Optical crossed-beam investigation of local sound generation in jets

Cross correlation of optical signals from two light beam sound detectors for jet noise source localizatio

The atom-molecule reaction D plus H2 yields HD plus H studied by molecular beams

Collisions between deuterium atoms and hydrogen molecules were studied in a modulated crossed beam experiment. The relative signal intensity and the signal phase for the product HD from reactive collisions permitted determination of both the angular distribution and HD mean velocity as a function of angle. From these a relative differential reactive scattering cross section in center-of-mass coordinates was deduced. The experiment indicates that reactively formed HD which has little or no internal excitation departs from the collision anisotropically, with maximum amplitude 180 deg from the direction of the incident D beam in center-of-mass coordinates, which shows that the D-H-H reacting configuration is short-lived compared to its rotation time. Non reactive scattering of D by H2 was used to assign absolute values to the differential reactive scattering cross sections

Excitation of Na D-line radiation in collisions of sodium atoms with internally excited H2, D2, and N2

Excitation of D-line radiation in collisions of Na atoms with vibrationally excited N2, H2 and D2 was studied in two modulated crossed beam experiments. In both experiments, the vibrational excitation of the molecules was provided by heating the molecular beam source to temperatures in the range of 2000 to 3000 K, which was assumed to give populations according to the Boltzmann expression. In the first experiment, a total rate coefficient was measured as a function of molecular beam temperature, with absolute calibration of the photon detector being made using the black body radiation from the heated molecular beam source. Since heating affects both the internal energy and the collisional kinetic energy, the first experiment could not determine the relative contributions of internal energy transfer versus collisional excitation. The second experiment achieved partial separation of internal versus kinetic energy transfer effects by using a velocity-selected molecular beam. Using two simple models for the kinetic energy dependence of the transfer cross section for a given change in vibrational quantum number, the data from both experiments were used to determine parameters in the models

Generation of coherent magnetic fields in sheared inhomogeneous turbulence: No need for rotation?

Coherent magnetic fields are often believed to be generated by the combination of stretching by differential rotation and turbulent amplification of magnetic field, via the so-called alpha effect. The latter is known to exist in helical turbulence, which is envisioned to arise due to both rotation and convection in solar-type stars. In this contribution, a turbulent flow driven by a nonhelical inhomogeneous forcing and its kinematic dynamo action are studied for a uniform magnetic field in the background of a linear shear flow. By using a quasilinear analysis and a nonperturbative method utilizing a time-dependent wave number, turbulence property and electromotive force are computed for arbitrary shear strength. Due to the large-scale shear flow, the turbulence is highly anisotropic, as a consequence, so is the electromotive force. The latter is found to exist even without rotation due to the combined effect of shear flow and inhomogeneous forcing, containing not only the alpha effect but also magnetic pumping (the gamma effect representing a transport of magnetic flux by turbulence). Specifically, without shear, only the magnetic pumping exists, aligned with the direction of inhomogeneity. For a weak but nonzero shear, the combined effects of shear and inhomogeneous forcing modify the structure of the magnetic pumping when the inhomogeneity is in the plane of the shear flow, the magnetic pumping becoming bidimensional in that plane. It also induces an alpha tensor which has nondiagonal components. When the inhomogeneity is perpendicular to the plane of the shear flow, the alpha effect has three nonzero diagonal components and one off-diagonal component. However, for a sufficiently strong shear, the gamma and alpha effects are suppressed due to shear stabilization which damps turbulence. A simplified dynamo model is then proposed where a large-scale dynamo arises due to the combined effect of shear flow and inhomogeneous forcing. In particular, the growth of a large-scale axisymmetric magnetic field is demonstrated in case of an inhomogeneity which is perpendicular to the plane of the shear flow. Interesting implications of these results for the structure of magnetic fields in star with slow rotation are discussed. (C) 2011 American Institute of Physics. [doi:10.1063/1.3551700

Transfer of excitation energy from nitrogen molecules to sodium atoms

Transfer of excitation energy from nitrogen molecules to sodium atom

Nonlinear dynamo action in a precessing cylindrical container

It is numerically demonstrated by means of a magnetohydrodynamics (MHD) code that precession can trigger the dynamo effect in a cylindrical container. This result adds credit to the hypothesis that precession can be strong enough to be one of the sources of the dynamo action in some astrophysical bodies.Comment: 5 pages, 5 figures including subfigure

Leading SU(3)-breaking corrections to the baryon magnetic moments in Chiral Perturbation Theory

We calculate the baryon magnetic moments using covariant Chiral Perturbation Theory ($\chi$PT) within the Extended-on-mass-shell (EOMS) renormalization scheme. By fitting the two available low-energy constants (LECs), we improve the Coleman-Glashow description of the data when we include the leading SU(3) breaking effects coming from the lowest-order loops. This success is in dramatic contrast with previous attempts at the same order using Heavy Baryon (HB) $\chi$PT and covariant Infrared (IR) $\chi$PT. We also analyze the source of this improvement with particular attention on the comparison between the covariant results.Comment: 4 pages, 2 figures, accepted for publication in PR

Active Brownian particles with velocity-alignment and active fluctuations

We consider a model of active Brownian particles with velocity-alignment in two spatial dimensions with passive and active fluctuations. Hereby, active fluctuations refers to purely non-equilibrium stochastic forces correlated with the heading of an individual active particle. In the simplest case studied here, they are assumed as independent stochastic forces parallel (speed noise) and perpendicular (angular noise) to the velocity of the particle. On the other hand, passive fluctuations are defined by a noise vector independent of the direction of motion of a particle, and may account for example for thermal fluctuations. We derive a macroscopic description of the active Brownian particle gas with velocity-alignment interaction. Hereby, we start from the individual based description in terms of stochastic differential equations (Langevin equations) and derive equations of motion for the coarse grained kinetic variables (density, velocity and temperature) via a moment expansion of the corresponding probability density function. We focus here in particular on the different impact of active and passive fluctuations on the onset of collective motion and show how active fluctuations in the active Brownian dynamics can change the phase-transition behaviour of the system. In particular, we show that active angular fluctuation lead to an earlier breakdown of collective motion and to emergence of a new bistable regime in the mean-field case.Comment: 5 figures, 22 pages, submitted to New Journal of Physic

Towards an experimental von Karman dynamo: numerical studies for an optimized design

Numerical studies of a kinematic dynamo based on von Karman type flows between two counterrotating disks in a finite cylinder are reported. The flow has been optimized using a water model experiment, varying the driving impellers configuration. A solution leading to dynamo action for the mean flow has been found. This solution may be achieved in VKS2, the new sodium experiment to be performed in Cadarache, France. The optimization process is described and discussed, then the effects of adding a stationary conducting layer around the flow on the threshold, on the shape of the neutral mode and on the magnetic energy balance are studied. Finally, the possible processes involved into kinematic dynamo action in a von Karman flow are reviewed and discussed. Among the possible processes we highlight the joint effect of the boundary-layer radial velocity shear and of the Ohmic dissipation localized at the flow/outer-shell boundary

The Cerebellar Fastigial Nucleus Contributes to CO\u3csub\u3e2\u3c/sub\u3e-H\u3csup\u3e+\u3c/sup\u3e Ventilatory Sensitivity in Awake Goats

The purpose of this study was to test the hypothesis that an intact cerebellar fastigial nucleus (CFN) is an important determinant of CO2-H+ sensitivity during wakefulness. Bilateral, stainless steel microtubules were implanted into the CFN (N = 9) for injection (0.5–10 μl) of the neurotoxin ibotenic acid. Two or more weeks after implantation of the microtubules, eupneic breathing and CO2-H+ sensitivity did not differ significantly (P \u3e 0.10) from pre-implantation conditions. Injection of ibotenic acid (50 mM) did not significantly alter eupneic PaCO2 (P \u3e 0.10). The coefficient of variation of eupneic PaCO2 was 4.0 ± 0.6 and 3.7 ± 0.4% over the 2 weeks before and after the lesion, respectively. CO2-H+ sensitivity expressed as inspired ventilation/PaCO2 decreased from 2.15 ± 0.17 pre-lesion to 1.58 ± 0.26 l/(min mmHg) 3–6 days post-lesion (P \u3c 0.02, −27%). There was no significant (P \u3e 0.10) recovery of sensitivity between 7 and 10 days post-lesion. The lesion also increased (P \u3c 0.05) the day-to-day variability of this index by nearly 100%. When CO2 sensitivity was expressed as elevated inspired CO2/room air VI, values at 7%, but not 3 and 5% inspired CO2, were reduced and more variable (P \u3c 0.05) after the ibotenic acid injections. We conclude that during wakefulness, the CFN contributes relatively more to overall ventilatory drive at high relative to low levels of hypercapnia