel

The prediction of roll motion of a ship with bilge keels is particularly difficult because of the nonlinear characteristics of the viscous roll damping. Flow separation and vortex shedding caused by bilge keels significantly affect the roll damping and hence the magnitude of the roll response. To predict the ship motion, the Slender-Ship Free-Surface Random-Vortex Method (SSFSRVM) was employed. It is a fast discrete-vortex free-surface viscous-flow solver developed to run on a standard desktop computer. It features a quasi-three-dimensional formulation that allows the decomposition of the three-dimensional ship-hull problem into a series of two-dimensional computational planes, in which the two-dimensional free-surface Navier–Stokes solver Free-Surface Random-Vortex Method (FSRVM) can be applied. In this paper, the effectiveness of SSFSRVM modeling is examined by comparing the time histories of free roll-decay motion resulting from simulations and from experimental measurements. Furthermore, the detailed two-dimensional vorticity distribution near a bilge keel obtained from the numerical model will also be compared with the existing experimental Digital Particle Image Velocimetry (DPIV) images. Next, we will report, based on the time-domain simulation of the coupled hull and fluid motion, how the roll-decay coefficients and the flow field are altered by the span of the bilge keels. Plots of vorticity contour and vorticity isosurface along the three-dimensional hull will be presented to reveal the motion of fluid particles and vortex filaments near the keels.

Effect of vortex shedding on the coupled roll response of bodies in waves

1988 ◽  
Vol 189 ◽  
pp. 243-261 ◽  
Author(s):  
M. J. Downie ◽  
P. W. Bearman ◽  
J. M. R. Graham
Keyword(s):  
Vortex Shedding ◽  
Degrees Of Freedom ◽  
Rectangular Cross Section ◽  
Vortex Method ◽  
The Body ◽  
Wave Radiation ◽  
Viscous Damping ◽  
Discrete Vortex ◽  
Hydrodynamic Damping ◽  
Main Component

Hydrodynamic damping of floating bodies is due mainly to wave radiation and viscous damping. The latter is particularly important in controlling those responses of the body for which the wave damping is small. The roll response of ship hulls near resonance in beam seas is an example of this. The present paper applies a discrete vortex method as a local solution to model vortex shedding from the bilges of a barge hull of rectangular cross-section and hence provides an analytic method for predicting its coupled motions in three degrees of freedom, including the effects of the main component of viscous damping. The method provides a frequency-domain solution satisfying the full linearized boundary conditions on the free surface.

scholarly journals Numerical Simulation of a Three-Dimensional Fish-like Body Swimming with Finlets

2012 ◽  
Vol 11 (4) ◽  
pp. 1323-1333 ◽  
Author(s):  
Shizhao Wang ◽  
Xing Zhang ◽  
Guowei He
Keyword(s):  
Three Dimensional ◽  
The Body ◽  
Hydrodynamic Performance ◽  
Rectangular Plates ◽  
Caudal Peduncle ◽  
Local Flow ◽  
Vortical Structures ◽  
Numerical Result ◽  
Simulation Based ◽  
3D Fish

AbstractThe swimming of a 3D fish-like body with finlets is numerically investigated at Re = 1000 (the Reynolds number is based on the uniform upstream flow and the length of the fish-like body). The finlets are simply modeled as thin rigid rectangular plates that undulate with the body. The wake structures and the flow around the caudal peduncle are studied. The finlets redirect the local flow across the caudal peduncle but the vortical structures in the wake are almost not affected by the finlets. Improvement of hydrodynamic performance has not been found in the simulation based on this simple model. The present numerical result is in agreement with that of the work of Nauen and Lauder [J. Exp. Biol., 204 (2001), pp. 2251-2263] and partially supports the hypothesis of Webb [Bull. Fish. Res. Bd. Can., 190 (1975), pp. 1-159].

Shape Effects on Viscous Damping and Motion of Heaving Cylinders1

2014 ◽  
Vol 136 (4) ◽  
Author(s):  
Ronald W. Yeung ◽  
Yichen Jiang
Keyword(s):  
Fluid Motion ◽  
Vortex Method ◽  
The Body ◽  
Floating Body ◽  
Navier Stokes ◽  
Inviscid Fluid ◽  
Fluid Viscosity ◽  
Viscous Damping ◽  
Viscous Effects ◽  
Shape Effects

Fluid viscosity is known to influence hydrodynamic forces on a floating body in motion, particularly when the motion amplitude is large and the body is of bluff shape. While traditionally these hydrodynamic force or force coefficients have been predicted by inviscid-fluid theory, much recent advances had taken place in the inclusion of viscous effects. Sophisticated Reynolds-Averaged Navier–Stokes (RANS) software are increasingly popular. However, they are often too elaborate for a systematic study of various parameters, geometry or frequency, where many runs with extensive data grid generation are needed. The Free-Surface Random-Vortex Method (FSRVM) developed at UC Berkeley in the early 2000 offers a middle-ground alternative, by which the viscous-fluid motion can be modeled by allowing vorticity generation be either turned on or turned off. The heavily validated FSRVM methodology is applied in this paper to examine how the draft-to-beam ratio and the shaping details of two-dimensional cylinders can alter the added inertia and viscous damping properties. A collection of four shapes is studied, varying from rectangles with sharp bilge corners to a reversed-curvature wedge shape. For these shapes, basic hydrodynamic properties are examined, with the effects of viscosity considered. With the use of these hydrodynamic coefficients, the motion response of the cylinders in waves is also investigated. The sources of viscous damping are clarified.

Direct Numerical Simulations of Planar and Cylindrical Density Currents

2005 ◽  
Vol 73 (6) ◽  
pp. 923-930 ◽  
Author(s):  
Mariano I. Cantero ◽  
S. Balachandar ◽  
Marcelo H. García ◽  
James P. Ferry
Keyword(s):  
Stokes Equations ◽  
Three Dimensional ◽  
Boussinesq Approximation ◽  
The Body ◽  
Density Current ◽  
Density Currents ◽  
Mean Flow ◽  
Viscous Effects ◽  
Spreading Velocity ◽  
Slip Boundary

The collapse of a heavy fluid column in a lighter environment is studied by direct numerical simulation of the Navier-Stokes equations using the Boussinesq approximation for small density difference. Such phenomenon occurs in many engineering and environmental problems resulting in a density current spreading over a no-slip boundary. In this work, density currents corresponding to two Grashof (Gr) numbers are investigated (105 and 1.5×106) for two very different geometrical configurations, namely, planar and cylindrical, with the goal of identifying differences and similarities in the flow structure and dynamics. The numerical model is capable of reproducing most of the two- and three-dimensional flow structures previously observed in the laboratory and in the field. Soon after the release of the heavier fluid into the quiescent environment, a density current forms exhibiting a well-defined head with a hanging nose followed by a shallower body and tail. In the case of large Gr, the flow evolves in a three-dimensional fashion featuring a pattern of lobes and clefts in the intruding front and substantial three-dimensionality in the trailing body. For the case of the lower Gr, the flow is completely two dimensional. The dynamics of the current is visualized and explained in terms of the mean flow for different phases of spreading. The initial phase, known as slumping phase, is characterized by a nearly constant spreading velocity and strong vortex shedding from the front of the current. Our numerical results show that this spreading velocity is influenced by Gr as well as the geometrical configuration. The slumping phase is followed by a decelerating phase in which the vortices move into the body of the current, pair, stretch and decay as viscous effects become important. The simulated dynamics of the flow during this phase is in very good agreement with previously reported experiments.

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