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.

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.

scholarly journals Investigations on Large-Scale Geometric Roughness Elements in Open Channels with Different Heights

2021 ◽  
Vol 28 (1) ◽  
pp. 07-14
Author(s):  
Iman A. Alwan ◽  
Riyadh Z. Azzubaidi
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Free Surface ◽  
Large Scale ◽  
Open Channel ◽  
Control Case ◽  
Hydraulic Characteristics ◽  
Roughness Elements ◽  
Computational Fluid Dynamics Cfd ◽  
Effective Case

Large-scale geometric roughness elements is one of the solutions that is used to protect openchannels from erosion. It is use to change the hydraulic characteristics of the flow. It may be concrete blocksor large stone placed at the bed of the channel to impose more resistance in the bed. The height of theseroughness elements is an important parameter that can affect the hydraulic characteristics of the flow. Usinga series of tests of T-shape roughness elements at three different heights, 3, 4.5, and 6cm, arranged in thefully rough configuration in order to investigate the velocity distributions along the flume. ANSYSParametric Design Language, APDL, and Computational Fluid Dynamics, CFD, were used to simulate theflow in an open channel with roughness elements. This simulation helps to find the best height of roughnesselements that can be used to change the hydraulic characteristics of the flow. The results showed that thevelocity values are decreased near the bed by about 61%, 58%, and 64% in case of 3cm, 4.5cm, and 6cmroughness heights consequently compared with the velocity of the control case. The velocity values areincreased near the free surface by about 32% and 19% in case of roughness elements height 6cm comparedwith 3cm and 4.5cm roughness heights respectively. The case of 6cm roughness height is considered to bethe effective case for decreasing the velocity values near the bed of the flume.

Virtual Model Basin for Resistance, Maneuvering, and Seakeeping: A RANS CFD Based Approach

2007 ◽  
Author(s):  
Balasubramanyam Sasanapuri ◽  
Viraj Suresh Shirodkar ◽  
Wesley Wilson ◽  
Samir Kadam ◽  
Shin Hyung Rhee
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Free Surface ◽  
Water Resistance ◽  
Primary Objective ◽  
Navier Stokes ◽  
Theory Method ◽  
Virtual Model ◽  
Calm Water ◽  
Computational Fluid Dynamics Cfd

A Virtual Model Basin (VMB) is developed based on a Computational Fluid Dynamics (CFD) approach to solving the Reynolds Averaged Navier-Stokes (RANS) equations along with the Volume of Fluid (VOF) method for predicting the free surface. The primary objective of this work is to develop methodologies for the VMB and to demonstrate the capabilities for a generic multi-hull ship geometry. The VMB is used to simulate various model basin tests for steady resistance, maneuvering and seakeeping. For a generic catamaran hull configuration, the methodologies are used for solving these problems and the results are discussed in this paper. VMB results are compared with the results of a benchmarked potential flow theory method for calm water resistance.

Numerical Simulation of the Fixed Barge Barge Interaction Under Irregular Wave Using HOS-StarCCM+ Coupling Tool With Experiment Validation

2021 ◽  
Author(s):  
Yali Zhang ◽  
Haihua Xu ◽  
Harrif Santo ◽  
Kie Hian Chua ◽  
Yun Zhi Law ◽  
...  
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Free Surface ◽  
Potential Flow ◽  
Computational Domain ◽  
Cfd Model ◽  
Flow Models ◽  
Irregular Wave ◽  
Computational Fluid Dynamics Cfd ◽  
Wave Conditions

Abstract The interaction between two side-by-side floating vessels has been a subject of interest in recent years due floating liquefied natural gas (FLNG) developments. The safety and operability of these facilities are affected by the free-surface elevation in the narrow gap between the two vessels as well as the relative motions between the vessels. It is common practice in the industry to use potential flow models to estimate the free-surface responses in the gap under various wave conditions. However, it is well-known that any potential flow models require calibration of viscous damping, and model tests are carried out to provide a platform to calibrate the potential flow models. To improve beyond the potential flow models, Computational Fluid Dynamics (CFD) models will be required. However, the large computational efforts required render the conventional CFD approaches impractical for simulations of wave-structure interactions over a long duration. In this paper, a developed coupled solver between potential flow and Computational Fluid Dynamics (CFD) model is presented. The potential flow model is based on High-Order Spectral method (HOS), while the CFD model is based on fully nonlinear, viscous and two phase StarCCM+ solver. The coupling is achieved using a forcing zone to blend the outputs from the HOS into the StarCCM+ solver. Thus, the efficient nonlinear long time simulation of arbitrary input wave spectrum by HOS can be transferred to the CFD domain, which can reduce the computational domain and simulation time. In this paper, we make reference to the model experiments conducted by Chua et al. (2018), which consist of two identical side-by-side barges of 280 m (length) × 46 m (breadth) × 16.5 m (draught) tested in regular and irregular wave conditions. Our intention is to numerically reproduce the irregular wave conditions and the resulting barge-barge interactions. We first simulate the actual irregular wave conditions based on wave elevations measured by the wave probes using the HOS solver. The outputs are subsequently transferred to the CFD solver through a forcing zone in a 2D computational domain for comparison of the irregular wave conditions without the barges present. Subsequently, a 3D computational domain is set up in the CFD with fixed side-by-side barges modelled, and the interaction under irregular waves is simulated and compared with the experiments. We will demonstrate the applicability of the HOS-StarCCM+ coupling tool in terms of accuracy, efficiency as well as verification and validation of the results.

scholarly journals Computational Fluid Dynamics Study of Water Entry Impact Forces of an Airborne-Launched, Axisymmetric, Disk-Type Autonomous Underwater Hovering Vehicle

Symmetry ◽  
2019 ◽  
Vol 11 (9) ◽  
pp. 1100 ◽  
Author(s):  
Chen-Wei Chen ◽  
Yi-Fan Lu
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Autonomous Underwater Vehicle ◽  
Water Entry ◽  
The Body ◽  
Velocity Versus ◽  
Computational Domain ◽  
Two Phase ◽  
Impact Forces ◽  
Two Dimensional Flow

An autonomous underwater hovering vehicle (AUH) is a novel, dish-shaped, axisymmetric, multi-functional, ultra-mobile submersible in the autonomous underwater vehicle (AUV) family. Numerical studies of nonlinear, asymmetric water entry impact forces on symmetrical, airborne-launched AUVs from conventional single-arm cranes on a research vessel, or helicopters or planes, is significant for the fast and safe launching of low-speed AUVs into the target sea area in the overall design. Moreover, a single-arm crane is one of the important ways to launch AUVs with high expertise and security. However, AUVs are still subject to a huge load upon impact during water entry, causing damage to the body, malfunction of electronic components, and other serious accidents. This paper analyses the water entry impact forces of an airborne-launched AUH as a feasibility study for flight- or helicopter-launched AUHs in the future. The computational fluid dynamics (CFD) analysis software STAR-CCM+ solver was adopted to simulate AUH motions with different water entry speeds and immersion angles using overlapping grid technology and user-defined functions (UDFs). In the computational domain for a steady, incompressible, two-dimensional flow of water with identified boundary conditions, two components (two-phase flow) were modeled in the flow field: Liquid water and free surface air. The variations of stress and velocity versus time of the AUH and fluid structure deformation in the whole water entry process were obtained, which provides a reference for future structural designs of an AUH and appropriate working conditions for an airborne-launched AUH. This research will be conducive to smoothly carrying out the complex tasks of AUHs on the seabed.

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

2014 ◽  
Author(s):  
Mohammad Saud Afzal ◽  
Hans Bihs ◽  
Arun Kamath ◽  
Øivind A. Arntsen
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Free Surface ◽  
Level Set Method ◽  
Level Set ◽  
Three Dimensional ◽  
Critical Shear Stress ◽  
Offshore Structures ◽  
Time Step ◽  
Pier Scour

Stability of offshore structures can be threatened by local scouring which could ultimately lead to their failure. As a consequence, knowledge of the scouring mechanism and the accurate prediction of the characteristic scour geometry is very important for the design of such structures. A three-dimensional computational fluid dynamics 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 computational fluid dynamics (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, 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 level set method is used. The numerical results for the local scour prediction are compared with physical experiments. The performance of the turbulence models, the formulations of the critical shear stress for the sloping bed and the effect of the variation of the sediment time stepping are investigated. 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.

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