Water Impact Loads Research Articles

Shear strength of sandwich core materials subjected to loading rates relevant to water slamming

Sandwich composite materials are widely used within the marine industry, particularly as hull panels. Water impact loads, known as slamming, can be very significant for these structures, particularly for high-speed craft. The transient nature of slamming loads means that the loads are applied very quickly, which can cause stress and strain rates that are high enough to affect the resulting strength of the core material, particularly for polymeric foams. The aim of this paper is to characterise the shear strength of cores at slamming relevant loading rates. Two testing approaches are used: a custom servo-hydraulic beam testing system and a drop-weight impact testing machine. Core materials studied included aramid honeycomb, cross-linked and linear polyvinyl chloride, polyethylene terephthalate and styrene acrylonitrile foams, representing a range of different levels of ductility and maximum elongation. For the moderate and high elongation core materials, there were significant increases in shear strength for dynamic loads; however, the strength of most of the materials does not appear to be sensitive to the exact loading rate.

Numerical Prediction of Symmetric Water Impact Loads on Wedge Shaped Hull Form Using CFD

Over the past two decades high-speed vessels have extended their service areas from protected waters to the open ocean where frequent and large water impacts can result in structural damage. The accurate prediction of slamming loads, and their consequences on light-weight high-speed vessels, is an essential element of efficient structural design. The aim of this work is to understand and accurately predict the behavior and local slam loads of quasi-2D wedge shaped hull forms impacting water. The computed results, using finite-volume Computational Fluid Dynamics (CFD), are validated against drop test experimental data and compared to a previously published numerical simulation using Smoothed Particle Hydrodynamics (SPH). The CFD results show good agreement with the experimental measurements.

Research on Ash Removal for the Water Wall of Coal Gasifier under Impact Loadings

Vibration responses of the water wall of coal gasifier under impact loads were investigated. The influences of different diameters of water walls and impact loads on the effectiveness of ash removal were further considered. The results showed that the peak acceleration along the axial direction of the water wall is greater than 80% of the peak acceleration at the impact point when rapping devices are installed at the 1/4, 1/2 and 3/4 heights. Moreover, rapping devices should be staggered 30 degree in the hoop direction at different heights to avoid the dead zone of ash removal. Furthermore, vibration accelerations increase with increasing the diameter of water wall, while the impact time should be as short as possible for higher efficiency of ash removal.

규칙파 중 갑판침입수 충격하중 감소를 위한 선수형상 설계

In this research, the relationship between the bow shape and green water phenomenon on the bow deck of an FPSO was studied using an experimental method. A 140,000 DWT FPSO was used as the objective hull form in the present research. The incident waves were regular types. The heights were 1.0 and 1.5 times the freeboard, and the length was equal in size to LBP. The wave heights and pressures on the deck were measured in experiments. Model tests were performed to determine the effects of bow flare angles, bow shapes, and a forecastle deck. The free heave and pitch conditions were applied to the models in these experiments. From the results of the model tests, an optimized bow shape was designed, which was found to decrease the green water impact loading. The results of this research could be used as fundamental data in the design of a bow shape.

Investigation of structural responses of breakwaters for green water based on fluid-structure interaction analysis

In the present study, the structural response of breakwaters installed on container carriers against green water impact loads was numerically investigated on the basis of the fluid-structure interaction analysis. A series of numerical studies is carried out to induce breakwater collapse under such conditions, whereby a widely accepted fluid- structure interaction analysis technique is adopted to realistically consider the phenomenon of green water impact loads. In addition, the structural behaviour of these breakwaters under green water impact loads is investigated simultaneously throughout the transient analysis. A verification study of the numerical results is performed using data from actual collapse incidents of breakwaters on container carriers. On the basis of the results of a series of numerical analyses, the pressure distribution of green water was accurately predicted with respect to wave mass and velocity. It is expected that the proposed analytical methodology and predicted pressure distribution could be used as a practical guideline for the design of breakwaters on container carriers.

CFD 기반 유체충격 해석에서 공기 압축성 효과

This paper describes the air compressibility effect in the CFD simulation of water impact load prediction. In order to consider the air compressibility effect, two sets of governing equations are employed, namely the incompressible Navier-stokes equations and compressible Navier-Stokes equations that describe general compressible gas flow. In order to describe violent motion of free surface, volume-of-fluid method is utilized. The role of air compressibility is presented by the comparative study of water impact load obtained from two different air models, i.e. the compressible and incompressible air. For both cases, water is considered as incompressible media. Compressible air model shows oscillatory behavior of pressure on the solid surface that may attribute to the air-cushion effect. Incompressible air model showed no such oscillatory behavior in the pressure history. This study also showed that the CFD simulation can capture the formation of air pockets enclosed by water and solid surface, which may be the location where the air compressibility effect is dominant.

NUMERICAL SIMULATION OF WATER IMPACT INVOLVING THREE DIMENSIONAL RIGID BODIES OF ARBITRARY SHAPE

As is well known, the design of coastal or offshore structures whether a ship, wave energy device or other fixed or floating structure, needs to consider its operation in a very hostile environment, including heavy storms. For example, an extremely high or steep wave impact on the bow or stern of a moored FPSO may result in a large amount of water on deck. Known as green water, this may cause severe damage to the deck house or other deckside equipment. Thus, there is great need for simulation tools to predict impact loadings and to provide more insight into the physics of local impact phenomena. 
 
 Published research or prediction work on the water impact problem has mostly related to studies in 2D. For example, Greehow& Lin (1983), Greenhow (1987), Zhao & Faltinsen (1993), Mei et al.(1999) have studied the hydrodynamics of rigid bodies entering water both theoretically and experimentally. More recently, a laboratory investigation of the pressure distribution on a free-falling wedge entering water by Yettou et al.(2006 has been compared a numerical and experimental study carried out by Campbell and Weynberg (1980). Water impact and green water loading in 3D has been simulated by Kleefsman et al. (2005) using a VOF method, which for dam break and water entry problems. In this study, we have developed the AMAZON-3D code for studies of water impact problems involving various 3D rigid solid bodies. 
 
 The in-house Cartesian cut cell approach has been used to simulate 3D water impact involving both moving rigid solid bodies and the free surface. The Cartesian cut cell method in the AMAZON-3D code is unrestricted in terms of boundary complexity or range of boundary movement. Solid objects are carved out of a background mesh, leaving a set of irregularly shaped cells aligned with the surface boundary. The advantages of the cut cell approach have been outlined previously by Causon et al. (2000, 2001) and Hu et al.(2009) including its flexibility for dealing with arbitrarily complex geometries and moving bodies. There is no requirement to re-mesh globally or even locally for the case of a moving body. All that is required is to update the cut cell data at the body contour for as long as the body motion continues.
 
 The AMAZON-3D finite volume code solves the incompressible Navier-Stokes equations in both air and water regions simultaneously treating the free surface as a contact surface in the density field that is captured automatically in a manner analogous to shock capturing in compressible flow. A time-accurate artificial compressibility method and high Godunov-type scheme replaces the pressure correction solver used in other methods (see Qian et al. 2006).
 
 We believe that the success of a study of water impact depends ultimately on the problem under consideration and the computer resources available and for each method there is a class of problem for which one method may perform better another. Each method has its own advantages and disadvantages and it is not possible to assert conclusively that one method is uniformly superior. However, we believe we can demonstrate that our method can be used successfully to study real local impact phenomena including the egress of an arbitrary rigid body from air to water or vice versa, the splash zone and entrapment of one fluid into the other. The code has been validated by recourse to a number of test cases including a cone undergoing forced oscillations and water impact of a rigid wedge with constant entry velocity where data and/or analytical results are available for comparison purposes. A range of results including the free surface elevation and force calculations will be presented for the water impact of various 3D rigid bodies.

Multiphysics Study of Structural Impact to Fluidic Media

The paper presents a fluid structure interaction based numerical study of impact loading for a hemispherical structure upon water and a space capsule water landing. The study has a strong relevance in the determination of the crashworthiness of aerospace structures upon water impact loading. Finite element based numerical techniques have been used for the analysis of the underlying transient dynamic and fluid-structure interaction. Smoothed Particle Hydrodynamics (SPH) and Arbitrary Lagrange-Eulerian (ALE) methods have been used to simulate the behaviour of the fluid (water) under impact conditions. The accelerations and velocities of the impacting objects have been validated with by experimental measurements and analytical results. Numerical analyses showed a strong potential for the use of developed computational fluid structure interaction models for analyses of water impact loading related problems.

Improved impact performance of marine sandwich panels using through-thickness reinforcement: Experimental results

This paper presents results from a test developed to simulate the water impact (slamming) loading of sandwich boat structures. A weighted elastomer ball is dropped from increasing heights onto rigidly supported panels until damage is detected. Results from this test indicate that honeycomb core sandwich panels, the most widely used material for racing yacht hulls, start to damage due to core crushing at impact energies around 550 J. Sandwich panels of the same areal weight and with the same carbon/epoxy facings but using a novel foam core reinforced in the thickness direction with pultruded carbon fibre pins, do not show signs of damage until above 1200 J impact energy. This suggests that these will offer significantly improved resistance to wave impact. Quasi-static test results cannot be used to predict impact resistance here as the crush strength of the pinned foam is more sensitive to loading rate than that of the honeycomb core.

Numerical Research on FPSOs With Green Water Occurrence

Green water loads on sailing ships or floating structures occur when an incoming wave significantly exceeds freeboard and water runs onto the deck. In this paper, numerical programs developed based on the platform of the commercial software Fluent were used to numerically model green water occurrence on floating structures exposed to waves. The phenomena of the fixed floating production, storage, and offloading unit (FPSO) model and oscillating vessels in head waves have been simulated and analyzed. For the oscillating floating body case, a combination idea is presented in which the motions of the FPSO are calculated by the potential theory in advance and computional fluid dynamics (CFD) tools are used to investigate the details of green water. A technique of dynamic mesh is introduced in a numerical wave tank to simulate the green water occurrence on the oscillating vessels in waves. Numerical results agree well with the corresponding experimental results regarding the wave heights on deck and green water impact loads; the two-dimensional fixed FPSO model case conducted by Greco (2001), and the three-dimensional oscillating vessel cases by Buchner (2002), respectively. The research presented here indicates that the present numerical scheme and method can be used to actually simulate the phenomenon of green water on deck, and to predict and analyze the impact forces on floating structures due to green water. This can be of great significance in further guiding ship design and optimization, especially in the strength design of ship bows.

ALE 유한 요소법을 적용한 위그선의 착수하중 해석

고속 및 고효율의 해양 수송수단에 대한 필요성이 점차 증가되면서 위그선(WIG: Wing in Ground effect)이 차세대 수송수단으로 관심이 집중되고 있다. 이러한 고속 해양 수송선의 구조설계 시 수면에 대한 충격하중은 중요하게 고려되어야 할 하중요소 중의 하나이며 착수하중에 의한 동적거동은 손상을 초래하는 중요한 요소이다. 본 연구에서는 위그선 착수 시 수면 충격하중에 대한 수치해석을 통해 선체의 동적거동을 평가하였다. 착수환경에서의 수면충격하중을 ALE (Arbitrary Lagrangian-Eulerian) 유한요소법을 적용하여 해석을 수행하였으며 3차원 쉘요소를 적용한 전기체 모델을 개발하여 위그선의 착수환경에서 선체의 동적거동을 모사하였다. 착수환경은 정상 착수조건과 측풍에 의한 비정상 착수조건 두 가지를 고려하여 해석을 수행하였으며 이러한 착수환경이 위그선 구조의 정적 구조 안정성에 미치는 영향을 평가하였다 Demand for high speed sea transportation modes has been increased dramatically last few decades. The WIG(Wing-in-ground effect) is considered as next generation maritime transportation system. In the structural design of high speed marine vessels, an estimation of water impact loads is essential. The dynamic structural responses of the WIG excited by the water impact loads may bring an important contribution to their damage process. The work presented in this paper is focused on the numerical simulation of the water impact on the WIG craft when it lands. It is aimed to study the structural responses of the WIG craft subjected to the water impact loads. The Arbitrary Lagrangian-Eulerian (ALE) finite element method is used to simulate the water impact of the WIG craft during a landing phase. A full 3D shell element is used to model the WIG craft in carbon composites, and a developed FE model is used to investigate the effect of the water impact loads on the structural responses of the WIG craft. In the analysis, two different landing scenarios are considered and their effects on the structural responses are investigated.

A Volume-of-Fluid based simulation method for wave impact problems

In this paper, some aspects of water impact and green water loading are considered by numerically investigating a dambreak problem and water entry problems. The numerical method is based on the Navier–Stokes equations that describe the flow of an incompressible viscous fluid. The equations are discretised on a fixed Cartesian grid using the finite volume method. Even though very small cut cells can appear when moving an object through the fixed grid, the method is stable. The free surface is displaced using the Volume-of-Fluid method together with a local height function, resulting in a strictly mass conserving method. The choice of boundary conditions at the free surface appears to be crucial for the accuracy and robustness of the method. For validation, results of a dambreak simulation are shown that can be compared with measurements. A box has been placed in the flow, as a model for a container on the deck of an offshore floater on which forces are calculated. The water entry problem has been investigated by dropping wedges with different dead-rise angles, a cylinder and a cone into calm water with a prescribed velocity. The resulting free surface dynamics, with the sideways jets, has been compared with photographs of experiments. Also a comparison of slamming coefficients with theory and experimental results has been made. Finally, a drop test with a free falling wedge has been simulated.

Water impact loads on circular structural members

When members of an offshore structure pass through the free surface, an impact load results which can not be calculated by use of Morison's equation. The force is characterized as an impulsive load which is also not treatable through static analyses since even though the member may be very stiff, its elastic properties have a profound effect on the resulting stress. A new design method for computing stress in cylindrical members due to impact is developed based on the energy principle which is shown to agree well with experiments. Subsequent to initial impact the next peak in the stress is shown to be primarily a function of the Froude number. The method is quite general and is convenient for application in design.

Water Impact Test of a Wedge with Rectangular Plates and its Analysis

Water impact of a wedge is a very primitive but an essential phenomenon to wave impact loads on ships in waves.Investigations have been carried out extensively on water impact loads on ships and so many features are known today on water impact on wedges and ships.But, a few aspect is lack in them. It is a scaling law of water impact pressure. In reality, Froude scaling law is applied to utilize the model test results for full scale ships, implicitly or explicitly.On the other hand, many test results put a question to the appropriateness of Froude scaling law for the water impact pressure.Response of a panel to the water impact pressure is another problem. Some papers and reports proposed methods to estimate the impact pressure on full-scale ships and to calculate the response of structures to the impact pressure, in which impact pressure is applied to the structure as a stationary uniform impact load.But, actual impact pressure on a wedge moves on the surface of the wedge at very high velocity.The response of the structure to the different two types of loads must be different.In this context, Water impact test of a wedge was carried out to study the scaling law of water impact pressure and to study the response of a panel to the water impact pressure.The test result showed that non-dimensional peak pressure is affected by the distance between the pressure gage and the keel of the wedge. The farther and farther the distance is, the larger and larger the peak value is. But, there seems to be an upper bound to them, which is almost equal to Wagner's theoretical impact pressure for the wedge.This implies that the water impact pressure for a wedge might obey Wagner's theory, but a pressure gage might modify it. A simulation of a pressure gage by finite element analysis with Wagner's theoretical impact pressure indicates that the impact pressure is modified by the pressure gage to be an analogous one to the test results. The result of this analysis explains well the tendency of the test results. The effect of a pressure gage on the peak pressure depends on the ratio of the diameter of the pressure gage to the distance between the pressure gage and the keel of the wedge.A finite element analysis of the panel with Wagner's theoretical impact pressure as external loads, gave results which showed very good agreements with the test results.Furthermore, the analysis indicated that the response of the panel to the moving impact pressure, as is the actual one, is about one half or one-third of that to the stationary uniform impact pressure.

Economics of the solid rocket booster for space shuttle

Solid Rocket Boosters (SRB's) became viable contenders as the booster for the Space Transportation System (STS) early in the concept studies of Space Shuttle because of their low development cost compared with equivalent liquid propellant boosters. Program risks and costs have been held down by scaling and adapting existing technology to the 146 in. SRB selected for development. To retain this low cost edge for the operational phase, NASA has concentrated on maintaining or reducing the cost of expendables and has demonstrated the feasibility of reusing the expensive nonexpendable SRB hardware. Drop tests of Titan III motor cases and nozzles in 1973 proved that boosters could survive water impact at vertical velocities of approx. 100 ft/s. SRB components have been designed with reuse in mind. In most cases, hardware designed for ascent will withstand water impact loads with little modification. Cost effective refurbishment is a foremost design consideration. Continuing review of each component assures that design for reusability and/or cost of refurbishment does not become so costly that a low-cost expendable approach may be more cost effective. The cost of expendables has been minimized by selecting proven propellants, insulations, and nozzle ablatives whose costs are well known. The propellant, which is approx. 95% of the expendables, is the lowest cost composite formulation available. As lower cost ablative materials such as pitch carbon fibers become available in quantity and are reliably demonstrated, they will be introduced to reduce operations costs. Thus, by use of proven technology, low cost expendables and reuse of more expensive non-expendables, the development and operations costs of SRB's are held to a level that make the SRB an economical booster for the Space Shuttle.