K. Murali

Circulation Modelling in the Strait of Singapore

Abstract The current system in the Strait of Singapore is fairly complicated and variable because it is under the influence of major currents driven by trade winds as well as the Asian monsoons. In addition to tidal forcing, circulation in the Strait is governed by a strong hydrodynamic pressure gradient which reverses direction semi-annually and coincides with seasonal monsoon changes. In this investigation, we examine the mechanism that controls the circulation in the Strait of Singapore by using a three-dimensional circulation model forced by hydrodynamic pressure gradient, tides and winds. Model results delineate in detail the characteristics of circulation and hydrodynamic structure in the strait. These results compare favourably with the available field measurements and agree, in general, with observations. It is shown that: (1) the low frequency net transport across the strait correlates well with the seasonal hydrodynamic pressure gradient, and (2) the tidal variation across the strait have a significant effect on the circulation in the Strait.

Fabric and mineralogical studies on lime treated marine clays

The use of lime to improve the engineering properties of weak marine clays is a common method from the past. Recent studies indicate that the various foundation problems occurred with passage of time for offshore structures due to hostile wave conditions and adverse climatic conditions. Hence, there is an urgent need to improve the engineering properties of these soft deposits using well established ground improvement techniques. In the present investigation, an attempt has been made on two marine clays to investigate the microchanges that occurred at particulate level due to the addition of lime and sodium hydroxide chemicals. The influence of sodium hydroxide additive on the fabric of lime treated marine clays has also been studied using scanning electron microscopy (SEM) technique. The formation of various new reaction products due to soil-lime reactions and their stability in marine environment were also studied and reported using x-ray diffraction (XRD) technique. The test results indicate that lime stabilization is effective for improving the properties of marine clays and the presence of sea water increases the efficiency of lime stabilization.

Rans Predictions of Free Surface Effects on Axisymmetric Underwater Body

Abstract The aim of the current investigation is an assessment of the influence of free surface effects on hydrodynamic coefficients of an axisymmetric underwater body using Reynolds averaged Navier-Stokes (RANS) solver. This paper mainly focuses on the capabilities of turbulence models (two high-Re models, k-ε Re-Normalised Group (RNG) and k-ε Realizable and one low-Re model, k-ε Abe-Kondoh-Nagano (AKN) together with volume of fluid (VOF) method for hydrodynamic investigation of an axisymmetric underwater body (Afterbody1) near the free surface. The current numerical investigations were carried out to simulate measured hydrodynamic coefficients for several experimental parameters, including velocity variations from 0.4 m/s (Reynolds number Rev = 2.12x 105) to m/s (Reynolds number Rev = 7.42 x 105), depth of submergence from 0.75D to 4.0D, and angle of attack 0° to 15° on a vertical plane for a model scale of 1:2. Numerical simulations were conducted at a grid density of 0.46 million cells based on grid independence study using a commercial flow solver, Fluent. Free surface effects on hydrodynamic coefficients (drag, lift and pitching moment) are studied from the calculated hydrodynamic forces and moments. Validation of numerical results is done by comparing with measured values. The present investigation found that the k-ε Realizable model together with VOF method consistently provided superior performance in predicting the free surface wave effects on hydrodynamic coefficients for Afterbody1 model. This study is expected to aid the selection of apposite RANS model for simulation of hydrodynamic forces over underwater bodies near the free surface.

Thermal distribution calculations for block level placement in embedded systems

Abstract Reliability is a very important concern for the embedded systems. Thermal distribution has become an important reliability concern for today’s integrated circuits and these circuits are being used increasingly in embedded systems. In traditional design flows, the temperature of the chip is assumed to be uniform across the substrate. However, non-uniform thermal distribution can be a major source of inaccuracy in delay and clock skew computations, and can have an impact on elctromigration reliability and self-heating effects for today’s very deep submicron technology. Hence, it has become necessary to obtain design with uniform temperature distribution to ensure minimum temperature gradient and avoid hot spots across the chip area. This will minimise reliability problems during the operation of the chip. The uniform temperature distribution can be achieved by appropriate placement of circuit blocks during the physical design. In this paper, thermal distribution of single chip embedded system on silicon is discussed. The thermal distribution calculations require evaluation of switching activity factor of circuit blocks. This factor is determined by computing activities of the blocks based on the application software of embedded system.

Role of Vegetation on Beach Run-up due to Regular and Cnoidal Waves

Abstract The effectiveness of measures in protecting the coast against natural hazards depends on the extent of run-up by ocean waves, which is an essential parameter in their planning and design. Although, several formulae have been derived in the past for estimation of run-up in laboratory studies for a variety of wave front structures, certain uncertainties still remain pertaining to flat slopes and long waves. To understand the performance characteristics of vegetation in attenuating run-up, a comprehensive experimental investigation was taken up. The studies were carried out with regular and cnoidal waves propagating over a plane slope of 1 ∶ 30 in the presence and absence of vegetation of different characteristics. The results on the variation of dimensionless run-up as a function of wave and vegetal parameters are presented and discussed in this paper.

Performance of flexible emergent vegetation in staggered configuration as a mitigation measure for extreme coastal disasters

The performance characteristics of emergent greenbelt vegetation has received considerable attention in the recent years, post-Great Indian Ocean Tsunami (26 December 2004). In the present work, a comprehensive laboratory study on the hydraulic resistance characteristics due to a group of slender cylindrical members representing flexible plantation has been carried out in a laboratory open channel. The model stem groups comprised of individual members of different sizes and concentrations in staggered configuration. The sizes and concentrations were chosen in such a way that they would fall into the practical ranges of vegetations in coastal forestry. The results indicate that the flow resistance varies with stem concentration, stem diameter and elastic properties of the individual members and the flow velocity. Based on the results, the Darcy friction factor, f, for various flow and vegetative parameters was derived. A new empirical equation is proposed for evaluating f for partially submerged vegetation in staggered configuration for a wide range of depths of flow in relation to un-deflected plant heights. It is expected that the f thus determined will be useful in modelling the shallow flows using shallow water equations.

Effect of the Tidal Currents at the Amphidromes on the Characteristics of an N-Wave-Type Tsunami

The disturbance due to a tsunami is always coupled with permanent harmonic ocean tides. The tsunami based on its time of occurrence will combine with a particular tidal phase. Thus, a coast will experience a modified water level due to a high- or low-tide situation. On the other hand, the tidal current at the amphidromes may act as an attenuator or amplifier of the tsunami prior to its arrival at the shore. A one-dimensional numerical study has been carried out to demonstrate the effect of the amphidromic current on tsunami characteristics. A realistic N-wave profile of the initial perturbation has been considered for the initiation of a tsunami. The interaction of the amphidromic current with the phase velocity of an N-wave-type tsunami and the subsequent run-up along the shore has been investigated in this paper.

Effect of vegetation on run-up and wall pressures due to cnoidal waves

A comprehensive experimental investigation was carried out to understand the effect of vegetation in attenuating run-up over a plane slope of 1 : 30 due to the propagation of cnoidal waves. Two different types of vegetation configurations, namely tandem and staggered, are considered. Dynamic pressures exerted along a vertical wall fronted by these two vegetation types were measured, and the typical variation of dimensionless peak pressures for different relative depths of submergence of the pressure ports is reported as a function of dimensionless parameters involving the vegetation and wave characteristics. Further, typical results on the variation of the dimensionless peak run-up versus the surf similarity parameter only for the staggered configuration are also herein reported. Finally, the percentage reduction in wave run-up and pressures on the wall due to the presence of wall fronted by vegetation are reported.

Estimation of hull-propeller interaction of a self-propelling model hull using a RANSE solver

Viscous flow around the self-propelled hull was investigated at model scale with a RANSE solver using k-ϵ turbulence model by coupling the flow around the hull model with the flow generated by the rotating propeller. Hence, the challenges involved in the coupling of flow past ship hull with propeller flow of lower-order scale have been brought out and addressed here. Propeller rotation is accounted for by mesh motion; the solution domain is represented by a rotating cylindrical domain encompassing the propeller and a fixed rectangular domain around the hull for the remaining part of the solution domain; both these domains are interfaced by sliding interfaces. Comparisons are made between the simulated results and corresponding experimental results for hull resistance, propeller thrust and propeller RPM. It is shown that the flow field in the self-propelling condition is well reproduced in the simulation, and the estimated thrust deduction factor and RPM agree well with the measured ones. Among the various turbulence modelling schemes available, the present study uses the Realizable k-ϵ turbulence model.

A comparative study of dynamic mesh updating methods used in the simulation of fluid-structure interaction problems with a non-linear free surface

Abstract This paper compares the performance of three mesh movement algorithms: Laplacian smoothing, linear spring analogy and torsion spring analogy for a fluid mesh update in staggered fluid-structure interaction (FSI) simulations with a non-linear free surface. The mesh updating schemes are applied to simulate three representative cases of the above-stated dual moving boundary problem. The performances of the algorithms are gauged on the basis of their ability to delay the initiation of a complete remesh of the fluid domain while maintaining solution accuracy. To satisfy this dual objective, the mesh-updating algorithm should not only prevent mesh failure but should also maintain well-shaped triangles. The reasons for the failure of different schemes are explained and suitable modifications are suggested/implemented to enhance thier performance. It is shown that these modifications prove to be very successful in improving the effectiveness of the algorithms.