Higher radial modes of azimuthal surface waves in magnetoactive cylindrical plasma waveguides

Azimuthal surface waves are eigenmodes of cylindrical plasma–dielectric–metal structures both in the presence of and without an axial static magnetic field. They are actively studied due to possible applications in plasma electronics, nanotechnologies and biomedical diagnostics. Higher radial modes are known to propagate at higher frequencies and shorter wavelengths compared to those of the zeroth mode, a feature which is of interest for practical applications. To gain the advantage of the excitation of higher radial modes of azimuthal surface waves one has first to know their dispersion properties. This paper generalizes the results of earlier papers by including a static axial magnetic field and considering the higher radial modes. The presence of the constant axial magnetic field removes the degeneracy in the wave spectrum with respect to the sign of the azimuthal wavenumber.

TERAHERTZ POLARIZATION CONTROLLER BASED ON ELECTRONIC DISPERSION CONTROL OF 2D PLASMONS

We numerically investigated the possibility of terahertz polarization controller based on electronic dispersion control of two dimensional (2D) plasmon gratings in semiconductor heterostructure material systems. Taking account of the Mikhailov's dispersive plasmonic conductivity model, the electromagnetic field emission properties of the gated 2D plasmon gratings were numerically analyzed with respect to the density (n) of electrons by using in-house Maxwell's FDTD (finite difference time domain method) simulator. When n is low under a constant drift-velocity condition, the fundamental plasmon mode is excited, being coupled with the radiative zeroth mode of transverse electric (TE) waves. When n exceeds a threshold level, the second harmonic mode of plasmon is predominantly excited, being coupled with the non-radiative first mode of TE waves. We numerically demonstrated that if a grating mesh of 2D plasmons is formed where two independent 2D plasmon gratings are combined orthogonally, the structure can act as a polarization controller by electronically controlling the two axial plasmonic dispersions.

Generalization of the RANS Equations Using Mean Modal Decomposition of the Navier Stokes Equations

A generalization in the Reynolds decomposition and averaging are proposed in this paper. The method is directly applied to the Navier Stokes (N-S) equations to construction of a generalized Reynolds Averaged Navier Stokes (RANS) equations. The formulation which is presented for the fields realized in a suitable ensemble, is based on a two part decomposition. One part is an approximate unique representation of the field and when reconstruction of the field, will repeat in all ensemble elements. The other part represents deviation of the real field from the approximate part and therefore is different in any mode and each ensemble element. The decomposition is applied in both spatial and temporal fashions. In the temporal decomposition, a system of Partial Differential Equations (PDEs) is obtained that is nonclosed, coupled and second order in space and its zeroth mode is the classical Reynolds averaged values of the field. In the spatial decomposition whereas, a first order system of nonclosed PDEs is obtained which could be seen as an alternative version of the Proper Orthogonal Decomposition (POD) or the Coherent Vortex Simulation (CVS) methods. In both fashions however, there are some terms that must be modeled just like as the classical closure problem in the RANS method. The method is applied on a one dimensional mixed random-nonrandom field and successfully extracted the coherent part of the field.

Bjerknes forces between two bubbles. Part 2. Response to an oscillatory pressure field

The motion of two gas bubbles in response to an oscillatory disturbance in the ambient pressure is studied. It is shown that the relative motion of bubbles of unequal size depends on the frequency of the disturbance. If this frequency is between the two natural frequencies for volume oscillations of the individual bubbles, the two bubbles are seen to move away from each other; otherwise attractive forces prevail. Bubbles of equal size can only attract each other, irrespective of the oscillation frequency. When the Bond number, Bo (based on the average acceleration) lies above a critical region, spherical-cap shapes appear with deformation confined on the side of the bubbles facing away from the direction of acceleration. For Bo below the critical region shape oscillations spanning the entire bubble surface take place, as a result of subharmonic resonance. The presence of the oscillatory acoustic field adds one more frequency to the system and increases the possibilities for resonance. However, only subharmonic resonance is observed because it occurs on a faster timescale, O(1/ε), where ε is the disturbance amplitude. Furthermore, among the different possible periodic variations of the volume of each bubble, the one with the smaller period determines which Legendre mode will be excited through subharmonic resonance. Spherical-cap shapes also occur on a timescale O(1/ε). When the bubbles are driven below resonance and for quite large amplitudes of the acoustic pressure, ε ≈ 0.8, a subharmonic signal at half the natural frequency of volume oscillations is obtained. This signal is primarily associated with the zeroth mode and corresponds to volume expansion followed by rapid collapse of the bubbles, a behaviour well documented in acoustic cavitation experiments.