On the global structure and asymptotic stability of low-stretch diffusion flame: Forced convection

The present work analyzes cylindrical diffusion flames (Tsuji burner) under low stretch condition, considering fuel injection also from the backward region of the burner. To highlight the fundamental aspects of this flame, some assumptions are imposed, like constant thermodynamic and transport coefficients, unitary Lewis number and no radiative heat loss. It is also considered potential flow model and incompressible Navier–Stokes model. Despite the simplicity of the former model, results from both models show good agreement. Also, an asymptotic analysis describing the problem far from the burner is able to capture the most important mechanisms controlling the flame, then the flame shape is determined and the dependence of the characteristic length scales on Peclet number (based on the burner properties), free stream velocity and stoichiometry is revealed. The results show that the flame width is proportional to the mass stoichiometric coefficient and reciprocal to the Peclet number the 1/4 power and free stream velocity the 3/4 power, and that the flame height is proportional to the square of the mass stoichiometric coefficient and to the square root of the ratio of Peclet number to free stream velocity. In addition, an asymptotic stability analysis reveals low-stretch flame extinction to be caused by reduction in fuel and oxidizer concentrations, which provides the range of the stoichiometric coefficient for stable regime, and at the same time the range of heat released.     

Forced convection in nanoparticles doped nematics without reorientation

The problem of forced convection in the cell of nanoparticles doped nematic liquid crystal with both boundaries being free, plane and isotherm is considered. These boundary conditions, offered by Rayleigh, allow us to get simple and exact solution for boundary-value problem, from which its most important peculiarities can be clearly seen. Particularly, there appears a possibility to induce convection without reorientation of liquid crystal director. It was shown that nanoparticles could have significant influence on the convection.     

Bursty events and incremental diffusion in a local diffusion and multi—scale convection system

A one-dimensional cellular automaton is defined without the critical gradient rule (Δh>Δh_c) which is essential to the existence of avalanches in self-organized criticality (SOC) models. Instead, only the local diffusion rule is used, however, the characteristics of SOC, such as the bursty behaviour, power-law decay in fluctuation spectra, self-similarity over a broad range of scales and long-time correlations, are still observed in these numerical experiments. This numerical model is established to suggest that the bursty events and the incremental diffusion observed universally in fusion experiments do not necessarily imply the submarginal dynamics.     

Velocity intermittency in a buoyancy subrange in a two-dimensional soap film convection experiment

In a two-dimensional soap film convection experiment, the velocity fields are found to be strongly intermittent in the buoyancy subrange, which was reported to be nonintermittent in a recent numerical simulation. The structure functions Sq(l)(= ) exhibit self-similar structures and can be described by power laws l(zetaq) for integers 8 > or = q < or = 1. By extending Kolmogorov's refined similarity hypothesis to our system, an analytical form is derived for the scaling exponent zeta(q) = q/2 + (mu/18)(3q - q2). Our measurements yield mu = 0.42, which is significantly greater than 0.2 found in high Reynolds number turbulence in wind tunnels.     

Estimation of Lagrangian parameters from a diffusion experiment

Summary Taylor's statistical theory in the limits of small and large diffusion times is used to assess the Lagrangian parameters of a local atmospheric turbulence. Experimental data, representing the disperison of tracers in the atmosphere, are examined in order to derive the values of the local turbulence parameters pertaining to the Hanford-67 (Nickola P. W., 1977) experiment. Once determined, these Lagrangian parameters allow for a renormalization of the lateral dispersion as a function of time, that exhibits data dispersion smaller by two to three orders of magnitude, as compared to that obtained by normalization through the use of corresponding Eulerian parameters. This closer packing of the experimental data allows for a more precise fitting of a theoretical curve that is basic to the determination of local Lagrangian velocity autocorrelation coefficients. The authors of this paper have agreed to not receive the proofs for correction.     

Phonon-mediated electron-spin phase diffusion in a quantum dot

An effective spin relaxation mechanism that leads to electron spin decoherence in a quantum dot is proposed. In contrast with the common calculations of spin-flip transitions between the Kramers doublets, we take into account a process of phonon-mediated fluctuation in the electron spin preces-sion and subsequent spin phase diffusion. Specifically, we consider modulations in the longitudinal g factor and hyperfine interaction induced by the phonon-assisted transitions between the lowest electronic states. Prominent differences in the temperature and magnetic field dependence between the proposed mechanism and the spin-flip transitions are expected to facilitate its experimental verification. Numerical estimation demonstrates highly efficient spin relaxation in typical semiconductor quantum dots.     

Off-diagonal geometric phase in a neutron interferometer experiment

Off-diagonal geometric phases acquired by an evolution of a 1/2-spin system have been observed by means of a polarized neutron interferometer. We have successfully measured the off-diagonal phase for noncyclic evolutions even when the diagonal geometric phase is undefined. Our data confirm theoretical predictions and the results illustrate the significance of the off-diagonal phase.     

Forced diffusion in magnetic fluids under the influence of a strong magnetic field gradient

We have investigated the forced diffusion of magnetic nanoparticles suspended in a carrier liquid under the influence of a magnetic field gradient. A cylindrical layer of the suspension was exposed to an azimuthal magnetic field with radial gradient. The radial distribution of the concentration of magnetic particles was determined for different times. The obtained experimental data are compared with a numerical solution of the diffusion equation and good agreement has been observed.     

The effect of HF field on diffusion and convection in a plasma column

A cylindrical plasma model with both axial and azimuthal steady magnetic field is used. Nonlinear excitation of time averaged electric currents by the HF field is analyzed. Ponderomotive forces of the HF field also lead to translational and rotational plasma motions. Radial plasma diffusion can be modified by the presence of the HF field at sufficiently low frequencies. The analysis can be applied to problems of anomalous transport.     

Quantum phase diffusion in a small underdamped Josephson junction

Quantum phase diffusion in a small underdamped Nb/AlO(x)/Nb junction (～0.4 μm(2)) is demonstrated in a wide temperature range of 25-140 mK where macroscopic quantum tunneling (MQT) is the dominant escape mechanism. We propose a two-step transition model to describe the switching process in which the escape rate out of the potential well and the transition rate from phase diffusion to the running state are considered. The transition rate extracted from the experimental switching current distribution follows the predicted Arrhenius law in the thermal regime but is greatly enhanced when MQT becomes dominant.     

Forced vibrations of a beam with one-sided amplitude constraint: Theory and experiment

An elastic beam with one-sided amplitude constraint subjected to periodic excitation is considered. Experimental results are obtained and compared with results given by a theoretical model based on a single mode analysis of the beam following the work of Moon and Shaw [1]. This model has been studied in some detail by the author in previous works [2–4], it is a single degree of freedom oscillator with periodic excitation and a piecewise linear restoring force. This single mode model is shown to provide good overall qualitative information about the actual physical system. It predicts the multiple subharmonic resonances, period doublings, and some chaotic regimes found experimentally.     

Phase space structure and anomalous diffusion in a rotational fluid experiment

The transport of passive scalars is considered in a model of rotating annulus experiments. The system has a chain of vortices and a jet, separated by a stochastic layer. For special values of the control parameters, the boundary of the stochastic layer can contain self-similar structures of islands with regular trajectories. Two such values are identified, with the structure being on the jet boundary and on the vortex boundary, respectively. The transport properties for both cases were studied by high-precision direct numerical integration of the equations of motion. The presence of such structures is found to significantly affect the statistical properties of the trajectories and the transport exponent. The results of the computations are compared with various theoretical models of anomalous diffusion. The particle behavior was found to depend significantly on the time scale, with different theories being applicable on different time intervals. Some regimes do not match any of the existing theories. (c) 2000 American Institute of Physics.     

Forced convection heat transfer around large two-phase bubbles condensing in an immiscible liquid

In this investigation, an equation is theoretically developed to predict the Nusselt number and hence the collapse rate of large spherical two-phase bubbles condensing in quiescent immiscible liquid. Heat transfer from the thin film of condensate and in the wake is determined. Theoretical prediction and experimental data show satisfactory agreement. A parametric study is also carried out for different parameters affecting the collapse rate.     

Forced and mixed convection heat transfer from an array of cylinders to a liquid submerged jet

The results of an experimental investigation concerning the heat transfer from three cylindrical heaters to a water jet are reported in the form of correlating equations, which express the Nusselt number versus the Reynolds, Prandtl and Grashof numbers and some dimensionless ratios characterising the configuration. As the experienced range of the thermal flux is wide (2·10⁴ ≤ q̇ ≤ 6·10⁵ W·m⁻²), the influence of the free convection, which was shown to be negligible in previous studies, is carefully investigated in the present one. This influence appears still negligible up to the maximum value of q̇ for the heater impinged by the jet; on the contrary it is remarkable for the heaters lying in its wake. Another aspect which is carefully studied is the influence both of the ratios characterising the configuration and of the impingement direction: accordingly the values of these ratios and the kind of impingement which yield the maximum Nusselt number are clearly singled out. The investigation is completed by some visualization experiments which allow us to qualitatively clarify some aspects of the interaction between the dynamic and thermal fields.     

Magnetic field diffusion during a metal-insulator phase transition

An analysis is made of the diffusion of a static (slowly varying) magnetic field in a conductor in which a first-order phase transition to the insulating state takes place under the action of the Joule heating. An investigation is made of the case of subsonic propagation of the phase boundary. A (V_1−X Cr_X)₂O₃ solid solution is analyzed as a model substance. The application of this effect in pulsed high-current circuit breakers is discussed. Zh. Tekh. Fiz. 68, 43–48 (December 1998)