1,720,965 research outputs found
The simulation of free surface flows with Computational Fluid Dynamics
No full textComputational fluid dynamics is a powerful and versatile tool for the analysis of flow problems encountered in themaritime environment. The University of Southampton Fluid-Structure Interactions research group use ANSYS CFX tomodel a wide variety of flow problems; to gain insight into flow physics, improve designs and increase the efficiencyand safety of marine vehicles. A series of three case studies from on-going research looks at: loads applied on liquefiednatural gas tanks due to sloshing, slamming pressures experienced by high speed craft as well as the influence ofpropellers on the resistance characteristics of autonomous underwater vehicles. The presence of the free surface,complex shapes and the unsteady nature of these applications make their simulation with computational fluid dynamicsparticularly challenging. The successful validation of the computational models has resulted in the development of aselection process for suitable multiphase models as well as cost-effective meshing strategies
A rapid sloshing model for liquefied natural gas carriers
No full textThe significant rise in demand for liquefied natural gas (LNG) and the economic aspects of its transportation have resulted in an increase in the number and size of LNG carriers. One of the principal design issues for LNG carriers is sloshing as the containment systems have no internal structures to damp out the liquid motion. Furthermore, because the mass of ship and cargo are comparable, the coupling effect between ship motions and LNG sloshing requires carefu investigation. Considerable increases in the capacity of LNG carriers have renewed interest in the assessment of sloshing loads, and analysis of floating liquefaction and re-gasification installations (floating LNG) requires the inclusion of the sloshing dynamics in a seakeeping model. Recent incidents of sloshing damage onboard LNG carriers1 have added further urgency to the improvement of sloshing analysis in LNG carriers and floating LNG design. The costs of repair of sloshing damage to the containment system and consequent loss of revenue can be significant.Design optimisation or the use of a numerical wave tank to gather statistical sloshing data requires sloshing simulations with long durations. The full assessment of loading times for offshore LNG (approximately 12–18 hours per condition) with computational fluid dynamics (CFD) is not feasible with currently available computational resources and methods. Membrane containment systems are considered to be at greater risk from sloshing damage than spherical tanks, and detailed sloshing studies are required to determine the sloshing characteristics of a new tank design or vessel operating profile.Model testing at the experimental scale is often used for the comparative assessment of sloshing, but the scaling of impact pressures between model and full scale is often problematic. Full-field numerical techniques such as CFD can capture strongly non-linear sloshing at full scale, but large computational requirements restrict their application and they are unsuitable for the analysis of longer time series due to excessive computational requirements and their susceptibility to growth of numerical errors. Analytical approaches such as multimodal analysis can be computed in faster than realtime, but they are limited to linear and some cases of weakly non-linear sloshing
A rapid method for the simulation of sloshing using a mathematical model based on the pendulum equation
No full textA mathematical model for the rapid assessment of sloshing in faster than real time is developed using a phenomenological modelling approach with a pendulum equation. Based on observations of the consistent trajectory of the centre ofmass of a sloshing fluid, the imbalance force due to the displacement of the sloshing fluid is linked to the restoring force in the pendulum equation. The damping characteristics are replicated using a first and third-order damping model and impact dynamics are included using a modified impact potential. The equations are solved using a variable-order Adams–Bashforth–Moulton scheme and adequate error tolerances of the numerical scheme are established by reversing the direction of time marching. Solutions are obtained within 0.1% of real time. The proposed methodology is considered suitable for the fast time assessment of sloshing on Liquefied Natural Gas carriers, reduction of test matrices during gas carrier design and the simulation of coupled vessel-sloshing dynamics
The effect of an internal pump tower on fluid sloshing in a rectangular container
No full textThe complexity of the sloshing analysis for liquefied natural gas carriers is usually reduced by neglecting the pump tower. This study examines the effect of the pump tower, located near the aft bulkhead of an LNG tank, on the sloshing flow evolution by comparing the results of surge-induced sloshing in a rectangular tank with pump tower to a clean 3D and 2D tank using an inhomogeneous multiphase CFD model implemented in the commercial CFD code ANSYS CFX-11. The results are validated against experimental data. A simplified pump tower consisting of a single vertical tube is developed using the total fluid force on a real pump tower, which reduces the required mesh size by an order of magnitude. The simplified pump tower is included in the sloshing simulation. It is found that the effect of the pump tower on the sloshing flow is small. Reductions of impact pressures by up to 50% are observed and the sloshing flow with the simplified pump tower develops a lag compared to the clean tank. The inclusion of a simplified pump tower in the CFD simulation gives a force history similar to the Morison equation with the flow field from the clean tank
ISOPE 2009 Sloshing Comparative Study: simulation of lateral sloshing with multiphase CFD
No full textAs part of the ISOPE 2009 sloshing comparative study, the eight
specified test problems are analyzed using the CFD code CFX-11. The computational models and meshes are based on the methods developed in previous studies. 20 oscillations are simulated for each case and the mean, maximum and the mean of the 1/10th highest pressures are computed for each specified pressure sensor. The flow features and their influence on the impact pressure magnitude are then considered and both hydrodynamic impacts and impacts with air entrapment are
observed. It is found that the peak pressures are up to 170 kN/m², with time durations of the order of one millisecond. The resolution of such small time scales and the occurrence of wave breaking and air entrapment, which influence the pressure significantly, require a robust multiphase model capable of simulating phase mixing
A verification and validation study of the application of computational fluid dynamics to the modelling of lateral sloshing
No full textAn understanding of liquid sloshing is of primary concern to the design and operation of Liquefied Natural Gas (LNG) carriers. Safe operation of LNG carriers requires the knowledge of global and local pressures imposed by the sloshing liquid. The most general method available to quantify such sloshing loads is the solution of the Navier Stokes system of equations using Computational Fluid Dynamics (CFD). Given the wide variety of modelling options available, as yet there is no consensus on the best modelling practice for such sloshing flows.This report seeks to address this issue, examining various models and identifying the most suitable combination. The work uses the commercial CFD code ANSYS superscript TM CFX-10.0 superscript TM but most of the findings are also relevant for similar other commercial codes. The physics of the sloshing problem are considered in order to identify the key modelling aspects. The correct application of CFD and how it can be used to model sloshing is considered. A suitable experimental dataset is described for use as a validation test case. The sloshing problem simulated is in a 1.2 m long and 0.6 m high tank with a 60 % filling level; excited at 95% of the first natural frequency with a maximum displacement of 1.25 % of the tank length.A space and time discretisation independence study is carried out to ascertain the applicability of the results. Subsequently, the effect of including either a k ? ? or Reynolds stress turbulence model as opposed to forcing laminar flow is examined. The choice of fluid (water and air) compressibility is investigated to determine its effects on model accuracy as well as the associated computational cost. Results are compared to experimental data and a computational reference case.It is found that a grid of 6000-7000 elements with an initial node wall offset of 1 mm is sufficient to achieve effective grid independence for sloshing in. The necessary time discretisation scheme was determined to be second order with a dynamic timestep adaptation scheme controlled by a root mean square Courant Number of 0.2. The flow regime should be considered as turbulent and the standard k ? ? turbulence model is suitable. Finally it is observed that a compressible-incompressible model combination for air and water respectively gives a near identical result to a fully compressible model with a 20% reduction in computational time
Evaluation of a rapid method for the simulation of sloshing in rectangular and octagonal containers at intermediate filling levels
No full textThe Rapid Sloshing Model methodology developed by Godderidge et al. [13], is used for the simulation of sloshing in longitudinal and transverse cross sections of membrane liquefied natural gas tanks near the critical depth. Sloshing is induced by periodic translatory and rotational tank motions at and near the first resonant period. Subsequently irregular translatory motions obtained with a realistic wave spectrum and simultaneous translatory and rotational motions are applied to the tank cross sections. The validated Computational Fluid Dynamics (CFDs) methodology from Godderidge et al. (2009) is applied and it is found that the results from the Rapid Sloshing Model are typically within 5–10% of the corresponding CFD solution for linear, weakly nonlinear and strongly nonlinear sloshing with sloshing impacts. Simulation times are typically 0.1% of real time on a desktop PC. A similar level of agreement between Rapid Sloshing Model and CFD solution is observed when an irregular motion profile from a realistic seaway is applied to the tank for a duration corresponding to 35 min on a liquefied natural gas carrier. Compared to an existing phenomenological modelling approach the RSM methodology reduces the error by up to an order of magnitude in sloshing scenarios of practical interest.<br/
A simplified pump tower approach for realistic CFD simulation of sloshing in LNG tanks
No full textThe complexity of the sloshing analysis for liquefied natural gas carriers can be reduced by neglecting the pump tower. The validity of this assumption is examined by studying the effect of the pump tower, located near the aft bulkhead of a typical LNG tank on the sloshing flow evolution. Results are compared for surge-induced sloshing in a rectangular tank with pump tower to that without such an obstruction. A commercial flow solver is used to solve the unsteady Reynolds Averaged Navier Stokes equations for an inhomogeneous multiphase flow. Initial validation of the sloshing flow uses the experimental data of Hinatsu. It was found that a simplified pump tower consisting of a single vertical tube was suitable to capture the effect of the pump tower without the necessity of discretising the fine geometric detail of the pump tower structure. A suitable size for the simplified tower diameter was found using the total fluid force on a real pump tower in a steady flow for a similar range of Reynolds Number. This reduces the required mesh size by an order of magnitude. Although it is found that the effect of the pump tower on the overall force levels is small reductions of local impact pressures of up to 50% are observed and the sloshing flow develops a phase lag compared to the unobstructed tank
Grid resolution for the simulation of sloshing using CFD
No full textSloshing occurs when a tank is partially filled with a liq-uid and subjected to an external excitation force [1]. Ships with large ballast tanks and liquid bulk cargo carriers, such as very large crude carriers (VLCCs), are at risk of expo
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