1,720,985 research outputs found
Suction force and multiple frequencies subjected to oscillating cylinder on water surface
No full textThe flow field characteristics of modulation instability were widely
investigated by many researchers. In marine and offshore engineering
involved hydrodynamics, engineers generally care about the forces and
then the motions of the structures. In this paper, numerical simulation is
presented to investigate the modulation instability due to the forced
oscillating cylinder on the water surface and then the force
characteristics is focused on and analyzed. The numerical results show
that the lift coefficients would be negative with minimum values, which
mean that the suction forces would be presented, when the nondimensional
parameter is ac >1. However, the lift coefficients would be
always positive with ac <1. A constant which is the multiplication of
various ac and reciprocal of the averaged lift coefficients is obtained in
all cases regardless of the value ac. The motion of the cylinder is
monochrome, but the lift force coefficient is not monochrome but with
multiple frequencies, which is analyzed using Fast Fourier Transform
(FFT)
A Technique For Lock-In Prediction On A Fluid Structure Interaction Of Naca 0012 Foil With High Re
Full text linkA numerical lock-in prediction technique of a NACA 0012 hydrofoil, immersed in a flow having a Re of 3.07x106 is proposed in this paper. The technique observes the foil’s response as part of a fluid-structure interaction analysis. The response is modelled by foil’s vibration which is represented by spring and damper components. The technique identifies and predicts the foil’s lock-in when it vibrates. The prediction is examined using the Phase Averaged Method which employs the Hilbert Transform Method. The aim of this paper is to propose a numerical way to identify a lock-in condition experienced by a NACA 0012 foil in a high Reynolds number flow. The foil’s mechanical properties are selected and its motions are restricted in two modes which are in the pitch and heave directions. The rotational and transverse lock-in modes are identified in the model. The existence of lock-in is verified using pressure distribution plot, the history of trailing edge displacement and fluid regime capture. The history of total force coefficients is also shown to justify the result. The result shows that the technique can predict reliably the lock-in condition on the foil’s interaction. Three main fluid induced vibration frequencies are generated in the interaction. None of them are close to natural frequency of the foil and lock-in is apparently not found in the typical operational condition
AxV: An autonomous vehicle concept capable of operating throughout the ocean space: air, surface and subsea
No full textThis paper presents a concept design for an Autonomous Vehicle (AxV) capable of operating throughout the ocean space; air, surface and subsea. With current autonomous platforms limited in their operation, for example aerial platforms only operate in air, ASVs only operate on the ocean surface and AUVs operate subsea, a platform which can operate, transiting and transitioning in and between air, surface and subsea, providing increased mobility is very attractive. In this paper, a novel AxV platform is described. The governing equations are presented, describing each operational mode and the transitions between modes (air to surface, surface to subsea, subsea to surface, surface to air). Results are presented based on the dimensions of existing vehicles, showing that the system is theoretically feasible.</p
CFD simulation of orifice flow for the flooding of damaged ships
No full textThe aim of this research is to investigate parameters that affect the behaviour of a damaged ship and also to assess the ability of RANS codes to accurately model the physical processes occurring in the flooding of a damaged ship. Initially, the flooding of a damaged ship has been divided into smaller components to assess the accuracy of the predictions for these flow-fields when validated against experiment or other simulation methods. Damage size affects the frequency and amplitude of floodwater oscillation compared to wave oscillation and for certain conditions this can lead to maximum floodwater being present as a wave trough passes amidships. When the ship length is equal to the wave length this can increase hull girder loads significantly. Full simulation of this condition requires accurate simulation of flooding rates into the ship. Typical state of the art flooding models use Torricelli’s formula (equation 1) to calculate flooding rates using a constant coefficient of discharge (de Kat, et al., 2002). Q=C_d A_0 ?2gh, (1)where, Cd is the discharge coefficient, with typical values between 0.6-0.8, A0 is the area of the hole, g is gravitational acceleration and h is the height difference between the internal and external water levels. Based on Bernoulli’s theorem, turbulence and viscosity effects are included in this equation using a Cd which is independent of damage size or shape. It is believed this assumption could potentially over-simplify the problem to an extent where the calculated flooding rates are in error<br/
Two-dimensional fluid–structure interaction analysis of a vertical axis tidal turbine blade using periodic inflow equivalence model
No full textFluid structure response of vertical axis tidal turbine blades using NACA 0012 and periodic inflow equivalence model are predicted in this work. The response is investigated numerically by developing a two-dimensional computational fluid dynamics model at high Reynolds number (3.07 × 106). The Periodic Inflow Equivalence Model is conducted by modeling the rotation of the turbine as a time-dependent incoming fluid velocity magnitude and angle of attack current entering the two-dimensional computational fluid dynamics domain. The blade response is modeled by a vibrational system with spring damper components which are attached at the blade fluid dynamic center point. The aim of this study is to predict a resonant condition or a lock-in frequency induced by wake generation at a vertical axis turbine blade during the turbine operation. The model is generated using a dynamic mesh construction in OpenFOAM 2.2, and the mesh is refined using snappyHexMesh utility. The mesh has seven added boundary layers around the blade surface and simulated using k-ω shear stress transport turbulence model. Drag, lift, and moment force coefficient are observed during 12 s, which is equal to 3.3 revolutions, and extracted using fast Fourier transform method to obtain its predominant frequency. The predominant frequency determines the dynamic condition of the blade and is used for predicting a resonance based on the turbine’s natural frequency. The result shows that the vertical axis tidal turbine which is manufactured from a composite material with pitch stiffness of 200 (N m)/rad, heave stiffness of 1000 N/s, and operates at tidal velocity of 0.656 m/s is found to experience a resonance or lock-in phenomena induced by wake generation in the pitch mode response.</p
An investigation on fatigue crack growth rates through a designed nonsymmetric crack growth test
No full textA nonsymmetric fatigue crack growth (FCG) test is specially designed and performed to study whether FCG test data from nonsymmetric cracking which is unavoidable in practice can be used to determine valid FCG rate information for fatigue life evaluation. Two types of alloys were used in this test and the FCG data were obtained via visual measurements. The effects of boundary conditions and central hole were examined using FEA software and the results showed that within engineering allowable error margin, the theoretical solution of stress intensity factors (SIFs) under the remote uniform tension is valid for nonsymmetric crack in this test. As a result, a method for analyzing the FCG rate of nonsymmetric through-thickness crack is proposed. It is found that the FCG rate parameters in the stable stage have no difference between two crack tips and the crack growth data can be used together to determine the Paris parameters. The proposed method, if supported by further research, can be adopted to validate FCG test data involving nonsymmetric cracking in standard tests.<br/
Investigations of boundary treatments in incompressible smoothed particle hydrodynamics for fluid-structural interactions
No full textTwo boundary treatment methods were developed for incompressible flow simulations and fluid-structural interaction problems using Smoothed Particle Hydrodynamics (SPH): 1) To apply repulsive force on the boundary particles while keeping the same particle spacing for inner fluid particles and wall boundary particles; 2) To use denser wall particles without any additional force. The dam-breaking problem and another testing example are used to demonstrate the performance of this method. Results obtained from the present approach show reasonable agreement with experimental data. The fluid pressure values obtained with SPH method is investigated. Based on the result of the study, it can be concluded that the present approach is reliable to simulate incompressible fluid and the pressure value obtained can be used to solve fluid-structural interaction problems
Forward speed prediction of a free-running wave-propelled boat
No full textWave-propelled boats utilize submerged flapping foils to convert wave energy directly into propulsion. For platforms that are solely propelled using submerged flapping foils, predicting the forward speed is challenging as it is time varying and dependent on the coupled responses of the wave-induced hull motions (surge, heave, and pitch) and the foil flapping motion (driven by the wave induced hull motions and incident wavy flow). To ascertain the free-running response of wave-propelled boats, this article presents a hybrid discrete time-domain numerical model and experimental results from a prototype wave-propelled autonomous surface vehicle (ASV) with forward and aft (tandem) flapping foils. Results from a series of free-running experiments in regular head waves, over a range of wave frequencies for three different foil locations, are presented and used to validate the numerical model. The model was found to show good agreement with the experimental results, capturing the coupled dynamics of the vessel and foils and oscillating forward speed, over a range of wave frequencies and foil locations. The model and results provide a valuable insight for the design of wave-propelled boats
Air-water two phase flow simulation using smoothed particle hydrodynamics
No full texta new method is proposed for air-water two phase flowsimulation using Smoothed Particle Hydrodynamics method: thetwo different fluid phases are treated separately within the same time step. Air is solved using weakly compressible SPH and water is solved using incompressible SPH. No special treatment is required for the interface. It is the first time these two algorithms are combined together. The dam-break case with air-water two phase fluids is used to demonstrate the performance of the proposed algorithm. Results obtained from single phase flow and air-water two phase flow simulations are compared. It is shown that the consideration of air does not change the water movementsignificantly
Hydrodynamic coefficients for a 3-dimensional uniform flexible barge using weakly compressible smoothed particle hydrodynamics (WCSPH)
No full textThe numerical modelling of the interactions between water waves and floating structures is significant for different areas of the marine sector, especially seakeeping and prediction of wave-induced loads. Seakeeping analysis involving severe flow fluctuations is still quite challenging even for the conventional RANS method. Particle method has been viewed as alternative for such analysis especially those involving deformable boundary, wave breaking and fluid fragmentation around hull shapes. In this paper, the Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH), a fully Lagrangian particle method, is applied to simulate the symmetric radiation problem for a stationary barge treated as a flexible body. This is carried out by imposing prescribed forced simple harmonic oscillations in heave, pitch and the 2- and 3-node distortion modes. The resultant, radiation force predictions, namely added mass and fluid damping coefficients, are compared with results from 3-D potential flow boundary element method and 3-D RANS CFD predictions, in order to verify the adopted modelling techniques for WCSPH. WCSPH were found to be in agreement with most results and could predict the fluid actions equally well in most cases
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