Santanu Koley

Bio: Santanu Koley is an academic researcher from Birla Institute of Technology and Science. The author has contributed to research in topics: Boundary value problem & Eigenfunction. The author has an hindex of 11, co-authored 40 publications receiving 376 citations. Previous affiliations of Santanu Koley include Indian Institute of Technology Kharagpur & Indian Institutes of Technology.

Oblique Wave Trapping by Porous Structures Near a Wall

Abstract: The current study deals with the oblique wave trapping by bottom-standing and surface-piercing porous structures of finite width placed at a finite distance from a vertical rigid wall Using the Sollitt and Cross model for wave motion within the porous structure, the problems are analyzed based on the small-amplitude water wave theory in water of finite depth The solutions of the associated boundary value problems are obtained analytically using the eigenfunction expansion method and numerically using a multidomain boundary-element method In the boundary-element method, the boundary value problems are converted into integral equations over the physical boundaries The physical boundaries are discretized into a finite number of elements to obtain a system of linear algebraic equations Various aspects of structural configurations, in trapping surface gravity waves, are analyzed from the computed results on the reflection coefficients and the hydrodynamic forces Suitable arrangements of the rigid

Wave interaction with a submerged semicircular porous breakwater placed on a porous seabed

Abstract: In the present study, a coupled eigenfunction expansion-boundary element method is developed and used to analyze the interaction of surface gravity waves with a submerged semicircular porous breakwater placed on a porous seabed in water of finite depth. Two separate cases: (a) wave scattering by the semicircular breakwater, and (b) wave trapping by the semicircular breakwater placed near a porous sloping seawall are studied. Further, as a special case, wave trapping by a semicircular breakwater placed on a rubble mound foundation near a sloping seawall is analyzed in water of uniform depth having an impermeable bed. The wave motion through the semicircular permeable arc of the breakwater is modeled using the Darcy's law of fine pore theory, whilst the wave motion through the porous seabed, rubble mound foundation and the porous seawall are modeled using the Sollitt and Cross model. The friction coefficient defined in Sollitt and Cross model is computed by approximating the spatial dependency of the seepage velocity with the average velocity within the porous media. An algorithm for determining the friction coefficient f is provided. Various physical quantities of interests are plotted and analyzed for various values of waves and structural parameters.

Oblique Wave Scattering by a Vertical Flexible Porous Plate

Abstract: In the present study, oblique surface wave scattering by a submerged vertical flexible porous plate is investigated in both the cases of water of finite and infinite depths. Using Green's function technique, the boundary value problem is converted into a system of three Fredholm type integral equations. Various integrals associated with the integral equations are evaluated using appropriate Gauss quadrature formulae and the system of integral equations are converted into a system of algebraic equations. Further, using Green's second identity, expressions for the reflection and transmission coefficients are obtained in terms of the velocity potential and its normal derivative. Energy balance relations for wave scattering by flexible porous plates and permeable membrane barriers are derived using Green's identity and used to check the correctness of the computational results. From the general formulation of the submerged plate, wave scattering by partial plates such as (i) surface-piercing and (ii) bottom-standing plates are studied as special cases. Further, oblique wave scattering by bottom-standing and surface-piercing porous membrane barriers are studied in finite water depth as particular cases of the flexible plate problem. Various numerical results are presented to study the effect of structural rigidity, angle of incidence, membrane tension, structural length, porosity and water depth on wave scattering. It is found that wave reflection is more for a surface-piercing flexible porous plate in infinite water depth compared to finite water depth and opposite trend is observed for a submerged flexible porous plate. For a surface-piercing nonpermeable membrane, zeros in transmission coefficient are observed for waves of intermediate water depth which disappear with the inclusion of porosity. The study reveals that porosity has small influence on the wave-induced excitation of the structure with higher flexibility but it tends to reduce the deflection of a stiffer structure. In case of partial flexible plates and membrane barriers, irrespective of the gap length, full transmission occurs due to wave diffraction through the gap in the very long wave regime while, full reflection occurs by complete flexible impermeable barriers for similar wave condition.

Interaction of gravity waves with bottom-standing submerged structures having perforated outer-layer placed on a sloping bed

Abstract: The present study deals with the interaction of oblique surface gravity waves by bottom-standing submerged structures having perforated outer-layers with the structure being placed on a sloping sea bed. Both the cases of wave scattering by a submerged structure and wave trapping by a submerged structure located near a wall are studied in water of finite depth under the assumption of small amplitude water wave theory. Sollitt and Cross model is used to analyze the wave motion within the perforated layer of the structure. The mathematical model of the physical problem is handled for solutions using a suitable combination of the eigenfunction expansion method and the boundary element method. Various aspects of structural configurations and wave characteristics on scattering and trapping of surface gravity waves are analyzed from the computed results on the reflection and transmission coefficients and the hydrodynamic forces acting on the structure and the rigid wall. It is observed that suitable configurations and location of the submerged structure can provide long-term and cost-effective solutions for creating a tranquility zone and in protecting various marine facilities from wave attack during extreme wave climate. Further, the outer perforated layer of the structure plays a significant role in reducing the wave forces on the submerged structure by dissipating a part of the wave energy, which in turn increases the service life of the structure as a wave barrier. The method does not require the solution of the complex dispersion relation and can be applied to analyze a large class of complex wave-structure interaction problems of practical interest in ocean engineering and other branches of mathematical physics.

Wave transmission by partial porous structures in two-layer fluid

Abstract: The present study deals with oblique surface gravity wave scattering and trapping by bottom-standing and surface-piercing porous structures of finite width in two-layer fluid. The problems are analyzed based on the linearized water wave theory in water of uniform depth. Both the cases of interface piercing and non-piercing structures are considered to analyze the effect of porosity in attenuating waves in surface and internal modes. Eigenfunction expansion method is used to deal with wave past porous structures in two-layer fluid assuming that the associated eigenvalues are distinct. Further, the problems are analyzed using boundary element method and results are compared with the analytic solution derived based on the eigenfunction expansion method. Efficiency of the structures of various configuration and geometry on scattering and trapping of surface waves are studied by analyzing the reflection and transmission coefficients for waves in surface and internal modes, free surface and interface elevations, wave loads on the structure and rigid wall. The present study will be of significant importance in the design of various types of coastal structures used in the marine environment for reflection and dissipation of wave energy at continental shelves dominated by stratified fluid which is modeled here as a two-layer fluid.

Review of recent research and developments on floating breakwaters

Abstract: Floating breakwaters are an effective solution for protecting fragile beaches from being washed away, coastlines from erosion, floating structures, marinas and ports from strong wave action. This paper presents a literature review on the research and developments of floating breakwaters. Floating breakwaters may be categorized into seven main types, namely, the box-type, the pontoon-type, the frame-type, the mat-type, the tethered float type, the horizontal plate type and other types. The research and developments as well as the performance of these different types of floating breakwaters and wave attenuating devices are reviewed and discussed.

Investigation on the effects of Bragg reflection on harbor oscillations

Abstract: Periodic undulating topographies (such as sandwaves and sandbars) are very common in coastal and estuarine areas. Normally incident water surface waves propagating from open sea to coastal areas may interact strongly with such topographies. The wave reflection by the periodic undulating topography can be significantly amplified when the surface wavelength is approximately twice the wavelength of the bottom undulations, which is often called as Bragg resonant reflection. Although the investigations on the hydrodynamic characteristics related to Bragg reflection of a region of undulating topography have been widely implemented, the effects of Bragg reflection on harbors have not yet been studied. Bragg resonant reflection can effectively reduce the incident waves. Meanwhile, however, it can also significantly hinder the wave radiation from the harbor entrance to the open sea. Whether Bragg reflection can be utilized as a potential measure to alleviate harbor oscillations is unknown. In the present study, Bragg reflection and their interactions with the harbor are simulated using a fully nonlinear Boussinesq model, FUNWAVE 2.0. For the purpose, an elongated harbor with constant depth is considered, and a series of sinusoidal bars with various amplitudes and numbers are deployed outside the harbor. The incident waves considered in this paper include regular long waves and bichromatic short wave groups. It is revealed for the first time that for both kinds of incident waves, Bragg resonant reflection can significantly alleviate harbor resonance. The influences of the number and the amplitude of sinusoidal bars on the mitigation effect of harbor resonance and on the optimal wavelength of sinusoidal bars that can achieve the best mitigation effect are comprehensively investigated, and it is found that the former two factors have remarkable influences on the latter two parameters. The present research provides a new option for the mitigation of harbor oscillations via changing the bottom profile, which is feasible as long as the navigating depth is guaranteed.

Iterative multi-domain BEM solution for water wave reflection by perforated caisson breakwaters

Abstract: This study develops a full solution for water wave reflection by a partially perforated caisson breakwater with a rubble mound foundation using multi-domain BEM (boundary element method). Regular and irregular waves are both considered. A quadratic pressure drop condition on caisson perforated wall is adopted, and direct iterative calculations are performed. Due to the use of quadratic pressure drop condition, the effect of wave height on the energy dissipation by the perforated wall is well considered. This study also develops an iterative analytical solution for wave reflection by a partially perforated caisson breakwater on flat bottom using matched eigenfunction expansion method. The reflection coefficients calculated by the multi-domain BEM solution and the analytical solution are in excellent agreement. The present calculated results also agree reasonably well with experimental data from different literatures. Suitable values of discharge coefficient and blockage coefficient in the quadratic pressure drop condition are recommended for perforated caissons. The effects of the wave steepness, the blockage coefficient of perforated wall and the relative wave chamber width on the reflection coefficient are clarified. The present BEM solution is simple and reliable. It may be used for predicting the reflection coefficients of perforated caisson breakwaters in preliminary engineering design.

Water-wave scattering and energy dissipation by a floating porous elastic plate in three dimensions

Abstract: An eigenfunction-matching method is developed for the problem of linear water-wave scattering by a circular floating porous elastic plate, and a coupled boundary-element and finite-element method is developed for the problem in which the plate is of arbitrary shape. The methods are shown to produce the same solutions for a circular plate, and their convergence properties are established. The impact of porosity on the far field (the wave field away from the plate) is investigated using integral representations for the Bessel and Hankel functions. It is shown that wave-energy dissipation due to porosity initially increases as the plate becomes more porous, but reaches a maximum and then slowly decreases as the porosity increases further.

Wave interaction with a submerged semicircular porous breakwater placed on a porous seabed

Abstract: In the present study, a coupled eigenfunction expansion-boundary element method is developed and used to analyze the interaction of surface gravity waves with a submerged semicircular porous breakwater placed on a porous seabed in water of finite depth. Two separate cases: (a) wave scattering by the semicircular breakwater, and (b) wave trapping by the semicircular breakwater placed near a porous sloping seawall are studied. Further, as a special case, wave trapping by a semicircular breakwater placed on a rubble mound foundation near a sloping seawall is analyzed in water of uniform depth having an impermeable bed. The wave motion through the semicircular permeable arc of the breakwater is modeled using the Darcy's law of fine pore theory, whilst the wave motion through the porous seabed, rubble mound foundation and the porous seawall are modeled using the Sollitt and Cross model. The friction coefficient defined in Sollitt and Cross model is computed by approximating the spatial dependency of the seepage velocity with the average velocity within the porous media. An algorithm for determining the friction coefficient f is provided. Various physical quantities of interests are plotted and analyzed for various values of waves and structural parameters.