Showing papers by "Vallam Sundar published in 2022"

Wave Energy Potential

Abstract: A brief introduction to Ocean energy followed by an outlook on the wave energy potential around the globe and a comprehensive discussion on the distribution of wave energy potential along the coast of Indian peninsula are presented in this chapter. The wave energy distribution along the coastal region is expected to facilitate the understanding and planning process of installation of new energy devices. Further the importance of wave energy and its impact on the environment are reported.

A comprehensive review on structural tsunami countermeasures

Abstract: Abstract Tsunamis pose a substantial threat to coastal communities around the globe. To counter their effects, several hard and soft mitigation measures are applied, the choice of which essentially depends on regional expectations, historical experiences and economic capabilities. These countermeasures encompass hard measures to physically prevent tsunami impacts such as different types of seawalls or offshore breakwaters, as well as soft measures such as long-term tsunami hazard assessment, tsunami education, evacuation plans, early-warning systems or coastal afforestation. Whist hard countermeasures generally aim at reducing the inundation level and distance, soft countermeasures focus mainly on enhanced resilience and decreased vulnerability or nature-based wave impact mitigation. In this paper, the efficacy of hard countermeasures is evaluated through a comprehensive literature review. The recent large-scale tsunami events facilitate the assessment of performance characteristics of countermeasures and related damaging processes by in-situ observations. An overview and comparison of such damages and dependencies are given and new approaches for mitigating tsunami impacts are presented.

Hydrodynamic Characteristics of Concave Front Pile-Supported Breakwaters with a Tubular Wave Screen

Abstract: Concave front pile-supported breakwaters, namely, Galveston wall-shaped pile-supported breakwater (GS-PSB) and circular cum parabola-shaped pile-supported breakwater (CPS-PSB), have proven...

Wave Energy Convertors

Abstract: A detailed review of the works carried out in connection with wave energy extraction devices are presented in this chapter. Although, there are a few methods of extracting the energy from waves, the emphasis is laid on Oscillating water column (OWC) devices due to its’ more attractive, simple, and proven technology. The classification of the OWC devices based on location, its principle and the different options for its installation is made. If one works out the cost-benefit ratio, it is rather not encouraging to plan for a device to extract energy. Hence, the possibilities of merging OWC with breakwaters would be more viable and cost-effective too. Here again, one should consider the purpose of the breakwater as it may be either for development harbor or coastal protection. With this background, the different options of multi-purposed wave energy devices and, in specific, the conceptual design of integrating the breakwater and OWC are presented in this chapter.

Performance Characteristics of an OWC in Regular and Random Waves

Abstract: Typically, the dimensions of OWC chamber have a single resonant frequency which captures efficiently wave energy over a narrow range of frequency of the waves. The high-frequency waves are reflected. Provision of a pair of additional harbour walls in front of the device creates an added resonance enabling it to capture substantial energy from a wider range of frequency of the waves. Although, there exists a substantial loss in energy due to wave deformation as the wave from the offshore approach shallow waters, in which case, the harbour walls prevent any further dissipation till it reaches the device. This chapter reports the influence of harbour walls on the hydrodynamic characteristics of a bottom - mounted OWC. The study was carried out with different configurations of harbour walls, i.e., the projecting length was varied in terms of the ratio of harbour wall length (c) to the breadth of OWC (b), (c/b = 1, 1.5 and 2). The opening angle of harbour wall was varied in the range of [π/2, 7π/8] in an interval of π/8 with respect to the front lip wall of OWC. The results reveal that the efficiency of the OWC with the harbour walls has been significantly improved. The details of such a behaviour are herein discussed.

Congregation of particles on a plane boundary due to the flow induced by an oscillating sphere

Abstract: The complex flow field associated with a spherical structure oscillating adjacent to a solid boundary is significant in ocean engineering. The dynamic interaction between the flow induced by an oscillating sphere and the boundary is evident not only in terms of flow-field deformation around the structures but also on the boundary in the form of shear stress modifications. The present study aims to understand the influence of a plane boundary on the shear stress distributions and particle motion near the boundary induced by the flow due to a submerged sphere oscillating in a viscous fluid. Experiments are conducted using a non-intrusive flow visualization technique and three-dimensional Direct Numerical Simulations are used to link the time-mean results of the experiments to the simulated Lagrangian particle drift on the plane boundary. The dye tracer in the experiments and the Lagrangian particles in the simulation congregate at the same location. This stagnation location coincides with a ring of zero mean shear stress centered on the point on the plane boundary through which the axis of oscillation of the sphere passes. This ring of zero mean stress is termed the congregation zone. The radius of the particle congregation zone is investigated as a function of the sphere oscillating amplitude, Reynolds number, and the distance between the center of the sphere and the plane boundary. Further, a systematic power scaling law is established for the radius of the congregation zone from the control parameters.

Numerical Prediction of Siltation Patterns at a Tidal Inlet in Karaikal, India

Abstract: Tidal inlets are an ecologically sensitive and significant constituent of the coastal environment, where an opening along the shoreline permits the free exchange of fresh and seawater. Predominant longshore currents along the shore result in the formation of sandbars, spits and shoals. Frequent occurrence of such sediment depositions acts as barriers, preventing the ingress of tidal flow. Conventionally training walls are constructed at the inlet to prevent sandbar and spit formations. The volume of water exchanged at the inlet mouth and ebb tidal currents primarily govern the inlet dimensions and the rate of littoral transport, respectively. The case study of a trained micro-tidal inlet at Karaikal (10[Formula: see text]52[Formula: see text]N; 79[Formula: see text]E), India is discussed in this paper. A numerical model using the finite volume method is applied to estimate the siltation rate and distribution within the study domain, driven by tide-induced currents and riverine discharge. A revetment structure is proposed to combat siltation within the inlet to facilitate smooth navigation. The study highlights the notable changes in the presence and absence of the proposed revetment.

Hydrodynamic Performance Characteristics of U-OWC Devices

Abstract: OWC is reported to perform well when exposed to low frequency waves in narrow frequency band. On several occasions, the trough of the potential long wave might reach below the front lip wall of OWC, inducting air into the air chamber. To solve this problem, Boccotti (Boccotti, P. Ocean Engineering 34(5–6), 806–819 (2007a)) modified the conventional OWC concept by incorporating a duct in front of the opening with an intention was to capture the maximum magnitude of dynamic pressure available for oscillation inside the air chamber. This is widely referred to as U-OWC. The performance of U-OWC is explored in a laboratory wave flume. A Froude model scale of 1:20 was adopted to carry out a comprehensive experimental study to investigate the effect of water plane area on pneumatic damping with two aspect ratios (W/B, ratio of chamber width parallel to the wave crest to the width of the chamber in the wave direction) of 0.50 and 1.67 in the model geometry. To investigate the effect of dynamic pressure intensity variation with depth, three h/di of 0.15, 0.30, and 0.45 were considered in the study, where di is the water depth in the chamber. At each of the h/di, the effect of relative bottom opening (O/di) for better phase control and energy conversion was studied. The maximum impact of pressure excitation had occurred at h/di of 0.30 and b/B of 0.50. The performance of the U-OWC is found to be better over a relatively wider frequency bandwidth near the natural frequency of the system.

Numerical Investigation on the Mean Flow Fields Generated by an Oscillating Sphere

Abstract: This work represents an attempt to provide an in-depth understanding of the three-dimensional flow physics related to the characterization of streaming flows due to the oscillating spherical body. The presence of any structure at sea will impact the local hydrodynamics of the site and therefore the local sediment dynamics. When a body oscillates while immersed in a quiescent fluid, the effects of their dynamic interaction are evident not only on the forces acting on the body but also on the flow field deformation by generating a non-zero mean flow. Thus, in many ocean engineering applications, it becomes necessary to understand not only the instantaneous events such as the wave-generated hydrodynamic forces acting on the structures but also the mean hydrodynamic pressure fields and fluid velocity fields. The non-linear streaming flows produced by an oscillating sphere driven in unidirectional simple harmonic motion in a quiescent fluid have been investigated. A comprehensive parametric study has been undertaken over a range of oscillatory Reynolds number and amplitude ratio and a quantitative assessment of the relation between the mean flows and the Lagrangian particle drift associated with the flow has been carried out. It is shown that while an inner mean viscous boundary layer flow is generated that is confined to a very small region near the sphere, a large mean outer vortex flow is generated that can extend a long way from the sphere along the axis of oscillation.

Hydrodynamic Performance Assessment of a Geo-Synthetic Seawall

Abstract: In the recent years, various geo-synthetic components find an extensive application in civil and coastal engineering practice. Commonly, geo-synthetics have a wide application for secondary or tertiary purposes, such as filtration, separation, barrier, reinforcement, whereas, they can be potentially exploited for various other applications in the coastal engineering practice. A sustainable seawall cross-section comprising different geo-synthetic products was proposed to be erected along the Pallana Beach ([Formula: see text] N and [Formula: see text] E), located in Alleppey district of Kerala along the south-west Indian coastline. Since the subsoil at the location is poorly graded, it was decided to replace the conventional materials like rock boulders and concrete armour units with geo-synthetic products. A comprehensive physical model study was conducted to assess the hydrodynamic performance (i.e. reflection, run-up, and pressure distribution) of the geo-synthetic seawall cross-section for a wide range of random wave characteristics and two water depth conditions. The relative overtopping rates for the seawall at varying water levels are computed conservatively from the guidelines prescribed by the EurOtop Manual and XGB Overtopping model.

Seasonal Flow Characteristics of Kandla Creek Through In-Situ Measurements

Abstract: The measurement of tidal stream discharge is a complicated assignment in complex flow conditions that may be occasional or seasonal. The efficient discharge measurement in tidal streams is required with accurate real-time measurements over a longer-period. The discharge of tidal streams is computed as the product of the cross-sectional area and mean velocity. At present, the data on the discharge in tidal creeks and its behavior over time of the ingress of tide (i.e., rising or falling tide) which are seasonal along the Indian coast is inadequate, the understanding of which is considered in the present paper. This study has been carried out with the interest of exploring the seasonal flow pattern in the Kandla creek, Gulf of Kutch, Gujarat. The seasonal, temporal and spatial variation of Kandla Creek discharge of both vertical and lateral distributions of velocity was carried out by repetitive measurements by moving boat method throughout the 8 days of measurement during daylight of 10 to 12 hours a day. The magnitude and direction of the velocities relative to the boat were measured throughout the cross sections (east to west and/or west to east) at two locations (Location-1: North end of the Kandla creek and Location-2: South (near the creek mouth), by downward looking Acoustic Doppler Current Profiler (ADCP) equipment fixed at the side of the measurement vessel/boats with known draft and headings. The measurements were made at about 94 vertical traverses at each cross section. As soon as one traverse completion (left to right), the opposite side (right to left) of the creek traverse was measured for comparison. The maximum net discharge was recorded around 920 m3/sec at Location-1 and around 1900 m3/sec at Location-2, both during pre-monsoon with the ebb tide. During monsoon and post-monsoon season, the net discharge is observed during flood tide with relatively lesser magnitude than during the pre-monsoon ebb tide. The results on the discharge measurement confirm that the ebb tide is dominant during the pre-monsoon and the flood tide is dominant during the monsoon and post-monsoon seasons.

Wave Induced Pressures and Forces on an OWC Device

Abstract: The research over several decades on the Oscillating Water Column has been on the enhancement of its efficiency by optimizing its configuration and its wave power absorption. However, the stability of such a device is of great concern, which requires a detailed evaluation of wave induced loadings on such structures. This chapter details a comprehensive experimental investigation to understand the dynamic pressures on the different components of the OWC including the total force on the structure induced by both regular and random waves. The design of the lip wall of OWC is dictated by the seaward exerted pressure it experiences, which is a critical parameter, as it governs the stability of the structure towards the seaside. The total horizontal wave force is noted to be 2.5 to 3 times the total vertical wave force. The pressures on the rear wall of the OWC chamber and the air pressure over the roof of the caisson exhibit a reverse trend in their variation compared to the pressures on the front of the lip wall. A comparison of the measured force with the formula of (Goda Y Random seas and design of maritime structures. University of Tokyo press (1985)) has revealed that the formula overestimates the shoreward force and underestimates the seaward force in the low frequency zone.

Hydrodynamic Performance of an Array of OWC Devices Integrated with Breakwater

Abstract: Marine renewable energy has potential benefits like the enhancement of the livelihood and can solve the energy-security. For cost-effective solution, a multipurpose wave energy system with an integration into coastal protection has been explored in detail. Having evaluated the performance of an OWC in isolation in the previous chapters, an effort is made to understand the performance of the combination of wave energy technology with detached offshore breakwaters. The said multi-purpose option, termed as OWCBW system is investigated for optimizing the spacing between an array of OWC devices subjected to regular and random waves in normal and oblique wave incidences. It is observed that the OWCBW system with thrice the width of the model spacing exhibited a better performance. The OWC system is found to absorb more power when being placed in an array of devices than compared to an isolated device. The extent of decrease in relative capture width at the resonant frequency of the system for oblique wave incidence compared to that of normal wave incidence is found to be about 30%.

Development of Extreme Wave Maps for Indian Territorial Waters

Abstract: Extreme wave environments have an effect on all marine engineering applications. Hence, the estimation of extreme wave conditions is a prerequisite for offshore and coastal developments like the optimization of marine energy converters, design of port and harbor infrastructures, flood risk analysis and management, and planning of beach management schemes. Given this. extreme wave maps are developed for the Indian territorial waters using the Generalized Extreme Value distribution method. ERA-interim wave hindcast data for 36 years are utilized for this analysis obtained from the European Centre for Medium-Range Weather Forecasts (ECMWF) global atmospheric reanalysis. However, the oceans are highly dynamic, with the wave climate and sea states varying in both space and time. These extreme wave maps may aid as a quick guide in identifying regions of extreme wave heights lying within the design structural criteria of the marine structures to be constructed.

Experimental investigation on the characteristics of directional focusing waves

Abstract: The analysis on fixed or floating structures exposed to directional extreme waves has been a topic of keen interest in ocean engineering. In this context, an attempt is made to study the characteristics of laboratory generated directional rogue waves, termed also as focusing waves. The first-order wave paddle displacements were generated using the linear superimposition principle by applying a constant steepness spectrum associated with directional components. The phase angles are set to be constant at a predefined location to create an extreme event. The experimental test cases span from narrow to broadband spectra having two central frequencies, namely, 0.68 and 0.85 Hz. The bandwidth ratios of 0.5 and 0.75 were considered with five discrete directions ranging from −25°to 25°. The tests include spilling breakers and non-breaking waves. The investigation involves studying the variations in the Atiltness parameter (ATP), Crest height ratio, and the nonlinearity parameter provided by Hastings, an index to measure the effect of nonlinearity. In addition, directional studies using the maximum likelihood method (MLM) are also presented. The critical combination of frequency spectrum and bandwidth is identified from the focusing wave characteristics and discussed in this paper for the tested conditions. The shift in directional spectrum due to nonlinearity was noticed in the present study.

Longshore sediment transport rate from the field measured wave and sediment characteristics along the coast of Karaikal, India

Abstract: The change in the shoreline greatly influences the coastal developments such as maritime trade, fishing activities, coastal transport, and tourism. Due to the requirements of vast developments along the coast near major habitat zones, such as the Karaikal coast along the east coast of India, it is essential to understand the longshore sediment transport (LST) movement to plan for any coastal systems to protrude. A detailed LST estimate has been made with the predicted wave climate using the wind-wave model, WAM. The wave characteristics are compared with the field measurements off the coast considered herein. The LST rates computed using five different sediment transport formulae and the method of integrating the distribution of sediment transport rates within the surf zone have been computed to understand the variations of the estimate in each method. Although the predominant direction of sediment movement is towards the north, there is a seasonal variability of LST based on the prevailing monsoon wind direction. Monitoring and analysis of the LST are important as it is required in designing coastal structures.