P. Krishnankutty

Ship Form Effects on the Forces and Moment on a Stationary Ship Induced by a Passing Ship

Abstract The hydrodynamic interaction forces and moment acting on a moored ship due to the passage of another ship in its proximity is studied here by considering the influence of ship form against the idealized approach of the use of parabolic sectional area distribution. Comparisons with experimental results have shown that the interaction effects are predicted by theory in a better way by the inclusion of ships form in it.

Study of manoeuvrability of container ship by static and dynamic simulations using a RANSE-based solver

The numerical study of manoeuvrability of surface ships necessitates the determination of the hydrodynamic derivatives in the equations of motion. Standard manoeuvring tests are simulated to evaluate the ships manoeuvring qualities. This paper deals with the estimation of linear, nonlinear and roll-coupled hydrodynamic derivatives of a container ship by numerically simulating static and dynamic tests at different roll angles using a RANSE solver. The mathematical model suitable for the nonlinear roll-coupled steering model for high-speed container ships is considered here. In order to include the effect of roll on the ship, the roll-dependent derivatives are estimated by using static and dynamic tests numerically performed at discrete heel angles. Standard definitive manoeuvres such as turning circle and zig-zag tests are numerically simulated by solving the equations of motion and the results are verified with those obtained by using experimental values.

Single input fuzzy logic controller tuning for steering control of autonomous underwater vehicle: Genetic algorithm approach

Autonomous underwater vehicles (AUV) are robotic devices which perform tasks underwater without operator interference. The paper presents, a simple genetic algorithm (sGA) is employed to tune gains for single input fuzzy logic controller (SIFLC) used for steering control of AUV. SIFLC is a computationally optimized form of conventional fuzzy logic controller (CFLC). In contrast to PD-like CFLC which requires two inputs, error and change in error, SIFLC uses only one input, signed distance. The universe of discourse for the input is tuned using a simple genetic algorithm (GA). GA is an optimization algorithm which mimics biological evolution to find the optimum solution to the problem. Population of likely solution is initialized and fitness of the population is calculated. The fit member of population reproduce while the unfit die out. The reproduction is replicated with help of arithmetic crossover. The population is subjected to mutation, to bring diversity to the population. The elite members of the population are preserved from extinction. The optimum value of universe of discourse are used to simulate yaw control of steering subsystem of NPS AUV II.

Study of Maneuverability of Container Ship With Nonlinear and Roll-Coupled Effects by Numerical Simulations Using RANSE-Based Solver

The examination of maneuvering qualities of a ship is necessary to ensure its navigational safety and prediction of trajectory. The study of maneuverability of a ship is a three-step process, which involves selection of a suitable mathematical model, estimation of the hydrodynamic derivatives occurring in the equation of motion, and simulation of the standard maneuvering tests to determine its maneuvering qualities. This paper reports the maneuvering studies made on a container ship model (S175). The mathematical model proposed by Son and Nomoto (1981, “On Coupled Motion of Steering and Rolling of a High Speed Container Ship,” J. Soc. Nav. Arch. Jpn., 150, pp. 73–83) suitable for the nonlinear roll-coupled steering model for high-speed container ships is considered here. The hydrodynamic derivatives are determined by numerically simulating the planar motion mechanism (PMM) tests in pure yaw and combined sway–yaw mode using an Reynolds-Averaged Navier–Stokes Equations (RANSE)-based computational fluid dynamics (CFD) solver. The tests are repeated with the model inclined at different heel angles to obtain the roll-coupled derivatives. Standard definitive maneuvers like turning tests at rudder angle, 35 deg and 20 deg/20 deg zig-zag maneuvers are simulated using the numerically obtained derivatives and are compared with those obtained using experimental values.

Numerical Investigation on the Influence of Froude Number on the Hydrodynamic Derivatives of a Container Ship

Evaluation of maneuverability of a ship at the early design stages is necessary for ensuring safety of its voyage. IMO recommends the test speed or approach speed for the maneuvering predictions as 90–100% of the service speed of the vessel. The confined model tests for ship maneuvering assessment are usually conducted at low speeds and the hydrodynamic derivatives obtained from these tests are used in the equation of motion even when vessel operates at much higher speeds. But the hydrodynamic derivatives and consequently the trajectory predicted using these derivatives differ substantially from the actual maneuvering conditions. Hence the dependency of the derivatives on vessel speed needs to be understood properly to get the correct estimate of the vessel trajectory prediction. This paper investigates the effect of vessel speed (Fn) on hydrodynamic characteristics of a container ship. Straightline test and horizontal planar motion mechanism (HPMM) tests are conducted for a container ship model for different speeds in a CFD environment.Copyright

Sensitivity Study of Hydrodynamic Derivative Variations on the Maneuverability Prediction of a Container Ship

The study of maneuverability of a ship involves the determination of the hydrodynamic derivatives in the equations of motion. The standard maneuvers are simulated by integrating the equations of motion and the maneuvering parameters are checked for compliance with appropriate standards set by IMO. The numerically or experimentally predicted hydrodynamic derivatives may differ from actual values of the built and operated ship. Hence, it is worth to understand the sensitivity of these variations on the actual maneuvering performance of the ship. This paper deals with a study on the sensitivity of the hydrodynamic derivatives in the equations of motion of a container ship (S175). The sensitivity analysis of all the hydrodynamic derivatives is performed by deviating each derivative in the range of −50% to +50% from the experimentally derived values, in steps of 10%. The standard maneuvering tests like turning tests at rudder angle, δ = 35° and 20°/20° zig-zag maneuvers are performed for each case and their effects on the standard maneuvering parameters are estimated. The hydrodynamic derivatives that are important and which have to be estimated with high level of accuracy in maneuvering studies for a container ship are identified through this study.Copyright

Hydrodynamic study of flapping foil propulsion system fitted to surface and underwater vehicles

ABSTRACT The engineering translation of the aquatic animal propulsion systems and its appropriate application to marine vehicles help them to achieve movement with less power and hence resulting in the CO2 emission reduction. In this paper, the design of a robotic fish with pectoral and caudal fins, which operates at subsurface, is considered. The robofish body shape and fin geometrical parameters are also important with regard to the resistance and power aspects. Numerical studies are conducted with the robotic fish to determine its resistance in the bare hull and also for the case fitted with fins. Experimental studies carried out on a remotely operated surface ship with thunniform mode of fish propulsion, which operates at subsurface, are considered. Model tests are performed to analyse the propulsive performance of fishtail propulsion for the remotely operated surface ship in bollard pull condition and in self-propulsion mode.

Numerical study on the manoeuvring of a container ship in regular waves

ABSTRACT Ships operate in seaways, but ship manoeuvrability is usually studied in calm water conditions, thus ignoring the effects of wave on the ship hydrodynamic behaviour. So, the assessment of surface ship manoeuvrability in wave environment is more realistic and accurate than its estimation in calm water condition. In the present study, captive dynamic ship model tests are numerically simulated in a computational fluid dynamics environment in regular head sea waves and the hydrodynamic derivatives are derived from the estimated force/moment time series using the Fourier series expansion method. These derivatives and wave excitation forces are fitted in the manoeuvring equations of motion and are solved to simulate the ship standard manoeuvres in head sea waves. Parameters of these definitive manoeuvres in wave condition are compared with those in still-water condition.

Study of water wave diffraction around cylinders using a finite-element model of fully nonlinear potential flow theory

Numerical models employing the fully nonlinear potential flow theory have been studied for over three decades. Adopting the mixed Eulerian–Lagrangian formulation, solution over the fluid domain has been obtained using both the boundary integral/element and finite-element (FE) methods, the latter being the main focus of the present paper. The FE model is derived employing a variational formulation. The main highlight of the FE model developed pertains to the calculation of fluid particle velocities using a C0-type FE solution. The velocity calculation procedure is analogous to the traditional stress calculation approach used in FE analysis of solid mechanics problems and is applicable to unstructured isoparametric hexahedral meshes. The numerical model has been applied to evaluate nonlinear diffraction forces on single cylinder problems and Fourier decomposition has been applied to the pressure force time history obtained. The first-order diffraction forces and moments derived from the present nonlinear model compare very well with literature results. The second-order oscillating and mean forces as well as the second-order mean moments compare fairly well with literature results. However, significant deviations are seen in the second-order oscillating moments. It is conjectured that side wall reflections of higher-order wave components in the numerical wave tank might be responsible for this. This aspect merits further investigation.

Experimental and Numerical Study of Penguin Mode Flapping Foil Propulsion System for Ships

The use of biomimetic tandem flapping foils for ships and underwater vehicles is considered as a unique and interesting concept in the area of marine propulsion. The flapping wings can be used as a thrust producing, stabilizer and control devices which has both propulsion and maneuvering applications for marine vehicles. In the present study, the hydrodynamic performance of a pair of flexible flapping foils resembling penguin flippers is studied. A ship model of 3 m in length is fitted with a pair of counter flapping foils at its bottom mid-ship region. Model tests are carried out in a towing tank to estimate the propulsive performance of flapping foils in bollard and self propulsion modes. The same tests are performed in a numerical environment using a Computational Fluid Dynamics (CFD) software. The numerical and experimental results show reasonably good agreement in both bollard pull and self propulsion trials. The numerical studies are carried out on flexible flapping hydrofoil in unsteady conditions using moving unstructured grids. The efficiency and force coefficients of the flexible flapping foils are determined and presented as a function of Strouhal number.