Experimental laminar Rayleigh-Bénard convection in a cubical cavity at moderate Rayleigh and Prandtl numbers

Rayleigh-Bénard convection in a cubical cavity with adiabatic or conductive sidewalls is experimentally analyzed at moderate Rayleigh numbers (Ra ≤ 8 × 10⁴) using silicone oil (Pr=130) as the convecting fluid. Under these conditions the flow is steady and laminar. Three single-roll-type structures and an unstable toroidal roll have been observed inside the cavity with nearly adiabatic sidewalls. The sequence from the conductive state consists of a toroidal roll that evolves to a diagonally oriented single roll with increasing Rayleigh number. This diagonal roll, which is stabilized by the effect of the small but finite conductivity of the walls, shifts its axis of rotation towards to two opposite walls, and back to the diagonal orientation to allow for the increase in circulation that occurs as the Rayleigh number is further increased. Conduction at the sidewalls modifies this sequence in the sense that the two initial single rolls finally evolve into a four-roll structure. Once formed, this four-roll structure remains stable when decreasing the Rayleigh number until the initial single diagonally oriented roll is again recovered. The topology and the velocity fields of all structures, characterized with visualization and particle image velocimetry, respectively, are in good agreement with numerical results reported previously for the cavity with adiabatic walls, as well as with the numerical predictions obtained in the present study for perfectly conducting lateral walls. Received: 10 August 1998/Accepted: 1 August 2000     

Non-local symmetries of first-order equations

It is shown how one can transform scalar first-order ordinarydifferential equations which admit non-local symmetries of theexponential type to integrable equations admitting canonicalexponential non-local symmetries. As examples we invoke theAbel equation of the second kind, the Riccati equation and naturalgeneralizations of these. Moreover, our method describes howa double reduction of order for a second-order ordinary differentialequation which admits a two-dimensional Lie algebra of generatorsof point symmetries can be affected if the second-order equationis first reduced in order once by a symmetry which does notspan an ideal of the two-dimensional Lie algebra.     

Identification of vortex cores of three-dimensional large-vortical structures

In this paper, we propose a nonlocal method to identify vortex cores in three-dimensional flows as a complement to the existing list of local and nonlocal methods of the bibliography. The method is based on the vector field of the instantaneous rotation of a particle around a center. This center is defined using the Darboux vector field along the path-particle lines; the vortex core is detected using their Frenet–Serret frame. We illustrate the application of the method to identify the core of large-vortical structures in analytical and numerically simulated laminar and turbulent natural convection flows.     

Magnon excitation by spin injection in thin Fe/Cr/Fe films

The technique of Brillouin light scattering is used to observe strong excitation of magnons in antiferromagnetically coupled trilayers of Fe/Cr/Fe at room temperature. The magnons are driven out of equilibrium by a microwave current applied in the trilayer through point contacts. The magnitude of the scattering intensity is investigated as a function of the magnon wave number and applied magnetic field. Confirming recent theoretical predictions, the observations provide strong evidence of electronic spin injection in the rf driving field.     

Comparative study of turbulent mass transfer in the viscous sublayer using electrochemical method and direct numerical simulations

In this study we present, analyze and compare the power spectral density of the wall shear stress in a turbulent plane channel flow obtained with different techniques. Experimentally the instantaneous wall shear stress was measured with the electrochemical technique using different probes, which give approximately the same results after applying the transfer function for correction of the probe??s inertia. Numerically, the time evolution of the wall shear stress has been determined using direct numerical simulations (DNS) and large eddy simulations (LES). The results of DNS are in a good agreement with the electrochemical flow measurements. However the power spectra of the wall shear stress obtained with LES shows deviations with respect to DNS at high frequencies because of the spatial filtering inherent to the LES technique.     

Effect of localized traps on the anomalous behavior of the transconductance in nanocrystalline TFTs

It has been reported that nanocrystalline and microcrystalline devices show an anomalous behavior in the transconductance where several rates of increase of the transconductance with applied gate voltage, not present in amorphous TFTs are observed. In this paper we show that the anomalous effect of the transconductance is observed for an acceptor tail states activation energy similar to the normal values for hydrogenated silicon amorphous devices, (a-Si:H), provided that some conditions are met regarding the density of trapped charge in tail and deep states and the density of free charge in the material, which does not necessarily suggest a behavior in between amorphous and polycrystalline. The effect appears if the density of deep tail states, is smaller (higher) than the typical values in a-Si:H. The localized state distribution present in a nanocrystalline TFT prepared by hot wire deposition technique is estimated by comparison of experimental and simulated transconductance curves. In our case a lower density of deep states is obtained, which corresponds with their better light and bias stability.