Subroto Kumar Bhattacharya

The dynamics of flexible slender circular cylinders in axial flow having regard to their fluidelastic instability characteristics is an extensively studied problem in the literature. A finite element formulation of the problem is presented in this paper. The formulation has been validated with the analytical results of supported cylinders available in the literature. Additional numerical examples are presented for which analytical methods are difficult to formulate and do not exist in the literature, in order to bring out the breadth of application of the finite element method. © 1997 Academic Press Limited.

The dynamics of underwater towing of flexible cylindrical structures belongs to the class of fluid-structure interaction problems commonly referred to as "cylinders in axial flow". The serious concern in such towing operations is the various types of hydroelastic instabilities exhibited by the structure at certain critical tow speeds. In practice, reliable prediction of tow configurations and stability characteristics of such towed systems can lead to optimum deployment of cable scope and control of tow speed. The present investigation is concerned with the development of a comprehensive linear finite element method for the dynamics of the flexible towed cylinder with focus on the stability behaviour. The finite element approximation is derived from a variational statement of the problem based on Hamilton's principle. The various structure- and fluid-related matrices as well as matrices resulting from boundary terms have been derived, resulting in a complex unsymmetric eigenvalue problem. Exhaustive validation and convergence studies show that the comparisons between finite element and analytical results are almost exact. Using the finite element code, the hydroelastic instability of a ship-towed array system has been analyzed. The effect of cable scope and shape of the downstream end on stability have been examined. © 2000 Academic Press.

Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping.

The design of 'Autonomous Underwater Vehicles (AUVs)' is mission specific and it is getting more and more complex with time because of higher demands of range, endurance, payload, operation flexibility, navigational capabilities for deep and restricted water depths, energy efficiency and special mission requirements as placed by the user agencies. These requirements of design are primarily conflicting in nature and hence they can be efficiently satisfied or at least a best design suiting a set of requirements can be computed with the application of optimization techniques. At present, the design process for AUVs is dominated by 'ad-hoc' approaches or empirical formulations. Recent developments in computational science and engineering, optimization techniques and 'Computational Fluid Dynamics (CFD)' have the capability to enable the designer to compute better estimates of the drag and other parameters that are important in design. This premise has motivated the present work and in this paper we present a design optimization framework for design of an AUV. In the proposed model, 'Computer Aided Geometric Definition (CAGD)' is integrated with the CFD along with the optimization framework for optimizing a given parameter (i.e. resistance). In our work, CFD is used in place of empirical formulations for the estimation of drag as with CFD one can accurately estimate the drag and thus increasing the fidelity of the optimization model. Furthermore, the use of CFD by integrating it with CAGD and optimization method allows the study of parametric hull form generations and analysis. With the genetic algorithm driven optimization technique, CFD simulation, the automatic generation of geometry based on the design parameters, automatic generation of mesh and automatic analysis of fluid flow, the objective function is computed and optimized efficiently and the results are presented in this work. In this paper, one of the most popular genetic algorithm, 'Non-dominated Sorting Genetic Algorithm NSGA-II' is implemented with MATLAB∗TM and the CFD analysis is implemented with SHIPFLOW∗∗TM. And, in the design process GA and CFD are integrated and used to optimize the design variables for minimization of an objective function (i.e. viscous resistance) and finally, we present a design example motivated by the real world applications and show that the integration of NSGA-II with CFD and CAGD is effective for AUV hull form optimization. Furthermore, the effectiveness of the design variables considered for the optimization process on the design of the low drag hull forms for the design of AUVs is critically examined in this work.

This paper deals with the application of nonparametric system identification to the nonlinear maneuvering of ships using neural network method. The maneuvering equations contain linear as well as nonlinear terms, and one does not attempt to determine the parameters (or hydrodynamic derivatives) associated with nonlinear terms, rather all nonlinear terms are clubbed together to form one unknown time function per equation, which are sought to be represented by neural network coefficients. The time series used in training the network are obtained from simulated data of zigzag and spiral maneuvers. The neural network has one middle or hidden layer of neurons and the Levenberg-Marquardt algorithm is used to obtain the network coefficients. Using the best choices for number of hidden layer neurons, length of training data, convergence tolerance, and so forth, the performances of the proposed neural network models have been investigated and conclusions drawn.

A modular and integrated optimisation model for the design of underwater vehicles is presented. In the proposed optimisation model two modules (i.e. low fidelity and high fidelity) are incorporated and the basic geometric definition of computer aided design (CAD) is integrated with computational fluid dynamic (CFD) analysis. The hydrodynamic drag is considered as single objective with constraints on the geometric parameters of dimension, space and volume. The CAD model is implemented in MATLAB∗™ and CFD model is implemented in Shipflow∗∗™. A real-world design example of an existing underwater vehicle is presented. The applicability of proposed optimisation model is shown. The presented results show that within given set or sets of constraints the application of optimisation model in design results into an efficient hull form.

Bodies under impulsive motion, immersed in an infinite acoustic fluid, severely put to test any numerical method for the transient exterior acoustic problem. Such problems, in the context of the finite element method (FEM), are not well studied. FE modeling of such problems requires truncation of the infinite fluid domain at a certain distance from the structure. The volume of computation depends upon the extent of this domain as well as the mesh density. The modeling of the fluid truncation boundary is crucial to the economy and accuracy of solution and various boundary dampers have been proposed in the literature for this purpose. The second order damper leads to unsymmetric boundary matrices and this necessitates the use of an unsymmetric equation solver for large problems. The present paper demonstrates the use of FEM with zeroth, first and second order boundary dampers in conjunction with an unsymmetric, out of core, banded equation solver for impulsive motion problems of rigid bodies in an acoustic fluid. The results compare well with those obtained from analytical methods. (C) 2000 Acoustical Society of America.

In a domain method of solution of exterior scalar wave equation, the radiation condition needs to be imposed on a truncation boundary of the modelling domain. The Bayliss, Gunzberger and Turkel (BGT) boundary dampers of first- and second-orders, which require a circular cylindrical truncation boundary in the diffraction-radiation problem of water waves, have been particularly successful in this task. However, for an elongated body, an elliptic cylindrical truncation boundary has the potential to reduce the modelling domain and hence the computational effort. Grote and Keller [On non-reflecting boundary conditions. Journal of Computational Physics 1995; 122: 231-243] proposed extension of the first- and second-order BGT dampers for the elliptic radiation boundary and used these conditions to the acoustic scattering by an elliptic scatterer using the finite difference method. In this paper, these conditions are implemented for the problem of diffraction of water waves using the finite element method. Also, it is shown that the proposed extension works well only for head-on wave incidence. To remedy this, two new elliptic dampers are proposed, one for beam-on incidence and the other for general wave incidence. The performance of all the three dampers is studied using a numerical example of diffraction by an elliptic cylinder. Copyright © 2001 John Wiley & Sons, Ltd.

The structural performance of a launch barge in load-out and launching of an offshore steel jacket is the crucial criterion of its designs. The overall bending of the hull girder subjected to the design load form the main theme of the study. In this paper, approach of physical modelling has been adopted to study the problem in the content of a case study which is oriented to establish, in general terms, the model design principles, the role of similitude and the simulation of the various loading cases. With considerable sophistication of the techniques of experimental stress analysis, large scale problems (of which the present study is an example in the area of offshore structures) can be efficiently tackled. It is seen that such experimental approach can efficiently serve as an adjunct to computational model, in addition to the visual qualities of this approach which is of much interest to the practising engineer. © 1985.