27,270 research outputs found
On the determination of the boundary impedance from the far field pattern
We consider the Helmholtz equation in the half space and suggest two methods
for determining the boundary impedance from knowledge of the far field pattern
of the time-harmonic incident wave. We introduce a potential for which the far
field patterns in specially selected directions represent its Fourier
coefficients. The boundary impedance is then calculated from the potential by
an explicit formula or from the WKB approximation. Numerical examples are given
to demonstrate efficiency of the approaches. We also discuss the validity of
the WKB approximation in determining the impedance of an obstacle.Comment: 10 pages, 4 figure
The inverse electromagnetic scattering problem in a piecewise homogeneous medium
This paper is concerned with the problem of scattering of time-harmonic
electromagnetic waves from an impenetrable obstacle in a piecewise homogeneous
medium. The well-posedness of the direct problem is established, employing the
integral equation method. Inspired by a novel idea developed by Hahner [11], we
prove that the penetrable interface between layers can be uniquely determined
from a knowledge of the electric far field pattern for incident plane waves.
Then, using the idea developed by Liu and Zhang [21], a new mixed reciprocity
relation is obtained and used to show that the impenetrable obstacle with its
physical property can also be recovered. Note that the wave numbers in the
corresponding medium may be different and therefore this work can be considered
as a generalization of the uniqueness result of [20].Comment: 19 pages, 2 figures, submitted for publicatio
Design and performance of duct acoustic treatment
The procedure for designing acoustic treatment panels used to line the walls of aircraft engine ducts and for estimating the resulting suppression of turbofan engine duct noise is discussed. This procedure is intended to be used for estimating noise suppression of existing designs or for designing new acoustic treatment panels and duct configurations to achieve desired suppression levels
Analysis, design, and test of acoustic treatment in a laboratory inlet duct
A suppression prediction program based on the method of modal analysis for spinning mode propagation in a circular duct was used in the analytical design of optimized, multielement, Kevlar bulk-absorber treatment configurations for an inlet duct. The NASA-Langley ANRL anechoic chamber using the spinning mode synthesizer as a sound source was used to obtain in-duct spinning mode measurements, radial mode measurements, and far-field traverses, as well as aerodynamic measurements. The measured suppression values were compared to predicted values, using the in-duct, forward-traveling, radial-mode content as the source for the prediction. The performance of the treatment panels was evaluated from the predicted and measured data. Although experimental difficulties were encountered at the design condition, sufficient information was obtained to confirm the expectation that it is the panel impedance components which are critical to suppression at a single frequency, not the particular construction materials. The agreement obtained between measurement and prediction indicates that the analytical program can be used as an accurate, reliable, and useful design tool
Impedance matching and emission properties of optical antennas in a nanophotonic circuit
An experimentally realizable prototype nanophotonic circuit consisting of a
receiving and an emitting nano antenna connected by a two-wire optical
transmission line is studied using finite-difference time- and frequency-domain
simulations. To optimize the coupling between nanophotonic circuit elements we
apply impedance matching concepts in analogy to radio frequency technology. We
show that the degree of impedance matching, and in particular the impedance of
the transmitting nano antenna, can be inferred from the experimentally
accessible standing wave pattern on the transmission line. We demonstrate the
possibility of matching the nano antenna impedance to the transmission line
characteristic impedance by variations of the antenna length and width
realizable by modern microfabrication techniques. The radiation efficiency of
the transmitting antenna also depends on its geometry but is independent of the
degree of impedance matching. Our systems approach to nanophotonics provides
the basis for realizing general nanophotonic circuits and a large variety of
derived novel devices
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