Convection in vertical Bridgman configurations

We are interested in tracing the convective profiles of vertical Bridgman growth in two configurations, the pure Rayleigh convective mode and the combined Rayleigh-Marangoni mode. In order to do so, we conducted a numerical investigation that involved a finite volume calculation. The governing equations were integrated about a cell volume, using the Gauss Theorem and the volume variables like temperature and velocity were related to the surface variables. In order to solve for the pressure field, we employed the continuity equation and the residuals resulted in a Poisson equation. Results and comments for the Rayleigh and Marangoni problems in a vertical cylinder or Bridgman configuration are given

OBSERVATIONS ON SOME MULTI-LIQUID LAYER PROBLEMS IN CONVECTION

The main result from our calculation is that the solid thickness of the lower phase does not affect the flow structure at '1 g' but does affect the flow at low gravity levels. In other words the coupling is only one-way at hi'gh gravity. We surmise that even though the solid thickness destabilizes the flow, it is insignificant compared to the overwhelming stabilization of gravity. As a result of our calculations it appears that liquid encapsulated crystal growth is better conducted under earth's gravity conditions as the configuration would be more stable. In summary, we may state that the specific mode into which the convective state settles largely depends on the type of system that is studied but we can predict the sequential change in modes as certain key parameters such as gravity level, total depths and depth ratios are changed

Margin Rules, Informed Trading in Derivatives, and Price Dynamics

We analyze the impact of option trading and margin rules on the behavior of informed traders and on the micro structure of stock and option markets. In the absence of binding margin requirements, the introduction of an options market causes informed traders to exhibit a relative trading bias towards the stock because of its greater information sensitivity. In turn, this widens the stock's bid-ask spread. But when informed traders are subject to margin requirements, their bias towards the stock is enhanced or mitigated depending on the leverage provided by the option relative to the stock, leading to wider or narrower stock bid-ask spreads. The introduction of option trading, with or without margin requirements, unambiguously improves the informational efficiency of stock prices. Margin rules improve market efficiency when stock and option margins are sufficiently large or small but not when they are of moderate size

Margin Rules, Informed Trading in Derivatives, and Price Dynamics

We analyze the impact of option trading and margin rules on the behavior of informed traders and on the microstructure of stock and option markets. In the absence of binding margin requirements, the introduction of an options market causes informed traders to exhibit a relative trading bias towards the stock because of its greater information sensitivity. In turn, this widens the stock's bid-ask spread. But when informed traders are subject to margin requirements, their bias towards the stock is enhanced or mitigated depending on the leverage provided by the option relative to the stock, leading to wider or narrower stock bid-ask spreads. The introduction of option trading, with or without margin requirements, unambiguously improves the informational efficiency of stock prices. Margin rules improve market efficiency when stock and option margins are sufficiently large or small but not when they are of moderate size

Margin Rules, Informed Trading in Derivatives and Price Dynamics

We analyze the impact of option trading and margin rules on the behavior of informed traders and on the microstructure of stock and option markets. In the absence of binding margin requirements, the introduction of an options market causes informed traders to exhibit a relative trading bias towards the stock because of its greater information sensitivity. In turn, this widens the stock's bid-ask spread. But when informed traders are subject to margin requirements, their bias towards the stock is enhanced or mitigated depending on the leverage provided by the option relative to the stock, leading to wider or narrower stock bid-ask spreads. The introduction of option trading, with or without margin requirements, unambiguously improves the informational efficiency of stock prices. Margin rules improve market efficiency when stock margins and options margins (relative to stock margins) are sufficiently large or small but not when they are of moderate size

Faraday Forcing of High-Temperature Levitated Liquid Metal Drops for the Measurement of Surface Tension

In this work, a method for the measurement of surface tension using continuous periodic forcing is presented. To reduce gravitational effects, samples are electrostatically levitated prior to forcing. The method, called Faraday forcing, is particularly well suited for fluids that require high temperature measurements such as liquid metals where conventional surface tension measurement methods are not possible. It offers distinct advantages over the conventional pulse-decay analysis method when the sample viscosity is high or the levitation feedback control system is noisy. In the current method, levitated drops are continuously translated about a mean position at a small, constant forcing amplitude over a range of frequencies. At a particular frequency in this range, the drop suddenly enters a state of resonance, which is confirmed by large executions of prolate/oblate deformations about the mean spherical shape. The arrival at this resonant condition is a signature that the parametric forcing frequency is equal to the drops natural frequency, the latter being a known function of surface tension. A description of the experimental procedure is presented. A proof of concept is given using pure Zr and a Ti(sub 39.5)Zr(sub 39.5)Ni(sub 21) alloy as examples. The results compare favorably with accepted literature values obtained using the pulse-decay method

Comments on the numerical investigation of Rayleigh and Marangoni convection in a vertical circular cylinder

Convection in a cylindrical container was simulated with a three dimensional, time-dependent code. For the case of purely Rayleigh convection, a completely rigid cylinder with adiabatic vertical walls and conducting horizontal walls was considered. The calculations showed that the individual velocity components could be in a transient state, while the total heat transfer was steady. This occurred in cases where the maximum azimuthal component of velocity was very small in magnitude, in comparison to the other two components, and this component decreased in time. On the other hand, the total kinetic energy along with the heat transfer reached a steady value. Consequently the present results have been shown to be at variance with the calculations of Neumann [I. Fluid Mech. 214, 559 (1990)]. Marangoni convection was modeled with a free flat surface on the upper side, assuming the superimposed second layer to be passive. The numerically obtained critical Marangoni numbers and flow patterns were compared favorably to earlier results from linearized stability. In addition, flow structural changes for supercritical Marangoni numbers were illustrated. Interestingly, axisymmetric disturbances led to nonsymmetric bifurcation diagrams, but three-dimensional disturbance calculations led to symmetry in the bifurcation plots. Another very interesting result was the observed transition from three-dimensional to two-dimensional patterns as the Marangoni number was increased. Large computational requirements precluded a detailed parametric study. The special case of Prandtl number equal to 6.7 (corresponding to water), and in the case of Marangoni convection, a surface Biot number of unity was assumed

Etude expérimentale et numérique de l'instabilité de Faraday entre deux liquides miscibles

Nous avons étudié l'instabilité paramétrique de Faraday dans le cas particulier d'une interface entre deux liquides miscibles. Dans cette configuration l'instabilité développée est transitoire et ses caractéristiques physiques dépendent de l'épaisseur de l'interface. Nous présentons ici ces caractéristiques et les mécanismes derrière cette instabilité grâce à une étude à la fois expérimentale et numérique