Study of Water Impact and Entry of a Free Falling Wedge Using CFD Simulations

2016 ◽  
Author(s):  
Arun Kamath ◽  
Hans Bihs ◽  
Øivind A. Arnsten
Keyword(s):  
Free Surface ◽  
Numerical Model ◽  
Level Set ◽  
Free Fall ◽  
Water Entry ◽  
Cfd Model ◽  
Water Impact ◽  
Surface Deformations ◽  
Computational Performance ◽  
Free Falling

Many offshore constructions and operations involve water impact problems such as water slamming onto a structure or free fall of objects with subsequent water entry and emergence. Wave slamming on semi-submersibles, vertical members of jacket structures, crane operation of a diving bell and dropping of free fall lifeboats are some notable examples. The slamming and water entry problems lead to large instantaneous impact pressures on the structure, accompanied with complex free surface deformations. These need to be studied in detail in order to obtain a better understanding of the fluid physics involved and develop safe and economical design. In the special case of free-fall lifeboats, model testing can be expensive and time consuming. Here, numerical modelling can make useful contributions to the design process. The slamming of a free falling body into water involves several complex hydrodynamic features after its free-fall such as water entry, submergence into water and resurfacing. The water entry and submergence lead to formation of water jets and air cavities in the water resulting in large impact forces on the object. In order to evaluate the forces and hydrodynamics involved, the numerical model should be able to account for the complex free surface features, the instantaneous pressure changes around the lifeboat and accurately evaluate the loads on the lifeboat. As a step towards simulating free-fall lifeboats, water entry of a free-falling wedge into water is studied in this paper using a CFD model. The vertical velocity of the wedge during the process of free fall and water impact are calculated for different cases and the free surface deformations are captured in detail. Numerical results are compared with experimental data and a good agreement is seen. The open-source CFD model REEF3D is used in this study. The model solves the Reynolds-Averaged Navier-Stokes equations to evaluate the fluid flow. The convective terms are discretized using a 5th-order conservative finite difference WENO scheme. Time discretization is carried out using a 3rd-order Runge-Kutta scheme. Pressure discretization is carried out using Chorins projection method. The Poisson pressure equation is solved using a pre-conditioned BiCGStab algorithm. A sharp representation of the free surface is obtained using the level set method. The falling wedge is represented using the level set paradigm as well, avoiding the need for re-meshing during the simulation. Turbulence modeling is carried out using the k-ω model. Computational performance of the numerical model is improved by parallel processing using the MPI library.

Study of Water Impact and Entry of a Free Falling Wedge Using Computational Fluid Dynamics Simulations

2017 ◽  
Vol 139 (3) ◽  
Author(s):  
Arun Kamath ◽  
Hans Bihs ◽  
Øivind A. Arntsen
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Free Surface ◽  
Free Fall ◽  
Water Entry ◽  
Water Impact ◽  
Surface Deformations ◽  
Jacket Structures ◽  
Wave Slamming ◽  
Free Falling

Many offshore constructions and operations involve water impact problems such as water slamming onto a structure or free fall of objects with subsequent water entry and emergence. Wave slamming on semisubmersibles, vertical members of jacket structures, crane operation of a diving bell, and dropping of free fall lifeboats are some notable examples. The slamming and water entry problems lead to large instantaneous impact pressures on the structure, accompanied with complex free surface deformations. These need to be studied in detail in order to obtain a better understanding of the fluid physics involved and develop safe and economical design. Numerical modeling of a free falling body into water involves several complex hydrodynamic features after its free fall such as water entry, submergence into water and resurfacing. The water entry and submergence lead to formation of water jets and air cavities in the water resulting in large impact forces on the object. In order to evaluate the forces and hydrodynamics involved, the numerical model should be able to account for the complex free surface features and the instantaneous pressure changes. The water entry of a free falling wedge into water is studied in this paper using the open source computational fluid dynamics (CFD) model REEF3D. The vertical velocity of the wedge during the process of free fall and water impact are calculated for different cases and the free surface deformations are captured in detail. Numerical results are compared with experimental data and a good agreement is seen.

scholarly journals STUDY ON WATER ENTRY PROBLEM USING A LEVEL-SET IMMERSED BOUNDARY METHOD

2014 ◽  
Vol 34 ◽  
pp. 1460376
Author(s):  
WEI BAI ◽  
CHENGZHONG HUO
Keyword(s):  
Free Surface ◽  
Level Set ◽  
Immersed Boundary Method ◽  
Surface Profile ◽  
Water Entry ◽  
Navier Stokes ◽  
Immersed Boundary ◽  
Interface Evolution ◽  
Boundary Method ◽  
Moving Bodies

Water entry of a solid through the free surface is a persisting field of research in ship hydrodynamics applications. Indeed, the knowledge of pressure forces acting on structures is necessary to ensure the verification of safety criteria in the design and operation. However, in water entry problems, jets can be generated, thus an effective numerical model is needed to capture this complicated breaking water surface. In this paper, the level set method is adopted, which has been shown to be capable of capturing interface evolution when the topological change of shape is extremely large, or merging, breaking and pinching occur. Moreover, the incorporation of an immersed boundary method with this free surface capture scheme implemented in a Navier-Stokes solver allows the interaction between fluid flow with free surface and moving bodies of almost arbitrary shape to be modeled. The developed Level-Set Immersed Boundary Method is applied to simulate the water entry of a rectangular body with different velocities into the still water. The complicated surface profile, velocity field and pressure are obtained. The simulation is also carried out for the same body exiting the water, and some preliminary results are presented.

Slamming Loads on a Wedge Elastically Suspended on a Marine Structure

2015 ◽  
Author(s):  
Hui Sun ◽  
Jens B. Helmers
Keyword(s):  
Free Surface ◽  
Impact Force ◽  
Water Entry ◽  
The Body ◽  
Water Impact ◽  
Surface Conditions ◽  
Marine Structure ◽  
Nonlinear Free Surface ◽  
Stiffness And Damping ◽  
Water Exit

Slamming loads on a two-dimensional wedge elastically suspended on a marine structure are analyzed by using either a combined Wagner and von Karman method (W-vK) or a boundary element method (BEM). Fully nonlinear free surface conditions are satisfied in the BEM. Hydroelasticity effects are considered in both methods. A sinusoidal free surface motion relative to the marine structure is specified for the slamming event. Both the water entry phase and the water exit phase are simulated. The numerical results by the two different methods are compared. The W-vK method can generally predict the same trend of the variation of the body motions and the water forces, although the predicted maximum forces are larger than those by the BEM. The influence of the stiffness and damping of the elastic connection on the water impact force are discussed.

scholarly journals Fluid Structure Interaction of Buoyant Bodies with Free Surface Flows: Computational Modelling and Experimental Validation

Water ◽  
2019 ◽  
Vol 11 (5) ◽  
pp. 1048 ◽  
Author(s):  
Andrea Luigi Facci ◽  
Giacomo Falcucci ◽  
Antonio Agresta ◽  
Chiara Biscarini ◽  
Elio Jannelli ◽  
...  
Keyword(s):  
Free Surface ◽  
Numerical Model ◽  
Fluid Structure Interaction ◽  
Free Surface Flow ◽  
Surface Flow ◽  
Water Impact ◽  
Ocean Engineering ◽  
Fluid Structure ◽  
Structure Interaction ◽  
Wide Range

In this paper we present a computational model for the fluid structure interaction of a buoyant rigid body immersed in a free surface flow. The presence of a free surface and its interaction with buoyant bodies make the problem very challenging. In fact, with light (compared to the fluid) or very flexible structures, fluid forces generate large displacements or accelerations of the solid and this enhances the artificial added mass effect. Such a problem is relevant in particular in naval and ocean engineering and for wave energy harvesting, where a correct prediction of the hydrodynamic loading exerted by the fluid on buoyant structures is crucial. To this aim, we develop and validate a tightly coupled algorithm that is able to deal with large structural displacement and impulsive acceleration typical, for instance, of water entry problems. The free surface flow is modeled through the volume of fluid model, the finite volume method is utilized is to discretize the flow and solid motion is described by the Newton-Euler equations. Fluid structure interaction is modeled through a Dirichlet-Newmann partitioned approach and tight coupling is achieved by utilizing a fixed-point iterative procedure. As most experimental data available in literature are limited to the first instants after the water impact, for larger hydrodynamic forces, we specifically designed a set of dedicated experiments on the water impact of a buoyant cylinder, to validate the proposed methodology in a more general framework. Finally, to demonstrate that the proposed numerical model could be used for a wide range of engineering problems related to FSI in multiphase flows, we tested the proposed numerical model for the simulation of a floating body.

CFD Simulations of Non-Linear Sloshing in a Rotating Rectangular Tank Using the Level Set Method

2016 ◽  
Author(s):  
Erlend Liavåg Grotle ◽  
Hans Bihs ◽  
Eilif Pedersen ◽  
Vilmar Æsøy
Keyword(s):  
Free Surface ◽  
Numerical Model ◽  
Time Evolution ◽  
Level Set Method ◽  
Level Set ◽  
Turbulence Energy ◽  
Limiting Factor ◽  
Surface Time ◽  
Non Linear ◽  
Multiple Grids

In this paper, numerical simulations of non-linear sloshing in rectangular tanks are presented. Model implementations in the open source software REEF3D are tested and results compared with experimental data. Three different conditions are compared with experiments in 2D. First, the free surface time-evolution is compared for both linear and non-linear sloshing. In the last case, video images from the SPHERIC project are compared with simulations images of the free surface. A condition with lateral wave impacts in sloshing, with a frequency closer to the natural frequency of the first mode, can be found in this case. The non-linear sloshing, case 2, is also simulated in 3D. The numerical model is solving the RANS equations with the k-ω turbulence model. The level set method is used to capture the interface. Higher order discretization schemes are implemented to handle time-evolution and convective fluxes. A ghost cell method is used to account for solid boundaries and multiple grids for parallel computations. It is found that the limiting factor for the eddy-viscosity has significant influence in case 2 and 3. As the sloshing becomes more violent, the increased strain at the gas-liquid interface overproduces turbulence energy with unrealistically high damping of the motion. 3D simulations are only performed in case 2, which shows slightly better comparison than with 2D. Due to non-linearities and small damping, the time to reach steady-state may take several cycles, but no information is given in the paper [1]. The last case shows promising results for the global motion. As expected, the break up of the liquid surface makes it difficult to resolve each phase. But overall, the numerical model predicts the sloshing motion reasonably well.

scholarly journals Numerical simulation of water entry of different arbitrary bow sections

2014 ◽  
Vol 11 (2) ◽  
pp. 117-129 ◽  
Author(s):  
Parviz Ghadimi ◽  
Mohammad A. Feizi Chekab ◽  
Abbas Dashtimanesh
Keyword(s):  
Free Surface ◽  
Pressure Distribution ◽  
Surface Profile ◽  
Water Entry ◽  
Intersection Point ◽  
Critical Event ◽  
Water Impact ◽  
Ansys Cfx ◽  
Free Surface Profile ◽  
Volume Method

Water impact phenomenon of general bow section is a critical event for planning hulls. In this paper, the water entry of several arbitrary bow sections is investigated. For this purpose, arbitrary bow shapes which are introduced by Lewis form approximation are considered. In order to obtain pressure distribution and free surface profile, volume of fluid (VOF) method coupled with finite volume method (FVM) are utilized in Ansys-CFX solver. The main feature of present study is consideration of some new arbitrary bow sections which have not been previously studied. Another motivation of the current work is investigation of water entry of arbitrary bow sections using a coupled numerical solution of FVM/VOF. Pressure distribution, free surface, and evolution of intersection point on bow sections are presented, while secondary water impact is demonstrated. Comparison of selected current findings against the results of previous studies indicates favorable agreement.DOI: http://dx.doi.org/10.3329/jname.v11i2.18724

Three-Dimensional Numerical Modeling of Pier Scour Under Current and Waves Using Level-Set Method

2015 ◽  
Vol 137 (3) ◽  
Author(s):  
Mohammad Saud Afzal ◽  
Hans Bihs ◽  
Arun Kamath ◽  
Øivind A. Arntsen
Keyword(s):  
Free Surface ◽  
Level Set Method ◽  
Level Set ◽  
Three Dimensional ◽  
Three Dimensions ◽  
Navier Stokes ◽  
Reasonable Assumption ◽  
Cfd Model ◽  
Time Step ◽  
Pier Scour

A three-dimensional (3D) computational fluid dynamics (CFD) model is used to calculate the scour and the deposition pattern around a pier for two different boundary conditions: constant discharge and regular waves. The CFD model solves Reynolds-Averaged Navier–Stokes (RANS) equations in all three dimensions. The location of the free-surface is represented using the level-set method (LSM), which calculates the complex motion of the free-surface in a very realistic manner. For the implementation of waves, the CFD code is used as a numerical wave tank. For the geometric representation of the moveable sediment bed, the LSM is used. The numerical results for the local scour prediction are compared with physical experiments. The decoupling of the hydrodynamic and the morphodynamic time step is tested and found to be a reasonable assumption. For the two situations of local pier scour under current and wave conditions, the numerical model predicts the general evolution (geometry, location, and maximum scour depth) and time development of the scour hole accurately.

Analysis of Wave Interaction With Cylinders Using a 3D Numerical Wave Tank

2014 ◽  
Author(s):  
Arun Kamath ◽  
Hans Bihs ◽  
Øivind A. Arntsen
Keyword(s):  
Free Surface ◽  
Numerical Model ◽  
Flow Field ◽  
Stokes Equations ◽  
Total Force ◽  
Numerical Wave Tank ◽  
Offshore Structure ◽  
Wave Tank ◽  
Computational Performance ◽  
Numerical Wave

Evaluation of flow around a cylinder placed in waves is a challenging task due to the complex nature of the flow. A good understanding of the flow physics involved here is important as coastal and offshore structures consist of horizontal and vertical cylindrical elements. This paper explores the use of Computational Fluid Dynamics (CFD) to evaluate the flow field around cylindrical structures. A 3D numerical wave tank is employed to study the free surface and fluid velocities around a vertical cylinder placed in waves and the total force acting on the cylinder is evaluated. The numerical results are compared with experimental data. Further, a simple representation of an offshore structure modelled as multiple cylinders in proximity is also simulated in the numerical wave tank. The presence of neighbouring cylinders has an effect on the flow field. This affects the force acting on each of the cylinders in the group. The forces acting on every cylinder in the group are evaluated and the free surface elevation in the flow field is also visualised. The numerical result is compared with the result from an analytical formula. The numerical model uses the Reynolds-Averaged Navier-Stokes equations to evaluate the flow field. The convective terms are discretized using a 5th-order conservative finite difference WENO scheme. Time discretization is carried out using a 3rd-order Runge-Kutta scheme. Pressure discretization is carried out using Chorin’s projection method. The Poisson pressure equation is solved using a pre-conditioned BiCGStab algorithm. A sharp representation of the free surface is obtained using the level set method. Turbulence modeling is carried out using the k-ω model. Computational performance of the numerical model is improved by parallel processing using the MPI library.

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