26,400 research outputs found
Accurate metasurface synthesis incorporating near-field coupling effects
One of the most promising metasurface architectures for the microwave and
terahertz frequency ranges consists of three patterned metallic layers
separated by dielectrics. Such metasurfaces are well suited to planar
fabrication techniques and their synthesis is facilitated by modelling them as
impedance sheets separated by transmission lines. We show that this model can
be significantly inaccurate in some cases, due to near-field coupling between
metallic layers. This problem is particularly severe for higher frequency
designs, where fabrication tolerances prevent the patterns from being
highly-subwavelength in size. Since the near-field coupling is difficult to
describe analytically, correcting for it in a design typically requires
numerical optimization. We propose an extension of the widely used
equivalent-circuit model to incorporate near-field coupling and show that the
extended model can predict the scattering parameters of a metasurface
accurately. Based on our extended model, we introduce an improved metasurface
synthesis algorithm that gives physical insight to the problem and efficiently
compensates for the perturbations induced by near-field coupling. Using the
proposed algorithm, a Huygens metasurface for beam refraction is synthesized
showing a performance close to the theoretical efficiency limit despite the
presence of strong near-field coupling
The design of a numerically controlled drilling system using fluidic elements 1966/7
This report outlines the aims of the
project and procedures used to bring
it to a successful conclusion. It
also summarises the results of an
investigation into the parameters
which govern the specification of a
numerically controlled point to point
drilling machine
The prediction of nonlinear three dimensional combustion instability in liquid rockets with conventional nozzles
An analytical technique is developed to solve nonlinear three-dimensional, transverse and axial combustion instability problems associated with liquid-propellant rocket motors. The Method of Weighted Residuals is used to determine the nonlinear stability characteristics of a cylindrical combustor with uniform injection of propellants at one end and a conventional DeLaval nozzle at the other end. Crocco's pressure sensitive time-lag model is used to describe the unsteady combustion process. The developed model predicts the transient behavior and nonlinear wave shapes as well as limit-cycle amplitudes and frequencies typical of unstable motor operation. The limit-cycle amplitude increases with increasing sensitivity of the combustion process to pressure oscillations. For transverse instabilities, calculated pressure waveforms exhibit sharp peaks and shallow minima, and the frequency of oscillation is within a few percent of the pure acoustic mode frequency. For axial instabilities, the theory predicts a steep-fronted wave moving back and forth along the combustor
The prediction of the nonlinear behavior of unstable liquid rockets
Analytical technique for solving nonlinear combustion problems associated with liquid propellant rocket engine
Development of a spinning wave heat engine
A theoretical analysis and an experimental investigation were conducted to assess the feasibility of developing a spinning wave heat engine. Such as engine would utilize a large amplitude traveling acoustic wave rotating around a cylindrica chamber, and it should not suffer from the inefficiency, noise, and intermittent thrust which characterizes pulse jet engines. The objective of this investigation was to determine whether an artificially driven large amplitude spinning transverse wave could induce a steady flow of air through the combustion chamber under cold flow conditions. In the theoretical analysis the Maslen and Moore perturbation technique was extended to study flat cylinders (pancake geometry) with completely open side walls and a central opening. In the parallel experimental study, a test moel was used to determine resonant frequencies and radial pressure distributions, as well as oscillatory and steady flow velocities at the inner and outer peripheries. The experimental frequency was nearly the same as the theoretical acoustic value for a model of the same outer diameter but without a central hole. Although the theoretical analysis did not predict a steady velocity component, simulaneous measurements of hotwire and microphone responses have shown that the spinning wave pumps a mean flow radially outward through the cavity
Towards mechanomagnetics in elastic crystals: insights from [Cu(acac)]
We predict that the magnetic properties of \cuacac, an elastically flexible
crystal, change dramatically when the crystal is bent. We find that unbent
\cuacac\ is an almost perfect Tomonaga-Luttinger liquid. Broken-symmetry
density functional calculations reveal that the magnetic exchange interactions
along the chains is an order of magnitude larger than the interchain exchange.
The geometrically frustrated interchain interactions cannot magnetically order
the material at any experimentally accessible temperature. The ordering
temperature (), calculated from the chain random phase approximation,
increases by approximately 24 orders of magnitude when the material is bent. We
demonstrate that geometric frustration both suppresses and enhances the
sensitivity of to bending. In \cuacac, is extremely sensitive to
bending, but remains too low for practical applications, even when bent.
Partially frustrated materials could achieve the balance of high and good
sensitivity to bending required for practical applications of mechanomagnetic
elastic crystals
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Hemidactylus mabouia
Number of Pages: 11Integrative BiologyGeological Science
A short note on useful codes for fluidic control circuits
This note gives a survey of codes particularly useful for computational
and analogue to digital circuits. Also included are logic circuits capable
of converting the code into natural binary form as at some stage in a control
circuit this is normally required
The effect of combined glutamate receptor blockade in the NTS on the hypoxic ventilatory response in awake rats differs from the effect of individual glutamate receptor blockade.
Ventilatory acclimatization to hypoxia (VAH) increases the hypoxic ventilatory response (HVR) and causes persistent hyperventilation when normoxia is restored, which is consistent with the occurrence of synaptic plasticity in acclimatized animals. Recently, we demonstrated that antagonism of individual glutamate receptor types (GluRs) within the nucleus tractus solitarii (NTS) modifies this plasticity and VAH (J. Physiol. 592(8):1839-1856); however, the effects of combined GluR antagonism remain unknown in awake rats. To evaluate this, we exposed rats to room air or chronic sustained hypobaric hypoxia (CSH, PiO2 = 70 Torr) for 7-9 days. On the experimental day, we microinjected artificial cerebrospinal fluid (ACSF: sham) and then a "cocktail" of the GluR antagonists MK-801 and DNQX into the NTS. The location of injection sites in the NTS was confirmed by glutamate injections on a day before the experiment and with histology following the experiment. Ventilation was measured in awake, unrestrained rats breathing normoxia or acute hypoxia (10% O2) in 15-min intervals using barometric pressure plethysmography. In control (CON) rats, acute hypoxia increased ventilation; NTS microinjections of GluR antagonists, but not ACSF, significantly decreased ventilation and breathing frequency in acute hypoxia but not normoxia (P < 0.05). CSH increased ventilation in hypoxia and acute normoxia. In CSH-conditioned rats, GluR antagonists in the NTS significantly decreased ventilation in normoxia and breathing frequency in hypoxia. A persistent HVR after combined GluR blockade in the NTS contrasts with the effect of individual GluR blockade and also with results in anesthetized rats. Our findings support the hypotheses that GluRs in the NTS contribute to, but cannot completely explain, VAH in awake rats
Determination of the effects of nozzle nonlinearities upon nonlinear stability of liquid propellant rocket motors
The research is reported concerning the development of a three-dimensional nonlinear nozzle admittance relation to be used as a boundary condition in the nonlinear combustion instability theories for liquid propellant rocket engines. The derivation of the nozzle wave equation and the application of the Galerkin method are discussed along with the nozzle response
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