Individual Velocity Components Research Articles

Resolved Simulation of the Clarification and Dewatering in Decanter Centrifuges

Solid–liquid separation is a fundamental operation in process engineering and thus an important part of many process chains in the preparation of slurries in the chemical industry and other parts of the industrial environment. For the separation of micron-sized particles which, due to their size, do not settle or settle very slowly in the earth’s gravity field, centrifuges are often used. The preferred choice are often decanter centrifuges because they work continuously and stabilize the process against product fluctuations due to their adjustment possibilities. The design of the apparatus is complex: The main components of the apparatus are the cylindrical-conical bowl, which rotates at a high speed, and a screw located inside the bowl, which rotates in the same direction at a low differential speed to transport the separated solids out of the apparatus. Geometrical properties of the apparatus, as well as the adjustable operating parameters, such as rotational speed or differential speed, have a significant influence on the separation. In practice, analytical models and the experience of the manufacturers form the basis for the design. Characteristics of the disperse phase, interactions with the liquid, as well as the influence of the flow on the separation, are not taken into account. As a consequence, the transfer to industrial scale always requires a large number of pilot-scale experiments, which are time-consuming and expensive. Due to the increasing computational power, computational fluid dynamics (CFD) provides one possibility to minimize the experimental effort in centrifuge design. In this work, the open-source software OpenFOAM is used to simulate the multi-phase flow in a laboratory decanter centrifuge. For validation, experiments were carried out on a laboratory scale and the main operating parameters, such as speed, differential speed, and volume flow rate, were varied. The simulation results show a good agreement with the experimental data. Furthermore, the numerical investigations show the influence of the flow on the separation of the particles. To evaluate the transportability of a material, the transport efficiency was introduced as a dimensionless parameter. In addition, the simulation allows the consideration of the individual velocity components, making it possible to generate an impression of the complex three-dimensional flow in the apparatus for the first time.

Modeling wind-driven seed dispersal using a coupled Lagrangian particle tracking and 1-D [formula omitted]-[formula omitted] turbulence model

We present a one-dimensional k-ɛ model and a Markov chain stochastic Lagrangian particle tracking model to study wind-driven dispersal of forest and grassland seeds. The turbulence model within and above the canopy is implemented for transient winds so that time series of profiles of mean velocity, turbulence kinetic energy, dissipation rate, and the root-mean-square of fluctuating velocity components can be modeled. We validate the model for steady state conditions using observations in a wide range of canopies, and apply the model for predicting time series of seed dispersal of Andropogon gerardii in a tall grass prairie in Missouri, United States. The stochastic seed dispersal model is validated against experimental data from a Quercus-Carya (oak-hickory) forest where seeds were released from different heights for a range of wind speeds. Using the model, we explore the detailed seed movement trajectories, instantaneous velocities, and investigate the relationship between dispersal distance and the seed falling duration and the mean fluctuating velocity during the falling. Moreover, examining the role of individual turbulent velocity components, we elucidate the different roles of horizontal and vertical velocity fluctuations, i.e., the horizontal component mainly contributes to the seed spreading in the wind direction, while the vertical component enhances the long tail of the distribution in the downstream direction, i.e., promoting long-distance-dispersal of seeds by wind.

Decomposing magnetic fields in three dimensions over the central molecular zone

ABSTRACT Measuring magnetic fields in the interstellar medium and obtaining their distribution along line-of-sight (LOS) is very challenging with the traditional techniques. The Velocity Gradient Technique (VGT), which utilizes anisotropy of magnetohydrodynamic turbulence, provides an attractive solution. Targeting the central molecular zone (CMZ), we test this approach by applying the VGT to $\rm ^{12}CO$ and $\rm ^{13}CO$ (J = 1–0) data cubes. We first used the scousepy algorithm to decompose the CO line emissions into separate velocity components, and then we constructed pseudo-Stokes parameters via the VGT to map the plane-of-the-sky magnetic fields in three-dimension. We present the decomposed magnetic field maps and investigate their significance. While the LOS integrated magnetic field orientation is shown to be consistent with the polarized dust emission from the Planck survey at 353 GHz, individual velocity components may exhibit different magnetic fields. We present a scheme of magnetic field configuration in the CMZ based on the decomposed magnetic fields. In particular, we observe a nearly vertical magnetic field orientation in the dense clump near the Sgr B2 and a change in the outflow regions around the Sgr A*. Two high-velocity structures associated with an expanding ring in the CMZ show distinct swirling magnetic field structures. These results demonstrate the potential power of the VGT to decompose velocity or density-dependent magnetic structures.

FEEDBACK from the NGC 7538 H II region

Context. The interaction of expanding H II regions with their environmental clouds is one of the central questions driving the Stratospheric Observatory for Infrared Astronomy (SOFIA) legacy program FEEDBACK. Aims. We want to understand the interaction of the prototypical NGC 7538 H II region with the neighboring molecular cloud hosting several active star-forming regions. Methods. Using the SOFIA, we mapped an area of ~210′2 (~125 pc2) around NGC 7538 in the velocity-resolved ionized carbon fine-structure line [CII] at 1.9 THz (158 μm). Complementary observed atomic carbon [CI] at 492 GHz and high-J CO(8–7) data, as well as archival near- and far-infrared, cm continuum, CO(3–2), and HI data are folded into the analysis. Results. The ionized carbon [CII] data reveal rich morphological and kinematic structures. While the overall morphology follows the general ionized gas that is also visible in the radio continuum emission, the channel maps show multiple bubble-like structures with sizes on the order of ~80–100″ (~1.0–1.28 pc). While at least one of them may be an individual feedback bubble driven by the main exciting sources of the NGC 7538 H II region (the O3 and O9 stars IRS6 and IRS5), the other bubble-like morphologies may also be due to the intrinsically porous structure of the H II region. An analysis of the expansion velocities around 10 km s−1 indicates that thermal expansion is not sufficient but that wind-driving from the central O-stars is required. The region exhibits a general velocity gradient across, but we also identify several individual velocity components. The most blue-shifted [CII] component has barely any molecular or atomic counterparts. At the interface to the molecular cloud, we find a typical photon-dominated region (PDR) with a bar-shape. Ionized C+, atomic C0 and molecular carbon CO show a layered structure in this PDR. The carbon in the PDR is dominated by its ionized C+ form with atomic C0 and molecular CO masses of ~0.45 ± 0.1 M⊙ and ~1.2 ± 0.1 M⊙, respectively, compared to the ionized carbon C+ in the range of 3.6−9.7 M⊙. This bar-shaped PDR exhibits a velocity-gradient across, indicating motions along the line of sight toward the observer. Conclusions. Even if it is shown to be dominated by two nearby exciting sources (IRS6 and IRS5), the NGC 7538 H II region exhibits a diverse set of substructures that interact with each other as well as with the adjacent cloud. Compared to other recent [CII] observations of H II regions (e.g., Orion Veil, RCW120, RCW49), the bubble-shape morphologies revealed in [CII] emission that are indicative of expanding shells are recurring structures of PDRs.

Excitation and evolution of coronal oscillations in self-consistent 3D radiative MHD simulations of the solar atmosphere

Context. Solar coronal loops are commonly subject to oscillations. Observations of coronal oscillations are used to infer physical properties of the coronal plasma using coronal seismology. Aims. Excitation and evolution of oscillations in coronal loops is typically studied using highly idealised models of magnetic flux tubes. In order to improve our understanding of coronal oscillations, it is necessary to consider the effect of realistic magnetic field topology and evolution. Methods. We study excitation and evolution of coronal oscillations in three-dimensional (3D) self-consistent simulations of solar atmosphere spanning from the convection zone to the solar corona using the radiation-magnetohydrodynamic (MHD) code Bifrost. We use forward-modelled extreme-ultraviolet emission and 3D tracing of magnetic field to analyse the oscillatory behaviour of individual magnetic loops. We further analyse the evolution of individual plasma velocity components along the loops using wavelet power spectra to capture changes in the oscillation periods. Results. Various types of oscillations commonly observed in the corona are present in the simulation. We detect standing oscillations in both transverse and longitudinal velocity components, including higher-order oscillation harmonics. We also show that self-consistent simulations reproduce the existence of two distinct regimes of transverse coronal oscillations: rapidly decaying oscillations triggered by impulsive events and sustained small-scale oscillations showing no observable damping. No harmonic drivers are detected at the footpoints of oscillating loops. Conclusions. Coronal loop oscillations are abundant in self-consistent 3D MHD simulations of the solar atmosphere. The dynamic evolution and variability of individual magnetic loops suggest that we need to re-evaluate our models of monolithic and static coronal loops with constant lengths in favour of more realistic models.

C II] 158 μm self-absorption and optical depth effects

Context. The [C II] 158 μm far-infrared fine-structure line is one of the most important cooling lines of the star-forming interstellar medium (ISM). It is used as a tracer of star formation efficiency in external galaxies and to study feedback effects in parental clouds. High spectral resolution observations have shown complex structures in the line profiles of the [C II] emission. Aims. Our aim is to determine whether the complex profiles observed in [12C II] are due to individual velocity components along the line-of-sight or to self-absorption based on a comparison of the [12C II] and isotopic [13C II] line profiles. Methods. Deep integrations with the SOFIA/upGREAT 7-pixel array receiver in the sources of M43, Horsehead PDR, Monoceros R2, and M17 SW allow for the detection of optically thin [13C II] emission lines, along with the [12C II] emission lines, with a high signal-to-noise ratio. We first derived the [12C II] optical depth and the [C II] column density from a single component model. However, the complex line profiles observed require a double layer model with an emitting background and an absorbing foreground. A multi-component velocity fit allows us to derive the physical conditions of the [C II] gas: column density and excitation temperature. Results. We find moderate to high [12C II] optical depths in all four sources and self-absorption of [12C II] in Mon R2 and M17 SW. The high column density of the warm background emission corresponds to an equivalent Av of up to 41 mag. The foreground absorption requires substantial column densities of cold and dense [C II] gas, with an equivalent Av ranging up to about 13 mag. Conclusions. The column density of the warm background material requires multiple photon-dominated region surfaces stacked along the line of sight and in velocity. The substantial column density of dense and cold foreground [C II] gas detected in absorption cannot be explained with any known scenario and we can only speculate on its origins.

GAUSSPY+: A fully automated Gaussian decomposition package for emission line spectra

Our understanding of the dynamics of the interstellar medium is informed by the study of the detailed velocity structure of emission line observations. One approach to study the velocity structure is to decompose the spectra into individual velocity components; this leads to a description of the data set that is significantly reduced in complexity. However, this decomposition requires full automation lest it become prohibitive for large data sets, such as Galactic plane surveys. We developed GAUSSPY+, a fully automated Gaussian decomposition package that can be applied to emission line data sets, especially large surveys of HI and isotopologues of CO. We built our package upon the existing GAUSSPY algorithm and significantly improved its performance for noisy data. New functionalities of GAUSSPY+ include: (i) automated preparatory steps, such as an accurate noise estimation, which can also be used as stand-alone applications; (ii) an improved fitting routine; (iii) an automated spatial refitting routine that can add spatial coherence to the decomposition results by refitting spectra based on neighbouring fit solutions. We thoroughly tested the performance of GAUSSPY+ on synthetic spectra and a test field from the Galactic Ring Survey. We found that GAUSSPY+ can deal with cases of complex emission and even low to moderate signal-to-noise values.

Kinematics of Circumgalactic Gas: Feeding Galaxies and Feedback

Abstract We present observations of 50 pairs of redshift z ≈ 0.2 star-forming galaxies and background quasars. These sightlines probe the circumgalactic medium (CGM) out to half the virial radius, and we describe the circumgalactic gas kinematics relative to the reference frame defined by the galactic disks. We detect halo gas in Mg ii absorption, measure the equivalent-width-weighted Doppler shifts relative to each galaxy, and find that the CGM has a component of angular momentum that is aligned with the galactic disk. No net counter-rotation of the CGM is detected within 45° of the major axis at any impact parameter. The velocity offset of the circumgalactic gas correlates with the projected rotation speed in the disk plane out to disk radii of roughly 70 kpc. We confirm previous claims that the Mg ii absorption becomes stronger near the galactic minor axis, and we show that the equivalent width correlates with the velocity range of the absorption. We cannot directly measure the location of any absorber along the sightline, but we explore the hypothesis that individual velocity components can be associated with gas orbiting in the disk plane or flowing radially outward in a conical outflow. We conclude that centrifugal forces partially support the low-ionization gas and galactic outflows kinematically disturb the CGM producing excess absorption. Our results firmly rule out schema for the inner CGM that lack rotation and suggest that angular momentum as well as galactic winds should be included in any viable model for the low-redshift CGM.

The effect of magnetic field on perturbation evolution in homogeneously sheared flows

The goal of this paper is to identify and characterize the fundamental mechanisms that contribute toward flow stabilization in sheared plasma flows. Toward that end, we investigate the evolution of velocity and magnetic field perturbations in homogeneously sheared magnetohydrodynamic (MHD) flows subjected to an imposed streamwise magnetic field. The influences of magnetic field strength ($B_{0}$) and perturbation wavevector orientation ($\unicode[STIX]{x1D6FD}$) are characterized using linear analysis and direct numerical simulations. The linear analysis of ideal MHD indicates that the perturbation evolution is governed by four processes: pressure redistribution, kinetic energy production, kinetic–magnetic energy exchange and magnetic energy production due to magnetic stretching. The interplay between these processes can be characterized by the ratio of shear-to-magnetic timescales ($R_{A}\equiv V_{A}\unicode[STIX]{x1D705}/S$) and $\unicode[STIX]{x1D6FD}$, where $V_{A}$, $\unicode[STIX]{x1D705}$ and $S$ are Alfvén wave speed, initial wavenumber and mean flow shear, respectively. For cases with low values of $\boldsymbol{R}_{\boldsymbol{A}}~(\ll \mathbf{1})$, a three-stage perturbation evolution is seen. At the first stage, pressure redistribution and production dominate leading to hydrodynamic-type behaviour. In the second stage, the onset of magnetic stretching process leads to an increase in magnetic energy. At late stages, production subsides and the dynamics is dominated by harmonic exchange between velocity and magnetic fields. For cases of $\boldsymbol{R}_{\boldsymbol{A}}\approx \mathbf{1}$, the magnetic field reacts rapidly enough that hydrodynamic and magnetic production stages occur simultaneously followed by harmonic exchange. In the case of $\boldsymbol{R}_{\boldsymbol{A}}\gg 1$, all three stages occur simultaneously leading to harmonic exchange between kinetic and magnetic energies without any perturbation growth. For all cases considered, the late stage harmonic exchange results in equipartition between perturbation magnetic and kinetic energies. For a given $R_{A}$, the effect of increasing $\unicode[STIX]{x1D6FD}$ is to reduce the intensity of coupling and progressively slow down the three stages of evolution. For spanwise wavevector perturbations, the velocity–magnetic field interaction mechanism vanishes and there is no effect of pressure or magnetic field on individual velocity components.

Turbulent flow structure at a 90-degree open channel confluence: Accounting for the distortion of the shear layer

City channels often have a smaller width-to-depth ratio in comparison to natural rivers due to the limited land availability. The penetration of the tributary into the main channel can cause the distortion of the shear layer. The main purpose of the present study is to investigate the mean and turbulent flow structure in the distorted shear layer in a discharge-adjustable plexiglass circulating flume. Three-dimensional velocities were collected and hydrodynamics and turbulence characteristics such as mean velocity field, turbulent kinetic energy, Reynolds shear stress, turbulence spectrum, and occurrence probabilities of quadrant events were analyzed. The results showed that a stronger helical cell was formed and extended for a longer distance downstream when the tributary channel had a higher flow rate than the main channel. The maximum Reynolds shear stress and the ejection and sweep events were mainly distributed at the middle zone of the water depth, rather than near the water surface, which were coincident with the shear layer as indicated by the turbulence kinetic energy. No obvious energy concentration was observed, and the power law relations for individual velocity components all had an exponent slightly larger than −5/3 in the flow frequency. The distortion of the shear layer resulted in an increase in occurrence probabilities of ejection and sweep events within the shear layer, which were related to the turbulence presenting vortices induced by wall. If the discharge ratio remained unchanged, an increase in the discharge of both channels resulted in an increase in some parameters, such as velocity, turbulent kinetic energy, and the absolute values of Reynolds shear stress, while the shear layer was distorted to a larger extent as the discharge of each channel decreased. All these results suggested that sediment transport, bed morphology and contaminant transport in the distorted shear layer at city channel confluences may differ significantly from that at natural river confluences.

Velocity and energy distributions in microcanonical ensembles of hard spheres.

In a microcanonical ensemble (constant NVE, hard reflecting walls) and in a molecular dynamics ensemble (constant NVEPG, periodic boundary conditions) with a number N of smooth elastic hard spheres in a d-dimensional volume V having a total energy E, a total momentum P, and an overall center of mass position G, the individual velocity components, velocity moduli, and energies have transformed beta distributions with different arguments and shape parameters depending on d,N,E, the boundary conditions, and possible symmetries in the initial conditions. This can be shown marginalizing the joint distribution of individual energies, which is a symmetric Dirichlet distribution. In the thermodynamic limit the beta distributions converge to gamma distributions with different arguments and shape or scale parameters, corresponding respectively to the Gaussian, i.e., Maxwell-Boltzmann, Maxwell, and Boltzmann or Boltzmann-Gibbs distribution. These analytical results agree with molecular dynamics and Monte Carlo simulations with different numbers of hard disks or spheres and hard reflecting walls or periodic boundary conditions. The agreement is perfect with our Monte Carlo algorithm, which acts only on velocities independently of positions with the collision versor sampled uniformly on a unit half sphere in d dimensions, while slight deviations appear with our molecular dynamics simulations for the smallest values of N.

Robust attitude control design for spacecraft under assigned velocity and control constraints

A novel robust nonlinear control design under the constraints of assigned velocity and actuator torque is investigated for attitude stabilization of a rigid spacecraft. More specifically, a nonlinear feedback control is firstly developed by explicitly taking into account the constraints on individual angular velocity components as well as external disturbances. Considering further the actuator misalignments and magnitude deviation, a modified robust least-squares based control allocator is employed to deal with the problem of distributing the previously designed three-axis moments over the available actuators, in which the focus of this control allocation is to find the optimal control vector of actuators by minimizing the worst-case residual error using programming algorithms. The attitude control performance using the controller structure is evaluated through a numerical example.

A method for characterising the sensitivity of turbulent flow fields to the structure of inlet turbulence

The specification of inlet conditions for eddy-resolving simulations is an important research question in turbulence research. A large number of schemes have been proposed but comparisons to a benchmark simulation are usually undertaken in such a way that it is ambiguous as to whether the source of the improved preservation of the properties of the benchmark case is due to the physics captured in the inlet generation algorithm or to differences in the precise values or spectra generated at a particular position. This article uses gradual wavelet reconstruction to degrade a precursor simulation such that the time series input at each cell has the same histogram and Fourier spectrum as the original precursor case, thereby removing this source of variability. A parameter, ρ, is used to index the nature of the degraded inlets and as ρ increases from 0 to 1 the cross-correlative and nonlinear properties of the precursor are increasingly preserved too. We compare the results of four large-eddy simulations for the flow over a wall-mounted square rib and show that there are clear differences between these simulations for the flow before and over the rib, and in the far-field. However, in the wake, the intense shearing and mixing means that any differences are harder to attribute to the nature of the inlet. Hence, for flows that are mixed strongly, there is less of a need to preserve the cross-correlative structure of the inlet condition as long as the individual velocity components are modelled with an appropriate set of values and Fourier spectrum. By comparing particular inlet generation algorithms in terms of the value for ρ that they can attain, their suitability for modelling flows of particular configurations may be determined.

Fragmentation and dynamics in Massive Dense Cores in Cygnus-X

AbstractA systematic, high angular-resolution study of IR-quiet Massive Dense Cores (MDCs) of Cygnus-X in continuum and high-density molecular tracers is presented. The results are compared with the quasi-static and the dynamical evolutionary scenario. We find that the fragmentation properties are not compatible with the quasi-static, monolithic collapse scenario, nor are they entirely compatible with the formation of a cluster of mostly low-mass stars. The kinematics of MDCs shows individual velocity components appearing as coherent flows, which indicate important dynamical processes at the scale of the mass reservoir around high-mass protostars.

Study on the Flow Field around Two Parallel Moving Bubbles and Interaction Between Bubbles Rising in CMC Solutions by PIV

The flow fields surrounding two parallel moving bubbles rising from two identical orifices submerged in non-Newtonian fluid of carboxymethylcellulose (CMC) solution of three different mass concentration were measured experimentally by the use of particle image velocimetry (PIV). The influences of gas flowrate, solution mass concentration, orifice interval and the angle between two bubble centers line and vertical direction on the flow field surrounding bubbles were discussed respectively by analyzing the velocity vector, velocity contours as well as individual velocity components. The results show that the liquid velocity both in front of two bubbles and behind increases with gas flowrate duo to shear-thinning effect of previous bubbles, whereas decreases with the increase of CMC concentration due to the increase of drag force acting on bubbles. The effect of the orifice interval on the flow field around two moving bubbles becomes gradually obvious as the interval becomes closer. Moreover, two adjacent side-by-side bubbles repulse each other during rising, leading to the practical interval between them increased somewhat above the orifice interval. When the distance between bubbles is less than the orifice interval l0 mm, the interaction between two neighboring bubbles changed from mutual repellence to attraction with the decrease of the angle of the line of linking two bubble centers to the vertical direction.

Cloud Structure and Physical Conditions in Star‐forming Regions from Optical Observations. II. Analysis

To complement the optical absorption-line survey of diffuse molecular gas in Paper I, we obtained and analyzed far ultraviolet H$_2$ and CO data on lines of sight toward stars in Cep OB2 and Cep OB3. Possible correlations between column densities of different species for individual velocity components, not total columns along a line of sight as in the past, were examined and were interpreted in terms of cloud structure. The analysis reveals that there are two kinds of CH in diffuse molecular gas: CN-like CH and CH$^+$-like CH. Evidence is provided that CO is also associated with CN in diffuse molecular clouds. Different species are distributed according to gas density in the diffuse molecular gas. Both calcium and potassium may be depleted onto grains in high density gas, but with different dependences on local gas density. Gas densities for components where CN was detected were inferred from a chemical model. Analysis of cloud structure indicates that our data are generally consistent with the large-scale structure suggested by maps of CO millimeter-wave emission. On small scales, the gas density is seen to vary by factors greater than 5.0 over scales of $\sim$ 10,000 AU. The relationships between column densities of CO and CH with that of H$_2$ along a line of sight show similar slopes for the gas toward Cep OB2 and OB3, but the CO/H$_2$ and CH/H$_2$ ratios tend to differ which we ascribe to variation in average density along the line of sight.

The Complex Interstellar Na i Absorption toward h and χ Persei

ABSTRACTRecent high spatial and spectral resolution investigations of the diffuse interstellar medium (ISM) have found significant evidence for small‐scale variations in the interstellar gas on scales ≤1 pc. To better understand the nature of small‐scale variations in the ISM, we have used the Kitt Peak National Observatory (KPNO) WIYN Hydra multiobject spectrograph, which has a mapping advantage over the single‐axis, single‐scale limitations of studies using high proper motion stars and binary stars, to obtain moderate‐resolution (∼12 km s−1) interstellar Na i D absorption spectra of 172 stars toward the double open cluster h and χ Persei. All of the sight lines toward the 150 stars with spectra that reveal absorption from the Perseus spiral arm show different interstellar Na i D absorption profiles in the Perseus arm gas. Additionally, we have utilized the KPNO coudé feed spectrograph to obtain high‐resolution (∼3 km s−1) interstellar Na i D absorption spectra of 24 of the brighter stars toward h and χ Per. These spectra reveal an even greater complexity in the interstellar Na i D absorption in the Perseus arm gas and show individual components changing in number, velocity, and strength from sight line to sight line. If each of these individual velocity components represents an isolated cloud, then it would appear that the ISM of the Perseus arm gas consists of many small clouds. Although the absorption profiles vary even on the smallest scales probed by these high‐resolution data (∼30″, ∼0.35 pc), our analysis reveals that some interstellar Na i D absorption components from sight line to sight line are related, implying that the ISM toward h and χ Per is probably composed of sheets of gas in which we detect variations due to differences in the local physical conditions of the gas.

Ionized Gas in the First 10 Kiloparsecs of the Interstellar Galactic Halo

We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of the post-asymptotic giant branch star von Zeipel 1128 (l = 425, b = +787; d = 10.2 kpc; z = 10.0 kpc; vLSR = -140 ± 8 km s-1), located in the globular cluster Messier 3. The FUSE observations cover the wavelength range 905-1187 Å at ~20 km s-1 (FWHM) resolution. These data exhibit many photospheric and interstellar absorption lines, including absorption from ions associated with the warm neutral, warm ionized, and highly ionized phases of the interstellar medium along this sight line. We derive interstellar column densities of H I, P II, Ar I, Fe II, Fe III, S III, and O VI, with lower limits for C II, C III, N I, O I, and Si II. Though the individual velocity components within the absorption profiles are unresolved by FUSE, a comparison of the velocity distribution of depleted or ionized species with the neutral species suggests that the thick-disk material along this sight line is infalling onto the Galactic plane, while material near the plane is seen closer to rest velocities. Ionized hydrogen represents ≳12%, most likely ~45%, of the total hydrogen column along this sight line, most of it associated with the warm ionized phase. The warm ionized and neutral media toward von Zeipel 1128 have very similar gas-phase abundances and kinematics: the neutral and ionized gases in this region of the thick disk are closely related. Strong O VI absorption is seen with the same central velocity as the warm ionized gas, although the O VI velocity dispersion is much higher (σ ≡ b = 32 km s-1). Virtually all of the O VI is found at velocities where lower ionization gas is seen, suggesting that the O VI and WNM/WIM probes are tracing different portions of the same structures (e.g., the O VI may reside in interfaces surrounding the WNM/WIM clouds). We see no evidence for interstellar absorption associated with the globular cluster Messier 3 itself or with the circumstellar environment of von Zeipel 1128. Neither high-velocity cloud absorption (with |vLSR|≳125 km s-1) nor high velocity dispersion gas (with σ ~ 60 km s-1) is seen toward von Zeipel 1128.

Error propagation bounds in dual and triple beam vector Doppler ultrasound.

Measurement of the velocity components along two (three) directions enables the two (three)-dimensional velocity vector to be estimated exactly. However, in practical systems employing such multiple beam techniques, there will usually be errors in the measured velocity components along each beam, which will lead to errors in the estimated velocity magnitude and direction. This error propagation problem is analyzed in both two and three dimensions by decomposition of the velocity estimation matrix, and exact upper and lower bounds are derived for both the magnitude and angle bias as a function of the angle between the beams. The bias in triple beam systems is shown to have identical bounds to that in dual beam systems with an equivalent interbeam angle. It is found that small errors in the individual beam velocity components can be magnified in the final determination of velocity magnitude and angle. Plots are presented to assist system designers to specify the interbeam angle(s) to avoid gross velocity estimation errors.

Ultra-high-resolution observations of interstellar Na i and K i towards the Scorpius OB1 association

I present ultra-high-resolution (R 9 × 10 5 ) observations of interstellar NaI and K I absorption lines towards three members of the Scorpius OB1 association (HD 152235, 152236 and 152249). These observations have, for the first time, resolved the intrinsic linewidths of most of the discrete absorption components present along these complex sightlines. The aims of the project were twofold: (i) to constrain the physical conditions prevailing in the highly blueshifted Sco OB 1 shell components, and especially to search for evidence of active shocks within them; and (ii) to further constrain the low-velocity structure, where the Na I spectra are fully saturated but the unsaturated K I lines allow a clear resolution of the individual velocity components. The results of these analyses are discussed. Perhaps the most surprising result is the lack of any obvious correlation between the velocity dispersion of a velocity component (b-value) and its velocity. Specifically, the high-velocity shell components are generally found to be no broader than the low-velocity components attributed to foreground (often molecular) clouds, and cannot therefore be any hotter or more turbulent. Thus, with the possible exception of the most blueshifted component towards HD 152236, there is no evidence for active shocks in the shell components at present. However, consideration of the relative time-scales for post-shock cooling and grain surface adsorption indicates that shock processing in the past may still account for the low Na I/Ca II ratios of these components found in previous work.