Mean Vertical Velocity Research Articles

The Implications of Local Fluctuations in the Galactic Midplane for Dynamical Analysis in the Gaia Era

Abstract Orbital properties of stars, computed from their six-dimensional phase-space measurements and an assumed Galactic potential, are used to understand the structure and evolution of the Galaxy. Stellar actions, computed from orbits, have the attractive quality of being invariant under certain assumptions and are therefore used as quantitative labels of a star’s orbit. We report a subtle but important systematic error that is induced in the actions as a consequence of local midplane variations expected for the Milky Way. This error is difficult to model because it is non-Gaussian and bimodal, with neither mode peaking on the null value. An offset in the vertical position of the Galactic midplane of ∼15 pc for a thin disk-like orbit or ∼120 pc for a thick disk-like orbit induces a 25% systematic error in the vertical action J z . In Feedback in Realistic Environments simulations of Milky Way-mass galaxies, these variations are on the order of ∼100 pc at the solar circle. From observations of the mean vertical velocity variation of ∼5–10 km s−1 with radius, we estimate that the Milky Way midplane variations are ∼60–170 pc, consistent with three-dimensional dust maps. Action calculations and orbit integrations, which assume the global and local midplanes are identical, are likely to include this induced error, depending on the volume considered. Variation in the local standard of rest or distance to the Galactic center causes similar issues. The variation of the midplane must be taken into account when performing dynamical analysis across the large regions of the disk accessible to Gaia and future missions.

Bilateral and unilateral load-velocity profiling in a machine-based, single-joint, lower body exercise.

BackgroundTo analyze the goodness of fit of the load-velocity relationship in a machine-based, single-joint exercise performed both in a bilateral and unilateral manner, as well as to study its accuracy to estimate one repetition maximum (1-RM).MethodsFifteen resistance trained males performed an incremental test in the bilateral and unilateral leg extension exercise up to the 1-RM in two separate occasions. Mean vertical velocity of the weight plates in the leg extension machine was measured for every repetition using a smartphone application (My Lift).ResultsLinear regression analyses showed a high goodness of fit (R2 > 0.93) and small standard errors of estimate (SEE < 5%1-RM) both in the bilateral and unilateral leg extension when individual load-velocity regressions for each participant were computed. Unilateral load-velocity relationships showed significant differences in the intercept of the regression line with the Y-axis and the velocity at each percentage of the 1-RM (Cohen’s d > 1.0, p< 0.05). Finally, non-significant differences were observed between actual and estimated 1-RM from the load-velocity relationships (r = 0.88.0–96, Cohen’s d < 0.2, p> 0.05).ConclusionsThis proof of concept highlights that computing load-velocity relationships in a machine-based, single-joint, angular exercise can be appropriately performed by measuring the mean vertical velocity of the weight plates. These results could help strength and conditioning researchers and coaches who wish to analyze load-velocity relationship in other common machine-based exercises.

Dynamical heating across the Milky Way disc using APOGEE and Gaia

Abstract The kinematics of the Milky Way disc as a function of age are well measured at the solar radius, but have not been studied over a wider range of Galactocentric radii. Here, we measure the kinematics of mono-age, mono-[Fe/H] populations in the low and high [α/Fe] discs between 4 ≲ R ≲ 13 kpc and |z| ≲ 2 kpc using 65 719 stars in common between APOGEE DR14 and Gaia DR2 for which we estimate ages using a Bayesian neural network model trained on asteroseismic ages. We determine the vertical and radial velocity dispersions, finding that the low and high [α/Fe] discs display markedly different age–velocity dispersion relations (AVRs) and shapes σz/σR. The high [α/Fe] disc has roughly flat AVRs and constant σz/σR = 0.64 ± 0.04, whereas the low [α/Fe] disc has large variations in this ratio that positively correlate with the mean orbital radius of the population at fixed age. The high [α/Fe] disc component’s flat AVRs and constant σz/σR clearly indicate an entirely different heating history. Outer disc populations also have flatter radial AVRs than those in the inner disc, likely due to the waning effect of spiral arms. Our detailed measurements of AVRs and σz/σR across the disc indicate that low [α/Fe], inner disc ($R \lesssim 10\, \mathrm{kpc}$) stellar populations are likely dynamically heated by both giant molecular clouds and spiral arms, while the observed trends for outer disc populations require a significant contribution from another heating mechanism such as satellite perturbations. We also find that outer disc populations have slightly positive mean vertical and radial velocities likely because they are part of the warped disc.

Automated movement analysis to predict motor impairment in preterm infants: a retrospective study.

To apply automated movement analysis to the general movements assessment (GMA) to build a predictive model for motor impairment (MI). A retrospective cohort study including infants ≤306/7 weeks GA or BW ≤1500 g seen at 3-5 months was conducted. Automated video analysis was used to develop a multivariable model to identify MI, defined as Bayley motor composite score <85 or cerebral palsy (CP). One hundred and fifty two videos were analyzed. Median GA and BW were 275/7 weeks and 955 g, respectively. MI and CP rates were 22% (N = 33) and 14% (N = 22). Minimum, mean, and mean vertical velocity of the infant's silhouette correlated significantly with MI. Sensitivity, specificity, positive and negative predictive values, and accuracy of automated GMA were 79%, 63%, 37%, 91%, and 66%, respectively. C-statistic indicated good fit (C = 0.77). Automated movement analysis predicts MI in preterm infants. Further refinement of this technology is required for clinical application.

Examination of the Expendable Digital Dropsonde–Derived Vertical Velocities from the Tropical Cyclone Intensity (TCI) Experiment

Abstract The newly developed expendable digital dropsonde (XDD) allows for high spatial and temporal resolution data collection in tropical cyclones (TCs). In 2015, a total of 725 XDDs were launched into Hurricanes Marty (27–28 September), Joaquin (2–5 October), and Patricia (20–23 October) as part of the Tropical Cyclone Intensity (TCI) experiment. These dropsondes were launched from a NASA WB-57 at altitudes above 18 km, capturing the full depth of the TCs to the tropopause. This study documents the vertical velocity distributions observed in TCI using the XDDs and examines the distributions altitudinally, radially, and azimuthally. The strongest mean or median XDD-derived vertical velocities observed during TCI occurred in the upper levels and within the cores of the three TCs. There was little azimuthal signal in the vertical velocity distribution, likely due to sampling asymmetries and noise in the data. Downdrafts were strongest in Joaquin, while updrafts were strongest in Patricia, especially within the eyewall on 23 October. Patricia also had an impressive low-level (&amp;lt;2 km) updraft that exceeded 10 m s−1 associated with a shallow, overturning, radial circulation in the secondary eyewall.

Reliability of Barbell Velocities Recorded from the GymAware Device during Squat and Bench Press Exercises

PURPOSE: To assess the reliability of velocities recorded with the GymAware linear position transducer (LTP) during the squat and bench press exercises. METHODS: Thirteen resistance-trained men (age: 21.7±0.4 years; height 1.74±0.07 m; mass: 82.9±9.5 kg; 1-repetition maximum (1RM) back squat: 149.9±20.7 kg; bench 1RM: 114.8±18.5 kg.) performed three trials of squat and bench press using the following percentages of 1RM: 20, 30, 40, 50, 60, 70, 80%. The order of the loads was counterbalanced across the participants. The mean vertical velocity of the barbell during the concentric phase of each repetition was recorded using a 3-D motion analysis system (Vicon; 200 Hz) and the GymAware LTP. Reliability of the GymAware was determined using intraclass correlations (ICC) and coefficients of variance (CV%). RESULTS: The GymAware showed high intersession reliability for both exercises with ICCs ranging from good to excellent (squat: 0.71-0.91; bench press: 0.83-0.91). CV% showed precision in the recorded velocities during both exercises (squat: 3.6%-5.8%; bench press: 4.9%-7.4%). CONCLUSION: The GymAware LTP shows high intersession reliability for recorded velocities during the squat and bench press exercises.

Can VHF radars at polar latitudes measure mean vertical winds in the presence of PMSE?

Abstract. Mean vertical velocity measurements obtained from radars at polar latitudes using polar mesosphere summer echoes (PMSEs) as an inert tracer have been considered to be non-representative of the mean vertical winds over the last couple of decades. We used PMSEs observed with the Middle Atmosphere Alomar Radar System (MAARSY) over Andøya, Norway (69.30∘ N, 16.04∘ E), during summers of 2016 and 2017 to derive mean vertical winds in the upper mesosphere. The 3-D vector wind components (zonal, meridional and vertical) are based on a Doppler beam swinging experiment using five beam directions (one vertical and four oblique). The 3-D wind components are computed using a recently developed wind retrieval technique. The method includes full non-linear error propagation, spatial and temporal regularisation, and beam pointing corrections and angular pointing uncertainties. Measurement uncertainties are used as weights to obtain seasonal weighted averages and characterise seasonal mean vertical velocities. Weighted average values of vertical velocities reveal a weak upward behaviour at altitudes ∼84–87 km after eliminating the influence of the speed of falling ice. At the same time, a sharp decrease (increase) in the mean vertical velocities at the lower (upper) edges of the summer mean altitude profile, which are attributed to the sampling issues of the PMSE due to disappearance of the target corresponding to the certain regions of motions and temperatures, prevails. Thus the mean vertical velocities can be biased downwards at the lower edge, and the mean vertical velocities can be biased upwards at the upper edge, while at the main central region the obtained mean vertical velocities are consistent with expected upward values of mean vertical winds after considering ice particle sedimentation.

Theory of light-activated catalytic Janus particles.

We study the dynamics of active Janus particles that self-propel in solution by light-activated catalytic decomposition of chemical "fuel." We develop an analytical model of a photo-active self-phoretic particle that accounts for "self-shadowing" of the light by the opaque catalytic face of the particle. We find that self-shadowing can drive "phototaxis" (rotation of the catalytic cap toward the light source) or "anti-phototaxis," depending on the properties of the particle. Incorporating the effect of thermal noise, we show that the distribution of particle orientations is captured by a Boltzmann distribution with a nonequilibrium effective potential. Furthermore, the mean vertical velocity of phototactic (anti-phototactic) particles exhibits a superlinear (sublinear) dependence on intensity. Overall, our findings show that photo-active particles exhibit a rich "tactic" response to light, which could be harnessed to program complex three-dimensional trajectories.

Three‐dimensional movements of harbour seals in a tidally energetic channel: Application of a novel sonar tracking system

Abstract Understanding how marine predators utilize habitats requires that we consider their behaviour in three dimensions. Recent research has shown that marine mammals often make use of tidally energetic locations for foraging, yet data are generally limited to observations of animals at the water surface. Such areas are also of interest to the renewable energy industry for the deployment of tidal‐stream energy turbines; this has led to concerns about potential impacts on marine mammals. Methods for measuring animal movements underwater are limited; however, active sonar can image marine mammals and could potentially measure three‐dimensional movements in tidally energetic locations. Here, a dual 720 kHz sonar system was developed to investigate the three‐dimensional movements of harbour seals (Phoca vitulina) in a tidally energetic channel. Estimated mean depth (distance from the surface) of seals was 12.0 m (95% confidence intervals [CIs]: 11.6–12.4 m), and the majority of time was spent at the surface and at approximately 10–12 m distance from the surface. When expressed as distances from the sea bed, mean distance was 18.5 m (95% CI: 18.0–18.9 m), and the majority of time was spent at 14 m from the sea bed. Seal movements were generally in the same direction as the tidal flow with mean horizontal speeds of between 0.51 and 3.13 m s−1 (95% CIs = 1.24–1.54 m s−1). Mean vertical velocities (where negative and positive values represent a descent and ascent respectively) for each seal track ranged between −1.76 and +0.88 m s−1 (95% CIs: −0.23 to +0.03 m s−1). These results provide a basis for understanding how seals utilize a dynamic tidal environment and suggest that harbour seal behaviour can be markedly different to less tidally energetic habitats. The results also have important implications for the prediction of risk associated with interactions between diving seals and tidal turbines in these dynamic habitats.

Impact of Mountainous Topography on Surface-Layer Parameters During Weak Mean-Flow Conditions

We investigate surface-layer characteristics over a mountainous ridge in the Central Himalayas, utilizing tower-based fast-response micrometeorological observations (at 12 and 27 m above ground level) for the winter months November 2013–January 2014. During this period, the site generally experienced clear skies and weak synoptic flow (wind speed < 2 m s−1), favouring a strong diurnal evolution of the atmospheric boundary layer. The observations show a regular change in wind direction from north-easterly during the night-time to westerly during the daytime throughout the season, indicating the systematic development of a mountain circulation due to changes in heating of the mountain slopes as the day advances. Considering the variations in wind direction and topography of the site, the tilt corrections are implemented sector-wise by segregating the data into three sectors, thus estimating three sets of coefficients for the tilt correction. Observations during fair-weather conditions (59 days only) are analyzed with the sensible heat flux (H) showing large diurnal variations, which are in-phase with the mean vertical velocity. The afternoon peak value of H is found to be ≈ 116 ± 80 W m−2. In contrast, diurnal variations of momentum flux and turbulent kinetic energy are less prominent with rather weak maxima occurring between 0900 and 1300 IST, the period when wind direction changes over the ridge. Variations of the dimensionless standard deviations of the vertical velocity component and temperature are found to scale with the stability parameter z/L under convective conditions, while taking into account the effect of self-correlation. The constancy of fluxes with height, slope-flow buoyancy and stress divergence are also analyzed to provide a rigorous evaluation of Monin–Obukhov similarity theory based on two-level turbulence measurements.

Kinematics and wake of freely falling cylinders at moderate Reynolds numbers

We investigated experimentally the motion of elongated finite-length cylinders (length $L$, diameter $d$) freely falling under the effect of buoyancy in a low-viscosity fluid otherwise at rest. For cylinders with densities $\unicode[STIX]{x1D70C}_{c}$ close to the density $\unicode[STIX]{x1D70C}_{f}$ of the fluid ($\overline{\unicode[STIX]{x1D70C}}=\unicode[STIX]{x1D70C}_{c}/\unicode[STIX]{x1D70C}_{f}\simeq 1.16$), we explored the effect of the body volume by varying the Archimedes number $Ar$ (based on the body equivalent diameter) between 200 and 1100, as well as the effect of their length-to-diameter ratios $L/d$ ranging from 2 to 20. A shadowgraphy technique involving two cameras mounted on a travelling cart was used to track the cylinders along their fall over a distance longer than $30L$. A dedicated image processing algorithm was further implemented to properly reconstruct the position and orientation of the cylinders in the three-dimensional space. In the range of parameters explored, we identified three main types of paths, matching regimes known to exist for three-dimensional bodies (short-length cylinders, disks and spheres). Two of these are stationary, namely, the rectilinear motion and the large-amplitude oscillatory motion (also referred to as fluttering or zigzag motion), and their characterization is the focus of the present paper. Furthermore, in the transitional region between these two regimes, we observed irregular low-amplitude oscillatory motions, that may be assimilated to the A-regimes or quasi-vertical regimes of the literature. Flow visualization using dye released from the bodies uncovered the existence of different types of vortex shedding in the wake of the cylinders, according to the style of path. The detailed analysis of the body kinematics in the fluttering regime brought to light a series of remarkable properties. In particular, when normalized with the characteristic velocity scale $u_{0}=\sqrt{(\overline{\unicode[STIX]{x1D70C}}-1)gd}$ and the characteristic length scale $l_{0}=\sqrt{dL}$, the mean vertical velocity $\overline{u_{Z}}$ and the frequency $f$ of the oscillations become almost independent of $L/d$ and $Ar$. The use of the length scale $l_{0}$ and of the gravitational velocity scale to build the Strouhal number $St^{\ast }=fl_{0}/u_{0}$ allowed us to generalize to short ($0.1\leqslant L/d\leqslant 0.5$) and elongated cylinders ($2\leqslant L/d\leqslant 12$), the result $St^{\ast }\simeq 0.1$. An interpretation of $l_{0}$ as a characteristic length scale associated with the oscillatory recirculation thickness generated near the body ends is proposed. In addition, the rotation rate of the cylinders scales with $u_{0}/L$, for all $L/d$ and $Ar$ investigated. Furthermore, the phase difference between the oscillations of the velocity component $u$ along the cylinder axis and of the inclination angle $\unicode[STIX]{x1D703}$ of the cylinder is approximately constant, whatever the elongation ratio $L/d$ and the Archimedes number $Ar$.

Measuring Area-Averaged Vertical Motions with Dropsondes

Abstract Measurements of vertical profiles of areal-mean mass divergence, vorticity, and vertical velocity, based on dropsondes distributed over an area of 25 000 km2, are presented. The dropsondes were released with high frequency along circular flight patterns during an airborne field campaign taking place over the tropical Atlantic near Barbados. Vertical profiles of the area-averaged mass divergence and vorticity were computed from the horizontal wind profiles, and the area-averaged vertical velocity was then inferred from the divergence. The consistency of measurements over pairs of circles flown within the same air mass demonstrated the reproducibility of the measurements, and showed that they characterize the environmental conditions on the scale of the measurement, rather than being dominated by measurement error or small-scale wind variability. The estimates from dropsondes were found to be consistent with the observed cloud field, with Lagrangian estimates of the mean vertical velocity inferred from the free-tropospheric humidity field, and with the mean vertical velocity derived from simulations using an atmospheric model representing kilometer-scale motions and initialized with meteorological analyses. In trade wind–like conditions, the divergence and vorticity profiles exhibit a rich vertical structure and a significant variability in space and time. Yet a few features appear to be robust, such as the presence of layers of mass convergence at the top of moist layers, extrema of the area-averaged vertical velocity at the top of the subcloud layer and in the midtroposphere, and minima around the trade inversion near 2 km. The analysis of spatial and temporal autocorrelation scales suggests that the divergent mass field measured from dropsondes is representative of the environment of shallow clouds.

The impact of hydrogen enrichment on the flow field evolution in turbulent explosions

The reactivity of fuel-air mixtures has a strong impact on the severity of obstacle-induced turbulent explosions. While the influence on the flame speed and pressure development has been studied for a wide range of mixtures at different physical scales, the experimental quantification of the resulting flow fields, e.g. in the critical recirculation zones behind obstacles, is typically absent. The lack of such data presents a serious impediment to the development of reliable predictive methods. The current study reports velocity measurements obtained from highly reproducible experiments performed in a flame tube with two staggered obstacles using fuel lean H2/CH4/Air and H2/CO/Air mixtures at a stoichiometry of 0.60. The mixture reactivity for both fuels was varied using H2 substitution levels of 50% and 80% with the pure hydrogen (100%) case used as a reference. The flow field was quantified using high-speed (10 kHz) particle image velocimetry (PIV), time-series PIV and Mie scattering. The time-resolved evolution of the recirculation zone behind the second obstacle was successfully captured with the explosion over-pressure and flame propagation speed also measured. Data is presented for the mean horizontal (u¯) and vertical (v¯) velocity components at 18 spatial locations for each mixture along with the translational velocities of the shear driven recirculating eddies formed behind the second obstacle. It is shown that the temporal evolution of the flows (not velocity magnitudes) can be approximately normalised based on the flame arrival at the second obstacle. The data provides a comprehensive quantitative description of the flow field evolution leading up to explosion events and is expected to facilitate model development.

The echo of the bar buckling: Phase-space spirals in Gaia Data Release 2

We present a high-resolution numerical study of the phase-space diversity in an isolated Milky Way-type galaxy. Using a single N-body simulation (N ≈ 0.14 × 109) we explore the formation, evolution, and spatial variation of the phase-space spirals similar to those recently discovered by Antoja et al. in the Milky Way disk with Gaia Data Release 2 (DR2). For the first time in the literature we use a self-consistent N-body simulation of an isolated Milky Way-type galaxy to show that the phase-space spirals develop naturally from vertical oscillations driven by the buckling of the stellar bar. Thus, we claim that the physical mechanism standing behind the observed incomplete phase-space mixing process can be internal and not necessarily due to the perturbation induced by a massive satellite. In our model, the bending oscillations propagate outward and produce axisymmetric variations of the mean vertical coordinate and vertical velocity component of about 100 − 200 pc and 1 − 2 km s−1, respectively. As a consequence, the phase-space wrapping results in the formation of patterns with various morphologies across the disk, depending on the bar orientation, distance to the galactic center, and time elapsed since the bar buckling. Once bending waves appear, they are supported for a long time via disk self-gravity. Such vertical oscillations trigger the formation of various time-dependent phase-space spirals in the entire disk. The underlying physical mechanism implies the link between in-plane and vertical motion that leads directly to phase-space structures whose amplitude and shape are in remarkable agreement with those of the phase-space spirals observed in the Milky Way disk. In our isolated galaxy simulation, phase-space spirals are still distinguishable at the solar neighborhood 3 Gyr after the buckling phase. The long-lived character of the phase-space spirals generated by the bar buckling instability cast doubts on the timing argument used so far to get back to the time of the onset of the perturbation: phase-space spirals may have been caused by perturbations originated several gigayearrs ago, and not as recent as suggested so far.

Application of through-building openings for wind energy harvesting in built environment

This study investigated mean flow characteristics and structure of flow inside through-building openings with five different layout configurations for the purpose of wind energy harvesting. Two-dimensional Particle Image Velocimetry (PIV) was employed and validated against a series of Cobra probe measurements. The mean flow characteristics including streamwise and vertical mean velocity were investigated at the wind direction parallel to the corridors of the through-building openings. The coherent structure of flow was studied, using two-point cross-correlation and a simple flow visualisation. The results showed that creating recessed regions at inlet and outlet of the openings and using curved walls at corners increase the mean velocity at the middle of the corridor by approximately 25%. When using converging-diverging passages for openings, the mean velocity increases by approximately 33%. The two configurations exhibit better performance in attuning the flow to be more inform and steady in a larger portion of the corridor, and hence these configurations have the capacity to enhance the performance of a potential wind turbine inside the corridor. The confined area of corridors contributes to the progressive reduction of undesirable characteristics of the cavity flow including vertical mean velocity, and streamwise and vertical velocity fluctuations along the corridor.

The Signature of Shallow Circulations, Not Cloud Radiative Effects, in the Spatial Distribution of Tropical Precipitation

AbstractRecent research suggests cloud–radiation interaction as key for intermodel differences in tropical precipitation change with warming. This motivates the hypothesis that intermodel differences in the climatology of precipitation, and in its response to warming, should reduce in the absence of cloud–radiation interaction. The hypothesis is explored with the aquaplanet simulations by the Clouds On-Off Klimate Intercomparison Experiment performed by seven general circulation models, wherein atmospheric cloud radiative effects (ACREs) are active (ACRE-on) and inactive (ACRE-off). Contrary to expectation, models’ climatology of tropical precipitation are more diverse in the ACRE-off experiments, as measured by the position of the intertropical convergence zone (ITCZ), the subtropical precipitation minima, and the associated organization of the tropical circulation. Also the direction of the latitudinal shift of the ITCZ differs more in simulations with inactive cloud radiative effects. Nevertheless, both in ACRE-on and ACRE-off, the same relationship between tropical precipitation and the mean vertical velocity (zonally, temporally, and vertically averaged) emerges in all models. An analysis framework based on the moist static energy budget and used in the moisture space is then developed to understand what controls the distribution of the mean vertical velocity. The results suggest that intermodel differences in tropical circulation and zonal-mean precipitation patterns are most strongly associated with intermodel differences in the representation of shallow circulations that connect dry and moist regions.

An Improved Second‐Moment Closure Model for Langmuir Turbulence Conditions: Model Derivation and Validation

AbstractIn this study, an improved second‐moment closure model, the so‐called k‐ω model, which takes into consideration the Langmuir turbulent effect in the transport equations of turbulent kinetic energy and turbulent frequency, was developed. The Langmuir turbulent effect was also included in the vertical eddy viscosity and diffusivity coefficient. Langmuir turbulence production, as well as buoyancy production, anisotropic production, and vorticity production induced by velocity shear, was included in the pressure covariance parameterization to derive the stability function. The Langmuir effect and relatively complete pressure covariance parameterization inclusion can account for the influences of Langmuir turbulence and stratification in the vertical eddy viscosity and vertical eddy diffusion coefficients. Based on the modified model, one‐dimensional numerical experiments for horizontally homogeneous flows were carried out. The results indicate that the modified model is more accurate than other models in terms of the vertical eddy viscosity, vertical turbulence velocity variance, kinetic energy, and mean velocity profile compared with the large eddy simulation results. The simulated sea surface temperature (SST) and mixed layer depth of the modified model were compared with observations at Ocean Station Papa. When considering Langmuir turbulent effects, the modified model overcomes the shortcoming that the simulated SST is higher than the observed SST. Numerical results indicate that the k‐ω model performs better than the k‐kl model, although both models consider the Langmuir turbulent effect in the simulation of upper mixing motions.

Performance of second-moment differential stress and flux models for natural convection in an enclosure

In this paper the relative performance of two different differential stress and flux models for computing natural convection in an enclosure is examined. One model is the low-Reynolds number differential stress and flux model and the other model is a recently developed elliptic blending based differential stress and flux model. The primary emphasis of the study is placed on investigating the accuracy of the both models for the turbulent natural convection in an enclosure. Both models are used to predict turbulent natural convections in a square cavity with hot and cold side walls and conducting top and bottom walls (Ra = 1.58 × 109). The relative performance between both models is examined through comparing their solutions with the experimental data. It is shown that both models predict the vertical mean velocity component and mean temperature well. It is also shown that the low-Reynolds number differential stress and flux model slightly under-predicts the turbulent kinetic energy and vertical turbulent heat flux. It is also noted that the implementation of the elliptic blending second moment model is easier since this model does not contain any wall-related parameters and damping functions.

Nocturnal Low-level Winds and Their Impacts on Particulate Matter over the Beijing Area

Three-month wind profiles, 260 m PM1 concentrations [i.e., particulate matter (PM) with an aerodynamic diameter ⩽ 1 μm], and carrier-to-noise ratio data at two Beijing sites 55 km apart (urban and suburban) were collected to analyze the characteristics of low-level nocturnal wind and PM in autumn and winter. Three mountain-plain wind events with wind shear were selected for analysis. The measurements indicated that the maximum wind speeds of the northerly weak low-level jet (LLJ) below 320 m at the suburban site were weaker than those at the urban site, and the LLJ heights and depths at the suburban site were lower than those at the urban site. The nocturnal 140 m mean vertical velocities and the variations in vertical velocity at the urban site were larger than those at the suburban site. A nocturnal breeze with a weak LLJ of ∼ 3 m s−1 noticeably offset nocturnal PM transport due to southerly flow and convergence within the northern urban area of Beijing. Characteristics of the nocturnal LLJ, such as start-up time, structure, intensity, and duration, were important factors in determining the decrease in the nocturnal horizontal range and site-based low-level variations in PM.

Vertical waves in the solar neighbourhood inGaiaDR2

The vertical structure and dynamics of stars in our local Galactic neighbourhood contains much information about the local distribution of visible and dark matter and of perturbations to the Milky Way disc. We use data on the positions and velocities of stars in the solar neighbourhood from \gaia\ DR2 and large spectroscopic surveys to investigate the vertical number counts and mean-velocity trend as a function of distance from the local Galactic mid-plane. We perform a detailed measurement of the wave-like North-South asymmetry in the vertical number counts, which reveals a number of deficits at heights $\approx 0.4\,\mathrm{kpc}$, $\approx 0.9\,\mathrm{kpc}$, and $\approx 1.5\,\mathrm{kpc}$, and peaks at $\approx 0.2\,\mathrm{kpc}$, $\approx 0.7\,\mathrm{kpc}$, and $\approx 1.1\,\mathrm{kpc}$. We find that the asymmetry pattern is independent of colour. The mean vertical velocity is almost constant to $<1\,\mathrm{km\,s}^{-1}$ within a few 100 pc from the mid-plane and then displays a North-South symmetric dip at $\approx0.5\,\mathrm{kpc}$ with an amplitude of $\approx 2\,\mathrm{km\,s}^{-1}$ that is a plausible velocity counterpart to the main number-count dip at a similar height. Thus, with \gaia\ DR2 we confirm at high fidelity that the local Galactic disc is undergoing a wave-like oscillation and a dynamically-consistent observational picture of the perturbed local vertical structure emerges for the first time. We also present the most precise and accurate determination of the Sun's height above the local Galactic mid-plane, correcting for any asymmetry in the vertical density: $z_\odot = 20.8 \pm 0.3\,\mathrm{pc}$.