THE SURFACE IMPEDANCE OF THE "THIN ICE - SEA" AND "THICK ICE - SEA" STRUCTURES IN VLF-VHF RADIO WAVE BANDS

To calculate the propagation of the electromagnetic field over the two-layer struc-ture "ice - sea", the areas of applicability of the impedance boundary conditions are determined depending on the thickness of the ice and the angle of incidence of the vertically polarized wave in the range of 0.01-420 MHz. The article examines the structures "thin ice - sea" and "thick ice - sea" (six models of ice cover). The ice thickness varied from 0.03 to 9 m. The results are needed to calculate the atten-uation function and the field level in the VLF, LF, MF, HF, VHF radio wave bands. It is shown that for the structure “ice - sea” it is important to take into account the dependence of the surface impedance on the angle of incidence for over-the-horizon radar systems in the HF radio wave range. In the VHF range the frequency bands are determined for ice 80 cm thick, in which the impedance boundary condi-tions are applicable.

On the Existence, Uniqueness and Application of the Finite Difference Method for Solving Robin Elliptic Boundary Value Problem

This paper refers to mathematical modelling and numerical analysis. The analysis to be presented through this paper deals with Robin’s problem which boundary equation is a linear combination of Dirichlet and Neumann-type boundary condi-tions. For this purpose we proved the existence and uniqueness of the solution. It is worth noting that the implementation of numerical simulations depends on the type of problem since it requires a search for explicit solution. Consequently, the motivation exists in this paper for choosing a classical method of variation of constants and employing a finite difference method to find the exact and numerical solutions, respectively so that numerical simulations were implemented in Scilab.

NUMERICAL CALCULATION OF CONVECTIVE FLOW OF GAS AT CIRCULAR-TYPE SCHEME OF HEATING

To investigate the convective flows of polytropic gas a complete system of Navier - Stokes equations is consid-ered. As the initial and boundary conditions the specific ratios are offered. The proposed initial and boundary condi-tions realization is carried out at construction of the numerical solution of the complete system of Navier - Stokes equations for modeling the unsteady state three-dimensional convection flows of the compressible viscous heat-conducting gas in the isolated cubic area. Three components of the velocity vector are calculated for the initial stage of the convective flow. It is shown that the velocity components are complex and depend essentially on the heating shape, height and time.

Natural Frequencies and Mode Shapes of Laminated Composite Plates: Experiments and FEA

The natural frequencies and mode shapes of several graphite-epoxy plates were determined using experimental modal analysis and finite-element analysis. The experimental and theoretical results are com pared. The samples tested included four types of layups: ±15°, ±30°, cross-ply, and quasi-isotropic plates. Each plate was tested in three configurations: free-hanging, cantilever, and fixed-fixed for a total of twelve test configurations. The material properties of the plates and the test methods used to obtain them are in cluded. There is a very good agreement between the experimental and theoretical results for the free-hanging and cantilever configurations. The agreement for the fixed-fixed results is poor. This indicates that the clamps for the fixed-fixed configuralion are not ideal and that they introduce some uncertainty in the boundary condi tions. The free-hanging results provide accurate experimental natural frequencies of several composite plates; they can be used to validate future theoretical developments. The fixed-fixed results are used to provide pos sible explanations for the discrepancies between the measured and calculated natural frequencies previously reported in the literature.

On boundary generation and detection in random walk problems

In the Monte Carlo solution of boundary value problems modeled by partial differential equations ,1,2,3 the bound aries of the regions in question and the boundary condi tions must be generated and detected. This note is a discussion of such techniques developed at the University of British Columbia, Hybrid Computer Laboratory. Only electronic comparators and diode function generators are required in implementing these techniques.