Antiparallel Flow Research Articles

Backward Guiding of Terahertz Radiation in Periodic Dielectric Waveguides

The guiding properties of periodic dielectric waveguides (PDWGs) are investigated theoretically at terahertz frequencies for both two- and three-dimensional model systems. It is shown that in a PDWG there may exist several bound modes, and among them often occur backward modes with antiparallel phase velocity and energy flow. The backward guiding behavior of the PDWG is demonstrated by its contra-directional coupling with a conventional dielectric waveguide (CDWG). For the coupler formed by a PDWG and a CDWG, the coupling direction of energy flow is selectable, since the PDWG supports either a forward or a backward mode at different frequencies.

Solitary and Shock Structures Induced by Poloidal Flow in Tokamaks

It is shown that solitary and shock structures can appear in tokamaks, when a poloidal flow velocity becomes close to e c s , where c s is a sound velocity and e is a small inverse aspect ratio. To describe asymptotic behavior of the flow velocity v p , a KdV (Korteweg de Vries)-Burgers equation with a sinusoidal force term is derived from the two-fluid MHD equations with small resistivity and parallel viscosity. A solitary flow velocity structure appears inside of the torus (at the poloidal angle θ=π) in the non-dissipative, dispersive case. The supersonic flow ( v p > e c s ) parallel to the poloidal magnetic field and the subsonic anti-parallel flow ( v p < e c s ) are stable. In both cases the solitary density perturbation is negative. For the non-dispersive, dissipative case or the one-fluid MHD limit the forced Burgers equation becomes appropriate to describe the flow dynamics. This case corresponds to an appearance of shock structure. By solving an initial value problem it is shown that the shock p...

Numerical experiments on helium flow in superconducting generators

Numerical experiments on helium flow patterns in a superconducting generator are reported. The simulations were conducted with a thermohydraulic analysis program which integrates the equations of motion for one dimensional coolant mass flow rates in a rotating channel network. The method includes frictional, buoyant, and inertial forces. Heat generation and transfer as well as thermophysical material properties are modeled realistically. As a result, a complete picture of the hydraulic and thermodynamic state in the cooling system during transients is obtained. A simplified five-channel model was investigated for the KWU/Siemens Test Rotor. The model includes a rotating supply bath and self-pumping exhaust system. A number of unexpected flow patterns detected are explained. It is concluded that in normal operation, refilling from the supply bath may occur discontinuously combined with flow oscillations in the supply channels. Also, in parallel heated channels, parallel inward flow, antiparallel flow, and flow oscillations may occur. >

Resistive tearing mode in coronal neutral sheets

The apparent stability of coronal neutral sheets with respect to the resistive tearing mode has been attributed by previous authors to the influence of a weak normal component of the confining magnetic field. To check this hypothesis a normal mode analysis is performed applying rigorously singular perturbation technique. Allowance is made for a value of the normal component which is large measured in the appropriate units deduced from the dynamics of the one-dimensional tearing mode. The structure of the eigenmodes is completely changed: the singular layer decays into a broad band of filaments with antiparallel flow directions and spatial oscillations in the perturbed current density appear. Surprisingly, the growth rate is not changed. If parameters for a typical neutral sheet in the middle corona (0.5 solar radii) are inserted, the result is that no stabilization by a normal component occurs, if the value of the growth time predicted by the one-dimensional theory is far shorter than ten minutes - independent of the values assumed for the width of the neutral sheet or the resistivity.

An Electrical Analogue for the Study of Tissue Saturation Processes with Oxygen and Inert Gases

A resistance-capacity analogue machine for the study of the saturation and desaturation of tissues with oxygen and inert gases is described. Two types of deviation from saturation time courses of exponential form are found, and the occurrence of these depends upon the blood flow rate in relation to the geometrical arrangement of capillaries in the tissue. The difference between the effects of parallel and antiparallel blood flow in adjacent capillaries upon the saturation time course is shown. The relevance of the results to the estimation of regional blood flow in tissues from saturation time courses is discussed.