19,348 research outputs found
Numerical study of surface tension driven convection in thermal magnetic fluids
Microgravity conditions pose unique challenges for fluid handling and heat transfer applications. By controlling (curtailing or augmenting) the buoyant and thermocapillary convection, the latter being the dominant convective flow in a microgravity environment, significant advantages can be achieved in space based processing. The control of this surface tension gradient driven flow is sought using a magnetic field, and the effects of these are studied computationally. A two-fluid layer system, with the lower fluid being a non-conducting ferrofluid, is considered under the influence of a horizontal temperature gradient. To capture the deformable interface, a numerical method to solve the Navier???Stokes equations, heat equations, and Maxwell???s equations was developed using a hybrid level set/ volume-of-fluid technique. The convective velocities and heat fluxes were studied under various regimes of the thermal Marangoni number Ma, the external field represented by the magnetic Bond number Bom, and various gravity levels, Fr. Regimes where the convection were either curtailed or augmented were identified. It was found that the surface force due to the step change in the magnetic permeability at the interface could be suitably utilized to control the instability at the interface.published or submitted for publicationis peer reviewe
Complete Coherent Control of a Quantum Dot Strongly Coupled to a Nanocavity
Strongly coupled quantum dot-cavity systems provide a non-linear
configuration of hybridized light-matter states with promising quantum-optical
applications. Here, we investigate the coherent interaction between strong
laser pulses and quantum dot-cavity polaritons. Resonant excitation of
polaritonic states and their interaction with phonons allow us to observe
coherent Rabi oscillations and Ramsey fringes. Furthermore, we demonstrate
complete coherent control of a quantum dot-photonic crystal cavity based
quantum-bit. By controlling the excitation power and phase in a two-pulse
excitation scheme we achieve access to the full Bloch sphere. Quantum-optical
simulations are in good agreement with our experiments and provide insight into
the decoherence mechanisms
Can the glass transition be explained without a growing static length scale?
It was recently discovered that SWAP, a Monte Carlo algorithm that involves
the exchange of pairs of particles of differing diameters, can dramatically
accelerate the equilibration of simulated supercooled liquids in regimes where
the normal dynamics is glassy. This spectacular effect was subsequently
interpreted as direct evidence against a static, cooperative explanation of the
glass transition such as the one offered by the random first-order transition
(RFOT) theory. We review several empirical facts that support the opposite
view, namely, that a local mechanism cannot explain the glass transition
phenomenology. We explain the speedup induced by SWAP within the framework of
the RFOT theory. We suggest that the efficiency of SWAP stems from a postponed
onset of glassy dynamics, which allows the efficient exploration of
configuration space even in the regime where the physical dynamics is dominated
by activated events across free-energy barriers. We describe this effect in
terms of `crumbling metastability' and use the example of nucleation to
illustrate the possibility of circumventing free-energy barriers of
thermodynamic origin by a change of the local dynamical rules.Comment: 15 pages, 3 figures; v2: improved discussions and clarification
Optical memory based on ultrafast wavelength switching in a bistable microlaser
We propose an optical memory cell based on ultrafast wavelength switching in
coupled-cavity microlasers, featuring bistability between modes separated by
several nanometers. A numerical implementation is demonstrated by simulating a
two-dimensional photonic crystal microlaser. Switching times of less than 10
ps, switching energy around 15--30 fJ and on-off contrast of more than 40 dB
are achieved. Theoretical guidelines for optimizing the performance of the
memory cell in terms of switching time and energy are drawn.Comment: to appear in Optics Letter
A numerical study of transient heat and mass transfer in crystal growth
A numerical analysis of transient heat and solute transport across a rectangular cavity is performed. Five nonlinear partial differential equations which govern the conservation of mass, momentum, energy and solute concentration related to crystal growth in solution, are simultaneously integrated by a numerical method based on the SIMPLE algorithm. Numerical results showed that the flow, temperature and solute fields are dependent on thermal and solutal Grashoff number, Prandtl number, Schmidt number and aspect ratio. The average Nusselt and Sherwood numbers evaluated at the center of the cavity decrease markedly when the solutal buoyancy force acts in the opposite direction to the thermal buoyancy force. When the solutal and thermal buoyancy forces act in the same direction, however, Sherwood number increases significantly and yet Nusselt number decreases. Overall effects of convection on the crystal growth are seen to be an enhancement of growth rate as expected but with highly nonuniform spatial growth variations
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