We explore the phase-space structure of nearby halo stars identified kinematically from the Gaia second data release (DR2). We focus on their distribution in velocity and in "integrals of motion" ...space, as well as on their photometric properties. Our sample of stars selected to be moving at a relative velocity of at least 210 km s−1, with respect to the Local Standard of Rest, contains an important contribution from the low rotational velocity tail of the disk(s). The VR-distribution of these stars depicts a small asymmetry similar to that seen for the faster rotating thin disk stars near the Sun. We also identify a prominent, slightly retrograde "blob" that traces the metal-poor halo main sequence reported by Gaia Collaboration et al. We also find many small clumps that are especially noticeable in the tails of the velocity distribution of the stars in our sample. Their Hertzsprung-Russell (HR) diagrams disclose narrow sequences characteristic of simple stellar populations. This stream-frosting confirms predictions from cosmological simulations, namely that substructure is most apparent among the fastest moving stars, typically reflecting more recent accretion events.
Coherent plane-wave compounding technique enables rapid ultrasound imaging with comparable image quality to traditional B-mode imaging that relies on focused beam transmission. However, existing ...methods assume homogeneity in the imaged medium, neglecting the heterogeneity in sound velocities and densities present in real tissues, resulting in noise reverberation. This study introduces the Reverse Time Migration (RTM) method for ultrasound plane-wave imaging to overcome this limitation, which is combined with a method for estimating the speed of sound in layered media. Simulation results in a homogeneous background demonstrate that RTM reduces side lobes and grating lobes by approximately 30 dB, enhancing the contrast-to-noise ratio by 20% compared to conventional delay and sum (DAS) beamforming. Moreover, RTM achieves superior imaging outcomes with fewer compounding angles. The lateral resolution of the RTM with 5-9 angle compounding is able to achieve the effectiveness of the DAS method with 15-19 angle compounding, and the CNR of the RTM with 11-angle compounding is almost the same as that of the DAS with 21-angle compounding. In a heterogeneous background, experimental simulations and in vitro wire phantom experiments confirm RTM's capability to correct depth imaging, focusing reflected waves on point targets. In vitro porcine tissue experiments enable accurate imaging of layer interfaces by estimating the velocities of multiple layers containing muscle and fat. The proposed imaging procedure optimizes velocity estimation in complex media, compensates for the impact of velocity differences, provides more reliable imaging results.
Abstract
We explore the dynamical structure of the protoplanetary disks surrounding HD 163296 and MWC 480 as part of the Molecules with ALMA at Planet-forming Scales (MAPS) large program. Using the
J
...= 2–1 transitions of
12
CO,
13
CO, and C
18
O imaged at spatial resolutions of ∼0.″15 and with a channel spacing of 200 m s
−1
, we find perturbations from Keplerian rotation in the projected velocity fields of both disks (≲5% of the local Keplerian velocity), suggestive of large-scale (tens of astronomical units in size), coherent flows. By accounting for the azimuthal dependence on the projection of the velocity field, the velocity fields were decomposed into azimuthally averaged orthogonal components,
v
ϕ
,
v
r
, and
v
z
. Using the optically thick
12
CO emission as a probe of the gas temperature, local variations of ≈3 K (≈5% relative changes) were observed and found to be associated with the kinematic substructures. The MWC 480 disk hosts a suite of tightly wound spiral arms. The spirals arms, in conjunction with the highly localized perturbations in the gas velocity structure (kinematic planetary signatures), indicate a giant planet, ∼1
M
Jup
, at a radius of ≈245 au. In the disk of HD 163296, the kinematic substructures were consistent with previous studies of Pinte et al. and Teague et al. advocating for multiple ∼1
M
Jup
planets embedded in the disk. These results demonstrate that molecular line observations that characterize the dynamical structure of disks can be used to search for the signatures of embedded planets. This paper is part of the MAPS special issue of the Astrophysical Journal Supplement.
Accurate ice flow velocity data are essential for studying the mass balance of the Antarctic ice sheet. However, there is a lack of ice velocity maps of 1960s–80s in basin-wide regions or the entire ...ice sheet. In this study, an enhanced hierarchical network densification approach is developed for basin-wide Antarctic velocity mapping using historical ARGON and Landsat images. The produced multiple historical velocity maps from 1963 to 1989 in the region of the Fimbul and Jelbart ice shelves, East Antarctica, achieved an accuracy better than 29 m a−1. They revealed that the ice flow velocity had no significant changes over the period. Combining the surface mass balance estimate with the ice discharge estimated from our historical velocity maps and recently published velocity maps, we estimated a positive mass balance of 8.6 ± 3.9 Gt a−1 in the study area from 1963 and 2015. Our results indicate that the region's positive mass balance, as estimated in recently published studies, has been maintained since the 1960s. It is also in concordance with the low level of mass balance from 1992 to 2017 in East Antarctica. This suggests that the study area has been stable since the 1960s.
Landslides in mining engineering mainly occur in tailings dams. The evolution of monitoring indexes is important for the improvement of warning accuracy in tailings dam breaks. The velocity field and ...outflow discharge behavior in overtopping dam-break of an iron mine tailings dam were studied. There were two stages of the tailings flow in the model test, corresponding to two types of flow state, granular flow and viscous flow, respectively. The absolute vorticity vertical line |
w
| vertical line was measured to reveal the influence of terrain on the velocity. An estimation equation for the maximum velocity was proposed, in which vorticity and the height difference of terrain were taken into account. The standard deviation of the velocity stack
e
¯
with minus on top was used to distinguish the dam-break flow into “collision” and “non-fluctuating” regions. The velocity profiles of the transversal section were concluded to be three patterns based on the positions of the peak velocities. The three patterns well described the velocity field at different flow states. Estimation equations were proposed for the transversal section velocity profile. Based on the velocity profile equations, the estimation method of the outflow discharge was developed. This study provides a reference for improving the accuracy of tailings dam break warnings by non-intrusive monitoring methods.
The flow of three non-Newtonian fluids, comprising polymer and surfactant additives, in a periodically constricted tube (PCT) are experimentally compared. The radius of the tube walls is sinusoidal ...with respect to the streamwise direction. The three fluids are aqueous solutions of flexible polymers, rigid biopolymers and surfactants, which are typically used for drag-reduction in turbulent flows. Steady shear viscosity measurements demonstrate that rigid and flexible polymer solutions are shear-thinning, while surfactant solutions have a Newtonian and water-like shear viscosity. Capillary driven extensional rheology demonstrates that only flexible polymer solutions produce elastocapillary thinning. Particle shadow velocimetry is used to measure the velocity of each flow within the PCT at five Reynolds numbers spanning roughly 0.5 to 300. Relative to the Newtonian flows, rigid polymer solutions exhibit a blunt velocity profile. Flexible polymer solutions demonstrate a distinct chevron-shaped velocity contour and zones of opposing vorticity when the Deborah number exceeds 0.1. Using the vorticity transport equation, it is revealed that the opposing vorticity zones are coupled with a non-Newtonian torque. The PCT reveals that the surfactant solutions have similar non-Newtonian features as flexible polymer solutions – those being a chevron velocity pattern, opposing vorticity and a finite non-Newtonian torque. This observation is of practical importance since conventional shear and extensional rheometric measurements are not capable of demonstrating non-Newtonian features of the surfactant solutions. The investigation demonstrates that the PCT serves as a viable geometry for showing the non-Newtonian traits of dilute surfactant solutions.
Ocean convergence and the dispersion of flotsam D’Asaro, Eric A.; Shcherbina, Andrey Y.; Klymak, Jody M. ...
Proceedings of the National Academy of Sciences - PNAS,
02/2018, Letnik:
115, Številka:
6
Journal Article
Recenzirano
Odprti dostop
Floating oil, plastics, and marine organisms are continually redistributed by ocean surface currents. Prediction of their resulting distribution on the surface is a fundamental, long-standing, and ...practically important problem. The dominant paradigm is dispersion within the dynamical context of a nondivergent flow: objects initially close together will on average spread apart but the area of surface patches of material does not change. Although this paradigm is likely valid at mesoscales, larger than 100 km in horizontal scale, recent theoretical studies of submesoscales (less than ∼10 km) predict strong surface convergences and downwelling associated with horizontal density fronts and cyclonic vortices. Here we show that such structures can dramatically concentrate floating material. More than half of an array of ∼200 surface drifters covering ∼20 × 20 km² converged into a 60 × 60 m region within a week, a factor of more than 10⁵ decrease in area, before slowly dispersing. As predicted, the convergence occurred at density fronts and with cyclonic vorticity. A zipperlike structure may play an important role. Cyclonic vorticity and vertical velocity reached 0.001 s−1 and 0.01 ms−1, respectively, which is much larger than usually inferred. This suggests a paradigm in which nearby objects form submesoscale clusters, and these clusters then spread apart. Together, these effects set both the overall extent and the finescale texture of a patch of floating material. Material concentrated at submesoscale convergences can create unique communities of organisms, amplify impacts of toxic material, and create opportunities to more efficiently recover such material.
•It is found that the smooth and abrupt transition patterns do exist at high and low Mach numbers.•The transition pattern is understood based on flame propagation.•The competition between the ...detonation velocity and the inflow velocity dominates the transition patterns.
The Oblique Detonation Wave Engine is among the promising choices for hypersonic propulsion systems. It is of great importance to investigate the initiation of the oblique detonation wave. Although it has been demonstrated that there exist two transition patterns from oblique shock wave to oblique detonation wave, i.e., abrupt and smooth, the key parameter that dominates the transition is not clear. A study on the transition patterns of the oblique detonation wave in hydrogen-air mixture with varying temperature is conducted. The oblique detonation wave angle is measured to calculate the detonation velocity. It is found that the effects of the inflow temperature on the detonation velocity are negligible when compared with the effects on the inflow velocity, which means that the inflow velocity dominates the transition patterns when changing the temperature of the mixture. Both smooth and abrupt transitions can exist for a given Mach number by changing the mixture temperature. It is further proved that what determines the smooth and abrupt transitions is the angle difference between the oblique detonation wave and the oblique shock wave (ODW-OSW). Due to a fact that the propagation velocity of a flame front equals to the normal component of the inflow velocity when the flame is stationary, the competition between the detonation velocity and the inflow velocity dominates the ODW angle. Thus, the competition between the detonation velocity and the inflow velocity can act as a criterion for transition patterns.
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