Free-Surface Wave-Induced Separation

1996 ◽  
Vol 118 (3) ◽  
pp. 546-554 ◽  
Author(s):  
Z. J. Zhang ◽  
F. Stern
Keyword(s):  
Fluid Dynamics ◽  
Free Surface ◽  
Surface Wave ◽  
Navier Stokes ◽  
Surface Boundary ◽  
Flow Conditions ◽  
Continuity Equations ◽  
Surface Boundary Conditions ◽  
Free Surface Wave ◽  
Wave Induced

Free-surface wave-induced separation is studied for a surface-piercing NACA 0024 foil over a range of Froude numbers (0, .2, .37, .55) through computational fluid dynamics of the unsteady Reynolds-averaged Navier-Stokes and the continuity equations with the Baldwin-Lomax turbulence model, exact nonlinear kinematic and approximate dynamic free-surface boundary conditions, and a body/free-surface conforming grid. The flow conditions and uncertainty analysis are discussed. A topological rule for a surface-piercing body is derived and verified. Steady-flow results are presented and analyzed with regard to the wave and viscous flow and the nature of the separation.

scholarly journals Linear pulse structure and signalling in a film flow on an inclined plane

1999 ◽  
Vol 396 ◽  
pp. 37-71 ◽  
Author(s):  
LEONID BREVDO ◽  
PATRICE LAURE ◽  
FREDERIC DIAS ◽  
THOMAS J. BRIDGES
Keyword(s):  
Free Surface ◽  
Saddle Point ◽  
Convective Instability ◽  
Stokes Equations ◽  
Three Dimensional ◽  
Numerical Results ◽  
Navier Stokes ◽  
Surface Boundary ◽  
Navier Stokes Equations ◽  
Surface Boundary Conditions

The film flow down an inclined plane has several features that make it an interesting prototype for studying transition in a shear flow: the basic parallel state is an exact explicit solution of the Navier–Stokes equations; the experimentally observed transition of this flow shows many properties in common with boundary-layer transition; and it has a free surface, leading to more than one class of modes. In this paper, unstable wavepackets – associated with the full Navier–Stokes equations with viscous free-surface boundary conditions – are analysed by using the formalism of absolute and convective instabilities based on the exact Briggs collision criterion for multiple k-roots of D(k, ω) = 0; where k is a wavenumber, ω is a frequency and D(k, ω) is the dispersion relation function.The main results of this paper are threefold. First, we work with the full Navier–Stokes equations with viscous free-surface boundary conditions, rather than a model partial differential equation, and, guided by experiments, explore a large region of the parameter space to see if absolute instability – as predicted by some model equations – is possible. Secondly, our numerical results find only convective instability, in complete agreement with experiments. Thirdly, we find a curious saddle-point bifurcation which affects dramatically the interpretation of the convective instability. This is the first finding of this type of bifurcation in a fluids problem and it may have implications for the analysis of wavepackets in other flows, in particular for three-dimensional instabilities. The numerical results of the wavepacket analysis compare well with the available experimental data, confirming the importance of convective instability for this problem.The numerical results on the position of a dominant saddle point obtained by using the exact collision criterion are also compared to the results based on a steepest-descent method coupled with a continuation procedure for tracking convective instability that until now was considered as reliable. While for two-dimensional instabilities a numerical implementation of the collision criterion is readily available, the only existing numerical procedure for studying three-dimensional wavepackets is based on the tracking technique. For the present flow, the comparison shows a failure of the tracking treatment to recover a subinterval of the interval of unstable ray velocities V whose length constitutes 29% of the length of the entire unstable interval of V. The failure occurs due to a bifurcation of the saddle point, where V is a bifurcation parameter. We argue that this bifurcation of unstable ray velocities should be observable in experiments because of the abrupt increase by a factor of about 5.3 of the wavelength across the wavepacket associated with the appearance of the bifurcating branch. Further implications for experiments including the effect on spatial amplification rate are also discussed.

Validation Study for Free-Surface Wave Flows Around Surface-Piercing Cylindrical Structures

2005 ◽  
Author(s):  
Shin Hyung Rhee ◽  
Boris Makarov
Keyword(s):  
Free Surface ◽  
Surface Wave ◽  
Breaking Waves ◽  
Navier Stokes ◽  
Test Cases ◽  
Ship Waves ◽  
Cylindrical Structures ◽  
Free Surface Waves ◽  
Free Surface Wave ◽  
Wave Flows

The present study is concerned with the free-surface wave flows around surface-piercing cylindrical structures. The volume of fluid method implemented in a Navier-Stokes computational fluid dynamics code is employed for test cases that involve general ship waves, spilling breaking waves, bubbly free-surface in separated regions, and interaction between free-surface waves and underlying viscous flow. The computational results are validated against existing experimental data, showing good agreement. The validation results suggest that the present computational approach provides a tool that is flexible and accurate enough to capture the outstanding flow physics associated with the free-surface wave flows around surface-piercing cylindrical structures.

Interaction of a deformable free surface with statistically steady homogeneous turbulence

2010 ◽  
Vol 658 ◽  
pp. 33-62 ◽  
Author(s):  
XIN GUO ◽  
LIAN SHEN
Keyword(s):  
Free Surface ◽  
Surface Deformation ◽  
Stokes Equations ◽  
Pseudospectral Method ◽  
Vertical Direction ◽  
Finite Amplitude ◽  
Navier Stokes ◽  
Surface Boundary ◽  
Navier Stokes Equations ◽  
Surface Boundary Conditions

Direct numerical simulation is performed for the interaction between a deformable free surface and the homogeneous isotropic turbulent flow underneath. The Navier–Stokes equations subject to fully nonlinear free-surface boundary conditions are simulated by using a pseudospectral method in the horizontal directions and a finite-difference method in the vertical direction. Statistically, steady turbulence is generated by using a linear forcing method in the bulk flow below. Through investigation of cases of different Froude and Weber numbers, the present study focuses on the effect of surface deformation of finite amplitude. It is found that the motion of the free surface is characterized by propagating waves and turbulence-generated surface roughness. Statistics of the turbulence field near the free surface are analysed in detail in terms of fluctuations of velocity, fluctuations of velocity gradients and strain rates and the energy budget for horizontal and vertical turbulent motions. Our results illustrate the effects of surface blockage and vanishing shear stress on the anisotropy of the flow field. Using conditional averaging analysis, it is shown that splats and antisplats play an essential role in energy inter-component exchange and vertical transport.

Modeling of Flow Around FPSO Hull Sections Subject to Roll Motions: Effects of Nonlinear Boundary Conditions

2005 ◽  
Author(s):  
Vimal Vinayan ◽  
Spyros A. Kinnas ◽  
Yi-Hsiang Yu
Keyword(s):  
Free Surface ◽  
Boundary Conditions ◽  
Nonlinear Boundary ◽  
Nonlinear Boundary Conditions ◽  
Navier Stokes ◽  
Surface Boundary ◽  
Surface Boundary Conditions ◽  
Forced Roll ◽  
Time Marching ◽  
Volume Method

The paper presents the development of BEM (Boundary Element Method) and FVM (Finite Volume Method) based models for the analysis of the flow around 2-D FPSO hull-sections fitted with bilge keels and subject to forced roll motions. Through these models an attempt is made to gain an insight into the two important aspects of the flow: separation around bilge keels and the effect of the free-surface. The effect of the free-surface and the resulting wave-body interaction is studied using a 2-D BEM model coupled with a Mixed-Eulerian-Lagrangian (MEL) time marching scheme for the free-surface boundary conditions. The separation around the bilge keels and viscous aspects of the flow are studied using a FVM based 2-D Navier Stokes (2DNS) solver with linear free-surface boundary conditions. The primary aim of the BEM model is to investigate the effects of the linear and nonlinear boundary conditions on the predicted flow within the scope of the parameters of the FVM model.

Sign in / Sign up

Export Citation Format

Share Document