Showing papers by "Vallam Sundar published in 2019"

Phase field lattice Boltzmann model for air-water two phase flows

Abstract: Two phase flows occur in different forms with liquid and gas in general, among which, the interaction of the flow of air and water is a common scenario However, modeling the two phase flow still remains a challenge due to the large density ratio between them and different space-time scales involved in the flow regimes In the present work, the lattice Boltzmann (LB) model capable of handling large density ratio (1000) and high Reynolds number (104) simultaneously is proposed The present model consists of two sets of LB equations, one for the flow field in terms of normalized pressure-velocity formulation and the other for the solution of the conservative Allen-Cahn equation to capture the interface The numerical tests such as stationary drop, bubble coalescence, and capillary wave decay have been performed, and the results exhibit excellent mass conservation property The capability of the present model to handle complex scenarios has been tested through test cases, for example, rise of an air bubble, splash of a water droplet on a wet bed, and Rayleigh-Taylor instability In all test cases, the simulation results agree well with the available reference data Finally, as an application of the present model, the breaking of a deep water wave with high Reynolds number (104) is simulated The plunging breaker with wave overturning and the generation of secondary jet and splashes are well described by the present LB model The evolution of wave energy dissipation during and after breaking is in agreement with the reference data

Hydrodynamic characteristics of a submerged trapezoidal artificial reef unit

Abstract: Submerged structures serve as wave attenuators and are widely adopted for protecting the coast against erosion or to reduce forces on structures situated on its leeward side. Conventional submerged...

Lattice Boltzmann simulation of free surface flow impact on a structure

Abstract: Liquid impact on a rigid wall is a common feature in the context of marine structures, as most of them are exposed continuously to breaking waves. In the present paper, a comparison and analysis of the impact load estimates obtained from free surface lattice Boltzmann (LB) simulation with the experimental measurements from available literature have been reported. Initially, two-dimensional simulation of the dam break impact on the wall is performed with two different LB models: BGK-F1: a Bhatnagar-Gross-Krook (BGK) collision operator with the force scheme of Buick and Greated [Phys. Rev. E 61, 5307 (2000)1063-651X10.1103/PhysRevE.61.5307], and MRT-F2: a multiple relaxation time (MRT) operator with the force scheme of Guo et al. [Phys. Rev. E 65, 046308 (2002)1063-651X10.1103/PhysRevE.65.046308]. The pressure estimates obtained from BGK-F1 over MRT-F2 are closer to the measurements, though the other key parameters, such as the waterfront evolution and the free surface profile, have not shown significant variations. Furthermore, the three-dimensional dam break simulation has been performed using BGK-F1 for three test cases: (i) impact on a wall, (ii) impact on a rectangular obstacle, and (iii) impact on a tall tower. In all the test cases, the load estimates are in agreement with the experimental measurements.

Numerical modeling of nonlinear sloshing of liquid in a container coupled with barge subjected to regular excitation

Abstract: The sloshing phenomenon of liquid in a partially filled tank mounted rigidly within a barge exposed to regular wave field has been considered for numerical investigation. This paper details the numerical scheme developed in time domain to study the interaction of wave induced barge motion with sloshing oscillations in a partially filled container. The sloshing time history results obtained from the numerical scheme adopted are validated with experimental measurements of Nasar et al. (2008) The harmonics present in the sloshing oscillation for the liquid fill levels (25%, 50% and 75%) have been analyzed.

Study on Stability of Eden Navigational Channel in Hooghly River Estuary

Abstract: Hooghly estuary is a complex dynamic estuary facing dredging maintenance and navigation-related problems due to the high rate of sediment load brought by the Hooghly River. The present study is to investigate the hydrodynamics and morphodynamics of the Hooghly estuary, with specific reference to stability of Eden navigational fairway and permanent operation of the channel as a possible main navigational route to HDC in the place of Auckland channel from the Bay of Bengal. Impact of stoppage of dredging at Auckland bar on the other channels (Eden and the Rangafalla channel that connect to Kolkata Dock System, KDS) is investigated. Simulations involving different scenarios like Auckland channel dredged and non-dredged conditions are considered to investigate the stability of the Eden channel and also to address the aspects relating to the maintenance of other channels. In the study, for the Auckland open condition, the predicted siltation levels are of about 8–12 cm over Auckland and about 4–8 cm over Eden bar, over 15 days of simulation. For Auckland closed condition and for Auckland with two tracks, the results indicate a marginal reduction in siltation over the entire Eden area, and it is also observed that the siltation in Jellingham and Haldi River confluence is significantly reduced. For the monsoon conditions in all the above scenarios, the results, as per the siltation patterns and as per the siltation levels on the edges of the channels, indicate that there will be a marginal increase in siltation, by about 20% when higher silt load is considered. The study suggests that Eden channel could continuously be used with little dredging in the longer term of more than 5 years with monitoring and realignment of the channel to cater for movement of sandbars. Further, non-dredging of Auckland channel may not have any bearing on the operation of Eden, Jellingham and Rangafalla channels.

Effects of Power Take-Off Damping and Model Scaling on the Hydrodynamic Performance of Oscillating Water Column Device

Abstract: A systematic numerical investigation on the hydrodynamic performance of an oscillating water column (OWC) system for different damping values from the power take-off (PTO) device and at different model scales has been studied. Further, an experimental programme was devised with a Froude model scale of 1:12 in a shallow wave flume by representing PTO damping by varying the air orifice ratio, a/A [ratio of air orifice area (a) to plan area (A)]. In the case of numerical investigation, simulation was carried out in the computational fluid dynamics (CFD) model, REEF3D. Herein, the equivalent PTO damping on the OWC chamber is represented by the porous media flow relation. First, the numerical results are validated by comparing with the laboratory measurements for five different PTO damping. Second, following the validation stage, the open-source numerical model is used to evaluate model scale effects on the hydrodynamic performance of the OWC by comparing hydrodynamic parameters of four different model scales from 1:12 to full scale. In this paper, the influence of PTO damping and model scales on the variation of pressure behind the lip wall, air pressure, and hydrodynamic efficiency is studied. The results are reported for different relative water depths. Since the air is modelled as incompressible, the effect of scaling on the hydrodynamic performance of OWC is found with negligible variation.

Influence of Harbour Wall on Pressure Variation in an Oscillating Water Column

Abstract: In the midst of a variety of wave energy converters (WECs), the oscillating water column (OWC) device that exploits ocean waves for its energy is an important class. It is a partially submerged structure with an opening to the sea below the water line that traps the air above the water column. Owing to the wave motion, the rise and fall of water level in a closed chamber pump the air thus causing its oscillation to drive a turbine coupled to electric generators. The objective of the present study is to analyse the influence of harbour walls on the pressure variation between the inside and front of the lip wall of a bottom mounted OWC. In contemplation of enumerating the influence of the harbour walls, a two-dimensional physical model study was conducted with a model scale of 1:20 in a 72.5 m long, 2 m wide and 2.5 m deep wave flume. The study was carried out with different configurations of harbour walls by changing its projecting length to examine the response of OWC. The dimensions of OWC have been chosen in such a way that the device can absorb more energy for a particular relative water depth (d/L). The model was exposed to the action of regular waves in order to evaluate the hydrodynamic performance characteristics of the device mainly the effect of the harbour walls. The details of test set-up, testing facility, parameters of models, harbour wall configurations, test procedure, analysis of results and discussion are reported in this paper.

Impact of Flow-Driven Debris on Coastal Structure During Tsunami Bore

Abstract: Tsunami impact on infrastructure along the coast causes severe destruction, loss of human lives and negative influence on the economy. When tsunami propagates towards the coastline, the flow often resembles a bore which propagates with a high velocity and takes everything on its way, including heavy objects. When reaching the structure, this water-driven debris induces a kind of impact force and magnifies the load on structures along the coast. The present study is aimed to measure the load of tsunami-borne debris on a building constructed near the shoreline. In many situations, tsunami or any flood nearshore resembles a surge caused by a dam-break event; therefore, to model this process, we conducted our experiments by setting up a dam-break arrangement in a wave flume of 72.5 m length, 2 m wide and 2.5 m deep at the Department of Ocean Engineering, IIT Madras, India. A Froude scale of 1:20 was adopted for modelling the coastal structure and the debris placed over a beach slope of 1:30. The hydraulic bore was generated by a sudden opening of the gate of the tank. We considered three water depths of 0.8, 0.9 and 1.0 m. The debris was modelled as a box-shaped structure weighing 4.2, 5.6 and 6.0 kg. A video camera was used to capture the surging of the hydraulic bore and to study the character of debris motion during impact. The impact forces acting on the structure due to debris were measured with a load cell. The acquired data were further analysed and discussed.

Estimation and Analysis of Extreme Maximum Wave Heights

Abstract: Extreme wave height is the most decisive parameter in the design and survival of variety of coastal engineering activities like port and harbour infrastructures, beach management schemes and flood risk analysis. Although the initial focus of this study was in the estimation of return values for significant wave heights, it is important to arrive at the design wave height with utmost caution after a rigorous analysis, as both safety, as well as the cost of investment, are involved. Hence, in the present study, the extreme wave heights for different return periods were estimated not only for significant wave heights (Hs) but also for the maximum individual wave heights (Hmax), assuming the individual wave height measurements are Rayleigh distributed. In order to carry out the analysis envisaged herein, long-time series of ERA-Interim significant wave height hindcast data covering a period of 36 years i.e., from 1979 to 2014 were employed along with the polynomial approximation method of extreme wave analysis.

An Experimental Study on Wave Forces and Pressures on an Oscillating Water Column Under Random Waves

Abstract: A partially submerged device, known as the Oscillating Water Column (OWC) device made up of a chamber with a bottom opening and air trapped at the top, can harness the wave energy. The studies on an OWC wave energy converter widely pertain to the optimization of its geometries to enhance the wave power absorption. On the other hand, limited studies deal with its survivability against wave conditions. For that purpose, a detailed experimental investigation is taken up to understand the nature and magnitude of dynamic pressures on the individual components and the total dynamic forces on the OWC device subjected to random wave incidence. A Froude scale of 1:12 is chosen for studying shoreward and seaward wave load distribution on the OWC device in terms of the non- dimensional pressure on the seaside of the front wall, rear wall pressure, air pressure, and the total horizontal and vertical wave forces. In general, force measurement increases with wavelength. In structural design of the lip wall of OWC device, the seaward exerted pressure should be considered as a critical parameter, since, it governs its stability against being pulled out towards the seaside. The ratio of total horizontal to vertical wave force is vary between 2.5 to 3. It is found that the neighborhood of the natural frequency of the system around d/Lp = 0.131, the measured wave force on the energy device is smaller due to high wave energy absorption.

Experimental Studies on Hydrodynamic Performance of an Artificial Reef

Abstract: A two-dimensional experimental study has been conducted on non-perforated and perforated artificial trapezoidal reefs in order to plan and prescribe suitable coastal protection measures against erosion. Multiple units of reef have been tested in order to reduce the wave transmission. However, the results for a single reef are presented in this paper. It mainly focuses on the influence of perforations and water depth on the artificial reef as a possible wave attenuator. The percentage of reduction in wave transmission is quantified. The results on the variations of transmission, reflection and loss coefficients along with the dynamic pressure exerted on the reef due to regular waves as a function of relative reef width for its different relative depths of submergence are reported in this paper.

Study on Maintenance Dredging for Navigable Depth Assurance in the Macro-tidal Hooghly Estuary

Abstract: Hooghly Estuary is a tide-dominated estuary in the east coast of India, experiencing heavy siltation in the navigation channel. This frequent siltation affects the movement of vessels, thus, hampering the efficiency of the port. The quantity of siltation is related to the physical and environmental conditions and the geometric configuration of the entrance as well as the navigation channels. This study aims at investigating the possibility of smooth navigable channel to the Haldia Dock Complex with a minimum quantity of maintenance dredging. The monitoring of the channel through bathymetry survey at regular intervals has been carried out. The filed survey involved identification of the zones of siltation, changes in the depth and estimation of sediment volumes at 10 different sections along the Eden bar. It was found in the study that the siltation depth varies between the spring and neap tide periods due to the variation in the tidal currents. From the field monitoring surveys, it is found that there is predominant erosion from neap-to-spring tidal cycle. These aspects are brought out in the manuscript along with the stability details of the Eden channel, throughout the year.

Study on Wave Amplification Using Circular and Elliptical Plates

Abstract: Bathymetry induced focusing can be used to improve the efficiency of different wave energy devices by amplifying the wave height at the focal region. Different wave focusing devices have been tested in the past to increase the power extracted from ocean waves. In the present study, wave amplification caused by submerged elliptical and circular plates is modeled using Delft3D. Submerged horizontal plates produce more amplification than the mounds of same width. Amplification produced by circular plates is equal to that of elliptical plates. Wave amplification is more when the depth of submergence of plate below the free surface is less, provided it does not induce wave breaking. Wave amplification increases with increase in plate width up to half the wave length. Beyond relative depth greater than 0.25, depth does not affect wave amplification. These circular plates can be used to increase the efficiency of all types of wave energy devices.

Vibration Control of Large Floating Offshore Structures by Means of Damping Plates: A Case Study

Abstract: Large floating multi-purpose platforms are interesting and emerging concepts in terms of envisaged future development in the offshore engineering. The investigation on the hydrodynamic behaviour of such structures plays an important role in their characterization. In particular, vibration control and motion reduction of the platform may be crucial for its operation since most of the renewable energy converters cannot cater to large support motions (e.g. wind turbines, photovoltaic plants and OWC wave energy converters). With this as the background, a small-scale open-sea experimental investigation on a barge structure has been carried out in the Natural Ocean Engineering Laboratory (NOEL) in Reggio Calabria, Italy. The barge was equipped with removable vertical plates in order to investigate their effects on the structure dynamics and their optimal configuration, in terms of its motion attenuation. In addition, a numerical model using the commercial software Ansys AQWA (v. 16.1) is used to investigate its dynamic effects for five different configurations of the plates both in the time and in the frequency domains.

Hydrodynamic Performance of Pile Supported Breakwaters—A Review

Abstract: This paper reviews the progress on the hydrodynamic characteristics of pile-supported breakwaters. Partially immersed vertical barriers in general are found to perform well in reducing the transmission of waves in deep-waters. Structures in the marine environment are often required to attenuate the wave energy, in particular to serve as berthing facility on their leeside and at the same time maintaining lesser reflection. Several attempts in this direction have been made in the past and in general, structures with a porous front face such as wave screens are preferred to satisfy this requirement. In addition, curved front face structures also are sought to dissipate incident wave energy. It is observed that profiles such as Circular cum Parabola (CPS), Galveston (GS) and Flaring Shaped (FSS) employed for seawalls exhibit significant wave energy dissipation. However, these seawall profiles are yet to be used as a breakwater supported on piles. The efficiency of these unconventional shapes in combination with wave screens in ensuring the required harbor tranquility with reduced reflection is an area requiring further research.