Phase Velocity Of Wave Propagation Research Articles

Application of nonlocal elasticity theory on the wave propagation of flexoelectric functionally graded (FG) timoshenko nano-beams considering surface effects and residual surface stress

This research deals with wave propagation of the functionally graded (FG) nano-beams based on the nonlocal elasticity theory considering surface and flexoelectric effects. The FG nano-beam is resting in Winkler-Pasternak foundation. It is assumed that the material properties of the nano-beam changes continuously along the thickness direction according to simple power-law form. In order to include coupling of strain gradients and electrical polarizations in governing equations of motion, the nonlocal non-classical nano-beam model containg flexoelectric effect is used. Also, the effects of surface elasticity, di-electricity and piezoelectricity as well as bulk flexoelectricity are all taken into consideration. The governing equations of motion are derived using Hamilton principle based on first shear deformation beam theory (FSDBT) and also considering residual surface stresses. The analytical method is used to calculate phase velocity of wave propagation in FG nano-beam as well as cut-off frequency. After verification with validated reference, comprehensive numerical results are presented to investigate the influence of important parameters such as flexoelectric coefficients of the surface, bulk and residual surface stresses, Winkler and shear coefficients of foundation, power gradient index of FG material, and geometric dimensions on the wave propagation characteristics of FG nano-beam. The numerical results indicate that considering surface effects/flexoelectric property caused phase velocity increases/decreases in low wave number range, respectively. The influences of aforementioned parameters on the occurrence cut-off frequency point are very small.

Studies of the wind filtering effect of gravity waves observed at Allahabad (25.45°N, 81.85°E) in India

Well-defined coherent wave sources associated with the passage of short-period gravity waves were observed in all-sky images of OH emission on a total 21 occasions during January to May 2008 at Allahabad (25.45°N, 81.85°E, dip lat ∼16.49°N) in India. The wave motions exhibited similar spatial and temporal properties during each month, but the north-east ward motions were distinctly dominant in April and May 2008. It is a well-known theory that the upward propagating gravity waves may be blocked or absorbed at a critical layer. We have computed the magnitude and direction of atmospheric gravity waves subject to blocking by horizontal winds, i.e., critical layer directional filtering. The HWM-93 model (Hedin et al., 1996) was used to compute the two components of neutral wind velocity at Allahabad for the period of observation of gravity waves during March and April 2008. Data from two components of wind velocity were then used to construct the blocking diagrams, which show the directions and apparent phase velocities of wave propagation blocked at a given altitude. The blocking diagrams were then compared with experimental observations of gravity waves in OH airglow to determine the accuracy of the wind model and explain the critical layer theory.

Scattering of ultrasonic waves in randomly layered materials

The goal of the paper is the proposal of a new macroscopic description of scattering of elastic waves at internal inhomogeneities of the material. Within the studies the microscopic inhomogeneity of the medium is modeled as the alternately arranged two kinds of elastic layers of random thicknesses and different mechanical properties. Calculations of the acoustical characteristics of such medium (reflection and transmission coefficients) are performed in two stages: (i) first the problem of interaction of plane harmonic wave with the half space of the randomly layered medium for the case of normal wave incidence is analyzed; (ii) then the interaction of the harmonic wave with the slice composed of randomly layered structure is considered. Such approach allowed to derive analytical relations for the phase velocity of wave propagation and attenuation as the explicit functions of frequency, stochastic structural parameters of the medium and material properties of layers. The obtained formulas are helpful for interpretation of experimental ultrasonic data and may serve for identification of the characteristic size of material inhomogeneity based on the measured wave parameters like frequency dependent phase velocity, attenuation as well as backscattering coefficient.

Analysis of infinite zig-zag antenna

An approximate expression is derived for the phase velocity of wave propagation along an infinite zig-zag antenna. It is theoretically proved that that the zig-zag antenna acts as a slow wave structure. Theoretical calculations have been made for radiation patterns and characteristic impedance

A helical infinite Yagi structure

An approximate expression is derived for the phase velocity of wave propagation along an infinite Yagi structure with its conventional constituent dipoles replaced by short helices operating in their normal mode. It is theoretically predicted that the proposed infinite Yagi structure acts as a slow wave structure.

Interaction between a travelling electromagnetic fleld and a plasma

We investigate the possibility of plasma confinement by a travelling high-frequency H wave. The experiment was performed in a toroidal pulse device. The maximal value of the high-frequency magnetic field without a plasma is 230 Oe, the frequency is 1 MHz, the phase velocity of wave propagation is 5.6 × 107 cm/sec. The high-frequency field was used for both confinement and excitation of plasma.From the distribution of the concentration of charged particles (Te = 60 000° K) and the Hz component of the high-frequency field along the chamber cross section we define the gradients of plasma pressure and magnetic pressure. Since the gradients are comparable in order of magnitude, concentration distribution differed perceptively from the corresponding distribution for a low level of high-frequency power when magnetic pressure is much smaller than plasma pressure.We have studied the conditions under which plasma confinement by the field of a travelling high-frequency wave is possible, taking into account plasma heating. It is shown that the high-frequency magnetic field that is necessary for plasma confinement increases about proportionally to Te, for which also charged-particle concentration must increase proportionally to Te3/4 For example, for Te = 105 °K there must be n = l016cm−3 and Hz = 3×103 Oe (for hydrogen plasma).Consequently, it is possible to satisfy the conditions for plasma confinement only at sufficiently large n and Hz.

On the phase velocity of wave propagation along an infinite yagi structure

An approximate expression is derived for the phase velocity of wave propagation along an infinite Yagi structure and its dependence on the various parameters of the structure is discussed in detail. The structure is at first studied qualitatively by applying the transmission line analogy. The problem is next treated from the viewpoints of linear antenna and field theories. It is assumed that a traveling wave is propagating along the axial direction and it induces an axially symmetric and sinusoidal current distribution in each element The electric field at any point due to this current distribution is calculated by the Hertz vector method. After applying boundary condition to the electric field, an expression for the propagation constant is derived. The results are compared with existing experimental values. The agreement between theory and experiment is found to be within 5 per cent. The accuracy of the expression given is sufficient for practical purposes.

The Helical Antenna

The helix is a fundamental form of antenna of which loops and straight wires are limiting cases. When the helix is small compared to the wavelength, radiation is maximum normal to the helix axis. Depending on the helix geometry, the radiation may, in theory, be elliptically, plane, or circularly polarized. When the helix circumference is about 1 wavelength, radiation may be maximum in the direction of the helix axis and circularly polarized or nearly so. This mode of radiation, called the axial or beam mode, is generated in practice with great ease, and may be dominant over a wide frequency range with desirable pattern, impedance, and polarization characteristics. The radiation pattern is maintained in the axial mode over wide frequency ranges because of a natural adjustment of the phase velocity of wave propagation on the helix. The terminal impedance is relatively constant over the same frequency range because of the large initial attenuation of waves on the helix. The conditions for circular polarization are analyzed, and the importance of the array factor in determining the radiation pattern of a long helix is discussed.

Characteristics of Helical Antennas Radiating in the Axial Mode

A helix is a fundamental form of antenna with many radiation modes. A recently reported mode, called an axial or beam mode, occurs for a relatively wide range of helix dimensions, in the region of 0.2 to 0.5 wave-lengths diameter and as high as 0.5 wave-lengths spacing between turns. The radiation is maximum in the direction of the helix axis and is nearly circularly polarized. This mode may persist with a given helix over a considerable frequency range. In this range the phase velocity of wave propagation along the helical conductor is reduced. An approximate expression for the field pattern of a single turn helix is developed. The pattern of a helix of a number of turns is then calculated as an array of such turns. Measured and calculated patterns show good agreement.