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Purpose As the penalized vortex-in-cell (pVIC) method is based on the vorticity-velocity form of the Navier–Stokes equation, the pressure variable is not incorporated in its solution procedure. This is one of the advantages of vorticity-based methods such as pVIC. However, dynamic pressure is an essential flow property in engineering problems. In pVIC, the pressure field can be explicitly evaluated by a pressure Poisson equation (PPE) from the velocity and vorticity solutions. How to specify far-field boundary conditions is then an important numerical issue. Therefore, this paper aims to robustly and accurately determine the boundary conditions for solving the PPE. Design/methodology/approach This paper introduces a novel non-iterative method for specifying Dirichlet far-field boundary conditions to solve the PPE in a bounded domain. The pressure field is computed using the velocity and vorticity fields obtained from pVIC, and the solid boundary conditions for pressure are also imposed by a penalization term within the framework of pVIC. The basic idea of our approach is that the pressure at any position can be evaluated from its gradient field in a closed contour because the contour integration for conservative vector fields is path-independent. The proposed approach is validated and assessed by a comparative study. Findings This non-iterative method is successfully implemented to the pressure calculation of the benchmark problems in both 2D and 3D. The method is much faster than all the other methods tested without compromising accuracy and enables one to obtain reasonable pressure field even for small computation domains that are used regardless of a source distribution (the right-hand side in the Poisson equation). Originality/value The strategy introduced in this paper provides an effective means of specifying Dirichlet boundary conditions at the exterior domain boundaries for the pressure Poisson problems. It is very efficient and robust compared with the conventional methods. The proposed idea can also be adopted in other fields dealing with infinite-domain Poisson problems.

For amphibious vehicles that are utilized in military services, high-speed ship-to-shore movement is an important requirement for enhancement of the operational capability in tactical environments. At high speeds, such a vehicle begins to sink at the stern, causing the bow to rise out of the water. Moreover, excessive trim has a deleterious effect on forward field of vision and powering performance. In this study, a hydrofoil was considered as a trim-control device to decrease the significant bow rise without compromising the performance. Experiments were conducted in a towing tank to investigate the running attitude and powering performance of a box-shaped amphibious vehicle at high speeds. Results for the hydrodynamic features show that a fixed hydrofoil under the stern of the vehicle successfully achieved an improved trim. It was also found that the interaction between the waterjet and hydrofoil can enhance the dynamic lift of the hydrofoil.

In a composite material, stiffness and strength in a desired direction can be controlled via the combination and stacking of materials. As such, the application of composite materials has spread to almost every industry. To utilize this advantage in the design of a marine propeller with complex geometry, accurate and practical analysis tools that consider both the hydroelastic behavior and ply-stacking structure of composite materials are required. Therefore, steady and unsteady BEM–FEM FSI algorithms of a composite propeller blade and a simple finite element model that considers the lamination modulus of fiber materials have been introduced in this study. In addition, a comparative study using CFD–FEM-based FSI analysis and numerical investigation of the hydroelastic behavior of a composite propeller in the ship wake field have been performed based on the present methodology.

This is the continuation of a numerical study on vortex shedding from a blunt trailing-edge of a hydrofoil. In our previous work (Lee et al., 2015), numerical schemes for efficient computations were successfully implemented; i.e. multiple domains, the approximation of domain boundary conditions using cubic spline functions, and particle- based domain decomposition for better load balancing. In this study, numerical results through a hybrid particle-mesh method which adopts the Vortex-In-Cell (VIC) method and the Brinkman penalization model are further rigorously validated through comparison to experimental data at the Reynolds number of 2 × 106. The effects of changes in numerical parameters are also explored herein. We find that the present numerical method enables us to reasonably simulate vortex shedding phenomenon, as well as turbulent wakes of a hydrofoil.

Tip vortex cavitation (TVC) is important in a number of practical engineering applications. The onset of TVC is a critical concern for navy surface ships and submarines that aim to increase their capability to evade detection. A flexible thread attachment at blade tips was recently suggested as a new method to delay the onset of TVC. Although the occurrence of TVC can be reduced using a flexible thread, no scientific investigation focusing on its mechanisms has been undertaken. Thus, herein, we experimentally investigated the use of the flexible thread to suppress TVC from an elliptical wing. These investigations were performed in a cavitation tunnel and involved an observation of TVC using high-speed cameras, motion tracking of the thread using image-processing techniques, and near-field flow measurements performed using stereoscopic particle image velocimetry. The experimental data suggested that the flexible thread affects the axial velocity field more than the circumferential velocity field around the TVC axis. Furthermore, we observed no clear dependence of the vortex core size, circulation, and flow unsteadiness on TVC suppression. However, the presence of the thread at the wing tip led to a notable reduction in the streamwise velocity field, thereby alleviating TVC.

Normally, tip vortex cavitation (TVC) is first observed at a certain location behind the tips of propeller blades. Therefore, TVC is important for naval ships and research vessels that require raising the cavitation inception speed to maximum possible values. The concepts for alleviating the tip vortex are summarized by Platzer and Souders (1979), who carried out a thorough literature survey. Active control of TVC involves the injection of a polymer or water from the blade tip. The main effect of such mass injection (both water and polymer solutions) into the vortex core is an increase in the core radius, consequently delaying TVC inception. However, the location of the injection port needs to be selected with great care to ensure that the mass injection is effective in delaying TVC inception. In the present study, we propose a semi-active control scheme that is achieved by attaching a thread at the propeller tip. The main idea of a semi-active control is that because of its flexibility, the attached thread can be sucked into the low-pressure region closer to the vortex core center. An experimental study using a scale model was carried out in the cavitation tunnel at the Seoul National University. It was found that a flexible thread can effectively suppress the occurrence of TVC under the design condition for a model propeller.

A hybrid particle-mesh method based on the vorticity-velocity formulation for solving the incompressible Navier-Stokes equations is a combination of the Vortex-In-Cell(VIC) method for convection and the penalization method for diffusion. The key feature of the numerical methods is to determine velocity and vorticity fields around a solid body on a temporary grid, and then the time evolution of the flow is computed by tracing the convection of each vortex element using the Lagrangian approach. Assuming that the vorticity and velocity field s are to be computed in time domain analysis, pressure fields are estimated through a complete set of solutions at present time step. It is possible to obtain vorticity and velocity fields prior to any pressure calculation since the pressure term is eliminated in the vorticity-velocity formulation. Therefore, pressure field is explicitly treated by solving a suitable Poisson equation. In this paper, we propose a simple way to numerically implement the vorticity-velocity-pressure formulation including a penalty term. For validation of the proposed numerical scheme, we illustrate the early development of viscous flows around an impulsive started circular cylinder for Reynolds number of 9500.

A hybrid particle-mesh method as the combination between the Vortex-In-Cell (VIC) method and penalization method has been achieved in recent years. The VIC method, which is based on the vorticity-velocity formulation, offers particle-mesh algorithms to numerically simulate flows past a solid body. The penalization method is used to enforce boundary conditions at a body surface with a decoupling between body boundaries and computational grids. The main advantage of the hybrid particle-mesh method is an efficient implementation for solid boundaries of arbitrary complexity on Cartesian grids. However, a numerical simulation of flows in large domains is still not too easy. In this study, a multi-domain approach is thus proposed to further reduce computation cost and easily implement it. We validate the implementation by numerical simulations of an incompressible viscous flow around an impulsively started circular cylinder.

Recently, rudder erosion due to cavitation has been frequently reported on a semi-spade rudder of a high-speed large ship. This problem raises economic and safety issues when operating ships. The semi-spade rudders have a gap between the horn/pintle and the movable wing part. Due to this gap, a discontinuous surface, cavitation phenomenon arises and results in unresolved problems such as rudder erosion. In this study, we made a rudder model for 2-D experiments using the NACA0020 and also manufactured gap flow blocking devices to insert to the gap of the model. In order to study the gap flow characteristics at various rudder deflection angles(, , ) and the effect of the gap flow blocking devices, we carried out the velocity measurements using PIV(Particle Image Velocimetry) techniques and cavitation observation using high speed camera in Seoul National University cavitation tunnel. To observe the gap cavitation on a semi-spade rudder, we slowly lowered the inside pressure of the cavitation tunnel until cavitation occurred near the gap and then captured it using high-speed camera with the frame rate of 4300 fps(frame per second). During this procedure, cavitation numbers and the generated location were recorded, and these experimental data were compared with CFD results calculated by commercial code, Fluent. When we use gap flow blocking device to block the gap, it showed a different flow character compared with previous observation without the device. With the device blocking the gap, the flow velocity increases on the suction side, while it decreases on the pressure side. Therefore, we can conclude that the gap flow blocking device results in a high lift-force effect. And we can also observe that the cavitation inception is delayed.

In this paper, we establish an analysis algorithm and a design procedure for a Vane-Wheel which is a freely rotating device behind a propeller, by using a panel method. Vane-Wheel`s function is to extract energy from the propeller slipstream in turbine part which is inner part of the Vane-Wheel, and convert this energy into an additional propulsive thrust in propeller part which is outer part of the Vane-Wheel. Two parts must satisfy torque balance and thrust has to act to the ship`s forward direction. A Vane-Wheel has large interaction effect with propeller since it is placed behind of the propeller. Therefore, in order to consider interaction effect correctly, incoming velocity to the Vane-Wheel in a circumferential mean wake was calculated considering induced velocity from propeller to the Vane-Wheel. Likewise, incoming velocity to the propeller was calculated considering induced velocity from the Vane-Wheel to the propeller. This process is repeated until a converged result is obtained.

The 3-D unsteady viscous flow around an impulsively started rotating marine propeller is simulated using VIC(Vortex-In-Cell) method which is adequate to analyze the strong vortical flow around complicatedly-shaped body. The computational procedure is governed by the vorticity transport equation in Lagrangian form. In order to solve the equation, a regular grid which is independent to the shape of a body is introduced and each term of the equation is evaluated numerically on the grid by applying immersed boundary concept. In this paper, the overall algorithm including the formulation of governing equations and boundary conditions is described and some computational results are presented with discussing their physical validity.

SUMMARY In this study, an immersed boundary vortex-in-cell (VIC) method for simulating the incompressible flow external to two-dimensional and three-dimensional bodies is presented. The vorticity transport equation, which is the governing equation of the VIC method, is represented in a Lagrangian form and solved by the vortex blob representation of the flow field. In the present scheme, the treatment of convection and diffusion is based on the classical fractional step algorithm. The rotational component of the velocity is obtained by solving Poisson's equation using an FFT method on a regular Cartesian grid, and the solenoidal component is determined from solving an integral equation using the panel method for the convection term, and the diffusion term is implemented by a particle strength exchange scheme. Both the no-slip and no-through flow conditions associated with the surface boundary condition are satisfied by diffusing vortex sheet and distributing singularities on the body, respectively. The present method is distinguished from other methods by the use of the panel method for the enforcement of the no-through flow condition. The panel method completes making use of the immersed boundary nature inherent in the VIC method and can be also adopted for the calculation of the pressure field. The overall process is parallelized using message passing interface to manage the extensive computational load in the three-dimensional flow simulations. Copyright © 2011 John Wiley & Sons, Ltd.

We optimize the interpolation of a piecewise G 1 biarc curve through a given sequence of points by means of an internal energy minimization model. A combination of bending and stretching energy, which we call internal energy, provides global control over the shape of the curve. Starting from an initial guess, we solve the minimization model using a quasi-Newton algorithm. The input data can include or exclude end tangents, or it may form a closed shape. Experiments indicate the effect of internal energy on the interpolation and demonstrate that the resulting curves are smooth and free of extraneous loops.

We generate cutter paths free of interference and gouging for manufacturing model propellers using a five-axis numerical control (NC) machine. Our approach is faster than using a general-purpose computer-aided design (CAD)/computer-aided manufacturing (CAM) system. A roughing cut is made using only three axes for efficiency, and the finishing cut is made using all five axes to avoid collisions. Elements of the cutter path that might produce gouging are eliminated and the pose of the cutter is adjusted to eliminate interference. A number of models, including surface-piercing propellers, have been manufactured.

We refine the geometry of a propeller by modifying the blade sections to align them with surface streamlines, obtained by the panel method. Redefinition of the blade sections aligned with the streamlines is provided together with surface modeling scheme by which model propellers were built. Numerical simulations and open-water tests on models suggest a possible increase of 1% in propeller efficiency.

The two dimensional PIV (particle image velocimetry) measurement technique is applied to water flow in a narrow cavitation tunnel. The nearly homogeneous and isotropic turbulent flows are generated by the honeycomb installed in the tunnel and visualized with a PIV technique. The velocities in the measurement plane at the tunnel centerline 184cm downward from the honeycomb were measured and calculated by an image correlation technique. The turbulent properties are evaluated and each term in the turbulent kinetic energy equation is calculated for the conditions with different internal pressures. Lowering the internal pressure gives an effect on the turbulent flow due to growing bubbles which are resolved in the water. The turbulent kinetic energy in the measurement plane is decayed much slower than those of other research results carried out with wind tunnels. With decreasing the tunnel internal pressures the turbulent intensities are increased about 1.5 times and the anisotropic tendency is also increased.

Contrary to the conventional single-point measuring devices such as LDV, pitot-tube, hot-wire, etc., it would be possible to measure instantaneously 3-D flow fields with a stereoscopic PIV system. In this paper, we present an analysis algorithm for a stereoscopic PIV system using the normalized cross-correlation (NCC) and a 3 -D calibration based reconstruction method. The evaluation method based on NCC is one of the most accurate correlation-based methods. We validated the developed algorithm through a benchmarking comparison with 3-D artificial SPIV images and calibration target images.

To better understand the vortex shedding mechanism and to assess the capability of our numerical methodology, we conducted numerical investigations of vortex shedding from truncated and oblique trailing edges of a modified NACA 0009 hydrofoil. The hybrid particle-mesh method and the vorticity-based subgrid scale model were employed to simulate these turbulent wake flows. The hybrid particle-mesh method combines the vortex-in-cell and the penalization methods. We have implemented numerical schemes to more efficiently use available computational resources. In this study, we numerically investigated vortex shedding from various beveled trailing edges at a Reynolds number of 106. We then compared the numerical results with the experimental data, which show good agreement. We also conducted numerical simulations of wakes behind the hydrofoil at rest in periodically varying flows. Results reveal that vortex shedding is affected by the periodicity of a free-stream flow, as well as the trailing-edge shape.

This paper describes a preliminary ship design method using deterministic approach and probabilistic approach in the process of hull form design. In the deterministic approach, an interdisciplinary ship design method integrates principal dimension decisions and hull form variations in the preliminary ship design stage. Integrated ship design, as presented in this paper, has the distinctive feature that these parameters are evaluated simultaneously. Conversely, in sequential design, which is based on the traditional preliminary ship design process, hull form designs and principal dimension decisions are determined separately and sequentially. The current study adopts the first method to enhance the design quality in the early design stage. Furthermore, a probabilistic approach is applied to ship design to resolve uncertainties in design information more efficiently than a deterministic approach would.

In this paper, we suggest a design concept of defining the propeller geometry by stacking up the blade sections aligned with propeller surface streamlines. Numerical and experimental propeller open water(P.O.W.) characteristics of a newly designed propeller are presented. The surface streamlines for a propeller are obtained by using the panel method. Redefinition of the blade sections aligned with the streamlines is provided together with 8-spline modeling, by which we manufacture model propellers. We carried out the P.O.W, tests in a towing tank in order to show the effect of the present method on P.O.W. characteristics.

Noise reduction and control is an important problem in the performance of underwater acoustic systems and in the habitability of the passenger ship for crew and passenger. Furthermore, sound generated by a propeller is critical in underwater detection and it is often related to the survivability of the vessel especially for military purpose. This paper presents a numerical study on the non-cavitating and blade sheet cavitation noises of the underwater propeller. A brief summary of numerical method with verification and results are presented. The noise is predicted using time-domain acoustic analogy. The flow field is analyzed with potential-based panel method, and then the time-dependent pressure and sheet cavity volume data are used as the input for Ffowcs Williams–Hawkings formulation to predict the far-field acoustics. Noise characteristics are presented according to noise sources and conditions. Through this study, the dominant noise source of the underwater propeller is analyzed, which will provide a basis for proper noise control strategies.

Non-cavitating noise of underwater propeller is numerically investigated. The main purpose is to analyze non-cavitating noise in various operating conditions with different configurations. The noise is predicted using time-domain acoustic analogy and boundary element method. The flow field is analyzed with potential based panel method, and then the time-dependent pressure data are used as the input for Ffowcs-Williams Hawkings formulation to predict the far-field acoustics. Boundary element method is also considered to investigate the effect of ducted propellers. Sound deflection and scattering effect on the duct is considered with the BEM. The governing equations are based on the assumption that all acoustic pressure is linear. A scattering approach is applied in which the acoustic pressure field is split into the known incident component and the unknown scattered component. Noise prediction results are presented for single propeller and ducted propeller in non-uniform flow conditions. The investigation reveals that the effect of a duct on the acoustic performance of propellers is small in the far field under non-cavitating situations since the noise directivities of single and ducted propeller are almost the same. Only the high order BPFs are influenced by the existence of the duct.

In the vortex-in-cell method combined with the penalization method, fluid particles are traced by continuously updating their positions and strengths from the grid solution. To evaluate particle velocity, the velocity field is computed by solving a Poisson equation for the vector potential, namely [Formula: see text], where [Formula: see text], and its computation can be greatly accelerated by the use of a fast Poisson solver. While this method offers an efficient way to simulate unsteady viscous flows, the computation of the boundary values when solving the Poisson equation can become a computation time bottleneck. Although adopting the fast multipole method can lead to saving further computation time, its disadvantage is that it requires complicated hierarchical data structures such as a quad-tree and oct-tree. In this paper, we introduce and assess an approximation method for specifying domain boundary values using splines. Using the proposed spline approximation method we achieve significant savings in both computation time and memory consumption.

This paper describes a vorticity-based integro-differential formulation for the numerical solution of the two-dimensional incompressible Navier–Stokes equations. A finite volume scheme is implemented to solve the vorticity transport equation with a vorticity boundary condition. The Biot–Savart integral is evaluated to compute the velocity field from a vorticity distribution over a fluid domain. The Green's scalar identity is employed to solve the total pressure in an integral approach. Global coupling between the vorticity and the pressure boundary conditions is considered when this integro-differential approach is employed. For the early stage development of the flow about an impulsively started circular cylinder, the computational results with our numerical method are compared with known analytical solutions in order to validate the present formulation.

A surface panel method treating a boundary-value problem of the Dirichlet type is presented to design a hydrofoil corresponding to a prescribed pressure distribution. An integral equation is derived from Green's theorem, giving a relation between the total potential of known strength and the unknown local flux. Upon discretization, a system of linear simultaneous equations is formed and solved for an assumed geometry. The pseudo local flux, present due to the incorrect positioning of the assumed geometry, plays a role of the geometry corrector, with which the new geometry is computed for the next iteration. Sample designs for a series of pressure distributions of interest are performed to demonstrate the fast convergence, effectiveness and robustness of the procedure. The method is shown equally applicable to designing two- and three-dimensional hydrofoil geometry.

Analytical and numerical solutions are presented for the steady flow of an inviscid fluid about symmetric lifting profiles at an angle of attack in a plane sheared onset flow for which conformal mapping plays a critical role. For uniform shear (i.e. the onset flow speed varies linearly with position) in two dimensions, the disturbance field is potential and hence a solution based on the conformal transformation technique may be constructed. The Moriya transformation, which employs a leading-term transformation coefficient that stretches and rotates the field at great distances from the foil (as distinct from other classical transformations which leave the far field unchanged) is used and, with a limited number of terms selected for the transformation, a simple elegant solution is obtained that may be easily evaluated at arbitrary points on the foil contour. An additional investigation is reported for the field solution — involving a locally similar but globally non-uniform sheared onset flow — about one of the foils for which a simple O-type grid is analytically generated from the mapping function. These data indicate that the uniform-shear solution overpredicts the lift and surface speed on the suction side of the foil relative to the more realistic onset flow: the numerical solution predicts surface speeds that generally lie between those for the uniform flow and the uniformly sheared flow solutions.

Development of rudder gap flow blocking device for lift augmentation and cavitation suppression is presented. In order to verify the performance of this device, cavitation visualization and surface pressure measurements were carried out in a cavitation tunnel. Numerical simulations were conducted using a computational fluid dynamics code for more rigorous verification. The new rudder system is equipped with cam devices, which effectively close the gap between the horn/pintle and movable wing parts. The experimental and computational results show that the proposed rudder system is superior to the conventional rudder systems in terms of the lift augmentation and cavitation suppression.

This paper discusses how three geometrically similar models are made to investigate the scale law governing the air lubrication on a small high speed boat. Air is supplied behind a step placed on the bottom of the model and relations between flow rates of air and the shapes and areas of the air cavity as well as the resistance reduction are also observed. The reduction of resistance with lubrication of an air cavity and the similarity relations involved are investigated with a series of towing tank tests on geometrically similar models. Three similar models of different size are tested in the towing tank and the results indicate that an introduction of air cavity by artificially supplying air beneath the bottom of a model ship with a backward-facing step is effective for reduction of model resistance. The areas of air cavity and the required flow rates of air, both of which are directly related to the effective wetted surface area and the overall energy saving are found do be dominated by the Froude number scaling. Both the traditional two-dimensional method and Telfer's three-dimensional method seem to be applicable to the extension of the model resistance in the tested range if corrections are made to account for the changes in the frictional resistance caused by the changes in the effective wetted surface area. Based on these finings, a small test boat of practical size is constructed and the effect of air cavity on the reduction of ship resistance is studied. The results confirm the results found in the model tests and assure practical applicability of the air cavity for the resistance reduction of a real ship.

Publisher Summary This chapter proposes a new method for construction of a fiber-reinforced plastic (FRP) ship. In the method, hull shape is defined only by the line elements, and conventional molds for FRP construction method are not used. A 1/3 scale and a full scale mock-ups of a bow portion is manufactured on a proposed pin jig production system to identify and solve the difficulties for practical use. The plates are linked together to complete the hull shape and additional laminas are stacked inside until necessary thickness is obtained. To verify the proposed method and identify the possible flaws, a 1/3 scale model is constructed on the adjustable mold. The hull surface is divided into several pieces and developed onto a plane surface. Hull form of a typical planing boat can be easily modified into a set of developable surfaces with minor modification. It is found that the new production procedure can be successfully applied to the construction of planning vessels made of FRPs. The shape of a developable hull can be represented by line elements placed along the ruling lines and supported by pin jigs attached on the precision bed. It is possible that the system can be used as an adjustable mold for deformation of plane plate to form a hull shape, if the number and rigidity of the elements are enough.

Procedures for the design, analysis, and model tests of a preswirl stator-propeller system for a 300,000 DWT VLCC are described Five stators were designed and tested using the existing propeller for a 300,000 DWT VLCC in order to verify the effectiveness of the stator-propeller system. Maximum 6.5% decrease in delivered power at the design speed (15.5 knots) would be achieved with the designed stator-propeller system.