Shin Hyung Rhee

Propeller Cavitation Study Using an Unstructured Grid Based Navier-Stoker Solver

The cavitating flow around a marine propeller is studied using an unstructured grid based Reynolds-averaged Navier-Stokes computational fluid dynamics method. A cavitation model based on a single-fluid multi-phase flow method is implemented in the Navier-Stokes solver. The proposed computational approach for cavitation is validated against a benchmark database for a cavitating hydrofoil as well as measured data for a cavitating marine propeller

Numerical analysis of the three-dimensional cloud cavitating flow around a twisted hydrofoil

Unsteady sheet cavitation on a three-dimensional twisted hydrofoil was studied using a Reynolds-averaged Navier–Stokes simulation technique based on a cell-centered finite volume method. As a verification test of the computational method, the leading edge and mid-chord cavitating flows over a two-dimensional modified NACA66 foil section were simulated for various cavitation numbers and validated against existing experimental data. The cavitation model parameters and numerical schemes were determined by a comparison with the measured pressure distribution. Non-cavitating and cavitating flows around a three-dimensional twisted hydrofoil were simulated with the selected computational method. The computed pressure on the foil and the cavity shedding patterns were validated by comparing these results with existing experimental data. The cavity shedding dynamics due to a re-entrant jet and a side entrant jet were investigated in terms of the cavity shedding cycles. The computed lift force and Strouhal number were compared against existing experimental data.

RANS model for spilling breaking waves

A RANS model for spilling breaking waves is developed, which can be implemented with ship hydrodynamics RANS CFD codes. The model is based on the Cointe & Tulin theory of steady breakers. The breaker cross section is assumed triangular with maximum height determined by the theoretical/experimental linear relationship with following wave height. Pressure and velocity boundary conditions are imposed on the dividing streamline between the breaker and underlying flow based on the hydrostatic and mixing layer models. An iterative solution procedure provides a unique solution for specified breaking criteria and simulation conditions. The model is implemented using CFDSHIP-IOWA and validated using spilling breaking wave benchmark data for two-dimensional submerged hydrofoils. As with other current RANS codes, wave elevations are under-predicted. However, for the first time in literature, the breaking wave wake is predicted

Analysis of a jet-controlled high-lift hydrofoil with a flap

A jet-controlled high-lift hydrofoil with a flap is investigated using both experimental and computational methods. Experiments were carried out in a cavitation tunnel to measure forces and moment acting on the hydrofoil, and surface pressure distribution. The measured data show the feasibility of such a device for marine applications. Computational studies have also been carried out in parallel with the measurements. The computational results are analyzed in terms of global and local quantities using available experimental data. The present computational results compare well with the well-known experimental data for circulation control flows. The results for flow around a hydrofoil with a blown flap further validate the concept behind the proposed device. The results of the study demonstrate the applicability of the technology to the design of practical control surfaces.

Numerical Simulation of Unsteady Turbulent Flow Around Maneuvering Prolate Spheroid

A three-dimensional Reynolds averaged Navier-Stokes method for unsteady turbulent flow around a maneuvering vehicle was developed and applied to a model problem concerning an extreme case of submarine maneuvers. A body force term is added in the momentum equations to take into account the inertial motion in the body-fixed coordinate system. The Spalart and Allmaras turbulence model is employed for turbulence closure. An artificial compressibility is introduced into the continuity equation for velocity-pressure coupling. The governing equations are discretized by second-order accurate finite volume method in space and second-order accurate backward scheme in time. The computational results are analyzed with global and local quantities and validated by comparison with experimental data. Overall, the present method performs quite well in predicting the unsteady flow phenomena associated with a maneuvering prolate spheroid, and the results compare well with available experimental data

Unsteady flow around a two-dimensional section of a vertical axis turbine for tidal stream energy conversion

The two-dimensional unsteady flow around a vertical axis turbine for tidal stream energy conversion was investigated using a computational fluid dynamics tool solving the Reynolds-Averaged Navier-Stokes equations. The geometry of the turbine blade section was NACA653-018 airfoil. The computational analysis was done at several different angles of attack and the results were compared with the corresponding experimental data for validation and calibration. Simulations were then carried out for the two-dimensional cross section of a vertical axis turbine. The simulation results demonstrated the usefulness of the method for the typical unsteady flows around vertical axis turbines. The optimum turbine efficiency was achieved for carefully selected combinations of the number of blades and tip speed ratios.

Investigation for the Characteristics of Cavitation Modeling for Computational Fluid Dynamics

Cavitation is one of the most difficult physical phenomena to understand and predict. Many experimental and computational studies have been conducted for better understanding of the phenomenon. Recently, with the rapid development of computing hardware capacity and numerical methods, considerable advancement is observed in prediction of cavitation using computational fluid dynamics. To that end, many cavitation models have been developed and reported. In the present paper, some of the distinguished cavitation models are categorized and reviewed in terms of the computational frame work and formulation of transport equations. Then those characteristics are compared with each other.

CFD Validation for a Marine Propeller Using an Unstructured Mesh Based RANS Method

Results of computational fluid dynamics validation for flow around a marine propeller are presented. Computations were performed for various advance ratios following experimental conditions. The objectives of the study are to propose and verify a hybrid mesh generation strategy, and to validate computational results against experimental data with advanced computational fluid dynamics tools. Computational results for both global and local flow quantities are discussed and compared with experimental data. The predicted thrust and torque are in good agreement with the measured values. The pressure distribution and pathlines on and around the blade surface well reproduce the physics of highly skewed marine propeller flow with tip vortex. The circumferentially averaged velocity components compare well with the measured values, while the velocity and turbulence quantities in the highly concentrated tip vortex region are under-predicted. The overall results suggest that the present approach is practicable for actual propeller design procedures.Copyright

Experimental Study on the Six Degree-of-Freedom Motions of a Damaged Ship Floating in Regular Waves

One of the most critical issues for ship owners, shipbuilders, and insurance companies is the operational safety. In particular, keeping damaged ships stable in waves is of great interest, because more nonconventional hull forms are being introduced for military and passenger vessels while international rules and regulations are becoming stricter. However, ship stability for damaged ships is quite different from that for intact ships as the assessment is very complicated and difficult due to the highly nonlinear behavior. Computational fluid dynamics (CFD) methods that solve the Navier-Stokes equations are acknowledged as the only viable approach to simulate and analyze these complex physical phenomena. Although there have been a number of research activities reported on damaged ship stability recently, most of them are not designed to validate CFD studies. For a data set to be valuable for CFD validation and development, model tests should eliminate unclear factors as much as possible. The main objective of this study is to establish an experimental database for CFD validation by collecting data from towing tank tests of a ship hulls six degree-of-freedom (6DOF) motion responses in regular waves for both intact and damaged conditions. A mooring system was designed to prevent drift motions of the ship model. Parametric roll was not observed when the ship was damaged, although it was observed for the intact ship in the same wave conditions. The mooring force acting on the ship model due to spring tension was also calculated.

Experimental Study on Free Roll Decay Motions of a Damaged Ship for CFD Validation Database

Among many factors to be considered for higher safety level requirements, the hull stability in intact and damaged conditions in seaways is of utmost importance. Since the assessment of a damaged ship is complicated due to the highly non-linear behavior, it is widely acknowledged that computational fluid dynamics (CFD) methods are one of the most feasible approaches. Although many research activities are being reported on the damaged ship stability recently, most of them are not designed for validation of CFD studies. In this study, well-designed model tests were performed to build a CFD validation database, which is essential in developing better CFD methods for the damage stability assessment. The geometry of the damaged compartment and test conditions were determined based on preliminary CFD simulations. Free roll decay tests in calm water with both intact and damaged ships were performed and the roll motion characteristics were compared. The damaged ship showed a larger roll damping coefficient and more rapid decrease of roll amplitude than the intact ship. The primary reason of these efforts can be explained by the movement of the flooded water.