Showing papers by "Delhi Technological University published in 1985"

On the stability of pipe-Poiseuille flow to finite-amplitude axisymmetric and non-axisymmetric disturbances

Abstract: The stability of fully developed pipe-Poiseuille flow to finite-amplitude axisymmetric and non-axisymmetric disturbances has been studied using the equilibrium-amplitude method of Reynolds & Potter (1967). In both the cases the least-stable centre-modes were investigated. Also, for the non-axisymmetric case the mode investigated was the one with azimuthal wavenumber equal to one. Many higher-order Landau coefficients were calculated, and the Stuart-Landau series was analysed by the Shanks (1955) method and by using Pade approximants to look for the existence of possible equilibrium states. The results show in both cases that, for each value of the Reynolds number R , there is a preferred band of spatial wavenumbers α in which equilibrium states are likely to exist. Moreover, in both cases it was found that the magnitude of the minimum threshold amplitude for a given R decreases with increasing R . The scales of the various quantities obtained agree very well with those deduced by Davey & Nguyen (1971).

Input impedance of a probe excited circular microstrip ring antenna

Abstract: The input impedance of a circular microstrip ring antenna excited by a coaxial probe is studied using cavity Green's-function technique. The model of an annular cavity with the boundary admittances at the edges is considered in the analysis. The boundary admittance is obtained by considering the radiated and reactive power flows through the apertures at the edges. Input impedance is formulated from the conservation of energy principle, taking into account the losses in feed pin, conducting patch and dielectric substrate. The expression for input impedance includes the feed location, the feed diameter, the ring dimensions, thickness and dielectric constant of the substrate. TM120 mode is considered for antenna applications. Experimental results of input reflection coefficients show good agreement with the theory.

Mechanical properties of cryolite from Ivigtut, South Greenland

Abstract: Cryolite made up about 75 per cent of the deposit in Ivigtut, South Greenland. Its mechanical properties seem important for the shaping and for the internal build-up of the deposit of which a major part, sideritecryolite with about 15 per cent siderite and a few per cent of sulphides, quartz etc., consolidated between 500 and 600°C. Thermal expansion, by volume, of cryolite is shown to be two to five times greater than the expansion of the accompanying minerals and rocks. The elastic constants of cryolite are found to be: E = 6.8 • lO' bars, G = 2.7 • 10* bars and Poisson's No. = 0.24. Compressive strength at room temperature 919 bars. Uniaxial compression tests at atmospheric pressure showed strength to increase to 1360 bars at 200°C and strain rate 0.05 mm per minute. CryoUte behaved in the tests up to 300°C as a brittle substance. Duc­ tility was noted at temperatures from 400°C and, depending on strain rate, cryolite was found to exhibit unrestricted plastic deformations above 450°C. Ductility of cryolite around 500°C was found to be about the same as found for halite, tested in the same instrument, at 150-200°C. It is to be noted that the plastic deformation observed in these uniaxial compression tests dominantly appeared as bending of lamellae developed parallel to the load axis both in cryolite and in halite. In the siderite-cryoUte, representing the earliest formed part of the deposit, siderite exhibits a pronoun­ ced cataclastic structure. This is attributed to the high ductiUty of cryolite at temperatures around S0O°C. The contraction on cooling of cryolite appears important in judging joint patterns of the intrusion as well as macro- and microfissures within the deposit.