Debabrata Sen

Performance and validation of a coupled parallel ADCIRC–SWAN model for THANE cyclone in the Bay of Bengal

An accurate prediction of near-shore sea-state is imperative during extreme events such as cyclones required in an operational centre. The mutual interaction between physical processes such as tides, waves and currents determine the physical environment for any coastal region, and hence the need of a parallelized coupled wave and hydrodynamic model. The present study is an application of various state-of-art models such as WRF, WAM, SWAN and ADCIRC used to couple and simulate a severe cyclonic storm Thane that developed in the Bay of Bengal during December 2011. The coupled model (ADCIRC–SWAN) was run in a parallel mode on a flexible unstructured mesh. Thane had its landfall on 30 December, 2011 between Cuddalore and Pondicherry where in-situ observations were available to validate model performance. Comprehensive experiment on the impact of meteorological forcing parameters with two forecasted tracks derived from WRF model, and JTWC best track on the overall performance of coupled model was assessed. Further an extensive validation experiment was performed for significant wave heights and surface currents during Thane event. The significant wave heights measured along satellite tracks by three satellites viz; ENVISAT, JASON-1 and JASON-2, as well in-situ near-shore buoy observation off Pondicherry was used for comparison with model results. In addition, qualitative validation was performed for model computed currents with HF Radar Observation off Cuddalore during Thane event. The importance of WRF atmospheric model during cyclones and its robustness in the coupled model performance is highlighted. This study signifies the importance of coupled parallel ADCIRC–SWAN model for operational needs during extreme events in the North Indian Ocean.

Time-domain computation of large amplitude 3D ship motions with forward speed

Time-domain computations of 3D ship motions with forward speed are presented in this paper. The method of computation is based upon transient Green function. Both linear and nonlinear (large-amplitude) computations are performed where the included nonlinearities are those arising from the incident wave, but the diffraction and radiation forces are otherwise retained as linear. The incident wave can be described by any explicit nonlinear model. Computations over a variety of wave and speed parameters establish the robustness of the algorithm, which include high speed and following waves. Comparison of linear and nonlinear computations show that nonlinearities have a considerable influence on the results, particularly in predicting the instantaneous location of the hull in relation to the wave, which is crucial in determining forefoot emergence and deck wetness.

Computation of solitary waves during propagation and runup on a slope

Abstract A numerical time-simulation algorithm for analysing highly nonlinear solitary waves interacting with plane gentle and steep slopes is described by employing a mixed Eulerian–Lagrangian method. The full nonlinear free surface conditions are considered here in a Lagrangian frame of reference without any analytical approximations, and thus the method is valid for very steep waves including overturning. It is found that the runup height is crucially dependent on the wave steepness and the slope of the plane. Pressures and forces exerted on impermeable walls of different inclinations (slopes) by progressive shallow water solitary waves are studied. Strong nonlinear features in the form of pronounced double peaks are visible in the time history of pressure and force signals with increasing heights of the oncoming solitary waves. The effect of nonlinearity is less pronounced as the inclination of the wall decreases with respect to the bottom surface.

Nonlinear heave radiation forces on two-dimensional single and twin hulls

Abstract A study of nonlinear heave radiation of two-dimensional single and double hulls has been carried out in the time domain. The problem is analyzed by means of a fully nonlinear mathematical model, referred to as the mixed Eulerian–Lagrangian (MEL) model, which is based on an integral relation formulation coupled with time-integration of the nonlinear free-surface boundary conditions. The integral equation solver is based on a cubic-spline boundary-element scheme in which both potential and velocity continuity conditions can be enforced through the intersection points. The body undergoes periodic forced heave oscillation. By implementing effective wave-absorbing beaches at the two ends of the rectangular numerical tank, long-term steady-state force-histories could be achieved consistently in all computations. Results in terms of radiation forces for rectangular and triangular single- and twin-hull geometries are presented and discussed. Linear hydrodynamic forces in terms of added-mass and damping are validated for the rectangular hull. The Fourier-analyzed results reveal the extent of nonlinear (higher-order) components in the force-signals over different parameters which include the amplitudes of oscillation, hull-spacing for the twin-hulls and water depth.

Complexes of nickel(III) with biguanide and N′-amidinoisoureas

The following complexes of nickel(III) with biguanide (Hbg), N′-amidinoisourea (aiu), and O-alkyl-N′-amidinoisoureas [alkyl = Me (Hmaiu) or Et (Heaiu)] have been prepared and isolated: [NiBr2L2]Br and [NiCl2L2]X (L = Hbg, X = F or Cl); [NiL2]Cl (L = aiu); and [NiL2]2[SO4]·2H2O (L = maiu or eaiu). Oxidimetric titrations and the values of the oxidation potentials indicate that the complexes are one-electron oxidizing agents, while polarographic reduction-potential studies suggest nickel(III) complexes rather than nickel (II)-stabilized radical-ligand compounds. Magnetic-susceptibility, e.s.r., and i.r. data also support the presence of NiIII in these complexes.

Time-Domain Simulation of Motions of Large Structures in Nonlinear Waves

In this paper, we discuss development of a time-domain motion simulation method for studying the interaction of nonlinear waves with large offshore structures. The computational algorithm follows a simplified numerical wave-tank approach based upon a boundary-integral method and time-integration of boundary conditions. The simplifying approximations include linearization of the interaction hydrodynamic effects (radiation and diffraction) while the incident wave effects are considered in full. The main aim is to develop a method that will consider all important nonlinear effects associated with a large-amplitude incident wave, and yet practical enough to be applied routinely by the industry. In the time-integration of motion equations, numerical instabilities usually arise if difference rules are applied for determining pressures, due to coupling between forces and motions. To avoid this, an algorithm has been developed for the pressure evaluation. The resulting computational scheme is numerically stable for all conditions. The method can incorporate effects of other forces such as Morison forces, forces from mooring lines etc. which can be nonlinear. After providing a description of computational scheme, force and motion results for the interaction of large amplitude regular waves as well as irregular waves with two practical semisubmersible configurations are presented.Copyright

Time-domain wave diffraction of two-dimensional single and twin hulls

Abstract The nonlinear diffraction of 2D single and twin hulls are studied by employing a mixed Eulerian–Lagrangian model based on a higher-order cubic-spline boundary element solver. Two types of simulations are considered. In the first, waves are generated by a piston-type wave-maker in a rectangular tank and in the second case a nonlinear incident wave is assumed to exist in the tank in which the body is introduced. For the application of this model, the full nonlinear diffraction problem is recast in terms of a perturbation wave-field. Computations are performed for rectangular and triangular hull geometries. Computed results show significant nonlinearities, particularly in the heave force. The twin hull results show the influence of wave interference on the diffraction forces. This interference influences the surge force considerably, but heave force is less affected.

Electrophilic substitution on metal biguanides and metal amidinoureas

Electrophilic substitution (halogenation, sulphonation, and nitration) has been carried out on copper(II), nickel(II), cobalt(II), and chromium(III) biguanides. The substitution takes place on nitrogen atoms attached to the metal ions via intermediate formation of higher oxidation state metal species. Several complexes [M(dcbg)2 or 3](dcbg = 2,4-dichlorobiguanide) have been prepared and characterized by elemental analysis and magnetic and spectral studies. X-Ray investigation of [Cu(cbg)2]Cl2·2OH2(cbg = 2-chlorobiguanide) indicates the N–Cl bond distance to be 1.682 A. Each of the biguanide ligands is planar, allowing delocalization of the π-electron system in the metal biguanide ring. I.r. spectra show lowering of ν(N–C–N) in the ring system due to halogenation. Halogen substitution is also possible on bis(amidinourea)-copper(II) and -nickel(II) complexes.

A B-Spline Time Domain Solution for the Forward Speed Diffraction Problems

A B-spline based panel method is developed for the solution of the forward speed diffraction problem in time-domain. The body geometry is defined by an open uniform B-spline, and the unknown potentials and the source strengths are described by the same B-spline basis functions. The 3D potential flow boundary value problem is formulated based on a transient (time domain) Green’s function. Computed results are validated by comparing them with a wide variety of available results, including 3D numerical computations and experimental results. The present method agrees well with published results. Many of the existing 3D codes for the forward speed ship motion problem require high computing resources. The present method is however capable of producing the time simulation results over long duration using only a presently available PC, which is the main advantage of the proposed development.Copyright

Magnetic studies of copper(II) complexes of benzoic acid and substituted benzoic acids

A study of the magnetic properties (80–310 K) of the complexes [Cu3(O2CC6H4R)6]·3,6NH-od (R = H, m-Me, or o-NO2; 3,6NH-od = 3,6-diazaoctane-1,8-diamine) indicates the presence of ferromagnetic interactions. In the cases of R = H and m-Me, feeble ferromagnetism appears in the range 80–150 K. The o-nitrobenzoato-complex shows only antiferromagnetic interactions leading to diamagnetism in the 80–150 K region. Comparison of the experimental and calculated susceptibilities with the Bleaney–Bowers model with –2J= 289.2 cm–1 for R = H. 311.4 cm–1 for R =m-Me, and 265.6 cm–1 for R =o-NO2(in each case g= 2.19 and N= 60 c.g.s. units), indicates that the complexes are polymeric with three binuclear units of benzoatocopper(II) joined by two 3,6NH-od molecules. A spectral band in the 333–425 nm region indicates the presence of metal–metal interactions. Lowering of a band at 1 695cm–1 to 1 620–1 640 cm–1 is indicative of carboxylate bridging, and a band in the 800–820 cm–1 region is attributed to M–N co-ordination.