951,400 research outputs found

    Semiconvection: numerical simulations

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    A grid of numerical simulations of double-diffusive convection is presented for the astrophysical case where viscosity (Prandtl number Pr) and solute diffusivity (Lewis number Le) are much smaller than the thermal diffusivity. As in laboratory and geophysical cases convection takes place in a layered form. The proper translation between subsonic flows in a stellar interior and an incompressible (Boussinesq) fluid is given, and the validity of the Boussinesq approximation for the semiconvection problem is checked by comparison with fully compressible simulations. The predictions of a simplified theory of mixing in semiconvection given in a companion paper are tested against the numerical results, and used to extrapolate these to astrophysical conditions. The predicted effective He-diffusion coefficient is nearly independent of the double-diffusive layering thickness dd. For a fiducial main sequence model (15 MM_\odot) the inferred mixing time scale is of the order 101010^{10} yr. An estimate for the secular increase of dd during the semiconvective phase is given. It can potentially reach a significant fraction of a pressure scale height.Comment: arXiv admin note: substantial text overlap with arXiv:1012.585

    Numerical simulations of sunspots

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    The origin, structure and evolution of sunspots are investigated using a numerical model. The compressible MHD equations are solved with physical parameter values that approximate the top layer of the solar convection zone. A three dimensional (3D) numerical code is used to solve the set of equations in cylindrical geometry, with the numerical domain in the form of a wedge. The linear evolution of the 3D solution is studied by perturbing an axisymmetric solution in the azimuthal direction. Steady and oscillating linear modes are obtained

    Numerical simulations of effects of faults in a vertical axis wind turbine’s performance.

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    Renewable sources of energy are being developed globally to overcome the present excessive dependence on fossil fuels. Wind energy is one of the important sources of renewable energy. Considerable amount of research is being carried out on the innovative designs for optimal performance of wind turbines. Furthermore a lot of research is being carried out on maintenance and condition monitoring of such systems. Torque output is one of the most important parameters in analysing the performance of a turbine; which in turn depends on a number of factors including the structural health and the performance of each blade. Cracks in a wind turbine blade affect the aerodynamic profile of the blade and consequently flow field around it, and may cause vibration in the blade further affecting its performance. In this paper Computational Fluid Dynamics (CFD) based technique has been used to study the effect of the presence of cracks in the blades on the torque output of Vertical axis wind turbine (VAWT). For this purpose, different cracks configurations have been simulated and results analysed which indicate variations in the amplitude of the torque output of the turbine due to the presence of cracks

    Numerical Simulations of Indian Ocean Tsunami by Tuna-m2

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    The Sumatra-Andaman earthquake of magnitude 9.3 on the Richter scale occurred on 26 December 2004. It triggered off a series of tsunami waves that caused tremendous damage to the properties and lives along the affected coastal areas. The earthquake was located where the India Plate dives under the Burma Plate, and was extremely large in geographical extent, beginning off the coast of Aceh and proceeding northwesterly over a period of about 100 seconds. An estimated fault length is about 800 km, with a fault width of about 85 km and an initial vertical displacement of 11 m. There were no tsunami warning systems in the Indian Ocean to detect tsunamis, nor to warn the general populace living around the ocean. Thus, there is a need for early warning systems to predict the characteristics of tsunami propagation, including tsunami wave heights and arrival times. There are three phases of tsunami evolution, which are generation, propagation and runup. Tsunami is generated by the disturbance associated with seismic activity, explosive volcanism, and submarine landslide phenomena. Propagation of tsunami waves transports seismic energy away from the earthquake source. During the deep ocean propagation stage, the wave height is small compared to the wavelength and the ocean depth. Therefore, the linear wave theory can be applied. Tsunami runup is the most destructive phase of tsunami evolution. The wave behavior at the shoreline depends on such characteristics as the relationships between wavelength and water depth and between the wavelength of the wave and its height. This paper will present the simulations of these tsunami propagations in the Indian Ocean and discuss wave height characteristics near the coast of Sri Lanka, Bangladesh and India to highlight tsunami hazards and coastal vulnerability. The need for an early warning system in the Indian Ocean would appear urgent. The simulation is performed by means of an in-house tsunami numerical simulation model TUNA-M2 that solves the shallow water equation by the staggered finite difference method

    Numerical simulations of generic singuarities

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    Numerical simulations of the approach to the singularity in vacuum spacetimes are presented here. The spacetimes examined have no symmetries and can be regarded as representing the general behavior of singularities. It is found that the singularity is spacelike and that as it is approached, the spacetime dynamics becomes local and oscillatory.Comment: typos correcte

    Numerical simulations of rotating sunspots

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    A numerical model of idealized, axisymmetric, rotating sunspots is presented. The model contains a compressible plasma described by the nonlinear MHD equations, with density and temperature gradients simulating the upper layer of the sun’s convection zone. The solution forms a central flux tube in the cylindrical numerical domain, with convection cells pushing the magnetic field to the axis. When the numerical domain is rotated with a constant angular velocity, the umbra rotates as a rigid body while the surrounding convection cells show a swirling, vortical flow. As a result, the azimuthal velocity and magnetic field have their maximum values close to the flux tube, inside the innermost convection cell