Influence of the transom immersion to ship resistance components at low and medium speeds

The transom stern offered some advantages over the traditional rounded cruiser stern reducing the resistance of a ship. This can only be achieved if the transom stern is carefully designed with suitable transom immersion ratio. In this study, the influence of different transom area immersion ratios on the resistance components was investigated for a semi-displacement hull and a full displacement hull. The base hull was based on NPL hull form and KCS hull form for a semi-displacement and full-displacement hull respectively. The transom immersion ratios for the NPL hull were varied at a ratio of 0.5, 0.7, 0.8 and 1.0. The resistance of each of the NPL hull form was simulated at Froude number 0.3 up to 0.6. The transom immersion ratios for the KCS hull were varied at a ratio of 0.05, 0.1, 0.15 and 0.3. The resistance of each of the KCS hull form was simulated at Froude number 0.195, 0.23, 0.26 and 0.28. The transoms of both hulls were modified or varied systematically to study the influence of the transom shape or immersion on the total and wave resistance components. The investigation was carried out using a CFD software named SHIPFLOW 6.3 based on RANSE solver. These results on the NPL hull shows that the larger the transom immersion, the higher the resistance will be for a semi-displacement vessel. The increased resistance is contributed by additional frictional and wave resistance components. The results for the KCS hull seems to contradict with the results obtained from the NPL hull. The larger and deeper transom for the case of KCS hull form sometimes can be beneficial at higher Froude number.

Propeller Hydrodynamic Characteristics in Oblique Flow by Unsteady Ranse Solver

Propellers may encounter oblique flow during operation in off-design conditions. Study of this issue is important from the design and ship performance points of view. On the other hand, a propeller operating in oblique flow may sometimes result in a better propulsion efficiency. The main goal of the present study is to provide an insight on the propeller characteristics in the oblique flow condition. In this research, the performance of the DTMB 4419 propeller is studied by the numerical method based on solving Reynolds Averaged Navier–Stokes (RANS) equations in several inflow angles. The sliding mesh approach is used to model the rotary motion of the propeller. Initially, the numerical method is verified by grid and time step dependency analysis at various inflow angles. Additionally, computed results at zero inflow angle are compared with the available experimental data and good agreement is achieved. Finally, the forces and moments acting on the propeller are obtained for 0° to 30° inflow angles. It is concluded that the inflow angle up to 10° has no significant influence on the thrust and torque coefficients as well as the propeller efficiency. However, at high angles up to 30°, the thrust and torque coefficients increase as the inflow angle increases, which may result in a significant improvement of propeller efficiency.

Energy Conservation Method Combining Anti spray Rail and Wedge Flap for High speed Displacement Hulls

The hydrodynamic mechanism and parametric influences of the wedge flap and the anti-spray rail in combination is investigates. A methodology with specific guidelines for incorporating these appendages with significant drag reduction is provided. Small crafts designs frequently require interventional changes to realise the desired guaranteed speed with their installed engine power. The appendages namely, the wedge, flap and anti-spray rails are used as retrofit measures or adapted in new hull forms, in isolation or in combination, to improve the drag and bring down the power requirement. A judicious combination of different appendages can result in significantly reduced drag and therefore power saving. The methodology combines the results of numerical and experimental investigations. The systematic study identifies the parameters for control namely, wedge flap size in terms of the chord length, its orientation vide the angle of the wedge flap, and the anti-spray rail location with respect to the water surface. The choice of the size of the wedge flap is a constrained problem since excessive wedge flap can cause problems related to length and hydrodynamic loading. This study establishes a solution by combination of a minimum integrated wedge flap with properly located anti-spray rail to reduce the drag. The study shows favourable influences due to local pressure and numerical results using a RANSE solver show good comparison with experimental test results. The methodology is a new approach towards drag reduction in new designs as well as drag control by retrofit.

Dynamic Response of Monopile Wind Turbine in Large Waves

A strongly coupled hydroelastic code based on OpenFOAM has been applied to evaluate dynamic responses of a bottom mounted monopile in regular waves at 3 different wave periods and 3 wave steepness. The code uses a free surface RANSE solver to determine accurately the hydrodynamic loads and a modal superposition approach to describe the dynamic structural responses. Numerical simulations include both rigid and elastic monopiles and the results are evaluated against publicly available model test experiments. The discussion includes a quality control of the numerically simulated incident wave fields, mesh dependency and the quality of the numerically predicted and experimentally measured third harmonic of the inline forces.

Investigation of the Unsteady Flow Behaviour on a Wind Turbine Using a BEM and a RANSE Method

Analyses of the unsteady flow behaviour of a 5 MW horizontal-axis wind turbine (HAWT) rotor (Case I) and a rotor with tower (Case II) are carried out using a panel method and a RANSE method. The panel method calculations are obtained by applying the in-house boundary element method (BEM) panMARE code, which is based on the potential flow theory. The BEM is a three-dimensional first-order panel method which can be used for investigating various steady and unsteady flow problems. Viscous flow simulations are carried out by using the RANSE solver ANSYS CFX 14.5. The results of Case I allow for the calculation of the global integral values of the torque and the thrust and include detailed information on the local flow field, such as the pressure distribution on the blade sections and the streamlines. The calculated pressure distribution by the BEM is compared with the corresponding values obtained by the RANSE solver. The tower geometry is considered in the simulation in Case II, so the unsteady forces due to the interaction between the tower and the rotor blades can be calculated. The application of viscous and inviscid flow methods to predict the forces on the HAWT allows for the evaluation of the viscous effects on the calculated HAWT flows.

Comparative Performance of Optimum High Speed SWATH and Semi-SWATH in Calm Water and in Waves

The hydrodynamic performance of unconventional SWATH and Semi-SWATH for high speed applications are analyzed and compared in this paper. Bare hull resistance in calm water is estimated by an inviscid boundary element method with viscous corrections and verified by a fully turbulent, multiphase unsteady RANSE solver. Motions response in head waves, calculated by a frequency domain 3D panel method with forward speed effects are also evaluated and compared. Both considered hulls are the best designs coming from full parametric hull form optimization procedures, based on CFD solvers for the estimation of their hydrodynamic performance and driven by evolutionary minimization algorithms. The SWATH has twin parabolic struts and an unconventional underwater shape, the semi-SWATH has a slender triangular waterline, a bulbous shape in the entrance body which gradually morph into a U-section with a shallow transom in the run body. In general, as expected, the Semi-SWATH hull shows a lower drag at high speeds while the single strut SWATH is superior at lower speeds. As regards seakeeping, the SWATH shows unbeatable lower pitch and heave motions in shorter waves, where the Semi-SWATH evidences a double peaked RAO. More detailed analysis and conclusion are drawn in the paper.

An Investigation on the Discrepancies Between RANSE and BEM Approaches for the Prediction of Marine Propeller Unsteady Performances in Strongly Non-Homogeneous Wakes

In the present work an analysis of the reliability of different numerical approaches, namely BEM and RANSE solver, for the prediction of unsteady performances of marine propellers is presented. To this aim, the well-known Seiun-Maru Highly Skewed Propeller operating in different wakes is considered. The results of this analysis show that, in correspondence to particularly challenging conditions, i.e. when very pronounced ship wakes are present, the two approaches may provide considerably different results in terms of propeller mechanical characteristics and pressure distributions on the blades. This problem, which is implicitly eliminated when the thrust identity approach is applied (as almost always performed, as an example, for propeller analyses aimed to the evaluation of cavitation extension and / or induced pressures), may become critical in case the codes are used for numerical self-propulsion tests, where a high accuracy in the prediction of the unsteady propeller performances in correspondence to a prescribed value of the advance coefficient, instead of the thrust coefficient, is mandatory. The analyses carried out allow to underline which are the potentially more problematic cases, in terms of wakes characteristics, and to suggest some possible reasons of the encountered discrepancies, which will need further analyses to enhance the prediction capability of numerical codes.

Prediction of Roll Damping Using Viscous Flow Solvers

This article illustrates the use of a RANSE solver coupled to a motion solver to predict the free roll decay and the associated damping coefficients of floating bodies. The necessary building blocks to perform such a prediction are described briefly. A sensitivity study for the convergence criterion, the time step, the domain size and the grid resolution is performed for a simple 2-dimensional barge. The results are compared to results from experiments. Furthermore a simulation for a free roll decay of a Navy Combatant is performed, considering the results of the parameter study for the Barge. Overall results indicate that the natural roll frequency can be well predicted, while the prediction of the roll damping coefficients is afflicted with some uncertainties.

Physical and Theoretical Modeling of Surface-Piercing Hydrofoils for a High-Speed Unmanned Surface Vessel

The objective of this work is to investigate the performance of two pairs of negative dihedral surface-piercing (SP) hydrofoils designed especially for an unmanned surface vessel with a top speed of 120 knots in sea state two. Physical modeling of a 1/6-scaled model of the SP hydrofoil was conducted at the free-surface cavitation tunnel at the Technical University of Berlin (TUB). The SP hydrofoil feature a new type of super-cavitating profile with an annex tapered trailing edge to achieve good efficiencies in foil born conditions (60–120 knots, super-cavitating/ super-ventilated regimes), as well as at take-off speeds (25–40 knots, wetted and/or partial-cavitating regimes). Preliminary results showed interesting anomalies in the trends of the measured forces with respect to the cavitation number and angle of attack for a wide range of inflow speeds. Details of the experimental study are presented along with numerical predictions obtained using finite volume RANSE solver with a volume of fluid technique to allow for a mixture flow with air/vapor and water phases. Explanation of the anomalies in the hydrodynamic performance is given.