Mean Horizontal Velocity Research Articles

Medium-scale traveling ionospheric disturbances over Taiwan observed with HF Doppler sounding

We analyzed horizontal velocities of medium-scale traveling ionospheric disturbances using HF Doppler sounding over Taiwan from January 2014 to January 2016. Meridional components of the observed horizontal velocities are in most cases larger than zonal components. In summer, poleward propagation prevails, whereas in winter, disturbances primarily propagate southward. Zonal components of the analyzed events are mostly eastward. Seasonal behaviors of the observed propagation directions are similar at other locations with HF Doppler sounding at middle and low latitudes. Horizontal velocities of the observed events are in the 50–400-ms−1 range. The upper limit is determined using the spacing between reflection points, making the uncertainty in velocity very large for events with velocities larger than this limit. The mean horizontal velocity is 210 ms−1. The range between the 20th and 80th percentile of horizontal velocities is 144–281 ms−1. Graphical abstract.

Simulation of device-scale unsteady turbulent flow in the Fundy Tidal Region

This paper specifically focuses on site-specific CFD modelling in the context of tidal energy development, and presents results for two sites from the Fundy Tidal Region (FTR) situated in Eastern Canada. The FTR is one of the best and most energetic areas in the world for tidal energy development. To efficiently meet the computational demands of modelling unsteady three-dimensional turbulent flow in complex tidal zone terrains, highly parallel GPU hardware are utilized. Strategically deployed Acoustic Doppler Current Profiler (ADCP) measurements at two tidal turbine locations provide validation of the predictions under ebb and flood flows. The processing methodology of ADCP devices is incorporated into the CFD simulation to improve predicted turbulence levels for validation. The simulations use detached eddy simulation (DES) turbulence modelling with engineered inlet boundary conditions and realistic bathymetry. The inlet boundaries use mean horizontal velocity values interpolated from regional tidal simulations and are perturbed by synthetic turbulence to accelerate boundary layer development. Simulations cover tidal developments from ∼1.5 to 3 square kilometres with spatial and temporal resolutions of 1–2 m and 0.1 s, respectively.

The effects of stance width on static standing balance

Objective To explore the effect of stance width on balance in static standing. Methods Thirty healthy young subjects were evaluated using the PRO-KIN balance training apparatus. They stood with their eyes open and closed in three stances: feet together, feet 50 cm apart, and feet a comfortable distance apart (measured as 18.63±3.67 cm between the heels and 29.60±4.93 cm between the toes). The excursions of the center of pressure (COP), the standard deviation of their longitudinal sway (SDLS), the standard deviation of their horizontal sway (SDHS), the mean longitudinal and horizontal sway velocities (MVLS and MVHS), sway length (SL) and sway area (SA) were recorded. Results With the eyes either open or closed, significant differences in all of the COP data were observed when standing in the different stances. The average SDHS, MVHS, SL and SA were all significantly better when standing comfortably than when standing with the feet 50cm apart whether the eyes were open or closed. Conclusion There is a proper distance between the feet where healthy youngsters best maintain static standing balance. Key words: Stance width; Standing; Balance

Lidar observations of stratospheric gravity waves from 2011 to 2015 at McMurdo (77.84°S, 166.69°E), Antarctica: 1. Vertical wavelengths, periods, and frequency and vertical wave number spectra

AbstractFive years of atmospheric temperature data, collected with an Fe Boltzmann lidar by the University of Colorado group from 2011 to 2015 at Arrival Heights, are used to characterize the vertical wavelengths, periods, vertical phase speeds, frequency spectra, and vertical wave number spectra of stratospheric gravity waves from 30 to 50 km altitudes. Over 1000 dominant gravity wave events are identified from the data. The seasonal spectral distributions of vertical wavelengths, periods, and vertical phase speeds in summer, winter, and spring/fall are found obeying a lognormal distribution. Both the downward and upward phase progression gravity waves are observed by the lidar, and the fractions of gravity waves with downward phase progression increase from summer ~59% to winter ~70%. The seasonal and monthly mean vertical wavelengths and periods exhibit clear seasonal cycles with vertical wavelength growing from summer ~5.5 km to winter ~8.5 km, and period increasing from summer ~4.5 h to winter ~6 h. Statistically significant linear correlations are found between the monthly mean vertical wavelengths/periods and the mean zonal wind velocities from 30 to 50 km. Assuming horizontal phase speeds independent of month, the monthly mean horizontal wavelengths, intrinsic periods, and group velocities are estimated for stratospheric gravity waves. The slopes of wave frequency spectra change from −1.9 at 30–60 km to −1.45 around 60–65 km. The vertical wave number spectra show the power spectral density at vertical wavelengths of 5–20 km decreasing from winter maximum to summer minimum. Several aforementioned features are observed for the first time in Antarctica.

ASB clinical biomechanics award winner 2016: Assessment of gaze stability within 24–48 hours post-concussion

BackgroundApproximately 90% of athletes with concussion experience a certain degree of visual system dysfunction immediately post-concussion. Of these abnormalities, gaze stability deficits are denoted as among the most common. Little research quantitatively explores these variables post-concussion. As such, the purpose of this study was to investigate and compare gaze stability between a control group of healthy non-injured athletes and a group of athletes with concussions 24–48hours post-injury. MethodsTen collegiate NCAA Division I athletes with concussions and ten healthy control collegiate athletes completed two trials of a sport-like antisaccade postural control task, the Wii Fit Soccer Heading Game. During play all participants were instructed to minimize gaze deviations away from a central fixed area. Athletes with concussions were assessed within 24–48 post-concussion while healthy control data were collected during pre-season athletic screening. Raw ocular point of gaze coordinates were tracked with a monocular eye tracking device (240Hz) and motion capture during the postural task to determine the instantaneous gaze coordinates. This data was exported and analyzed using a custom algorithm. Independent t-tests analyzed gaze resultant distance, prosaccade errors, mean vertical velocity, and mean horizontal velocity. FindingsAthletes with concussions had significantly greater gaze resultant distance (p=0.006), prosaccade errors (p<0.001), and horizontal velocity (p=0.029) when compared to healthy controls. InterpretationThese data suggest that athletes with concussions had less control of gaze during play of the Wii Fit Soccer Heading Game. This could indicate a gaze stability deficit via potentially reduced cortical inhibition that is present within 24–48hours post-concussion.

GPS detection of the ionospheric disturbances over China due to impacts of Typhoons Rammasum and Matmo

AbstractMedium‐scale traveling ionospheric disturbances (MSTIDs) possibly excited by typhoon have been presented, for the first time, by applying the two‐dimensional total electron content perturbation maps derived from the dense Global Position System (GPS) network in China. Two MSTIDs were observed on 19 July and 23 July 2014, respectively, during the landing phases of Typhoons Rammasun and Matmo. The MSTIDs excited by Rammasum were detected at 06:56–07:40 UT on 19 July 2014, with a mean horizontal phase velocity of 118 m/s and an amplitude of about 2 total electron content unit (TECU), 1 TECU = 1016 el m−2, it horizontally extended to about 600 km. The Matmo‐related MSTIDs were observed at 14:48–15:54 UT on 23 July 2014; the wavefront, perturbation amplitude, and the mean horizontal phase velocity were 1000 km, 1.8 TECU, and 132 m/s, respectively. Both MSTIDs propagated southwestward with a distance less than 200 km. The short propagation distances of the MSTIDs indicate that the MSTIDs may be caused by the body forces resulting from the typhoons.

Retraction Note to: Numerical simulation on pollutant dispersion from vehicle exhaust in street configurations.

The impact of the street configurations on pollutants dispersion from vehicles exhausts within urban canyons was numerically investigated using a computational fluid dynamics (CFD) model. Three-dimensional flow and dispersion of gaseous pollutants were modeled using standard κ − e turbulence model, which was numerically solved based on Reynolds-averaged Navier–Stokes equations by the commercial CFD code FLUENT. The concentration fields in the urban canyons were examined in three cases of street configurations: (1) a regular-shaped intersection, (2) a T-shaped intersection and (3) a Skew-shaped crossing intersection. Vehicle emissions were simulated as double line sources along the street. The numerical model was validated against wind tunnel results in order to optimize the turbulence model. Numerical predictions agreed reasonably well with wind tunnel results. The results obtained indicate that the mean horizontal velocity was very small in the center near the lower region of street canyon. The lowest turbulent kinetic energy was found at the separation and reattachment points associated with the corner of the down part of the upwind and downwind buildings in the street canyon. The pollutant concentration at the upwind side in the regular-shaped street intersection was higher than that in the T-shaped and Skew-shaped street intersections. Moreover, the results reveal that the street intersections are important factors to predict the flow patterns and pollutant dispersion in street canyon.

Influence of the Coriolis force on the structure and evolution of wind turbine wakes

In this study, large-eddy simulation combined with a turbine model is used to investigate the effect of vertical wind veer associated with the Coriolis force on the structure and evolution of wind turbine wakes. In order to isolate the Coriolis effect on the wake, two cases are considered. In the first case, atmospheric boundary-layer flow is driven by a geostrophic wind, including the effect of Earth's rotation and the Coriolis force. In the second case, the boundary-layer flow is unidirectional and is forced by an imposed pressure gradient. Both cases have the same mean horizontal velocity and turbulence intensity at the hub height. The simulation results show that the Coriolis force significantly affects the aerodynamics of the wake including the mean velocity deficit, turbulence statistics, and wake-meandering characteristics downwind of the turbine. In particular, when the flow is forced by a geostrophic wind, vertical wind veer causes a skewed spatial structure in the wake. Moreover, the presence of lateral wind shear, in addition to the vertical one, enhances the shear production of turbulent kinetic energy and the turbulent momentum flux. This leads to a larger flow entrainment and, thus, a faster wake recovery compared to the case forced by unidirectional pressure gradient. Consistent with this result, wake meandering is also stronger in both lateral and vertical directions in the case of geostrophic forcing compared to the case with pressure-gradient forcing.

Horizontal semicircular canalvideo head impulse test in normaladults:normal value and age distribution

Objective:To study the parameters of video head impulse test (vHIT) for horizontal semicircular canal and explore its application when evaluating angular vestibular-ocular reflex (VOR) in adults.Method:vHIT were applied to 80 adults without prior vertigo or dizziness history.Parameters provided by the software included instantaneous gain(40 ms,60 ms,80 ms),regression gain,asymmetry value,corrective saccades(latency,peak velocity,occurrence rate).Result:The mean horizontal VOR velocity gain of normal subjects was 0.877±0.171,0.944±0.133,0.967±0.130 at 40 ms,60 ms,80 ms respectively.Regression gain of horizontal VOR is 0.944±0.090.The lower limit of normal horizontal VOR velocity gain was 0.73 at 60 ms and 0.80 as regression gain.All velocity gains declined by age,although there were no statistically significant difference(P>0.05).Asymmetry value was 5.60±3.46.Corrective saccades occurred in 28±15 of normal subjects,while latency and peak velocity was (200.87±0.34)ms and (97.7±40.1)°/s.Conclusion:The study found that horizontal VOR instantaneous gain value and regression gain value decreases slightly with age,which should be considered in clinical application.

Effect of dual-task-induced uncertainty on gait biomechanics in patients with multiple sclerosis with 2–6.5 EDSS grade

The goal of this study was to assess the effect that uncertainty induced by dual task conditions has on reaction-response time parameters and gait patterns of patients with multiple sclerosis (MS) with a 2–6.5 EDSS grade. The study involved eleven patients – nine women and two men – diagnosed with multiple sclerosis (age, 48±10years; height, 1.65±0.1m; weight, 72±22kg) with capacity to walk five meters without any aid or assistance. We employed an intra-group repeated measures design. Each participant was asked to walk with and without task-related uncertainty. Reaction-response and gait cycle times, as well as center of mass (CM) dynamics were measured using three force plates synchronized with a video camera through an electronic device that also controlled the system of uncertainty. The results obtained reveal that uncertainty induced by dual tasking is related to a reduction in the mean stride length and mean displacement and horizontal velocity of the CM in patients with MS. The values obtained for CM parameters indicate that uncertainty affects balance, as compared to no-uncertainty situations. These results confirm the necessity of including controlled dual-task-induced uncertainty in physical training programs for MS patients.

Eddy diffusivity: a single dispersion analysis of high resolution drifters in a tidal shallow estuary

In an estuary, mixing and dispersion resulting from turbulence and small scale fluctuation has strong spatio-temporal variability which cannot be resolved in conventional hydrodynamic models while some models employs parameterizations large water bodies. This paper presents small scale diffusivity estimates from high resolution drifters sampled at 10 Hz for periods of about 4 h to resolve turbulence and shear diffusivity within a tidal shallow estuary (depth 0.9) on the horizontal mean velocity within the channel. Enhanced diffusivity caused by shear dispersion resulting from the interaction of large scale flow with the boundary geometries was observed. Turbulence within the shallow channel showed some similarities with the boundary layer flow which include consistency with slope of 5/3 predicted by Kolmogorov’s similarity hypothesis within the inertial subrange. The diffusivities scale locally by 4/3 power law following Okubo’s scaling and the length scale scales as 3/2 power law of the time scale. The diffusivity scaling herein suggests that the modelling of small scale mixing within tidal shallow estuaries can be approached from classical turbulence scaling upon identifying pertinent parameters.

Large Eddy Simulation of Unidirectional and Wave Flows through Vegetation

AbstractMassively parallel large eddy simulation (LES) experiments were conducted to study the flow fields developed by unidirectional flow over submerged vegetation and wave flow-through emergent vegetation. For the submerged vegetation, vertical profiles of mean and turbulent horizontal and vertical velocities were found to be in good agreement with laboratory experiments. Canopy-averaged bulk drag coefficient calculated from the depth-integrated forces on the cylinders compares well with empirical measurements. For the emergent vegetation, wave-induced drag forces were calculated, and the inertia and pressure-drag coefficients were compared with laboratory experiments, which were found to be in good agreement. Vertical variation of forces and moments about the stem base are presented and compared with a single stem case under a variety of wave conditions and Keulegan-Carpenter (KC) numbers. It is seen that at low KC numbers the effect of the inertia force is significant. The vertical variation of the v...

Wind Turbulence Effects on Dune Sand Transport

ABSTRACT Udo, K., 2016. Wind Turbulence Effects on Dune Sand Transport. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 333–337. Coconut Creek (Florida), ISSN 0749-0208. This study involved field experiment of instantaneous wind velocity, angle, and aeolian sand flux on a coastal dune and numerical simulation of spatial wind fields to analyze the effects of wind turbulence on aeolian sand transport. The field data demonstrate that horizontal turbulence intensity is approximated by a power equation of mean horizontal wind velocity and has a negative relationship with the mean wind velocity in various wind conditions. Horizontal turbulence intensity also has a negative relationship with the aeolian sand flux in various wind conditions, indicating the potential for introducing the turbulence intensity into the aeolian sand transport equation. The win...

Interhemispheric structure and variability of the 5-day planetary wave from meteor radar wind measurements

Abstract. A study of the quasi-5-day wave (5DW) was performed using meteor radars at conjugate latitudes in the Northern and Southern hemispheres. These radars are located at Esrange, Sweden (68° N) and Juliusruh, Germany (55° N) in the Northern Hemisphere, and at Tierra del Fuego, Argentina (54° S) and Rothera Station, Antarctica (68° S) in the Southern Hemisphere. The analysis was performed using data collected during simultaneous measurements by the four radars from June 2010 to December 2012 at altitudes from 84 to 96 km. The 5DW was found to exhibit significant short-term, seasonal, and interannual variability at all sites. Typical events had planetary wave periods that ranged between 4 and 7 days, durations of only a few cycles, and infrequent strongly peaked variances and covariances. Winds exhibited rotary structures that varied strongly among sites and between events, and maximum amplitudes up to ~ 20 m s−1. Mean horizontal velocity covariances tended to be largely negative at all sites throughout the interval studied.

Orthoptic and video-oculographic analyses in oculopharyngeal muscular dystrophy.

Mild ophthalmoparesis can be seen in oculopharyngeal muscular dystrophy (OPMD). Orthoptic analysis included assessment of phoria/tropia, eye excursion, saccades, pursuit, stereoacuity, and Hess-Lancaster screen test. Video-oculography included fixation, horizontal and vertical saccades, and pursuit. Orthoptic abnormalities were: tropia (4 of 6); abnormal eye excursion (4 of 6, 78% involved lateral or superior rectus muscles); abnormal horizontal or vertical saccades (2 of 6); abnormal pursuit (0 of 6); abnormal stereoacuity (2 of 6); and pathological Hess-Lancaster screen (4 of 6). Video-oculographic abnormalities were present for: fixation (1 of 6); saccade latency (1 of 6); horizontal pursuit (3 of 6); and vertical pursuit (0 of 6). For horizontal saccades, mean velocity, peak velocity, and gain were pathological in 5 of 6, 5 of 6 (61% of pathological mean and peak velocities involved abducting eye movements), and 3 of 6, respectively. For vertical saccades, mean velocity, peak velocity, and gain were pathological in 4 of 6, 4 of 6 (53% involved upward movements), and 3 of 6, respectively. The data indicate preferential involvement of lateral and (to a lesser degree) superior rectus muscles in OPMD.

Temporal variations in the flow of a large Antarctic ice stream controlled by tidally induced changes in the subglacial water system

Abstract. Observations show that the flow of Rutford Ice Stream (RIS) is strongly modulated by the ocean tides, with the strongest tidal response at the 14.77-day tidal period (Msf). This is striking because this period is absent in the tidal forcing. A number of mechanisms have been proposed to account for this effect, yet previous modelling studies have struggled to match the observed large amplitude and decay length scale. We use a nonlinear 3-D viscoelastic full-Stokes model of ice-stream flow to investigate this open issue. We find that the long period Msf modulation of ice-stream velocity observed in data cannot be reproduced quantitatively without including a coupling between basal sliding and tidally induced subglacial water pressure variations, transmitted through a highly conductive drainage system at low effective pressure. Furthermore, the basal sliding law requires a water pressure exponent that is strongly nonlinear with q = 10 and a nonlinear basal shear exponent of m = 3. Coupled model results show that sub-ice shelf tides result in a ∼12 % increase in mean horizontal velocity of the adjoining ice stream. Observations of tidally induced variations in flow of ice streams provide stronger constraints on basal sliding processes than provided by any other set of measurements.

Turbulent Rayleigh–Bénard convection in spherical shells

We simulate numerically Boussinesq convection in non-rotating spherical shells for a fluid with a Prandtl number of unity and for Rayleigh numbers up to $10^{9}$. In this geometry, curvature and radial variations of the gravitational acceleration yield asymmetric boundary layers. A systematic parameter study for various radius ratios (from ${\it\eta}=r_{i}/r_{o}=0.2$ to ${\it\eta}=0.95$) and gravity profiles allows us to explore the dependence of the asymmetry on these parameters. We find that the average plume spacing is comparable between the spherical inner and outer bounding surfaces. An estimate of the average plume separation allows us to accurately predict the boundary layer asymmetry for the various spherical shell configurations explored here. The mean temperature and horizontal velocity profiles are in good agreement with classical Prandtl–Blasius laminar boundary layer profiles, provided the boundary layers are analysed in a dynamical frame that fluctuates with the local and instantaneous boundary layer thicknesses. The scaling properties of the Nusselt and Reynolds numbers are investigated by separating the bulk and boundary layer contributions to the thermal and viscous dissipation rates using numerical models with ${\it\eta}=0.6$ and with gravity proportional to $1/r^{2}$. We show that our spherical models are consistent with the predictions of Grossmann &amp; Lohse’s (J. Fluid Mech., vol. 407, 2000, pp. 27–56) theory and that $\mathit{Nu}(\mathit{Ra})$ and $\mathit{Re}(\mathit{Ra})$ scalings are in good agreement with plane layer results.

Exact expressions for transient forced internal gravity waves and spatially-localized wave packets in a Boussinesq fluid

We re-examine a simple model describing the propagation of transient forced internal gravity waves in a Boussinesq fluid with constant horizontal mean velocity which was previously studied by Nadon and Campbell (Wave Motion, 2007). The waves are generated by a horizontally-periodic lower boundary condition and propagate upwards. We derive an alternative exact expression for the solution which more readily gives insight into the behaviour of the solution at high altitude. Some special cases of lower boundary conditions are considered to illustrate the features of the solution. This form of the solution allows us to use a Fourier transform to derive the solution for the more general situation where a wave packet is generated by a horizontally-localized lower boundary condition, comprising a continuous spectrum of horizontal wavenumbers or Fourier modes. This is a more realistic representation of internal gravity waves in the atmosphere and can be used as a starting point for investigating waves generated by an obstacle of finite horizontal extent such as an isolated mountain or a mountain range.

An improved particle counting method for particle volume concentration in aeolian sand transport

An improved particle counting method (denoted as M-count method) is proposed to calculate the particle volume concentration in aeolian sand transport in a wind tunnel. In the present M-count method, the thickness of the laser sheet of PIV (particle image velocimetry) system and the variation of particle diameter with height are considered. The laser sheet thickness is determined by an optical method, in which a full width at half maximum (FWHM) is used to express the width of laser sheet on the image. The mean particle diameter calculated by the pixel areas from PIV images at different height layer is also modified. The results show that the present M-count method is an available method for obtaining the particle volume fraction and also verified by the M-flux method in which the particle volume fraction is deduced from the sand mass flux and the mean horizontal particle velocity. The present M-count method is also helpful to study particle concentration in aeolian sand transport.

A New Parametrization of Mixing Length in an Urban Canopy Derived from a Large-Eddy Simulation Database for Tokyo

Average horizontal wind velocity in an urban canopy is mainly determined by a balance between flow deceleration caused by the drag force of buildings and flow acceleration from the momentum flux gradient in the canopy. To express the transport of momentum in an urban canopy, mixing length is often used to calculate diffusivity there. A new parametrization for mixing length is introduced for a one-dimensional multilayer urban canopy model (UCM). A database from large-eddy simulations using actual urban morphology for Tokyo is used for this parametrization. The derived mixing length is described as a function of the non-dimensional height raised to the power of \(q\), where \(q < 1\). The \(q\) value and constants of the function also depend on the selection of canopy height. The mixing length profile is closely related to that of the average plane area index of the buildings in the study area. Recalculation of mean horizontal wind velocity using the new parametrization of mixing length for Tokyo slightly improved the multilayer UCM results.