C.P. Vendhan

Stability of initially stressed square plates with square openings

Abstract The stability of a simply supported square plate with openings under in-plane loading is analysed using a Finite Element program BUCSAP (BUCkling Structural Analysis Program). The openings are considered as square and central for the main study but rectangular and central for comparison with other work. Different magnitudes of tension and compression are assumed as initial pre-stress in the transverse direction before the longitudinal stress is applied. Two load cases have been considered. The longitudinal stress is assumed as uniform compression for Case 1 and is assumed as trapezoidal compression representing hydrostatic loading for Case 2. The results have been used to plot design charts.

Dynamic pressure distribution on a cylinder due to wave diffraction

Abstract The experimental investigations on the dynamic pressure distribution around a large vertical cylinder resting on a flume bed and piercing the free surface subjected to regular waves have been carried out in a 4-m wide wave flume in a constant water depth of 2.5 m at Ocean Engineering Centre, Indian Institute of Technology, Madras, India. The cylinder of diameter 400 mm was fixed with diaphragm type pressure transducers at eight different locations below the still water level along with one at the still water level. In addition to this, to study the effect of nonlinearity, one pressure transducer was located above the still water level. The experimental results pertaining to mostly deep water conditions are compared with MacCamy and Fuchs theory and the agreement is found to be good. In order to account for the effects of nonlinearities the above said theory has been modified the results of which are found to be in better agreement.

Wave propagation in semi-infinite plane anisotropic thin circular shells

Abstract The problem of propagation of elastic waves in semi-infinite thin circular cylindrical shells has been investigated. The focus of the paper is to bring out the effect of a finite end which forms a boundary of the shell. The shell theory used is a linear theory due to Sanders incorporating a plane anisotropic material model. The frequency equation is viewed as an “end problem”, leading to an eigenvalue structure of the system which encompasses both propagating and attenuating modes for a prescribed input excitation frequency at the finite end. The existence of the attenuating modes implies trapping of the part of the energy in the end zone of the shell and thus essentially represents the dynamic counterpart of the well known static Saint Venant effect. The eigenvalue structure is investigated and illustrative numerical examples worked out. The amplitude ratios associated with the various modes of propagation have also been investigated.

An efficient algorithm for strapdown accelerometer-based motion measurement

Abstract The paper is concerned with the measurement of motion of free floating bodies using accelerometers strapped to a cross-arm mounted on the body. The measurement system has been studied with the help of a formulation involving a set of coupled nonlinear initial-value equations involving the angular acceleration components. A PC-based software using the Bulirsch-Stoer technique has been developed to solve the initial-value problem so as to deduce the body motions from the measured accelerations. Suitable filtering strategy has been employed at every stage of numerical integration. The reliability of the strapdown accelerometer system together with software developed has been validated using a “dry” test. Typical motion measurements have been done in all the six degrees of freedom of a tug model in a wave flume. The method is deemed to be an efficient and cost-effective technique suitable for free floating bodies and for large motions.

Diffracted wave field and dynamic pressures around a vertical cylinder

Abstract Investigations on the hydrodynamic pressures due to regular and random waves exerted on a large vertical cylinder in a constant water depth are reported in this paper. In the experimental investigation, the test cylinder embedded with diaphragm-type pressure transducers at nine different elevations was rotated about its axis to measure the dynamic pressure around its circumference. The wave field in the neighbourhood of the cylinder was measured at six different locations. The results of the experiments are compared with the linear diffraction theory of MacCamy, R. C. and Fuchs, R. A. [(1954) Wave forces on piles: a diffraction theory. U.S. Army Beach Erosion Board, Technical Memorandum No. 69]. In general, the agreement between the theoretical and experimental results is found to be satisfactory. A comparison between the regular and random wave test results is also presented and discussed.

Finite-element modeling of depth and range dependent acoustic propagation in oceanic waveguides

Finite-element models (FEMs) of ocean acoustic waveguides are capable of predicting the full wave solution including the effect of inhomogeneities and interfaces. However the method appears computationally feasible at present only for low to intermediate frequencies. The FEM discussed in this paper treats the radiation boundary condition involving multiple propagating modes using a penalty function approach. The effect of a point source has been represented as a pressure boundary condition on a small boundary surrounding the source. The FEM has been validated with a few examples of ideal waveguides. The FE results for a range and depth dependent parallel waveguide--an ASA benchmark problem [F. B. Jensen and C. M. Ferla, J. Acoust. Soc. Am. 87, 1499-1520 (1990)]--compare well with published results.

Dynamic pressures on a large vertical cylinder due to random waves

Abstract The hydrodynamic pressures due to random waves on a large vertical cylinder, resting on a flume bed and piercing above the free surface has been investigated experimentally in constant water depth. The test cylinder was fixed with diaphragm-type pressure transducers at nine different pressure ports with one of them at the still water level (S.W.L.) itself. The cylinder was rotated about its own axis to study the pressure distribution around its circumference. The time histories of the water surface elevation and the corresponding dynamic pressures exerted on the cylinder have been analysed in the frequency domain. The experimental results are compared with the theoretical results based on the linear diffraction theory of MacCamy and Fuchs and the agreement is satisfactory. Finally, the hydrodynamic pressure coefficients obtained from random wave tests are compared with those from regular wave tests.

Relief of Growth Stresses in Diametrical Planks

The technique of examining the strain relief of Strips cut from diametrical planks has been used extensively in experimental studies aimed at predicting growth Stresses in trees. This paper presents an analysis of the redistribution of growth Stresses associated with the preparation of diametrical planks. The analysis is based on a homogeneous, orthotropic elastic model. The process of stress relief is divided into two stages, one the transverse stress relief which may be treated äs a plane strain problem, and the other the longitudinal stress relief which may be treated äs a plane stress Problem. Ash, a typical hardwood, is chosen äs an example and the theoretical axisymmetric growth stress distribution is adopted. The results show that the relief of Stresses on the sides of the plank does not produce significant strains, which has also been predicted by Gillis using a simplified analysis. The relief of Stresses at the ends of the plank, in addition to causing a uniform strain, results in end effects with large strains which may be responsible for the violent Splitting of diametrical planks äs observed in previous experiments. The contribution of the end displacement, caused by strain relief in the end zones of the plank, to the measured length changes in the plank and strips is examined with the help of the numerical results. These theoretical observations may be of help in devising suitable experimental methods aimed at measuring growth strains using the planks and strips.

Coupled dynamics of deepwater structures – issues and challenges

ABSTRACT Offshore systems consisting of floaters linked to line structures such as risers and mooring lines are favoured for deepwater applications. The dynamic coupling analysis of a floater together with the line structures has been the subject of several studies in recent years. Powerful numerical tools have been developed implementing full-fledged time-domain approaches as well as more efficient frequency-domain approaches for the analysis of deepwater systems. The present article provides a brief overview of coupled analysis techniques employing both time-domain and frequency-domain approaches, focusing on the basic modelling issues and challenges. It is worth formulating compact but sufficiently accurate dynamic models of line structures. The consequence of truncation errors of numerical integration schemes in long-term time-domain simulations needs to be studied. There is a need to standardise the statistical linearisation approach applied to dynamic response which spreads over a very wide frequency range. Verification of the nonlinear drag force model in the Morison equation through basic computational fluid dynamics studies may be undertaken.

Three-dimensional finite element analysis of the diffraction-radiation problem of hydrodynamically compact structures

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.