Numerical investigation of cross-flow separation in three-dimensional supersonic flows around circular cones

The problem of supersonic three-dimensional flow over sharp cones is solved using the system of Navier-Stokes equations in a locally conical approximation. Numerical solutions are found using an implicit finite-difference scheme with second-order exponential approximation of the equations with respect to the cross-flow coordinate. The appearance and development of cross-flow separation on the leeward side of the cones in a laminar regime of gas flow in the shock layer are investigated. Zh. Tekh. Fiz. 68, 20–26 (October 1998)     

Poisson bracket scheme for vortex dynamics in superfluids and superconductors and the effect of the band structure of the crystal

Poisson brackets for the Hamiltonian dynamics of vortices are discussed for 3 regimes, in which the dissipation can be neglected and the vortex dynamics is reversible: (i) The superclean regime, in which the spectral flow is suppressed. (ii) The regime in which the fermions are pinned by the crystal lattice. This includes the regime of extreme spectral flow of fermions in the vortex core: these fermions are effectively pinned by the normal component. (iii) The case when the vortices are strongly pinned by the normal component. All these limits are described by the single parameter C ₀, the physical meaning of which is discussed for superconductors containing several bands of electrons and holes. The effect of the topology of the Fermi surface on the vortex dynamics is also discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 11, 794–800 (10 December 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Breakdown of the linear current flow regime in periodic structures

It is shown that the linear current flow regime in two-dimensional, two-component textures is unstable for some critical values of the ratio of the linear conductivities of the components. The critical current and critical field are studied as functions of the parameters of the system. The possibility of observing the predicted effect experimentally is discussed. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 495–499 (10 October 1996)     

Cylindrical MHD induction device in the “ideal pressure source” regime

The causes for the onset of a specific operating regime of MHD induction devices which arises after a loss of stability of the uniform flow are investigated. A modification of the original method of studying singularly perturbed dynamical systems in the neighborhood of a bifurcation point is used to construct the asymptotic behavior of the characteristic pressure for various limiting cases. Zh. Tekh. Fiz. 67, 5–9 (June 1997)     

Poiseuille problem for an ellipsoidal-statistical equation and nearly specular boundary conditions

An analytical solution of the Poiseuille problem is obtained over a wide range of Knudsen numbers for the case when the tangential momentum accommodation coefficients of the channel walls are much less than unity. An expression for the mass flux is derived that is valid for Knudsen numbers much smaller than the reciprocal of the accommodation coefficients. A new intermediate flow regime, for which the mass flux is given by an expression that differs from the classical (macroscopic) form, is found to exist. Zh. Tekh. Fiz. 68, 27–31 (November 1998)     

Experiments to Explore the Mechanisms of Heat Transfer in Nanocrystalline Alumina/Water Nanofluid under Laminar and Turbulent Flow Conditions

Abstract

Heat transfer characteristics of water-based nanocrystalline alumina (Al₂O₃) nanofluids flowing through a uniformly heated tube under a fully developed laminar and turbulent flow regime is investigated experimentally in the present work to explore the heat transfer mechanism in nanofluids. In a laminar flow, the increase in Nusselt number was attributed to the thermophysical properties of the nanofluid. The movement of nanoparticles, along with the turbulent eddies in the turbulent core region and diffusion mechanism, such as thermophoresis, in the laminar sublayer are believed to be the reasons for enhanced heat transfer in turbulent region. The compatibility of Al₂O₃/water nanofluids was also examined by monitoring its color.     

Towards a classification of models for the numerical simulation of premixed combustion based on a generalized regime diagram

The inner structure, and the physical behaviour of turbulent premixed flames are usually described, and classified by means of the regime diagram introduced by Borghi and Peters. Thereby properties related to both the flame and the (turbulent) flow are considered. In this work a diagram valid for all physical regimes, comprising suitable requirements for laminar simulations, direct numerical simulation (DNS), large-eddy simulation (LES), and Reynolds averaging based numerical simulation (RANS) is proposed. In particular the diagram describes essential situations within the validity limits of the “Borghi, Peters diagram” which physical phenomena are resolved by the simulation, and which have to be modelled. This information is used for systematic classification of various models by suggesting specific models that are appropriate depending on the regime and numerical resolution, and may provide guidance for numerical simulation methods and model development in turbulent premixed combustion. This might help users as a guideline in choosing appropriate models for a given device, and numerical effort available. The regime diagram suggested by Pitsch and Duchamp de Lageneste, which includes DNS and LES by explicitely accounting for the numerical related variable filterwidth, emerges here as one of the special two-dimensional cases possible. In contrast to the generalized regime diagram, their diagram does not include laminar simulations, and RANS based considerations, while transition between wrinkled and corrugated flamelets is not clearly established.     

Composite spin-triplet superconductivity in an symmetric lattice model

The two-channel Anderson lattice model which has SU (2) ⊗ SU (2) symmetry is of relevance to understanding of the magnetic, quadrupolar and superconducting phases in U_1-xTh_xBe₁₃ or Pr based skutterudite compounds such as PrFe₄P₁₂ or PrOs₄Sb₁₂. Possible unconventional superconducting phases of the model are explored. They are characterized by a composite order parameter comprising of a local magnetic or quadrupolar moment and a triplet conduction electron Cooper-pair. This binding of local degrees of freedom removes the entropy of the non Fermi-liquid normal state. We find superconducting transitions in the intermediate valence regime which are suppressed in the stable moment regime. The gap function is non analytic and odd in frequency: a pseudo-gap develops in the conduction electron density of states which vanishes as |ω| close to ω = 0. In the strong intermediate valent regime, the gap function acquires an additional -dependence. Received 28 February 2002 / Received in final form 18 April 2002 Published online 9 July 2002     

Correlation between flow characteristics and interfacial friction behaviour of a Zr-based metallic glass during micro-extrusion

The interfacial friction behaviour of Zr₃₅Ti₃₀Be_26.75Cu_8.25 metallic glass during micro-extrusion was investigated at various strain rates and temperatures in the supercooled liquid region. A friction mechanism map that distributes adhesion regime, furrow regime and mechanical engagement regime was constructed. These regimes respectively correspond to Newtonian flow, non-Newtonian flow and inhomogeneous flow by comparing with the typical deformation map. The correlation between flow characteristics and interfacial friction behaviour is well analysed by combining the viscosity theory with the finite-element simulations.     

Properties of IR-active lattice vibrations in the vicinity of kinks in the Frenkel-Kontorova model

The properties of infrared lattice vibrations in the presence of kinks in the Frenkel-Kontorova model are analyzed. Our results show that the vibration of particles involved in kink formation is very similar to that in a gap mode around a force-constant defect. We found that the IR phonon mode intensity possesses a universal dependence on a certain combination of system parameters and kink concentration. On the basis of these results a criterion is proposed for separating the regime of weakly interacting kinks in the system from the regime of a kink lattice. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 10, 750–754 (25 May 1997) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Localization effects in the charge density wave state [4]of the quasi-two-dimensional monophosphate tungsten bronzes {(P{O_2})_4}{(W{O_3})_{2m}}(m = 7,8,9)

The monophosphate tungsten bronzes (PO ₂ ) ₄ (WO ₃ ) _2m are quasi-two-dimensional conductors which show charge density wave type electronic instabilities. We report electrical resistivity and magnetoresistance measurements down to 0.30 K and in magnetic fields up to 16 T for the m = 7, 8 and 9 members of this family. We show that these compounds exhibit at low temperature an upturn of resistivity and field dependences of the magnetoresistance characteristic of localization effects. We discuss the dimensionality of the regime of localization as m is varied. We show that for m =7, the regime is quasi-two-dimensional and three-dimensional for m = 8, 9. Received 16 September 1999     

Measurement of the electron-atom collision integral in low-temperature helium plasma

An expression is obtained for the electron-atom collision integral for axisymmetric low-temperature plasmas. The Legendre components of the collision integral are determined experimentally by the probe method. Comparison of the measured Legendre components with their theoretical values shows that, depending on the plasma conditions, the collisional regime or a regime of dominant collective interactions is established in the system. Zh. Tekh. Fiz. 67, 19–24 (April 1997)     

Elastic waves in non-Newtonian (Maxwell) fluid-saturated porous media

Abstract

This paper studies the elastic waves in non-Newtonian (Maxwell) fluid-saturated porous media with the nonzero boundary slip velocity for pore size distribution. The coefficient bF _m (ω) that measures the deviation from Poiseuille flow friction in such media is presented. Based on this coefficient, we investigate the properties of elastic waves by calculating their phase velocities and attenuation coefficients as functions of frequency and the behaviour of the dynamic permeability. The study shows that the pore size distribution removes oscillations in all physical quantities in the non-Newtonian regime. Consideration of the nonzero boundary slip effect in non-Newtonian (Maxwell) fluid-saturated porous media results in (a) an overall increase of the dynamic permeability, (b) an increase of phase velocities of fast Biot waves and shear waves except in the low frequency domain and an overall increase of phase velocity of slow Biot waves and (c) an overall increase of the attenuation of three Biot waves in the intermediate frequency domain except in the deeply non-Newtonian regime. The study also shows that the attenuation coefficient of slow Biot waves is small in the deeply non-Newtonian regime at higher frequency, which encourages us to detect slow Biot waves in oil-saturated porous rock.     

Heat Transfers in Tubes Fitted with Single,Twin, and Triple Twisted Tapes

This experimental study performs heat transfer and pressure drop measurements in three test tubes fitted with single, twin, and triple twisted-tapes with Reynolds number (Re) varying from 1,500 to 14,000. Each element of these continuous twisted-tapes that formulates the single, twin, and triple twisted-tapes has identical width, pitch, and thickness. Heat-transfer augmentations from the plain-tube conditions in three test tubes decrease with the increase of Re, while the descending rate decreases with the increase of twisted-tape in the tube. These twisted-tapes generate the more effective heat transfer enhancements in laminar flow regime than those developed in turbulent flow regime. With 3,000 ≤ Re < 14,000, the local Nusselt numbers in the tubes fitted with single, twin, and triple twisted-tapes were, respectively, 1.5–2.3, 1.98–2.8, and 2.86–3.76 times of the Dittus-Boelter levels. The Fanning friction factors in three test tubes decrease with the increase of Reynolds number. Based on the same pumping power consumption, the tube fitted with single and triple twisted-tapes possess the highest performance factors for laminar and turbulent flows, respectively. Experimental correlations of heat transfer and Fanning friction factor were derived for three tubes fitted with single, twin, and triple twisted-tapes.     

Coupled-mode theory of a fiber laser-amplifier

Abstract

The behavior of a fiber laser-amplifier is considered, including the effects of mode coupling, radiation losses, and noise. In the linear regime, a competitive effect between mode coupling and losses is present, in the sense that mode coupling tends to equalize the gains of different modes, while radiation losses favor low-order modes. Thus, single-mode amplification and collective amplification are predominant, respectively, in the weak and strong coupling case. In the saturated regime a mode coupling effect is found that is independent of fiber imperfections and relies on the nonlinear nature of the process.     

Dynamics of the tunneling contact in a tunneling microscope during a pulsed increase in the voltage between the tip and sample

The current flow regime between the tip and the sample during the supply of voltage pulses of various duration, amplitude, shape, and polarity is investigated experimentally. Solid silver and films of other materials of nanometer thickness are employed as samples. It is shown that the tunneling gap decreases as the voltage across the tunneling contact increases due to deformation of the tip and the sample until they come into mechanical contact. The resistance of the contact is measured for various samples. Various mechanisms of deformation of the tip and the sample are considered. It is concluded on the basis of estimates that solid metals are deformed mainly as a result of thermal expansion. Experimental current-voltage characteristics are compared with theoretical plots for the case of field emission. Zh. Tekh. Fiz. 68, 104–109 (February 1998)     

Parametric number variance of disordered systems in the multifractal regime

Abstract

We study the effect of an external field X on the energy levels of a disordered system by evaluating the parametric number variance (PNV). The weak disorder regime is studied within the Gaussian random matrix theory, while the multifractal regime is studied by considering the q-deformed random matrices. The PNV at both small and large values of X has distinct features in the weak disorder and multifractal regimes that should be observable in numerical studies.     

Vortex dynamics of a trapezoidal bluff body placed inside a circular pipe

The influence of Reynolds number and blockage ratio on the vortex dynamics of a trapezoidal bluff body placed inside a circular pipe is studied experimentally and numerically. Low aspect ratio, high blockage ratio, curved end conditions (junction of pipe and bluff body), axisymmetric upstream flow with shear and turbulence are some of the intrinsic features of this class of bluff body flows which have been scarcely addressed in the literature. A large range (200:200,000) of Reynolds number (Re_D) is covered in this study, encompassing all the three pipe flow regimes (laminar, transition and turbulent). Four different flow regimes are defined based on the distinct features of Strouhal number (St)–Re_D relation: steady, laminar irregular, transition and turbulent. The wake in the steady regime is stationary with no oscillations in the shear layer. The laminar regime is termed as irregular owing to irregular vortex shedding. The vortex shedding in this regime is observed to be symmetric. The emergence of separation bubble downstream of the bluff body on either side is another interesting feature of this regime, which is further observed to be symmetric. Two pairs of mean streamwise vortices are noticed in the near-wake regime, which are termed as reverse dipole-type wake topology. Beyond the irregular laminar regime, the Strouhal number falls gradually and vortex shedding becomes more periodic. This regime is named transition and occurs close to the Reynolds number at which transition to turbulence takes place in a fully developed pipe. The turbulent regime is characterised by a nearly constant Strouhal number. Typical Karman-type vortex shedding is noticed in this regime. The convection velocity, wake width formation length and irrecoverable pressure loss are quantified to highlight the influence of blockage ratio. These results will be useful to develop basic understanding of vortex dynamics of confined bluff body flow for several practical applications.     

Energy balance in the tip-sample system of a tunneling microscope in the surface modification regime

An analysis is made of the contributions of various energy release mechanisms in the tip of a scanning tunneling microscope under conditions typical of the local modification of the surface of a solid with allowance for the specific characteristics of the microscope geometry. It is shown that the dominant heating mechanism depends on the parameters of the material and the current flow regime. The solution of the exact problem yields a formula to estimate the temperature of a tip having a model profile in the form of a paraboloid of revolution. Known solutions of problems in fluctuation electrodynamics are used to derive an expression for the rate of energy dissipation of the thermal electromagnetic field of the half space at the apex of a tunneling microscope tip. Zh. Tekh. Fiz. 67, 70–76 (November 1997)     

Lagrangian dynamics of drift-wave turbulence

The statistical properties of Lagrangian particle transport are investigated in dissipative drift-wave turbulence modelled by the Hasegawa-Wakatani system. By varying the adiabaticity parameter c, the flow regime can be modified from a hydrodynamic limit for c=0 to a geostrophic limit for c→∞. For c of order unity the quasi-adiabatic regime is obtained, which might be relevant to describe the edge turbulence of fusion plasmas in tokamaks. This particularity of the model allows one to study the change in dynamics when varying from one turbulent flow regime to another. By means of direct numerical simulation we consider four values for c and show that the Lagrangian dynamics is most intermittent in the hydrodynamic regime, while the other regimes are not or only weakly intermittent. In both quasi-adiabatic and quasi-geostrophic regimes the PDFs of acceleration exhibit exponential tails. This behaviour is due to the pressure term in the acceleration and not a signature of intermittency.