On the applicability of low-dimensional models for convective flow reversals at extreme Prandtl numbers

Constructing simpler models, either stochastic or deterministic, for exploring the phenomenon of flow reversals in fluid systems is in vogue across disciplines. Using direct numerical simulations and nonlinear time series analysis, we illustrate that the basic nature of flow reversals in convecting fluids can depend on the dimensionless parameters describing the system. Specifically, we find evidence of low-dimensional behavior in flow reversals occurring at zero Prandtl number, whereas we fail to find such signatures for reversals at infinite Prandtl number. Thus, even in a single system, as one varies the system parameters, one can encounter reversals that are fundamentally different in nature. Consequently, we conclude that a single general low-dimensional deterministic model cannot faithfully characterize flow reversals for every set of parameter values.     

微通道换热器流动和传热特性的研究

通过对微通道换热器流动和传热特性的研究,设计了实验方案并建立了相应的实验装置,结合流动、传热特性的相关准则,得出了雷诺数Re-摩擦系数f,雷诺数Re、普郎特数Pr-努谢尔特数Nu间关系的实验模型,并对该模型进行了分析。     

Poiseuille flow of phonons in solid hydrogen

The thermal conductivity of solid parahydrogen is investigated using the stationary method with a plane sample in the temperature range 1.5–6.0 K in order to reveal a Poiseuille flow in solid hydrogen. It is established that the thermal conductivity at temperatures below the low-temperature maximum decreases very rapidly in accordance with the law K ～ T ⁿ (3 < n < 8). This finding is a direct indication that the possibility exists of observing a Poiseuille flow in solid hydrogen. The results obtained are compared with those for solid helium, in which the Poiseuille flow was observed for the first time in dielectric solids. According to the estimates, the mean free path of phonons at a temperature of approximately 3 K exceeds the radius of a cylindrical sample (3 mm). The thermal conductivity in the vicinity of the low-temperature maximum is found to be two times higher than the value available in the literature.     

Shape diagram of vesicles in Poiseuille flow

Soft bodies flowing in a channel often exhibit parachutelike shapes usually attributed to an increase of hydrodynamic constraint (viscous stress and/or confinement). We show that the presence of a fluid membrane leads to the reverse phenomenon and build a phase diagram of shapes-which are classified as bullet, croissant, and parachute-in channels of varying aspect ratio. Unexpectedly, shapes are relatively wider in the narrowest direction of the channel. We highlight the role of flow patterns on the membrane in this response to the asymmetry of stress distribution.     

The effect of a magnetic field on the Knudsen gas flow

The effect of a magnetic field on the Knudsen gas flow in a flat channel has been considered. The effect of the change of the channel resistance in the field is due to the polarization of molecules when they collide with the surface and to the destruction of this polarization in the magnetic field.     

On the temperature behavior of second sound and Poiseuille flow

The linearized Peierls equation for the phonon densityN (k λ,r t) is solved by replacing the collision operator in the subspace orthogonal to the collision invariants byk-dependent relaxation rates. For the normal process relaxation time the behaviorτ _N (k λ)∝|k|^?p for smallk is assumed. Taking into account thisk-dependence ofτ _N explicitly and avoiding an expansion with respect toΩτ _N (kλ) before performing the necessary integration overk yields new, non-analytic, terms in the hydrodynamic equations describing second sound and Poiseuille flow. It is shown that this may lead to a temperature dependence of second sound damping and thermal conductivity in the Poiseuille flow region differing from the usual theoretical predictions and in better agreement with experiments.     

Magic numbers of atoms in surface-supported planar clusters

Surface-supported planar clusters can sprout active research and create numerous applications in the realm of nanotechnology. Exploitation of these clusters will be more extended if their properties on a supported substrate are thoroughly apprehended, and if they can be fabricated in a controllable way. Here we report finding the magic numbers in two-dimensional Ag clusters grown on Pb quantum islands. We demonstrate, with the images and energy spectra of atomic precision, the transition from electronic origin to a geometric one within the same system. Applying the magic nature, we can also produce a large array of planar clusters with well-defined sizes and shapes.     

Magnetic stabilization,transition and energy analysis in the Marangoni driven Full-Zone at low Prandtl numbers

The linear stability of the Marangoni-driven Full-Zone is investigated for low Prandtl number fluids. A constant and uniform magnetic field is applied along the axial axis of the liquid bridge to stabilize the axisymmetric base state. Dramatic contraction of the flow circulation in both radial and azimuthal directions is observed with moderate magnetic fields. The numerical solution utilizes a vorticity transport formulation and high resolution spectral collocation scheme with Chebyshev polynomial basis functions. Critical transitions to three-dimensional, stationary flows are observed up to Ha = 300 for Pr = 0.02 and Ha = 500 for Pr = 0.001. A hydrodynamically driven instability is suggested by the perturbation flows and confirmed through an energy analysis.     

Plane Poiseuille flow for arbitrary accommodation of the tangential momentum

Using the Bubnov-Galerkin method, the problem of plane Poiseuille flow with any arbitrary Knudsen numbers is solved for the linearized BGK model. The boundary conditions assume accommodation of the tangential momentum of the gas molecules on the surface, In the case of the purely diffuse scattering of the molecules by the surface, the results so obtained agree closely (%) with those derived from the numerical method of Cherchin'yan. The method used in this paper may be employed for solving a wide circle of boundary problems in the dynamics of rarefied gases.Translated from Izvestiya VUZ. Fizika, No. 6, pp. 107–110, June, 1973.     

Formation and synchronization of autocatalytic noise-sustained structures under Poiseuille flow

The formation and synchronization of 2D noise-sustained structures are investigated for Gray–Scott kinetics in packed-bed reactors under Poiseuille flows, when identical systems are submitted to independent spatiotemporal Gaussian white noise sources. A finite-wavelength instability is theoretically predicted and numerically confirmed for uncoupled reactors. In particular, noise-sustained structures that flow with viscous boundary conditions are numerically observed above threshold. When the systems are coupled in master–slave configuration, the numerical simulations show that the slave system replicates to a very high degree of precision the convective patterns arising in the master one due to the selective amplification of noise. The nature of the synchronization and the stability of the synchronization manifold are elucidated.     

Migration of active filaments under Poiseuille flow in a microcapillary tube

The European Physical Journal E - The buckling and twisting of slender, elastic fibers is a deep and well-studied field. A slender elastic rod that is twisted with respect to a fixed end will...     

Experimental determination of angular momentum polarizations produced in a Knudsen gas flow

The influence of a magnetic field on a Knudsen flow of N₂ between parallel gold surfaces was measured. Changes in drag of order 10^?5 were observed. The field orientation dependence of the effect unexpectedly shows that gas-surface scattering predominantly produces angular momentum polarization of first rank.     

Effect of an axial magnetic field on the Poiseuille flow of a nematic liquid crystal

The effect of an axial magnetic field on the Poiseuille flow of nematicp-azoxyanisole (PAA) has been computed using the Ericksen-Leslie continuum theory. The apparent viscosity decreases appreciably in the presence of the magnetic field. Orientation and velocity profiles for different shear rates and magnetic fields are presented.     

Density waves and the effect of wall roughness in granular Poiseuille flow: Simulationand linear stability

The formation of density waves and the effect of wall roughness on them are studied using molecular dynamics simulations of gravity-driven granular Poiseuille flow. Three basic types of structures are found in moderately dense flows: a plug, a sinuous wave and a slug; a new varicose wave mode has been identified in dense flows with channels of large widths at moderate dissipations; only clump-like structures appear in dilute flows. The simulation results are contrasted with the predictions of a linear stability analysis of the kinetic-theory continuum equations for granular Poiseuille flow. The theoretical predictions on the form of density waves are in qualitative agreement with simulations in denser flows, however, there are discrepancies between simulation and theory in dilute flows.