P Krishnankutty

Oceanographic research vessels (ORVs) are used to collect ocean data and analyse them to understand the physical, chemical and biological characteristics of seawater, seabed and other ocean-related factors affecting climate changes. These task require vessel to track a predefined trajectory at different speeds. It is crucial to understand the effect of different speed on manoeuvring characteristics of the ship so as to assign appropriate control parameter and techniques to make its position/track keeping ability to an acceptable level. This paper attempts to estimate hydrodynamic derivatives of ORV by performing planar motion mechanism experiments at different speeds to understand the effect of Froude number on the hydrodynamic forces and subsequently on the different coefficient appearing in equations of motion and then on its turning performance. Sensitivity analysis is carried out as part of current work to understand the effect of different hydrodynamic derivative value variation on different turning parameter of vessel.

A dynamic positioning (DP) system is a computer-controlled system which maintains the positioning and heading of ship by means of active thrust. A DP system consist of sensors, observer, controller and thrust allocation algorithm. The purpose of this paper is to investigate the performance of proportional derivative type fuzzy controller with Mamdani interface scheme for dynamic positioning of an oceanographic research vessel (ORV) by numerical simulation. Nonlinear passive observer is used to filter the noise from the position and orientation. A nonlinear mathematical model of the ORV is subjected to the wave disturbance ranging from calm to phenomenal sea. Robustness and efficiency of the fuzzy logic controller is analysed in comparison with the multivariable proportional integral derivative (PID) and the linear quadratic regulator (LQR) controller. A simplified constrained linear quadratic algorithm is used for thrust allocation. The frequency response of the closed loop system with different controllers is analysed using the bode plot. The stability of controller is established using the Lyapunov criteria.

Present study consists the effect of variation of the vehicle length on the hydrodynamic forces and moments of axisymmetric underwater vehicles due to angle of attack. Five different vehicle lengths are used for this study (L/D varying between 10 and 15) by experimental, numerical and empirical methods. Experimental investigation is done in a tow tank using a VPMM. Numerical analysis is carried out using commercial CFD code Fluent 6.2. The empirical estimation is based on method suggested by Allen & Perkins. Results show that the linear coefficients vary linearly with L/D, while the nonlinear coefficients vary nonlinearly with L/D for the range tested.

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.

Locomotion techniques employed by different biological animals are extremely diverse and fascinating from an engineering point of view. The explorations of planets such as Mars, Titan, Europa, Enceladus with the use of aerial, terrestrial, and underwater rovers are gaining significant interest from academia, industry, planetary scientists, robotic engineers, and international space agencies around the globe. This article presents an overview of the existing state-of-the-art investigations on recently developed flapping foil propulsion of UAVs (unmanned aerial vehicles) and AUVs (autonomous underwater vehicles) for the exploration of Earth's oceans and other terrestrial bodies such as Mars, Jupiter's moon Europa, and Saturn's moon Titan. The use of flapping foils further advances into Martian Atmospheres in the form of insect-inspired micro aerial vehicles working at low Reynolds numbers. The development of aerial vehicles mimicking insect flapping is essential in low Reynolds number environments to generate sufficient lift and thrust for carrying out future Mars missions. The Cassini mission to Titan, Voyage mission, and other flyby missions to Europa found that liquid atmosphere exists on the subsurface of Europa and on the surface of Titan in the form of liquid methane lakes. The ice-covered ocean under the Europa surface is analogous to the Antarctic ice. The developments of autonomous surface ships and underwater vehicles for the exploration of the planets in cryogenic conditions are discussed with suitable biomimetic propulsion systems. The design methodology, hydrodynamic stability, and resistance estimation in cryogenic atmospheres are presented which can act as a benchmark for future missions.

The finite element method for solving the linear floating body hydrodynamic problem is considered with a hierarchy of boundary damper options for modelling the far field. The computer program is validated using two simple examples for which added mass and damping coefficients are computed over a wide range of frequencies. The program is applied to the ISSC TLP, being a typical practical example for which a large scatter in numerical results is reported in the literature. © 1995.

This paper presents the hydrodynamic effects due to slenderness ratio and aft fins for an underwater axisymmetric body at incidence. The body is torpedo shaped with elliptical nose, tapered tail and cylindrical middle body. The slenderness ratio is varied from 10.8 to 14.4 and flow inclination varied from 0° to 18°. The estimation is done by experimental and numerical means. The axial force, normal force and pitch moment acting on the model was measured during the experiments. Numerical study based on Reynolds Averaged Navier Stokes was carried out using commercial Computational Fluid Dynamics (CFD) code. Sources of experimental and numerical uncertainties were identified and quantified. Pressure measurements and flow visualization were done and compared with CFD flow field predictions. An empirical formulation for estimation of normal force and pitch moment for an axisymmetric body at incidence is suggested based on experimental results.

Captive ship model tests are conducted to determine the hydrodynamic derivatives appearing in the maneuvering equations of motions of a ship. These hydrodynamic derivatives have an important role in maneuvering prediction of a ship at early design stages. Practically, surface ships operate at different speed conditions. Variation in vessel speed will affect the hydrodynamic derivatives and subsequently maneuvering characteristics of the ship. This paper investigates the effect of vessel speed on the derivatives and maneuvering characteristics of a ship. Captive model tests are numerically simulated in a CFD environment for a container ship at different Froude numbers to estimate the influence of Froude number on hydrodynamic derivatives and on the turning characteristics of the ship.

Many areas of seas, harbours and channels that were considered deep and safe for vessel navigation have been changed into dangerous shallow water regions for the new generation vessels due to their drastic increase in size and speed. Hence, the understanding of the safe speed of a vessel to prevent it from grounding when it operates in restricted waterways is gaining more importance. A catamaran vessel, being one of the new generation vessel configurations, is considered here to study the effect of its under-keel clearance of water and also its demi-hull spacing in the determination of a safe vessel speed. The numerical study using RANSE based CFD solver Fluent has been complemented by results obtained from experiments carried out in the towing tank of IIT Madras using the newly commissioned floating false bottom facility. © 2010 - IOS Press and the authors. All rights reserved.

Ocean wave is an important marine environmental phenomena which influences the performance of a marine structure. This necessitates a detailed hydrodynamic study of the wave-structure interaction problem. In the present work the finite element method has been demonstrated as an effective tool to solve the complex linearised three-dimensional hydrodynamic problem. The developed software has been validated for regular bodies and applied to a multi-body (twin-barge) system. © 1995 IOS. All Rights Reserved.