On the self-induced motion of vortex sheets

An accurate numerical scheme has been devised to study the self-induced motion of an infinitely thin, free vortex sheet of finite span in an unbounded, inviscid, incompressible fluid. The new numerical scheme has been tested against two vortex sheet problems for which exact solutions have also been obtained. The agreement between the numerical and exact solutions is excellent. The scheme has been further tested against two more examples for which analytical solutions for small times were available. Here too the agreement is excellent.     

High Reynolds number turbulent flow past a rotating cylinder

High Reynolds-number flow over a rotating cylinder is investigated by two-dimensional numerical computations. The Reynolds-Averaged Navier-Stokes (RANS) equations are solved via the finite-volume method and they are closed by a modified k-ε turbulence model. The spin ratio a is defined as the ratio between the cylinder’s circumferential speed velocity to the free-stream varies from 2 to 8. The flow is examined at Reynolds numbers from 5 × 10⁵ to 5 × 10⁶, which is considered to be an interesting range for industrial flows. Available experimental and numerical data were used to verify the validity of the implemented procedure.The results revealed stabilization of the acting forces at high spin rates, thus indicating a flowfield with suppressed vortex-shedding activity, as it is expected, in accordance with theoretical considerations in previous studies. Load coefficients were found to be inversely proportional to the Reynolds number for most of the examined rotational rates.     

Arrangement effects of triangular defects on magnetization reversal process in a permalloy dot

The arrangement effects of triangular defects on the magnetization configurations and switching process of a permalloy disk are investigated by micromagnetic simulations. For the case of one defect, the vortex is nucleated via the S state (W state) as the direction of the triangular defect is parallel (perpendicular) to the orientation of the external field. For the case of two defects, two types of switching processes are found dependent on their arrangement. For the two triangular defects with the same direction, the reversal occurs via formation, pinning, depinning and annihilation of the vortex state, however, for the two triangular defects with the opposite directions, the reversal is realized by formation and annihilation of the double-vortex state. The nucleation field for the disk with a triangular defect is more sensitive to the defect position than the case of a circular (square) defect, and it shows different variation trends for different triangular directions. The chirality of the vortex state nucleated in the reversal process can be controlled by the triangular defect.     

Vortical structures and turbulent bursts behind magnetic obstacles in transitional flow regimes

The present paper reports on numerical investigations of vortical structures in transient flow regimes generated by the local action of the Lorentz force on an electrically conductive fluid. The locally imposed non-uniform magnetic field generates similar effects as observed for flows over submerged solid obstacles. It is demonstrated that complex flow patterns can be generated by imposing magnetic fields of different strengths. The initial validation of the electromagnetically extended Navier-Stokes solver on unstructured numerical grids is performed in the low-Reynolds number range 100 ? Re ? 400 for different values of the magnetic interaction parameter. A generally good agreement is obtained in comparison with similar numerical studies of [Votyakov et al., 2007] and [Votyakov et al., 2008] for the low-Reynolds number cases. Then, a series of simulations are performed in transitional flow regimes (Re = 900) for different values of the interaction parameter (N = 3, … , 25). Simulations demonstrated the appearance of vortex-shedding phenomena similar to the flows behind solid obstacles. In contrast to the solid obstacles, the magnetic obstacles also generated the vortical flow patterns inside the magnetically affected regions. This feature can be used for the flow control of electrically conductive fluids, for efficient enhancements of the wall-heat transfer or for better mixing of passive scalars. Despite the laminar inflow conditions, turbulent bursts are observed in the magnetic wake region for the Re = 900 case. The velocity spectra and spatial distributions of the long-time averaged second-moments of the velocity field demonstrated that turbulence was locally sustained in the proximity of the magnetic wake edge.     

Digitally controlled high-precision thermostat for X-ray investigations on lyotropic and thermotropic liquid crystals

The construction of a special hot air generation system for X-ray diffraction investigations of lyotropic and thermotropic mesophases at different temperatures is described. The sample is positioned at the center of the X-ray diffractometer contained in a long cylindrical capillary, and resides at a small goniometer head used to adjust the precise position and inclination of the sample. It was decided to use air as the medium and initially the medium was intended to flow axially. A stream of air is blown from a small and high rectangular opening sideways toward the sample. The gas streaming out of the vessel has to pass through a vortex cell. The air leaving the vortex cell forms a core of whirling gas with constant temperature. A temperature sensor is mounted longitudinally above the sample and is positioned inside the same airflow core. The sensor provides the input signal for a microprocessor-based controller which regulates the power of the heating or cooling system in the inlet tube providing constant temperature in the gas core. Received: 15 January 1999 Accepted: 1 February 1999     

Some aspects of streamwise vortex behavior during oblique shock wave/vortex interaction

An experimental study of the flowfield generated by the interaction of a streamwise vortex having a strong wake-type axial Mach number profile and a two-dimensional oblique shock wave was conducted in a Mach 2.49 flow. The experiments were aimed at investigating the dynamics of supersonic vortex distortion and to study downstream behavior of a streamwise vortex during a strong shock wave/vortex encounter. The experiments involved positioning an oblique shock generator in the form of a two-dimensional wedge downstream of a semi-span, vortex generator wing section so that the wing-tip vortex interacted with the otherwise planar oblique shock wave. Planar laser sheet visualizations of the flowfield indicated an expansion of the vortex core in crossing a spherically blunt-nose shock front. The maximum vortex core diameter occurred at a distance of 12.7 mm downstream of the wedge leading edge where the vortex had a core diameter of more than double its undisturbed value. At distances further downstream the vortex core diameter remained nearly constant, while it appeared to become more diffused at distances far from the wedge leading edge. Measurements of vortex trajectory revealed that the vortex convected in the freestream direction immediately downstream of the bulged-forward shock structure, while it traveled parallel to the wedge surface at distances further downstream. The turbulent distorted vortex structure which formed as a result of the interaction, was found to be sensitive to downstream disturbances in a manner consistent with incompressible vortex breakdown. Physical arguments are presented to relate behavior of streamwise vortices during oblique and normal shock wave interactions. Received 7 September 1996 / Accepted 10 February 1998     

Temperature separation and friction losses in vortex tube

The process of energy separation and friction losses in a vortex tube is studied in detail. The hot and cold exit air temperatures were measured. Experiments have been conducted at inlet pressure of 3.5, 5, 7.5 and 9 bar, at inlet temperature of 292.15 and 298.15 K and at cold air mass ratio from 0 to1. The results demonstrate that the hot air temperature reaches its maximum value at a cold air mass ratio of nearly 0.82, while the minimum value of cold air temperature is found at a cold air mass ratio of 0.3. Based on energy and mass balances as well as on the definition of internal energy and on experimental results a new model for the determination of hot and cold exit gas temperature has been developed. The model includes the relevant primary parameters and predicts the experimental results as well as the data published in the literature sufficiently accurate for engineering purposes.A cross-section area m³ - D diameter of the pipe m - F model parameter - f friction factor - L length of the tube m - m mass flow rate kg/s - y cold air mass ratio - P static pressure Pa - T temperature K - t thickness of the orifice m - R gas constant J/kg K - v velocity of fluid m/s - density of the fluid kg/m³ - friction factor for pipe - friction factor for orifice and tee junction - 1 inlet of compressed gas - 2 exit of hot gas - 3 exit of cold gas - atm atmospheric pressure - c cold exit gas - f friction - h hot exit gas - o orifice plate - T tee junction     

Spin wave spectrum of magnetic nanotubes

We investigate the spin wave spectra associated to a vortex domain wall confined within a ferromagnetic nanotube. Basing our study upon a simple model for the energy functional we obtain the dispersion relation, the density of states and dissipation induced life-times of the spin wave excitations in presence of a magnetic domain wall. Our aim is to capture the basics spin wave physics behind the geometrical confinement of nobel magnetic textures.     

Vortex lattice ordering in the flux flow state of Nb thin films

We measure current–voltage characteristics at high driving currents for different magnetic fields and temperatures in Nb thin films of rather strong pinning. In a definite range of the B–T phase diagram we find that a current induced transition occurs in the flux flow motion of the vortex lattice, namely a dynamic ordering (DO). Contrary to the case of weaker pinning materials, DO is observed only at low fields, due to the stronger intrinsic disorder that can deform plastically the moving vortex lattice even for small applied fields.