5,579 research outputs found
A new chiral electro-optic effect: Sum-frequency generation from optically active liquids in the presence of a dc electric field
We report the observation of sum-frequency signals that depend linearly on an
applied electrostatic field and that change sign with the handedness of an
optically active solution. This recently predicted chiral electro-optic effect
exists in the electric-dipole approximation. The static electric field gives
rise to an electric-field-induced sum-frequency signal (an achiral third-order
process) that interferes with the chirality-specific sum-frequency at
second-order. The cross-terms linear in the electrostatic field constitute the
effect and may be used to determine the absolute sign of second- and
third-order nonlinear optical susceptibilities in isotropic media.Comment: Submitted to Physical Revie
Analytical and Numerical Predictions of Short Pulsed Elastic Waves on a Half-Space
The numerical modeling of ultrasonic wave propagation in elastic solids is particularly attractive for NDT applications because of the relative ease with which the boundaries of realistic defect shapes and testing geometries can be handled. A two-dimensional explicit finite element code [1] has been developed for this purpose
Transportation policy for campus climate action planning: Process and policy implications
This article discusses the innovative methods used to complete the transportation components of Cal Poly’s Climate Action Plan (CAP). The campus\u27s CAP was completed by a BSCRP studio during the fall and winter quarters (2015-2016AY) in collaboration with Facilities Planning and Capital Projects. Professors William Riggs and Adrienne Greve (instructors for the studio along with Chris Clark) developed the methods discussed here, and C. Kai Lord-Farmer was the graduate assistant who assisted in completing the technical analysis
Finite element analysis of the influence of a fatigue crack on magnetic properties of steel
Fatigue can affect the magnetic properties of materials due to microstructural changes. Previous investigations have shown that several structure sensitive magnetic properties, such as coercivityHc and remanenceBr, changed systematically as a result of fatigue. When approaching failure the accumulated changes in microstructure resulted in the occurrence of fatigue cracks and the magnetic properties showed dramatic changes which mainly resulted from the geometrical changes in samples due to the cracks. It was found that the remanenceBr followed the changes in stress, while the coercivityHc sometimes showed different trends. In this article the influence of the size and the position of a fatigue crack on magnetic field and magnetic induction were studied using finite element modeling. Models were constructed to simulate the geometry of the test sample and sensor. It was found that, for a given coil current in the exciting coil, the magnetic induction was mainly determined by the geometry of the crack, while the magnetic field was influenced by both the size and the position of the crack
The effect of pressure on statics, dynamics and stability of multielectron bubbles
The effect of pressure and negative pressure on the modes of oscillation of a
multi-electron bubble in liquid helium is calculated. Already at low pressures
of the order of 10-100 mbar, these effects are found to significantly modify
the frequencies of oscillation of the bubble. Stabilization of the bubble is
shown to occur in the presence of a small negative pressure, which expands the
bubble radius. Above a threshold negative pressure, the bubble is unstable.Comment: 4 pages, 2 figures, accepted for publication in Physical Review
Letter
Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows
The nature of dynamo action in shear flows prone to magnetohydrodynamic
instabilities is investigated using the magnetorotational dynamo in Keplerian
shear flow as a prototype problem. Using direct numerical simulations and
Newton's method, we compute an exact time-periodic magnetorotational dynamo
solution to the three-dimensional dissipative incompressible
magnetohydrodynamic equations with rotation and shear. We discuss the physical
mechanism behind the cycle and show that it results from a combination of
linear and nonlinear interactions between a large-scale axisymmetric toroidal
magnetic field and non-axisymmetric perturbations amplified by the
magnetorotational instability. We demonstrate that this large scale dynamo
mechanism is overall intrinsically nonlinear and not reducible to the standard
mean-field dynamo formalism. Our results therefore provide clear evidence for a
generic nonlinear generation mechanism of time-dependent coherent large-scale
magnetic fields in shear flows and call for new theoretical dynamo models.
These findings may offer important clues to understand the transitional and
statistical properties of subcritical magnetorotational turbulence.Comment: 10 pages, 6 figures, accepted for publication in Physical Review
3D Radiative Hydrodynamics for Disk Stability Simulations: A Proposed Testing Standard and New Results
Recent three-dimensional radiative hydrodynamics simulations of
protoplanetary disks report disparate disk behaviors, and these differences
involve the importance of convection to disk cooling, the dependence of disk
cooling on metallicity, and the stability of disks against fragmentation and
clump formation. To guarantee trustworthy results, a radiative physics
algorithm must demonstrate the capability to handle both the high and low
optical depth regimes. We develop a test suite that can be used to demonstrate
an algorithm's ability to relax to known analytic flux and temperature
distributions, to follow a contracting slab, and to inhibit or permit
convection appropriately. We then show that the radiative algorithm employed by
Meji\'a (2004) and Boley et al. (2006) and the algorithm employed by Cai et al.
(2006) and Cai et al. (2007, in prep.) pass these tests with reasonable
accuracy. In addition, we discuss a new algorithm that couples flux-limited
diffusion with vertical rays, we apply the test suite, and we discuss the
results of evolving the Boley et al. (2006) disk with this new routine.
Although the outcome is significantly different in detail with the new
algorithm, we obtain the same qualitative answers. Our disk does not cool fast
due to convection, and it is stable to fragmentation. We find an effective
. In addition, transport is dominated by low-order
modes.Comment: Submitted to Ap
Mechanical Instabilities of Biological Tubes
We study theoretically the shapes of biological tubes affected by various
pathologies. When epithelial cells grow at an uncontrolled rate, the negative
tension produced by their division provokes a buckling instability. Several
shapes are investigated : varicose, enlarged, sinusoidal or sausage-like, all
of which are found in pathologies of tracheal, renal tubes or arteries. The
final shape depends crucially on the mechanical parameters of the tissues :
Young modulus, wall-to-lumen ratio, homeostatic pressure. We argue that since
tissues must be in quasistatic mechanical equilibrium, abnormal shapes convey
information as to what causes the pathology. We calculate a phase diagram of
tubular instabilities which could be a helpful guide for investigating the
underlying genetic regulation
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