Calculations of transonic potential flow over cascades

Transonic flow through a cascade was studied by using the full potential equation and the finite volume method of Jameson and Caughey. The C-type computational grid is generated by an electrostatic analogy and simple shearing transformation. The solution algorithm includes an option of using either an artificial density or an artificial viscosity formulation of the dissipative term. Using the developed code, flows through a cascade of NACA 0012 airfoils and flows through a cascade of shockless blades were computed. It is found that the designed flow through the shockless blade is accurately predicted, the artificial density formulation shows more tolerance to the mesh irregularity, and the C-type mesh does not extend very far upstream for a small pitch-cord ratio

Singularity embedding method in potential flow calculations

The so-called H-type mesh is used in a finite-element (or finite-volume) calculation of the potential flow past an airfoil. Due to coordinate singularity at the leading edge, a special singular trial function is used for the elements neighboring the leading edge. The results using the special singular elements are compared to those using the regular elements. It is found that the unreasonable pressure distribution obtained by the latter is removed by the embedding of the singular element. Suggestions to extend the present method to transonic cases are given

Procedure for noise prediction and optimization of advanced technology propellers

The sound field due to a propeller operating at supersonic tip speed in a uniform flow was investigated. Using the fact that the wave front in a uniform stream is a convected sphere, the fundamental solution to the convected wave equation was easily obtained. The Fourier coefficients of the pressure signature were obtained by a far field approximation, and are expressed as an integral over the blade platform. It is shown that cones of silence exist fore and aft the propeller plane. The semiapex angles are shown. These angles are independent of the individual Mach components such as the flight Mach number and the rotation Mach number. The result is confirmed by the computation of the ray path of the emitted Mach waves. The Doppler amplification factor strengthens the signal behind the propeller while it weakens that upstream

Array concepts for solid-state and vacuum microelectronics millimeter-wave generation

The authors have proposed that the increasing demand for contact watt-level coherent sources in the millimeter- and submillimeter-wave region can be satisfied by fabricating two-dimensional grids loaded with oscillators and multipliers for quasi-optical coherent spatial combining of the outputs of large numbers of low-power devices. This was first demonstrated through the successful fabrication of monolithic arrays with 2000 Schottky diodes. Watt-level power outputs were obtained in doubling to 66 GHz. In addition, a simple transmission-line model was verified with a quasi-optical reflectometer that measured the array impedance. This multiplier array work is being extended to novel tripler configurations using blocking barrier devices. The technique has also been extended to oscillator configurations where the grid structure is loaded with negative-resistance devices. This was first demonstrated using Gunn devices. More recently, a 25-element MESFET grid oscillating at 10 GHz exhibited power combining and self-locking. Currently, this approach is being extended to a 100-element monolithic array of Gunn diodes. This same approach should be applicable to planar vacuum electron devices such as the submillimeter-wave BWO (backward wave oscillator) and vacuum FET

Ideal gas sources for the Lemaitre-Tolman-Bondi metrics

New exact solutions emerge by replacing the dust source of the Lem\^aitre-Tolman-Bondi metrics with a viscous fluid satisfying the monatomic gas equation of state. The solutions have a consistent thermodynamical interpretation. The most general transport equation of Extended Irreversible Thermodynamics is satisfied, with phenomenological coefficients bearing a close resemblance to those characterizing a non relativistic Maxwell-Bolzmann gas.Comment: 7 pages, Plain TeX with IOP macros, important corrections to previous version, 3 figures (to appear in Classical and Quantum Gravity, June 1998

Millimeter-wave diode-grid phase shifters

Monolithic diode grids have been fabricated on 2-cm square gallium-arsenide wafers with 1600 Schottky-barrier varactor diodes. Shorted diodes are detected with a liquid-crystal technique, and the bad diodes are removed with an ultrasonic probe. A small-aperture reflectometer that uses wavefront division interference was developed to measure the reflection coefficient of the grids. A Phase shift of 70° with a 7-dB loss was obtained at 93 GHz when the bias on the diode grid was changed from -3 V to 1 V. A simple transmission-line grid model, together with the measured low-frequency parameters for the diodes, was shown to predict the measured performance over the entire capacitive bias range of the diodes, as well as over the complete reactive tuning range provided by a reflector behind the grid, and over a wide range of frequencies form 33 GHz to 141 GHz. This shows that the transmission-line model and the measured low-frequency diode parameters can be used to design an electronic beam-steering array and to predict its performance. An electronic beam-steering array made of a pair of grids using state-of-the-art diodes with 5-Ω series resistances would have a loss of 1.4 dB at 90 GHz

Millimeter-Wave Diode-Grid Frequency Doubler

Monolithic diode grid were fabricated on 2-cm^2 gallium-arsenide wafers in a proof-of-principle test of a quasi-optical varactor millimeter-wave frequency multiplier array concept. An equivalent circuit model based on a transmission-line analysis of plane wave illumination was applied to predict the array performance. The doubler experiments were performed under far-field illumination conditions. A second-harmonic conversion efficiency of 9.5% and output powers of 0.5 W were achieved at 66 GHz when the diode grid was pumped with a pulsed source at 33 GHz. This grid had 760 Schottky-barrier varactor diodes. The average series resistance was 27 Ω, the minimum capacitance was 18 fF at a reverse breakdown voltage of -3 V. The measurements indicate that the diode grid is a feasible device for generating watt-level powers at millimeter frequencies and that substantial improvement is possible by improving the diode breakdown voltage

Optical thermal insulation via solar-energy harvesting photothermal nano coatings

The current technological advancement has enabled glass-based building facades with double- or triple-glazed transparent panels. However, the conventional glazing technologies cannot effectively reduce building thermal energy loss especially for large area transparent building skin. According to a report by the U.S. Department of Energy, building heating, ventilation, and air conditioning (HVAC) accounted for 14.0 % of primary energy consumption in the United States. Heat loss through windows in cold weather consumes about 3.9 quads, which is estimated to encompass 28.7 % of total HVAC energy consumption. [1] We have developed a novel concept of Optical Thermal Insulation (OTI) without any intervening medium (Fig. 1). Instead of applying a thermal insulator, a transparent photothermal (PT) film can selectively absorb photons near the UV and NIR regions and efficiently convert them to heat, therefore raising the window surface temperature (via free energy). As the inner surface temperature is raised relative to room temperature, the heat transfer at the window inner surface can be effectively reduced via the so-called OTI, especially in winter. It must be noted that the PT films are spectral selective with high absorptions near UV and IR, while allowing high visible transmittance, therefore transparent and ideal for façade engineering. Based on this concept, a so-called “Green Window” has been designed for single-pane applications that meet the U-factor specifications of Department of Energy for colder regions of the United Sates. The “Green Window” is composed of chlorophyll (Chl) retrieved from natural greens (by which the name “Green Window” is derived). [2] A thin film window coating of naturally occurring chlorophyll exhibits strong near UV and NIR absorptions and pronounced photothermal effect, while remaining highly transparent (Fig. 2). Upon collecting solar light, considerable heat is created, effectively raising the window surface temperature, leading to a reduced U-factor less than 1.7 W m-2 K-1, even below the values of double-panes. Based on these experimental results, we demonstrate of a new concept of “optical thermal insulation” that lifts the dependence on insulating materials making single-pane window highly possible. Fig. 2 shows the change in temperature (ΔTg) induced by simulated solar light as a function of time for the multilayer samples of chlorophyll. Consistently, thicker films (each layer is ~ 2 mm) gave greater ΔTg as expected. Conversely, the thicker films exhibit lower visible transmittance (VT). As shown in this figure, the temperature plateaus can be observed after 2 min.-irradiation by solar simulator. Fig. 2b shows ΔTg,max vs. VT for thin films of different layers (a maximum of 6 layers). ΔTg,max vs. VT displays a linear relationship extending to the point where no Chl film was applied (highest VT). Please click Additional Files below to see the full abstract