Observed Mean Velocity Research Articles

Radial and vertical flows induced by galactic spiral arms: likely contributors to our ‘wobbly Galaxy’

In an equilibrium axisymmetric galactic disc, the mean Galactocentric radial and vertical velocities are expected to be zero everywhere. In recent years, various large spectroscopic surveys have however shown that stars of the Milky Way disc exhibit non-zero mean velocities outside of the Galactic plane in both the Galactocentric radial and vertical velocity components. While radial velocity structures are commonly assumed to be associated with non-axisymmetric components of the potential such as spiral arms or bars, non-zero vertical velocity structures are usually attributed to excitations by external sources such as a passing satellite galaxy or a small dark matter substructure crossing the Galactic disc. Here, we use a three-dimensional test-particle simulation to show that the global stellar response to a spiral perturbation induces both a radial velocity flow and non-zero vertical motions. The resulting structure of the mean velocity field is qualitatively similar to what is observed across the Milky Way disc. We show that such a pattern also naturally emerges from an analytic toy model based on linearized Euler equations. We conclude that an external perturbation of the disc might not be a requirement to explain all of the observed structures in the vertical velocity of stars across the Galactic disc. Non-axisymmetric internal perturbations can also be the source of the observed mean velocity patterns.

Electronic Baropodometry in Patients Affected by Ocular Torticollis

Aims: To evaluate, by means of electronic baropodometry (EB), the postural findings in patients affected by ocular torticollis.Methods: Posturographic analysis (length of the sway path, sway area, and mean velocity) was made in 54 patients with IV palsy, Duane Syndrome, or rectus superior palsy (group A) and compared with a control group of 45 healthy subjects (group B). The test was performed with both eyes open, then both closed, then with the affected eye open, and finally with the healthy eye open.Results: With both eyes open or closed, the length of the sway path, the sway area, and mean velocity were significantly increased in group A compared with group B (P<0.0001). When the open eye was the one with the muscular paresis, the length of the sway path was significantly increased as compared with the healthy eye (P<0.0001), and the sway area was increased too (P<0.029). No statistical differences were observed mean velocity according to which eye was open (P=NS).Conclusions: EB is a useful instrument for studying secondary postural anomalies in patients affected by OT.

Stratification Effects in a Bottom Ekman Layer

Abstract A stratified bottom Ekman layer over a nonsloping, rough surface is studied using a three-dimensional unsteady large eddy simulation to examine the effects of an outer layer stratification on the boundary layer structure. When the flow field is initialized with a linear temperature profile, a three-layer structure develops with a mixed layer near the wall separated from a uniformly stratified outer layer by a pycnocline. With the free-stream velocity fixed, the wall stress increases slightly with the imposed stratification, but the primary role of stratification is to limit the boundary layer height. Ekman transport is generally confined to the mixed layer, which leads to larger cross-stream velocities and a larger surface veering angle when the flow is stratified. The rate of turning in the mixed layer is nearly independent of stratification, so that when stratification is large and the boundary layer thickness is reduced, the rate of veering in the pycnocline becomes very large. In the pycnocline, the mean shear is larger than observed in an unstratified boundary layer, which is explained using a buoyancy length scale, u*/N(z). This length scale leads to an explicit buoyancy-related modification to the log law for the mean velocity profile. A new method for deducing the wall stress based on observed mean velocity and density profiles is proposed and shows significant improvement compared to the standard profile method. A streamwise jet is observed near the center of the pycnocline, and the shear at the top of the jet leads to local shear instabilities and enhanced mixing in that region, despite the fact that the Richardson number formed using the mean density and shear profiles is larger than unity.

Are cascading flows stable?

The stability of flows cascading down slopes as dense inclined plumes is examined, with particular reference to flows observed in Lake Geneva during winter periods of severe cooling. A previous conjecture by Turner that the flow may be in a state of marginal stability is confirmed: the observed mean velocity and density profiles are unstable to Kelvin–Helmholtz instability, but only marginally so; the growth rates of the most unstable small disturbances to the cascading flow in Lake Geneva are small, with e-folding periods of about 2 h. A reduction in the maximum velocity by about 20% is required to stabilize the flow.The possibility that stationary hydraulic jumps may occur in the observed flow is also considered. Several plausible flow states downstream of transitions are examined, allowing for mixing and density changes to occur, ranging from one that preserves the shape of the density and velocity profiles to one in which, as a consequence of mixing, the velocity and density become uniform in depth within the cascading flow. Neither of these extreme states is found to conserve the fluxes of volume, mass and momentum through a transition in which the energy flux does not increase, and to be unique or ‘stable’ in the sense that no further transition is possible to a similar flow state without more entrainment. Stable transitions to intermediate downstream flows that conserve flow properties and reduce energy flux are, however, found, although the smallest value of the flow parameter, Fr≡ Umax2/gΔ h (where Umax is the maximum flow speed, g is the acceleration due to gravity, Δ is a fractional density difference within the flow and h is the flow thickness) at which transitions may occur is only slightly less than that of the cascading flow in Lake Geneva. In this sense, the observed flow is marginally unstable to a finite-amplitude transition or hydraulic jump. Velocity and density profiles of possible flows downstream of a transition are found. The amplitudes of possible transitions and the flux of water entrained from the ambient overlying water mass are limited to narrow ranges.

Surface drifter exchange between the North Atlantic subtropical and subpolar gyres

Surface drifters deployed in the subtropical and subpolar North Atlantic from 1990 to 2002 show almost no connection between the subtropical and subpolar gyres; only one drifter crosses the intergyre boundary even though other data types (e.g., dynamic topography and tracers) suggest a major connection. Two of several possible causes for the lack of intergyre connectivity in this two‐dimensional data set are examined: (1) undersampling and short drifter lifetime leading to underestimation of the northward flow, and (2) the southward mean Ekman velocity. Advection of a large number of long‐lived synthetic drifters through the observed mean velocity results in a 5% increase in cross‐gyre flux compared with that for synthetic drifters with realistic lifetimes. By further advecting synthetic drifters through the observed mean velocity field with and without the Ekman component, estimated from the wind field associated with the actual drifters, it is shown that removal of the Ekman component further increases the intergyre flux by up to 6%. With a turbulent component added to the mean velocity field to simulate the eddy field, there is a further increase in connection by 5%. Thus the Ekman and eddy contributions to the drifter trajectories nearly cancel each other. Consideration of three‐dimensional processes (subduction and obduction) is reserved for complete modeling studies.

Gas Kinematics and the Black Hole Mass at the Center of the Radio Galaxy NGC 4335

We investigate the kinematics of the central gas disk of the radio-loud elliptical galaxy NGC 4335, derived from Hubble Space Telescope (HST) long-slit spectroscopic observations of Hα + [N II] along three parallel slit positions. The observed mean velocities are consistent with a rotating thin disk. We model the gas disk in the customary way, taking into account the combined potential of the galaxy and a putative black hole with mass M•, as well as the influence on the observed kinematics of the point-spread function and finite slit width. This sets a 3 σ upper limit of 108 M⊙ on M•. The velocity dispersion at r 05 is in excess of that predicted by the thin rotating disk model. This does not invalidate the model if the excess dispersion is caused by localized turbulent motion in addition to bulk circular rotation. However, if instead the dispersion is caused by the black hole (BH) potential then the thin disk model provides an underestimate of M•. A BH mass M• ~ 6 × 108 M⊙ is inferred by modeling the central gas dispersion as due to an isotropic spherical distribution of collisionless gas cloudlets. The stellar kinematics for NGC 4335 are derived from a ground-based (William Herschel Telescope/ISIS) long-slit observation along the galaxy major axis. A two-integral model of the stellar dynamics yields M• 3 × 109 M⊙. However, there is reason to believe that this model overestimates M•. Reported correlations between black hole mass and inner stellar velocity dispersion σ predict M• to be ≥5.4 × 108 M⊙ in NGC 4335. If our standard thin disk modeling of the gas kinematics is valid, then NGC 4335 has an unusually low M• for its velocity dispersion. If, on the other hand, this approach is flawed and provides an underestimate of M•, then black hole masses for other galaxies derived from HST gas kinematics with the same assumptions should be treated with caution. In general, a precise determination of the M•-σ relation and its scatter will benefit from (1) joint measurements of M• from gas and stellar kinematics in the same galaxies and (2) a better understanding of the physical origin of the excess velocity dispersion commonly observed in nuclear gas disks of elliptical galaxies.

The Salt Fluxes in a Tidally-Energetic Estuary

Time series measurements of velocity and salinity have been used to examine the salt flux components in the tidally-energetic regime of the Conwy Estuary in North Wales. The instantaneous volume transport, up to 400m3s−1, was deduced from a current meter located in mid-channel using a frictional model with a correction for the observed mean velocity shear. Transport estimates obtained in this way have been compared with independent measurements of integrated cross-section transport using a boat-mounted ADCP. The two determinations of the flow are found to be in satisfactory agreement confirming the validity of the frictional model. Estimates of the covariance of the tidally varying transport Qtand salinity Stindicate that a consistently upstream transport of salt occurred over the 9 day period of the observations. This upstream tidal pumping flux (≈60kgs−1) was greater than the downstream transport (≈27kgs−1) estimated from the freshwater river discharge Qf(≈3m3s−1). The difference in the salt fluxes implies a net upstream movement of salt and was consistent in sign with the observed general increase in mean salinity which occurred in the upper estuary prior to a rapid salinity decrease due to a large increase in freshwater discharge at the end of the measurement period. The observations have been interpreted to illustrate the problems involved in trying to determine the net flux of a dissolved constituent from such as estuary. Using estimates of the net inflow from river gauging to determine the volume dischargeQ0 , estimates of the net flux are realistic but, in the absence of river gauging, large errors are likely to arise from the uncertainties in Q0, which is the small difference between large flood and ebb transports.

Instrumental lahar monitoring at Merapi Volcano, Central Java, Indonesia

More than 50 volcanic debris flows or lahars were generated around Mt Merapi during the first rainy season following the nuees ardentes of 22 November 1994. The rainfalls that triggered the lahars were analyzed, using such instruments as weather radar and telemetered rain gauges. Lahar dynamics were also monitored, using new non-contact detection instrumentation installed on the slopes of the volcano. These devices include real-time seismic amplitude measurement (RSAM), seismic spectral amplitude measurement (SSAM) and acoustic flow monitoring (AFM) systems. Calibration of the various systems was accomplished by field measurements of flow velocities and discharge, contemporaneously with instrumental monitoring. The 1994–1995 lahars were relatively short events, their duration in the Boyong river commonly ranging between 30 min and 1 h 30 min. The great majority (90%) of the lahars was recognized at Kaliurang village between 13:00 and 17:30 h, due to the predominance of afternoon rainfalls. The observed mean velocity of lahar fronts ranged between 1.1 and 3.4 m/s, whereas the peak velocity of the flows varied from 11 to 15 m/s, under the Gardu Pandang viewpoint location at Kaliurang, to 8–10 m/s at a section 500 m downstream from this site. River slopes vary from 28 to 22 m/km at the two sites. Peak discharges recorded in various events ranged from 33 to 360 m 3/s, with the maximum value of peak discharge 360 m 3/s, on 20 May 1995. To improve the lahar warning system along Boyong river, some instrumental thresholds were proposed: large and potentially hazardous lahars may be detected by RSAM units exceeding 400, SSAM units exceeding 80 on the highest frequency band, or AFM values greater than 1500 mV on the low-gain, broad-band setting.

Faecal firing in a skipper caterpillar is pressure-driven

Many leaf-rolling caterpillars have a rigid anal comb attached to the lower surface of the anal plate (or shield) situated above the anus. This comb is widely assumed to be a lever used to 'flick' away frass pellets. An alternative mechanism to explain pellet discharge is proposed on the basis of observations on the caterpillar of the skipper Calpodes ethlius. The model proposes that the underside of the anal plate serves as a blood-pressure-driven surface for the ejection of faecal pellets. Rather than acting as a lever, the anal comb serves as a latch to prevent the premature distortion of the lower wall of the anal plate until the anal haemocoel compartment is fully pressurized. The anal comb is swung into position during pellet extrusion by retractor muscles attached at its base and held in place by a catch formed by a blood-swollen torus of everted rectal wall. As the caterpillar raises the blood pressure in its anal compartment by contracting its anal prolegs, the comb eventually slips over the toral catch. This causes the underside of the anal plate to move rapidly backwards as the blood pressure is released, projecting the pellet resting against it through the air. Simulation suggests that a local blood pressure of at least 10 kPa (75 mmHg) would be required to accelerate the lower surface of the anal plate outwards at a rate fast enough to discharge a 10 mg pellet at an observed mean velocity of 1.3 m s-1.

Calculation of Fully-Developed Turbulent Flow in Rectangular Ducts With Nonuniform Wall Roughness

The predictive capabilities of four transport-type turbulence models are analyzed by comparing predictions with experimental data for fully-developed flow in (1) a rectangular duct with a step change in roughness on one wall (Case 1), and (2) a square duct with one rib-roughened wall (Case 2). The models include the Demuren-Rodi (DR) k-ε model, the Sugiyama et al. (S) k-ε model, the Launder-Li (LL) Reynolds stress transport equation model, and the differential stress (DS) model proposed recently by the authors. For the first flow situation (Case 1), the results show that the DS model yields improved agreement between predicted and measured primary and secondary mean velocity distributions in comparison to the DR and LL models. For the second flow situation (Case 2), the DS model is superior to the DR and S models for predicting experimentally observed mean velocity, turbulence kinetic energy, and Reynolds stress anisotropy behavior, especially in the vicinity of a corner formed by the juncture of adjacent smooth and rough walls. The results are analyzed in order to explain why the DR model leads to the formation of a spurious secondary flow cell near this corner that is not present in the experimental flow.

Studies on twin elliptic jets

Non circular jets are emerging as a new class of passive control mechanism of jet flow. Jets issuing from non circular nozzles, rectangular, square, triangular, elliptic etc, are found to entrain several times more ambient fluid than equivalent axisymmetric nozzles. Non circular jets are generally referred to as non axisymmetric or three-dimensional jets. The increased entrainment of these asymmetric jets, particularly that of elliptic jets, has attracted the attention of many researchers. As asymmetric jets, multiple jets of different geometric configurations have also been proved to be effective in passive control of the jet flowfield. Multiple asymmetric jets may be expected to emerge as an effective method of passively controlling the jets. Many researchers have studied the flow field of single three dimensional jets and multiple circular or plane jets. But studies on multiple threedimensional jets are very limited. Pani and Dash studied the flowfield of three-dimensional single and multiple free jets. Their observed mean velocity distribution results were in agreement with the theoretical profiles.

Water Particle Velocities in Regular Waves

The Eulerian water particle velocities under paddle‐generated regular waves in a closed channel are not always accurately predicted by a conventional application of Stokes' first, second or fifth order wave theory. Phenomena, which give rise to errqrs, include mass‐transport, the partial clapotis formed by reflection from the spending beach and the free second harmonic wave produced by the sinusoidal motion of the paddle. Measurements taken with a Laser Doppler Anemometer indicate that the amplitudes predicted for the second harmonics of the velocity components can be over 100% in error. Furthermore, there is a mean horizontal velocity which is often greater than the amplitude of the second harmonic and can be 20% of the amplitude of the first harmonic. The observed mean velocities are compared with predictions based on Longuet‐Higgins' conduction solution. Because the design of a wave facility influences the particle kinematics, it is concluded that local empirical data will generally be required to achieve accurate predictions.

Three‐Dimensional Single and Multiple Free Jets

A theoretical distribution for the streamwise mean velocity of a single three‐dimensional turbulent free jet can be derived by following Reichardt's inductive theory. Using an integral type approach, a relationship for the decay of the maximum velocity on the axis of the jet is found. Further, it is possible to predict both the mean momentum flux and mean velocity profiles for three‐dimensional multiple jets by superimposition. Experiments are conducted on three‐dimensional jets issuing from single and multiple holes placed in line. The observed mean velocity distributions are in agreement with the theoretical profiles.

Fission ofU238by 2.2-GeV Protons

Angular distributions and differential range curves at angles of 15\ifmmode^\circ\else\textdegree\fi{}, 90\ifmmode^\circ\else\textdegree\fi{}, and 165\ifmmode^\circ\else\textdegree\fi{} have been obtained by radiochemical methods for several representative products which recoil out of thin uranium targets irradiated with 2.2-GeV protons. Mean momenta observed at 90\ifmmode^\circ\else\textdegree\fi{} to the beam direction are 133, 125, and 125 ${(\mathrm{MeV}\mathrm{amu})}^{\mathrm{\textonehalf{}}}$ for the neutron-excess isotopes ${\mathrm{Ba}}^{140}$, ${\mathrm{Mo}}^{99}$, and ${\mathrm{Sr}}^{91}$, but only 96 and 104 ${(\mathrm{MeV}\mathrm{amu})}^{\mathrm{\textonehalf{}}}$ for the neutron-deficient products ${\mathrm{Ba}}^{131}$ and ${\mathrm{Pd}}^{103}$. The neutron-deficient isotopes have broader momentum distributions and more forward-peaked angular distributions than do the neutron-excess products. Results are discussed in terms of a two-step vector model, i.e., a fast nucleonic cascade followed by fission or another slower de-excitation step. The observed spectra and angular distributions for all the isotopes are consistent with such a model. Observed mean velocities are compared to those predicted by a liquid-drop theory of fission. It is concluded that products in the mass-90-to-140 range are produced primarily by a conventional fission mechanism. However, some contribution of a spallationlike process appears necessary to account for low-velocity fragments in the case of the neutron-deficient products (particularly ${\mathrm{Ba}}^{131}$).

Turbulence in the atmosphere of 31 Cygni.

view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Turbulence in the atmosphere of 31 Cygni. Spiegel, Edward A. ; Aller, Lawrence H. Abstract We have been attempting to utilize data gathered in the 1951 atmospheric eclipse of 31 Cygni to investigate the hydrodynamics in the atmosphere of the K star. From radial velocities measured during the egress of the 1951 eclipse1 we have calculated correlation coefficients as defined below. The observati9ns here used are velocities at separate points in space-time; hence the correlation coefficients depend on space-time intervals. Moreover, the measured velocities are not velocities at single points in the atmosphere, but are mean velocities along the line of sight, the means being weighted with respect to the contribution of each element of atmosphere to the line intensities. For our purposes, the correlation coefficient is T - ~ v5(t) Vz(t + n) R5 (n) = [~))%52~t) T 52(t + n)i-s' t=n where n is a time interval in days, P is the number of days over which observations are made, t is the index (from I to P), the z direction is the line of sight direction, and v is the observed velocity corrected for orbital motion. The correlation Co- efficient is expressed as a function of n alone, since the transverse velocity of the B star is very nearly constant during the observation period. In this case, n completely specifies the space-time interval on which the correlation depends. If there exists turbulence in the hydrodynamical sense in the giant's atmosphere, moderately large eddies, i.e., those whose characteristic lengths are comparable to half the thickness of the atmosphere, will dominate the observed velocities. Smaller eddies than these will have effects analogous to those of thermal broadening, and larger ones will, if they are much larger than the transverse distance spanned by the observations, alter the observed mean velocities. In terms of this turbulence model, the length scale of the energy-containing eddies can be obtained, if the eddies lie within the detectable range discussed in the preceding paragraph. This is accomplished by replacing n by a corresponding space interval. The space interval is linearly related to n through the transverse velocity of the B star. We have calculated the above correlation for velocities derived from the calcium K line.1 Twenty-two observations, taken over a 3~-day period, were used. The calculated correlation Coefficients imply that, on the basis of this provisional turbulence model, the length scale of the energy-containing eddies is of the order of 2 X I0~ km. I.D. B. McLaughlin, Ap. J. ~~6, 550, 1952. University of Michigan Observatory, Ann Arbor, Mich. Publication: The Astronomical Journal Pub Date: 1953 DOI: 10.1086/106905 Bibcode: 1953AJ.....58Q.229S full text sources ADS |