2,858 research outputs found
Liquid compressibility effects during the collapse of a single cavitating bubble
The effect of liquid compressibility on the dynamics of a single, spherical cavitating bubble is studied.
While it is known that compressibility damps the amplitude of bubble rebounds, the extent to which
this effect is accurately captured by weakly compressible versions of the Rayleigh–Plesset equation is
unclear. To clarify this issue, partial differential equations governing conservation of mass, momentum,
and energy are numerically solved both inside the bubble and in the surrounding compressible
liquid. Radiated pressure waves originating at the unsteady bubble interface are directly captured.
Results obtained with Rayleigh–Plesset type equations accounting for compressibility effects, proposed
by Keller and Miksis [J. Acoust. Soc. Am. 68, 628–633 (1980)], Gilmore, and Tomita and
Shima [Bull. JSME 20, 1453–1460 (1977)], are compared with those resulting from the full model.
For strong collapses, the solution of the latter reveals that an important part of the energy concentrated
during the collapse is used to generate an outgoing pressure wave. For the examples considered in
this research, peak pressures are larger than those predicted by Rayleigh–Plesset type equations,
whereas the amplitudes of the rebounds are smaller
Simulation of low-speed buoyant flows with a stabilized compressible/incompressible formulation: the Full Navier–Stokes approach versus the Boussinesq model
This paper compares two strategies to compute buoyancy-driven flows using stabilized methods. Both formulations are based on a unified approach for solving compressible and incompressible flows, which solves the continuity, momentum, and total energy equations in a coupled entropy-consistent way. The first approach introduces the variable density thermodynamics of the liquid or gas without any artificial buoyancy terms, i.e., without applying any approximate models into the Navier–Stokes equations. Furthermore, this formulation holds for flows driven by high temperature differences. Further advantages of this formulation are seen in the fact that it conserves the total energy and it lacks the incompressibility inconsistencies due to volume changes induced by temperature variations. The second strategy uses the Boussinesq approximation to account for temperature-driven forces. This method models the thermal terms in the momentum equation through a temperature-dependent nonlinear source term. Computer examples show that the thermodynamic approach, which does not introduce any artificial terms into the Navier–Stokes equations, is conceptually simpler and, with the incompressible stabilization matrix, attains similar residual convergence with iteration count to methods based on the Boussinesq approximation. For the Boussinesq model, the SUPG and SGS methods are compared, displaying very similar computational behavior. Finally, the VMS a posteriori error estimator is applied to adapt the mesh, helping to achieve better accuracy for the same number of degrees of freedom
A posteriori pointwise error computation for 2-D transport equations based on the variational multiscale method
This article presents a general framework to estimate the pointwise error of linear partial differential equations. The error estimator is based on the variational multiscale theory, in which the error is decomposed in two components according to the nature of the residuals: element interior residuals and inter-element jumps. The relationship between the residuals (coarse scales) and the error components (fine scales) is established, yielding to a very simple model. In particular, the pointwise error is modeled as a linear combination of bubble functions and Green’s functions. If residual-free bubbles and the classical Green’s function are employed, the technology leads to an exact explicit method for the pointwise error. If bubble functions and free-space Green’s functions are employed, then a local projection problem must be solved within each element and a global boundary integral equation must be solved on the domain boundary. As a consequence, this gives a model for the so-called fine-scale Green’s functions. The numerical error is studied for the standard Galerkin and SUPG methods with application to the heat equation, the reaction–diffusion equation and the convection–diffusion equation. Numerical results show that stabilized methods minimize the propagation of pollution errors, which stay mostly locally
Genomic Features Of A Bumble Bee Symbiont Reflect Its Host Environment
Here, we report the genome of one gammaproteobacterial member of the gut microbiota, for which we propose the name >Candidatus Schmidhempelia bombi,> that was inadvertently sequenced alongside the genome of its host, the bumble bee, Bombus impatiens. This symbiont is a member of the recently described bacterial order Orbales, which has been collected from the guts of diverse insect species; however, >Ca. Schmidhempelia> has been identified exclusively with bumble bees. Metabolic reconstruction reveals that >Ca. Schmidhempelia> lacks many genes for a functioning NADH dehydrogenase I, all genes for the high-oxygen cytochrome o, and most genes in the tricarboxylic acid (TCA) cycle. >Ca. Schmidhempelia> has retained NADH dehydrogenase II, the low-oxygen specific cytochrome bd, anaerobic nitrate respiration, mixed-acid fermentation pathways, and citrate fermentation, which may be important for survival in low-oxygen or anaerobic environments found in the bee hindgut. Additionally, a type 6 secretion system, a Flp pilus, and many antibiotic/multidrug transporters suggest complex interactions with its host and other gut commensals or pathogens. This genome has signatures of reduction (2.0 megabase pairs) and rearrangement, as previously observed for genomes of host-associated bacteria. A survey of wild and laboratory B. impatiens revealed that >Ca. Schmidhempelia> is present in 90% of individuals and, therefore, may provide benefits to its host.Center for Insect Science (University of Arizona)National Science Foundation NSF 1046153NIH Director's Pioneer 1DP1OD006416-01NIH R01-HG006677Swiss National Science Foundation 140157, 147881Integrative Biolog
Quantum control of spin-correlations in ultracold lattice gases
We demonstrate that it is possible to prepare a lattice gas of ultracold
atoms with a desired non-classical spin-correlation function using atom-light
interaction of the kind routinely employed in quantum spin polarization
spectroscopy. Our method is based on quantum non-demolition (QND) measurement
and feedback, and allows in particular to create on demand exponentially or
algebraically decaying correlations, as well as a certain degree of
multi-partite entanglement.Comment: 2 figure
Bulk-driven non-equilibrium phase transitions in a mesoscopic ring
We study a periodic one-dimensional exclusion process composed of a driven
and a diffusive part. In a mesoscopic limit where both dynamics compete we
identify bulk-driven phase transitions. We employ mean-field theory
complemented by Monte-Carlo simulations to characterize the emerging
non-equilibrium steady states. Monte-Carlo simulations reveal interesting
correlation effects that we explain phenomenologically.Comment: 4 pages, 3 figure
Enhanced photoluminescence emission from two-dimensional silicon photonic crystal nanocavities
We present a temperature dependent photoluminescence study of silicon optical
nanocavities formed by introducing point defects into two-dimensional photonic
crystals. In addition to the prominent TO phonon assisted transition from
crystalline silicon at ~1.10 eV we observe a broad defect band luminescence
from ~1.05-1.09 eV. Spatially resolved spectroscopy demonstrates that this
defect band is present only in the region where air-holes have been etched
during the fabrication process. Detectable emission from the cavity mode
persists up to room-temperature, in strong contrast the background emission
vanishes for T > 150 K. An Ahrrenius type analysis of the temperature
dependence of the luminescence signal recorded either in-resonance with the
cavity mode, or weakly detuned, suggests that the higher temperature stability
may arise from an enhanced internal quantum efficiency due to the
Purcell-effect
Dephasing of quantum dot exciton polaritons in electrically tunable nanocavities
We experimentally and theoretically investigate dephasing of zero dimensional
microcavity polaritons in electrically tunable single dot photonic crystal
nanocavities. Such devices allow us to alter the dot-cavity detuning in-situ
and to directly probe the influence on the emission spectrum of varying the
incoherent excitation level and the lattice temperature. By comparing our
results with theory we obtain the polariton dephasing rate and clarify its
dependence on optical excitation power and lattice temperature. For low
excitation levels we observe a linear temperature dependence, indicative of
phonon mediated polariton dephasing. At higher excitation levels, excitation
induced dephasing is observed due to coupling to the solid-state environment.
The results provide new information on coherence properties of quantum dot
microcavity polaritons.Comment: Figure 2, panel (b) changed to logarithmic + linear scal
A Correlation between the Emission Intensity of Self-Assembled Germanium Islands and the Quality Factor of Silicon Photonic Crystal Nanocavities
We present a comparative micro-photoluminescence study of the emission
intensity of self-assembled germanium islands coupled to the resonator mode of
two-dimensional silicon photonic crystal defect nanocavities. The emission
intensity is investigated for cavity modes of L3 and Hexapole cavities with
different cavity quality factors. For each of these cavities many nominally
identical samples are probed to obtain reliable statistics. As the quality
factor increases we observe a clear decrease in the average mode emission
intensity recorded under comparable optical pumping conditions. This clear
experimentally observed trend is compared with simulations based on a
dissipative master equation approach that describes a cavity weakly coupled to
an ensemble of emitters. We obtain evidence that reabsorption of photons
emitted into the cavity mode is responsible for the observed trend. In
combination with the observation of cavity linewidth broadening in power
dependent measurements, we conclude that free carrier absorption is the
limiting effect for the cavity mediated light enhancement under conditions of
strong pumping.Comment: 8 pages, 5 figure
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