Hereditary Polytopes

Every regular polytope has the remarkable property that it inherits all symmetries of each of its facets. This property distinguishes a natural class of polytopes which are called hereditary. Regular polytopes are by definition hereditary, but the other polytopes in this class are interesting, have possible applications in modeling of structures, and have not been previously investigated. This paper establishes the basic theory of hereditary polytopes, focussing on the analysis and construction of hereditary polytopes with highly symmetric faces.Comment: Discrete Geometry and Applications (eds. R.Connelly and A.Ivic Weiss), Fields Institute Communications, (23 pp, to appear

A DESIGN OF GAS MIXER FOR SYNGAS ENGINE USING THREE-DIMENSIONAL CFD MODELING.

A gas mixer prototype is developed for mixing air and synthesis gas or “syngas” as a fuel. Syngas is being recognized as a viable energy source worldwide, particularly for stationary power generation. Syngas has a very low energy density, so a mixer with λ (ratio of actual to stoichiometric air-fuel ratio) in the range of 1.1 to 1.7 is expected. In this study, three-dimensional computational fluid dynamics (CFD) modeling is used to design venturi mixer, coaxial mixer and coaxial mixer with vortex generator. CFD modeling is used to investigate and analyze the influence of the throat diameter, gas chamber thickness and gas exits diameter on mixer characteristics and performance of the venturi mixer. While on the coaxial mixer model, CFD is used to analyze the influences of the primary nozzle exit diameter, constant pressure mixing chamber geometry, constant area mixing chamber geometry, divergent passage geometry, syngas inlet position and primary nozzle exit position on the coaxial mixer characteristics and performance. To design appropriate vortex generator, computational models are used to analyze the influence of the mechanical tab angle, number of tabs and geometry on the mixing characteristics and performance of the coaxial mixer. Attention is focused on the effect of mixers and vortex generator tabs geometry on the air-fuel ratio, pressure loss and mixing quality. Based on the numerical results, an optimized design of venturi gas mixer, coaxial mixer and vortex generator is decided and made. The optimized design of the venturi mixer has λ in the range of 1.2 to 1.3, good mixing quality and pressure loss of 46 Pa on air flow rate 100 m 3 /h. The optimized design of the coaxial mixer has λ ranging from 1.1 to 1.7 corresponding to pressure losses from 28 to 19 Pa at 100 m 3 /h air-flow rate. The optimized design of coaxial mixer equipped with the proposed vortex generator has λ in the range of 1.1 to 1.7 corresponding to pressure loss in the range of 41.4 to 31.9 Pa at 100 m 3 /h air flow rate. At λ about 1.2 and 100 m 3 /h air flow rate, the mixing quality of the optimized venturi mixer, coaxial mixer and coaxial mixer equipped with vortex generator have coefficient of variation (CoV) of 0.67, 0.88 and 0.29 respectively

A 100-element planar Schottky diode grid mixer

The authors present a Schottky diode grid mixer suitable for mixing or detecting quasi-optical signals. The mixer is a planar bow-tie grid structure periodically loaded with diodes. A simple transmission line model is used to predict the reflection coefficient of the grid to a normally incident plane wave. The grid mixer power handling and dynamic range scales as the number of devices in the grid. A 10-GHz 100-element grid mixer has shown an improvement in dynamic range of 16.3 to 19.8 dB over an equivalent single-diode mixer. The conversion loss and noise figure of the grid are equal to those of a conventional mixer. The quasi-optical coupling of the input signals makes the grid mixer suitable for millimeter-wave and submillimeter-wave applications by eliminating waveguide sidewall losses and machining difficulties. The planar property of the grid potentially allows thousands of devices to be integrated monolithically

A 530-GHz balanced mixer

We report on the design and performance of a 530-GHz balanced SIS mixer, the first balanced mixer in this frequency range. This quasi-optical balanced mixer utilizes a cross-slot antenna on a hyperhemispherical substrate lens with eight superconductor-insulator-superconductor (SIS) junctions and a 180° lumped element IF hybrid circuit. The local oscillator (LO) and the radio frequency (RF) signal, orthogonal in polarization to each other, are coupled to the mixer using a wire-grid polarizer. The noise performance of the mixer is excellent, giving an uncorrected receiver noise temperature of 105 K (DSB) at 528 GHz

Development of Low Noise THz SIS Mixer Using an Array of Nb/Al-AlN/NbTiN Junctions

We report the development of a low noise and broadband SIS mixer aimed for 1 THz channel of the Caltech Airborne Submillimeter Interstellar Medium Investigations Receiver (CASIMIR), designed for the Stratospheric Observatory for Infrared Astronomy, (SOFIA). The mixer uses an array of two 0.24 mum^2 Nb/Al-AlN/NbTiN SIS junctions with the critical current density of 30-50 kA/cm^2 . An on-chip double slot planar antenna couples the mixer circuit with the telescope beam. The mixer matching circuit is made with Nb and gold films. The mixer IF circuit is designed to cover 4-8 GHz band. A test receiver with the new mixer has a low noise operation in 0.87-1.12 THz band. The minimum receiver noise measured in our experiment is 353 K (Y = 1.50). The receiver noise corrected for the loss in the LO injection beam splitter is 250 K. The combination of a broad operation band of about 250 GHz with a low receiver noise makes the new mixer a useful element for application at SOFIA