Azimuthal Velocity Distribution Research Articles

Constraints on Intracluster Gas Bulk Motions in Clusters of Galaxies with ASCA

The recent detection of Doppler shifts of X-ray spectral lines in the intracluster gas of the Centaurus and Perseus clusters of galaxies with ASCA has raised the question of the frequency of such velocity gradients and their relation to other cluster properties. To address this and improve the sample of ICM velocity gradient candidates we carried out a systematic search of the ASCA archive for clusters with suitable observing conditions that allow full azimuthal mapping of gas velocities near the central regions. Here we present the results of the azimuthal velocity distribution of a sample consisting of the best observed clusters for velocity analysis. Our sample encompasses clusters with different physical and morphological characteristics, including both well-behaved cooling flow and dynamically active clusters. The gas temperatures of the clusters in our sample range from 1.4 to 6.3 keV. We find velocity substructures in ~15% of the clusters in our sample on the order of a few ×103 km s-1. The significance of the velocity gradients' is greater than 99% in Abell 576 and RX J0419.6+0225, the clusters where velocity gradients are most significant. The velocity gradients are consistent with transitory and/or rotational bulk motion. The velocity gradients in these clusters are unlikely to be due to projected temperature anisotropies, ASCA PSF, intrachip gain variations, and flux contamination from other sources. Chandra images of these clusters do not show obvious cluster alignments within the field of view covered by ASCA. We also noticed a high frequency (~50%) of anisotropic temperature gradients in the core of cooling flow clusters, suggesting the presence of frequent dynamical activity in the core of galaxy clusters.

A versatile single-screw-extruder system designed for magnetic resonance imagingmeasurements

A versatile system has been developed for magnetic resonance imaging (MRI)measurements, in which a ceramic barrel/outer cylinder (0.04 m internal diameter)can be configured either as a single-screw extruder (polyetheretherketone (PEEK),length to diameter ratio 4.575, root diameter 0.03 m), or as a concentric-cylinderCouette device (PEEK, length 0.156 m, inner cylinder diameter 0.03 m). A secondchannel in the sample inlet allows two streams of fluid to be pumpedsimultaneously through the system for mixing. The shaft rotation speed can be setbetween 5 and 1200 revolutions per minute (rpm); the barrel and sample feedercan be separately thermostatted to ±0.2 °C in the range of−10 to +60 °Cvia coolant jacket systems; samples with viscosity up to 10 Pa scan be pumped at rates up to 36 l h−1.This enables studies to be conducted with the system configured as a Couettedevice to provide knowledge of the rheological properties of complex fluids beforemore complicated studies of their flow and mixing with the system configured as asingle-screw extruder.Bench and MRI measurements have been carried out to testthe thermostat function of the system. The bench tests showed that the internalvolume of the device reached thermal equilibrium after 1 h of running and could bemaintained at constant temperature (within ±0.2 °C) forperiods of over 6 h. The MRI tests were conducted with the device configured in a Couettegeometry for measurements of the flow velocities of pure glycerol and 1% aqueoussodium carboxymethylcellulose (CMC) in the range of 10–60 °C, and atvarious rotation speeds. Results showed that although the azimuthal velocity distributions versusthe radius (v(r))were independent of temperature for glycerol, there was strong temperaturedependence for the CMC solution. On the latter the power-law index(n) from MRIdata agreed well with the literature values for the same concentrations and temperatures,and showed nvalues increasing with increase of temperature; this demonstratedthat the temperature control was reliable. However, thenvalues increased with decrease of the average shear rate; this is incontrast to the results from conventional rheometry, for which thenvalue is constant, regardless of shear rates at each temperature. Thiswas attributed to the differences between conventional, very-narrow-gapCouette rheometers and the MRI, wide-gap device, and showed theadvantage of using MRI for wide-gap concentric-cylinder rheometry.Couette flow of an aqueous suspension of 35% (w/w)tapioca starch, made of 37% (w/w) sucrose and 1% (w/w)CMC solutions, was measured at 20 °Cand a rotation speed of 20 rpm. This showed that wall slip developed with time,especially at the inner wall of the rotating cylinder.Velocity measurements in threedirections were made for the flow of 1% CMC solution at 22 °Cthrough the single-screw extruder using the pulsed-field gradient spin-echo imagingsequence for two-dimensional transverse or coronal images. Two-dimensional MRI formixing of fluids (1% CMC) in the single-screw extruder at 22 °Cis also demonstrated.

Three-dimensional numerical simulation of Marangoni flow instabilities in floating zones laterally heated by an equatorial ring

Instability of Marangoni convection in floating zones (full zone configuration) of a low Prandtl number fluid under microgravity conditions is investigated by parallel supercalculus and direct three-dimensional and time-dependent simulation of the problem. A parametric analysis (still absent in literature) of the influence of the aspect ratio of the liquid column on the features of the three-dimensional bifurcation of Marangoni flow is carried out. A novel distribution is introduced for the surface heat flux corresponding to the radiative flux generated by a ring heater positioned around the equatorial plane of the full zone at a distance h from the free interface. Axisymmetric computations are used to obtain the steady basic state, then the three-dimensional Navier–Stokes equations are solved to investigate the evolution of azimuthal disturbances. These disturbances always exhibit antisymmetric behavior with respect to the equatorial plane. The mirror symmetry with respect to this plane is broken. Strong interaction occurs in fact between the toroidal convection rolls located in the upper part and lower part of the liquid column. This leads for some values of the aspect ratio to a heretofore unseen “apparent” doubling or quadrupling of the azimuthal wave number of the azimuthal velocity distribution in the midplane. The present analysis points out that the instability of the half zone flow is not relevant for the full zone configuration.

Non-parametric reconstruction of distribution functions from observed galactic discs

A general inversion technique for the recovery of the underlying distribution function for observed galactic discs is presented and illustrated. Under the assumption that these discs are axisymmetric and thin, the proposed method yields a unique distribution compatible with all the observables available. The derivation may be carried out from the measurement of the azimuthal velocity distribution arising from positioning the slit of a spectrograph along the major axis of the galaxy. More generally, it may account for the simultaneous measurements of velocity distributions corresponding to slits presenting arbitrary orientations with respect to the major axis. The approach is non-parametric, i.e. it does not rely on a particular algebraic model for the distribution function. Special care is taken to account for the fraction of counter-rotating stars, which strongly affects the stability of the disc. An optimization algorithm is devised — generalizing the work of Skilling &38; Bryan — to carry this truly two-dimensional ill-conditioned inversion efficiently. The performance of the overall inversion technique with respect to the noise level and truncation in the data set is investigated with simulated data. Reliable results are obtained up to a mean signal-to-noise ratio of 5, and when measurements are available up to 4 Re. A discussion of the residual biases involved in non-parametric inversions is presented. The prospects of application of the algorithm to observed galaxies and other inversion problems are discussed.

Generation of Supersonic Swirling Flow Using Asymmetric Nozzles.

For performance improvement of a disk-shaped MHD generator, it is desirable to give the working inert gas not only radial velocity but also azimuthal velocity at the exit of a supersonic nozzle. Swirl vanes have been installed in the nozzle section, which can induce nonuniform azimuthal velocity distribution. In order to reduce the nonuniform velocity, a new swirl vane using asymmetric nozzles is designed, and its effect is confirmed by wind-tunnel experiment and two-dimensional numerical simulation. Then, the optimized design of the swirl vane is proposed based on a combination of those results.

Behavior of streamwise rib vortices in a three-dimensional mixing layer

The structure and behavior of a streamwise rib vortex in a direct numerical simulation of a time-developing three-dimensional incompressible plane mixing layer is examined. Where the rib vortex is being stretched, the vorticity vector is primarily directed in the vortex axial direction and the radial and azimuthal velocity distribution is similar to that of a Burgers vortex. In the region where the vortex stretching is negative, there is a change in the local topology of the vortex. The axial flow is decelerated and a negative azimuthal component of vorticity is induced. These features are characteristic of vortex breakdown. The temporal evolution of the rib vortex is similar to the evolution of an axisymmetric vortex in the early stages of vortex breakdown. The effect of vortex breakdown on other parts of the flow is, however, not as significant as the interaction between the rib vortex and other vortical structures.

Theoretical model of discrete tone generation by impinging jets

It is well known that when a high subsonic (Mach number > 0.7) high Reynolds number (Re > 2 × 105) jet is directed normal to a wall intense discrete frequency sound waves called impingement tones are emitted. This phenomenon has been studied by a number of investigators in the past. It is generally accepted that the tones are generated by a feedback loop. Despite this general agreement critical difference in opinion as to how the feedback is achieved remains unresolved. Early investigators (e.g. Wagner 1971; Neuwerth 1973, 1974) proposed that the feedback loop is closed by acoustic disturbances which propagate from the wall to the nozzle exit inside the jet. Recent investigators (e.g. Ho & Nosseir 1981; Umeda et al. 1987), However, believed that the feedback is achieved by sound waves propagating outside the jet. In this paper a new feedback mechanism is proposed. It is suggested that the feedback is achieved by upstream-propagating waves associated with the lowest-order intrinsic neutral wave modes of the jet flow. These wave modes have well-defined radial and azimuthal pressure and velocity distributions. These distributions are dictated by the mean flow of the jet exactly as in the case of the well-known Kelvin-Helmholtz instability waves. The characteristics of these waves are calculated and studied. These characteristics provide a natural explanation of why the unsteady flow fields of subsonic impinging jets must be axisymmetric, whereas those for supersonic jets may be either axisymmetric or helical (flapping). In addition they also offer, for the first time, an explanation as to why no stable impingement tones have been observed for (cold) subsonic jets with Mach number less than 0.6. Furthermore, the new model allows the prediction of the average Strouhal number of impingement tones as a function of jet Mach number. The predicted results compare very favourably with measurements. For subsonic jets the pressure and velocity field of these upstream-propagating neutral waves are found to be confined primarily inside the jet. This is in agreement with the observations of Wagner (1971) and Neuwerth (1973, 1974) and their contention that the feedback disturbances actually propagate upstream inside the jet.

Measurement of Flow behind Stationary Cylinder Placed in Rotating Gas

In view of the physical relevance to the flow in the scoop chamber of a gas centrifuge, it is of importance to investigate the flow induced by the stationary cylinder located in a rotating gas field. In the present study, simulating the scoop chamber of a gas centrifuge, a stationary cylinder was placed between co-rotating disks and the azimuthal velocity distributions behind it were measured by using a Pitot-static tube. From these experiments the following results were obtained. (1) When the stationary cylinder is placed in a rotating gas, the angular velocity of the gas starts to change at the outer edge of the cylinder and decreases as the rotation axis is approached. (2) In the wake flow behind the stationary cylinder, there exists a variation of the angular velocity with an angular period of 180°.

Measurement of flow behind stationary cylinder placed in rotating gas.

In view of the physical relevance to the flow in the scoop chamber of a gas centrifuge, it is of importance to investigate the flow induced by the stationary cylinder located in a rotating gas field. In the present study, simulating the scoop chamber of a gas centrifuge, a stationary cylinder was placed between co-rotating disks and the azimuthal velocity distributions behind it were measured by using a Pitot-static tube. From these experiments the following results were obtained. (1) When the stationary cylinder is placed in a rotating gas, the angular velocity of the gas starts to change at the outer edge of the cylinder and decreases as the rotation axis is approached. (2) In the wake flow behind the stationary cylinder, there exists a variation of the angular velocity with an angular period of 180°.