Carl-Erik Janson

Grid Influence on the Propeller Open-Water Performance and Flow Field

Abstract A grid sensitivity study for a conventional propeller in open-water condition uses three grids and compares with experimental data. Thrust and torque are rather insensitive to the grid resolution, but local quantities such as pressure coefficient and velocity are grid sensitive. An uncertainty analysis method reveals that the data are not in the asymptotic range which might be partly due to the complexity of the propeller flow.

Numerical Computations and Comparison to Experiments for a propeller in Open-Water Condition

Abstract Three-dimensional steady RANS computations simulate the flow around a conventional propeller operating in open water conditions. Turbulence is modeled by the shear stress transport SST k-ω model. The influence of the turbulence model is investigated using the realizable k-ε model in combination with enhanced-wall treatment as an alternative. Computed forces and velocity distributions downstream the propeller are compared with the experimental data obtained from open water tests and PIV measurements for the design advance number J. The thrust and torque coefficients arc compared for several advance numbers. The SST k-ω model performs well in predicting the performance of the propeller and the main characteristics of the propeller flow.

A Comparison of Four Wave Cut Analysis Methods for Wave Resistance Prediction

Abstract Four different techniques that use wave cut analysis to predict the wave pattern resistance coeffcient of a ship hull are compared. Two of the techniques are based on longitudinal wave cuts and two are based on transverse wave cuts. The results are compared with values from pressure integration and with towing tank measurements of the residuary resistance. Three hulls were used for the comparison: the Dyne Tanker, the Series 60 and the SSPA Ro-Ro Ship model 2062 having four different fore bodies. The wave cuts were obtained from linear and non-linear solutions of the free surface waves. A technique to predict free-surface deformation at any distance downstream of a vessel quickly based on the information computed in one of the transverse wave cut methods is presented.

Auxiliary Kite Propulsion Contribution to Ship Thrust

Low speed transportation is more energy efficient than high speed transportation in general. Ships with lower design speeds provide possibilities for using auxiliary wind propulsion devices to further reduce energy consumption. The kite is one type of auxiliary wind propulsion device that can be used for this purpose. The aim of this study is to investigate the influence of auxiliary kite propulsion on the performance of a panamax tanker, specifically on engine power reduction, course-keeping ability in different headings and on pitch and roll motion of the ship. The study is carried out by computer based simulations. Scenario combinations are established for various ship speeds, headings and weather conditions. For the purpose of simulation, a simplified auxiliary kite propulsion model is built and verified against full scale data. The SSPA in-house time domain maneuvering and seakeeping software – SEAMAN II is used for the study. The kite model is created as a subroutine to the SEAMAN software. The kite flies on the spherical surface along a predefined track. It performs span-wise rotation to achieve an optimum angle of attack. A NACA 4415 wing profile is adopted as the cross section of the kite. Aerodynamic forces are calculated based on the effective relative wind velocity and the optimum angle of attack. The radial component of the aerodynamic forces provides a traction force in the connection line. The traction force contributes to forces and moments acting on the simulated tanker. The numerical kite model was verified against full scale data. The deviations of traction force and thrust force between simulation and full scale could be controlled within 7%. Throughout the simulations, forces, moments, and motions of the tanker are recorded, as well as rudder angle and propeller efficiency. These output variables are plotted and analyzed to describe the influence of the auxiliary kite propulsion to ship performance. The result shows that auxiliary kite propulsion plays a significant role in reducing engine power in beam and following sea conditions. According to the SEAMAN simulations, a reduction of at least 40% could be obtained in many cases. The course keeping ability of the simulated panamax tanker is not affected by using the kite. The present study is a first step towards a fully dynamic kite model, where a control mechanism will be introduced to find the most favorable flying trajectory.

Maritime Traffic Situations in Bornholmsgat

Maritime traffic situations is regulated in the Convention on the International Regulations for Preventing Collisions at Sea (COLREGs), but how well are these rules followed by officers on board vessels? When the world shipping fleet grow and the traffic becomes more intensive, the risk of collision increase. By analysing AIS data from vessels in the traffic separation scheme Bornholmsgat during 24 hours in December 2013, 421 traffic situations were found where the passing distance between the vessels were less than 1.5 nautical miles. The compliance with the Convention on the International Regulations for Preventing Collisions at Sea (COLREGs) seems to be good, but the average avoiding action is less than the recommended manoeuver.

A note on the distribution of pressure resistance on a ship hull

Abstract The distribution of hydrodynamic and hydrostatic pressure resistance on a ship hull is investigated for four variants of the Dyne tanker and for the Series 60 hull. A grid dependence study is carried out for a double-model of the tanker to investigate the discretization error from the fore and aft bodies. The hydrodynamic influence between the fore and aft bodies is investigated by a length variation of the midship part of the tanker. The distribution of hydrodynamic and hydrostatic pressure resistance is discussed for linear and non-linear computations. The study shows that it is important to investigate the discretization error from the fore and aft bodies separately, and that the fore and aft bodies may act as independent hydrodynamic systems at low Froude numbers.

Linear and non-linear potential-flow calculations of free-surface waves with lift and induced drag

Abstract A potential-flow panel method is used to compute the waves and the lift force from surface-piercing and submerged bodies. In particular the interaction between the waves and the lift produced close to the free surface is studied. Both linear and non-linear free-surface boundary conditions are considered. The potential-flow method is of Rankine-source type using raised source panels on the free surface and a four-point upwind operator to compute the velocity derivatives and to enforce the radiation condition. The lift force is introduced as a dipole distribution on the lifting surfaces and on the trailing wake, together with a flow tangency condition at the trailing edge of the lifting surface. Different approximations for the spanwise circulation distribution at the free surface were tested for a surface-piercing wing and it was concluded that a double-model approximation should be used for low speeds while a single-model, which allows for a vortex at the free surface, was preferred at higher speeds. The lift force and waves from three surface-piercing wings, a hydrofoil and a sailing yacht were computed and compared with measurements and good agreement was obtained.