4,588 research outputs found
Transformation Acoustics in Generic Elastic Media
In this work a transformation acoustics scheme for generic elastic media is
developed. Our approach starts form the decomposition of the elasticity tensor
in terms of its eigentensors, an idea previously used by Norris. While Norris'
transformation acoustics is restricted to the special class of so-called
pentamode materials, we show that a similar scheme can be defined for the most
general elasticity tensor. As in case of Norris' model (and in sharp contrast
to transformation optics), the compatibility equations of the transformation
medium are not purely algebraic and it is not guaranteed that solutions to
these equations exist for any choice of material parameters and coordinate
transformation. Nonetheless, it is shown that our scheme yields new cloaking
solutions for certain classes of materials. In particular, we present the first
application of a transformation based device for a non-scalar wave equation
outside of the field of electromagnetics.Comment: 14 pages, LaTe
Negative index of refraction, perfect lenses and transformation optics -- some words of caution
In this paper we show that a negative index of refraction is not a direct
implication of transformation optics with orientation-reversing
diffeomorphisms. Rather a negative index appears due to a specific choice of
sign freedom. Furthermore, we point out that the transformation designed lens,
which relies on the concept of spacetime folding, does not amplify evanescent
modes, in contrast to the Pendry-Veselago lens. Instead, evanescent modes at
the image point are produced by a duplicated source and thus no imaging of the
near field (perfect lensing) takes place.Comment: 13 pages, 3 figures, LaTe
Rotor instability due to a gear coupling connected to a bearingless sun wheel of a planetary gear
A 21 MW electric power generating unit comprises a gas turbine, a planetary gear, and a generator connected together by gear couplings. For simplicity of the design and high performance the pinion of the gear has no bearing. It is centered by the planet wheels only. The original design showed a strong instability and a natural frequency increasing with the load between 2 and 6.5 MW. In this operating range the natural frequency was below the operating speed of the gas turbine, n sub PT = 7729 RPM. By shortening the pinion shaft and reduction of its moment of inertia the unstable natural frequency was shifted well above the operating speed. With that measure the unit now operates with stability in the entire load range
Black Hole Thermodynamics and Hamilton-Jacobi Counterterm
We review the construction of the universal Hamilton-Jacobi counterterm for
dilaton gravity in two dimensions, derive the corresponding result in the
Cartan formulation and elaborate further upon black hole thermodynamics and
semi-classical corrections. Applications include spherically symmetric black
holes in arbitrary dimensions with Minkowski- or AdS-asymptotics, the BTZ black
hole and black holes in two-dimensional string theory.Comment: 9 pages, proceedings contribution to QFEXT07 submitted to IJMPA, v2:
added Re
Bianchi I effective dynamics in Quantum Reduced Loop Gravity
The effective quantum dynamics of Bianchi I spacetime is addressed within the
statistical regularization scheme in Quantum Reduced Loop Gravity. The case of
a minimally coupled massless scalar field is studied and compared with the
effective Loop Quantum Cosmology. The dynamics provided by the two
approaches match in the semiclassical limit but differ significantly after the
bounces. Analytical and numerical inspections show that energy density,
expansion scalar and shear are bounded also in Quantum Reduced Loop Gravity and
the classical singularity is resolved for generic initial conditions in all
spatial directions.Comment: 19 pages, 23 figures, 1 tabl
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