Mean Horizontal Velocity Research Articles

Velocity and energy of periodic travelling interfacial waves between two bounded fluids

For a periodic travelling irrotational wave propagating at the interface between two homogeneous, incompressible and inviscid fluids bounded by horizontal planes, we generalise the Stokes definitions for the velocity of the wave propagation. Under certain conditions imposed on the horizontal velocity of the motion at the interface and supposing that the horizontal components of the velocity in each layer never reach the wave speed, we prove that the mean horizontal velocity of propagation of the wave is greater than the generalised mean horizontal velocity of the mass of the fluid. We show that, for interfacial waves of small amplitude, the excess kinetic and potential energy of the fluid have the same magnitude, but different signs, and for the nonlinear setting, we prove that the excess kinetic energy is negative.

Kinematic mechanism of the rehabilitative effect of 4-channel NMES: post-hoc analysis of a prospective randomized controlled study

The sequential 4-channel neuromuscular electrical stimulation (NMES), based on the normal contractile sequences of swallowing-related muscles, is a new rehabilitative treatment. The objective of this study was to explore the mechanism of the rehabilitative effect of the 4-channel NMES using kinematic analysis of videofluoroscopic swallowing study (VFSS) data. For this post-hoc analysis, we included a subset of participants from the prospective randomized controlled study on the clinical effectiveness of the sequential 4-channel NMES compared with that of the conventional 2-channel NMES. Seventeen subjects (11 and six in the 4- and 2-channel NMES groups, respectively) were eligible for the kinematic analysis of VFSS data. The hyoid bone movement was analyzed by evaluating the distance and time parameters with four peak points (A, B, C, D). The 4-channel NMES group showed significant improvement in vertical distances (A–C), horizontal distance (A–B, A–C), time interval (A–B–C) and total time, compared with their pretreatment data. The 2-channel NMES group showed significant improvements in time interval (A–B); however, the Euclidean distance (A–D) and mean velocity of the Euclidean distance (A-C) were significantly decreased. When the two groups were directly compared, the 4-channel group showed significantly greater improvement in horizontal distance (A–B), Euclidean distance (A–D), time interval (A–B–C), and mean velocity the Euclidean distance (A–D). The results in this study suggest that the sequential 4-channel NMES might lead to the physiologic circular movement of the hyoid bone during swallowing, and therefore be an effective treatment for dysphagia.Trial registration: Clinicaltrials.gov, registration number: NCT03670498.

A Mesh Convergence Study on 2-D Air Bubble Barrier Simulation with Mean of Inlet Static Pressure and Horizontal Surface Velocity

Timing is vital for oil spill response operations. However, deployment of the traditional response equipment, unfortunately, takes much more time. Therefore, innovative solutions are needed to minimize time losses. One of these innovative solutions is the air bubble barrier. Air bubble barrier creates a barrier to anything floating in the water, especially keeping the floating oil and petroleum in the area where it is spilled. Computational Fluid Dynamics simulation has grown in importance as a resource for air bubble barrier studies in recent years. Despite the extraordinary success of Reynolds Averaged Navier-Stoke applications on air bubble barriers, just a few studies concentrate on mesh sensitivity, one of the most fundamental issues with CFD methods. The main purpose of this study is to perform a mesh convergence study by simulating an air bubble barrier in the Simcenter STAR CCM+ software. In this context, in this simulation, a 2D numerical model is considered. The mesh convergence study has been performed by calculating the aperture inlet mean static pressure and the mean horizontal surface velocity. As a result, it is evident that the mesh base size and number of elements in mesh in case 10 can be employed to maintain the solution time-optimal state in the upcoming numerical simulations on the 2D and 3D air bubble barrier. Case 10 represents the mesh base size of 0.015 and the number of elements in mesh of 99042. Findings from this parametric study will be incorporated as mesh control rules into the subsequent 2D and 3D simulations of the air bubble barrier.

Horizontal transport in the bouncing ball system with a sawtooth-shaped table

The system that consists of a ball bouncing off a sawtooth-shaped table vibrating vertically is considered. The horizontal motion in this system is caused by the table shape, and we plotted the mean horizontal velocity as a function of the asymmetry of the table shape. The ball is transported in the direction which the gentler slopes face for the high-bounce parameter region. To give a description to the net asymmetric transport, we derive a simplified model using assumptions of high bounce and probabilistic collision to the left or right slopes. The simplified model exhibits a horizontal transport qualitatively similar to that observed in the original model.

Detection of Crustal Uplift Deformation in Response to Glacier Wastage in Southern Patagonia

The Southern Patagonian Icefield (SPI) is the largest continuous ice mass in the Southern Hemisphere outside Antarctica. It has been shrinking since the Little Ice Age (LIA) period, with increasing rates in recent years. An uplift of crustal deformation in response to this deglaciation process has been expected. The goal of this investigation is to analyze the crustal deformation caused by ice retreat using time-series data from continuous GPS stations (2015–2020) in the northern area of the SPI. For this purpose, we installed two continuous GPS stations on rocky nunataks of the SPI (the GRCS near Greve glacier and the GBCS close by Cerro Gorra Blanca). In addition, ice elevation changes (2000–2019) were analyzed by the co-registration of the SRTM digital elevation model and ICESat elevation data points. The results of the vertical components are positive (36.55 ± 2.58 mm a−1), with a maximum at GBCS, indicating the highest rate of crustal uplift ever continuously recorded in Patagonia; in addition, the mean horizontal velocities reached 11.7 mm a−1 with an azimuth of 43°. The negative ice elevation changes detected in the region have also accelerated in the recent two decades, with a median Δh (elevation change) of −3.36 ± 0.01 m a−1 in the ablation zone. The seasonality of the GPS signals was contrasted with the water levels of the main Patagonian lakes around the SPI, detecting a complex interplay between them. Hence, the study sheds light on the knowledge of the crustal uplift as evidence of the wastage experienced by the SPI glaciers.

Enhanced extreme wave statistics of irregular waves due to accelerating following current over a submerged bar

We present experimental results of irregular long-crested waves propagating over a submerged trapezoidal bar with the presence of a background current in a wave flume. We investigate the non-equilibrium phenomenon (NEP) induced by significant changes of water depth and mean horizontal flow velocity as wave trains pass over the bar. Using skewness and kurtosis as proxies, we show evidence that an accelerating following current could increase the sea-state non-Gaussianity and enhance both the magnitude and spatial extent of the NEP. We also find that below a ‘saturation relative water depth’ $k_p h_2 \approx 0.5$ ( $k_p$ being the peak wavenumber in the shallow area of depth $h_2$ ), although the NEP manifests, the decrease of the relative water depth does not further enhance the maximum skewness and kurtosis over the bar crest. This work highlights the nonlinear physics according to which a following current could provoke higher freak wave risk in coastal areas where modulation instability plays an insignificant role.

Computation of turbulent natural convection in an enclosure with differential flux models

This study investigates the performance of the elliptic blending differential flux model (EBDFM) and the standard differential flux model (DFM) in reproducing turbulent natural convection in a square cavity. In both cases, the Reynolds stresses are computed with the Elliptic Blending Reynolds Stress models (EBRSM). The model coefficient associated with the production of dissipation are modified to promote turbulence in the boundary layer, without which the velocity is overestimated and boundary layer is too thin. The computed mean quantities are in good agreement with data from the experiment. The vertical velocity component, the mean temperature and the local Nusselt number along the walls are reproduced with good accuracy. EBDFM show improved prediction of the mean horizontal velocity component, although the numerical predictions are not very accurate. In, general the EBDFM produces a better prediction of the turbulent quantities. While both models overpredict the peaks of the turbulent fields, the EBDFM is closer to the experiment. The DFM also overpredicts the turbulent vertical components in the outer boundary layer. Numerical predictions of the horizontal component of turbulent heat flux show weak accuracy with both models overestimating the peak by about three times. However, despite some discrepancies, the EBDFM shows improved performance over the DFM in the reproduction of the turbulent quantities and proves the advantage of the elliptic blending approach in modelling near wall turbulence.

Effect of particle shape on the hydrodynamics of gas-solid fluidized bed

Non-spherical particles are common in industrial applications such as coal gasification and biomass combustion. The present work employs a coupled Computational Fluid Dynamics – Discrete Element Method (CFD-DEM) approach to numerically investigate the particle shape effect on the hydrodynamics of a rectangular gas-solid fluidized bed. The CFD-DEM model capability is tested by comparing the mean horizontal and vertical velocities and pressure drop against the National Energy Technology Laboratory (NETL) experimental data. The effect of different numerical parameters (time step, grid size, discretization technique) on the predicted data is also studied. A grid size of three particle diameters and a 2nd Superbee discretization scheme are selected to run the simulations based on the numerical accuracy and simulation time. Seven different drag models are utilized to model the particle-fluid interaction. Further, simulations are conducted using the Di Felice-Ganser drag model for five different sphericities. A strong relationship between sphericity and fluidization behavior is noted. At the constant superficial gas velocity of 2.19 m/s, the fluidization behavior is changed from bubbling to a turbulent regime when the sphericity decreases from 1 to 0.42.

Schlieren Image Velocimetry of Swirl Sprays

Schlieren image velocimetry (SIV) is based on light deflection through flow heterogeneities and image cross-correlations. This is a low-cost and relatively low complexity technique that allows measurement of the droplet velocity field in a large region of a spray. A Z-type Toepler schlieren system with a high-speed camera was used to determine mean vertical and horizontal droplet velocities, as well as the cone angles of sprays produced by a pressure swirl injector with characteristic geometric constant K = 2. Different LEDs and digital filters were evaluated for edge detection and improvement of image contrast. Open software was adopted for digital image processing and velocimetry. Interrogation windows and overlaps of different sizes were tested to obtain an appropriate correlation for determination of the velocity field. The digital images were obtained with 5 × 103 fps and a resolution of 2.77 pixels/mm. Since the swirl sprays analysed presented instabilities, a number of 100 cross-correlations of images was required to reduce mean velocity fluctuations. Injection pressures varied from 0.05 to 7 bar and mass flow rates varied from 1.389 to 13.89 g/s, using water as test fluid. The wideband warm white LED with Laplacian or high-pass filters provided velocity data for a larger range of injection pressures. Mean axial velocities varied from 3.3 to 11.3 m/s, approximately, with mean horizontal velocities varying from around 0.17 to 3.3 m/s for pressures from 0.05 to 3.22 bar. The velocity data were compared to microscopic shadowgraphy results, showing a good agreement. Spray cone angles ranged from about 32.5o to 69.5°, for injection pressures from 0.05 to 7 bar, and results of triangulation with a blue LED were closer to semi-empirical data.

Storm‐Time Observations of Traveling Ionospheric Disturbances and Ionospheric Irregularities in East Africa

AbstractIn this paper equatorward propagating large scale traveling ionospheric disturbance (LSTID) on 17 March 2015 were investigated using the Statistical Angle‐of‐Arrival and Doppler Method for GPS (SADM‐GPS) radio interferometry technique in data‐scarce East Africa. To apply the SADM‐GPS method, 5 GPS arrays each with 3 GPS receivers arranged in a triangular geometry were used. Our results show that during 15:00–18:00 UT on 17 March, TIDs with mean horizontal velocities between 161.9 and 464.4 m/s were observed. Using the wavelet analysis, the periods of TIDs in a range of 51–69 min that qualify to LSTIDs were revealed. The peak‐to‐peak phase shift of detrended total electron content (TEC) over latitudes in this study confirms the equatorward TID propagation, which was obtained by the SADM‐GPS technique. A pair of magnetometers were used to infer E × B drift and an adequate agreement was found with Swarm satellites derived plasma density that enabled us to explain the behaviour of ionospheric irregularity during the main phase of the geomagnetic storm. Moreover, significant TEC enhancement (reaching ∼20%–105%) were captured by GPS arrays during the period of TID propagation. Nevertheless, the rate of change of TEC (ROT) and ROT index (ROTI) show wavy structures that reflect TID effects over ionospheric modulations during the post noon to evening hours of 17 March 2015.

Characteristics of Wind-Generated Near-Inertial Waves in the Southeast Indian Ocean

Abstract This study presents the characteristics and spatiotemporal structure of near-inertial waves and their interaction with Leeuwin Current eddies in the eastern south Indian Ocean as observed by Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats. The floats sampled the upper ocean during July–October 2013 with a frequency of eight profiles per day down to 1200 m. Near-inertial waves (NIWs) are found to be the dominant signal in the frequency spectra. Complex demodulation is used to estimate the amplitude and phase of the NIWs from the velocity profiles. The NIW energy propagated from the base of the mixed layer downward into the ocean interior, following beam characteristics of linear wave theory. We visually identified a total of 15 near-inertial internal wave packets from the wave amplitudes and phases with a mean vertical wavelength of 89 ± 63 m, a mean horizontal wavelength of 69 ± 85 km, a mean horizontal group velocity of 3 ± 2 cm s−1, and a mean vertical group velocity of 9 ± 7 m day−1. A strong near-inertial packet with a kinetic energy of 20–30 J m−3 found propagating below 700 m suggests that the NIWs can contribute to deep ocean mixing. A blue shift of 10%–15% in the energy spectrum of the NIWs is observed in the upper 1200 m as the floats move toward the equator. The impacts of mesoscale eddies on the characteristics and propagation of the observed NIWs are also investigated. The elevated near-inertial shear variance in anticyclonic eddies suggests trapping of NIWs near the surface. Cyclonic eddies, in contrast, were associated with weak near-inertial shear variance in the upper 400 m.

Do Different Hurdle Heights Alter Important Spatiotemporal Variables in Hurdle Clearance?

The aim of this study was to determine whether important spatiotemporal variables in hurdle clearance change with different hurdle heights. Twelve male hurdlers (mean height, 1.75 ± 0.04 m) cleared hurdles set at different heights [10% higher (High) and 10% lower (Low) than the center of mass (CM)]; images were captured by six high-speed cameras, and each spatiotemporal variable was calculated. Thereafter, the difference in each spatiotemporal variable between High and Low and the relationship between the mean horizontal velocity from takeoff to landing [Hurdle clearance velocity (HC-v)] and each spatiotemporal variable were examined. Our findings indicated that values for flight time, flight time from hurdle to landing (2nd flight time), clearance time, release height on takeoff, peak height of the CM, and the difference in landing distance were greater in the High condition than in the Low condition. Moreover, a low rate of deceleration on takeoff and short 2nd flight time, clearance distance, and takeoff distance were more strongly related to HC-v in the condition High, whereas a low rate of deceleration on landing, short flight time from takeoff to hurdle (1st flight time), high release height on landing, and touchdown height on landing were more strongly related to HC-v in the Low condition. Therefore, coaches should consider these changes in spatiotemporal variables when changing hurdle heights based on age group or event. It should also be noted that, even when the hurdle heights are the same, the spatiotemporal variables that should be considered may differ depending on the height of the hurdler.

Effect of hurdling step strategy on the kinematics of the hurdle clearance technique

ABSTRACT Athletes use either an eight-step or a seven-step strategy to reach the first hurdle in the 110 m hurdles event. This study investigates the effect of step strategy on the hurdle clearance technique and spatio-temporal parameters of the four steps prior to hurdle clearance. Two-dimensional video data were collected in the sagittal plane from 12 male sprinters, grouped as seven-step (n = 6) or eight-step (n = 6) strategists. The take-off distance was 0.20 m further from the hurdle and the touchdown was 0.42 m closer to the hurdle for seven-step athletes. Additionally, seven-step athletes reduced the length of the final step before hurdle take-off by 0.14 m compared with the previous step, whereas the eight-step athletes extended their final step by 0.17 m. There was negligible difference between the mean horizontal velocities of the two groups throughout the hurdle clearance (0.02 m/s) or the approach time to the first hurdle from the block clearance (0.01 s). This presents an important first insight into the effect of the step strategy on the first hurdle kinematics. Our findings identify the take-off and touchdown distance parameters of the hurdle clearance technique, and approach step characteristics for a successful seven- or eight-step approach strategy to be employed.

Need for the Determination of the Nigerian Fault Lines

Earthquakes and Tremors are Tectonic activities traced to Crustal Motions with roots in the Continental Drift Theory. These activities occur mostly at Plate Boundaries and along or close to Fault Lines. Nigeria was erroneously considered aseismic in the past, but studies have shown that Nigeria was never aseismic as information on different Tremors and Earthquakes abound. The recent occurrences of Tremors in Abuja, the Nation’s Capital, call for a serious attention towards the definition of the Nigerian Fault Lines. An attempt towards the determination of the Fault Lines is made, using the Non-Geodetic and Geodetic Methods. The Non-Geodetic Method Seeks to connect the Epicenters of different Earthquakes and Tremors on record, in order to present a possible set of Fault Lines. Three Fault Lines, The Western Nigeria Fault Line, the Mid Nigeria Fault Line and the Eastern Nigeria Fault Lines were proposed. The Geodetic Method adopted the use of archived GNSS data from the Nigerian CORS stations between 2012 and 2014. Fourteen sites, ABUZ, BKFP, GCCT, CLBR, FPNO, FUTA, FUTY, GEMB, HUKP, MDGR, OSGF, RUST, ULAG and UNEC, were used for this experiment, and ten IGS stations, DEAR, ABPO, RAMO, DARK, MELI, ASCG, MOIU, ZAMB, NOT1, BHR4, were used for Network Stabilization in the experiment conducted with the GAMIT/GLOBK Software. The Mean Horizontal Velocity of Nigeria for the period was 22.5625±0.32583mm/yr East, 18.93±0.23417mm/yr North, found to agree with Bawa, Ojigi and Dodo (2018) and Altamimi et al (2011). The Analysis of the Position Solution, showed that stations along a particular line, share similar direction of motion, which agrees with the Non-Geodetic Proposal. It was recommended that in order to adequately and accurately determine the Fault System in Nigeria, about three times the existing number of CORS Stations must be established, and their data made readily available to researchers. Keywords: Earthquakes, Tremors, Fault Lines, GAMIT/GLOBK DOI: 10.7176/JEES/11-10-05 Publication date: October 31 st 2021

Contribution of Reynolds shear stress to near-wall turbulence in Rayleigh–Bénard convection

Direct numerical simulations of Rayleigh–Bénard convection flows are performed for Pr=0.7 with Ra=5×108 and 2×1010, and for Pr=0.021 with Ra=107 and 5×108, where Pr and Ra are the Prandtl number and the Rayleigh number, respectively. The velocity-gradient production based on the short-time averaging shows the near-wall intensive positive peak and negative region. The correlations between the Reynolds shear stresses and the wall-normal and transverse gradients of the horizontal mean velocities locally and temporally result in the positive and negative velocity-gradient production near the wall. The eigenvalues of the anisotropic Reynolds stress tensors on the barycentric map show that the short-lived anisotropy related to the one-directional stretching occurs along the wall-normal distance; this behavior is similar to that of a canonical turbulent channel flow. The results indicate that the local and temporal Reynolds shear stresses near the wall with the shear of the horizontal mean velocities play an important role in the near-wall velocity-gradient production, which leads to the turbulent regime in the Rayleigh–Bénard convection flow with an increase in Rayleigh number.

Effect of hurdling step strategy on the kinematics of the block start

ABSTRACT Athletes use either a seven-step or eight-step strategy to reach the first hurdle in the 110 m hurdle event. This study investigated the effect of step strategy on the start position, the block exit and the first four approach steps. Two-dimensional video data were collected in the sagittal plane from 12 male sprinters, grouped as seven-step (n = 6) or eight-step (n = 6) strategists. Mean block spacing was 0.08 m further apart, block contact time 0.06s longer, first step 0.25 m longer and first ground contact 0.03s longer for seven-step athletes compared with eight-step athletes. There was also a greater vertical displacement of the centre of mass (CoM) (0.04 m) for the seven-step athletes compared with the eight-step athletes. Additionally, the front hip mean angular acceleration was 197°/s2 slower for the seven-step athletes than the eight-step athletes. There was limited difference between groups for mean horizontal velocity at the moment of block exit (0.14 m/s). These technical alterations provide an important first insight into start kinematics. The findings of this study identify the position in the starting blocks, and the key parameters which pertain to the initial phases for a successful seven-step approach strategy to be employed.

The head is an excellent proxy for the whole body center of mass when measuring running velocity in competition

Whole body digitizing used to calculate whole body center of mass (CM) variables from competitions is particularly time-consuming, and “shortcut” methods that substitute for it could expediate the calculation of spatiotemporal variables. The aim of this study was to measure the appropriateness of using the head as a proxy for the CM when calculating running velocity in competition. Fifty-six athletes in the IAAF World Championship marathons were recorded using two high-definition cameras (50 Hz) on two laps so that 112 running sequences were analyzed. The video files were imported into SIMI Motion and manually digitized. The horizontal running velocity during one gait cycle was obtained using four methods: horizontal velocity of the CM; horizontal velocity of the head (raw data); horizontal velocity of the head (Butterworth filtered); and horizontal displacement of the head (a single measurement using SIMI Motion 3D still image measurement) divided by time taken. In comparison with the criterion CM measurements for mean horizontal velocity, the filtered head data had the best 95% confidence interval (95% CI) for intraclass correlation coefficient (ICC) (0.999 – 1.000), the least bias (–0.006 m/s), and the lowest root mean square difference (0.024 m/s). The filtered head condition also had the best 95% CI for ICC for maximum and minimum horizontal velocities during the stride (>0.988) and the lowest bias (–0.001 m/s and –0.003 m/s, respectively). With the application of an appropriate filter, the head is thus an excellent proxy for whole body CM velocity calculations.

Solar activity dependence of medium-scale traveling ionospheric disturbances using GPS receivers in Japan

In order to reveal solar activity dependence of the medium-scale traveling ionospheric disturbances (MSTIDs) at midlatitudes, total electron content (TEC) data obtained from a Global Positioning System (GPS) receiver network in Japan during 22 years from 1998 to 2019 were analyzed. We have calculated the detrended TEC by subtracting the 1-h running average from the original TEC data for each satellite and receiver pair, and made two-dimensional TEC maps of the detrended TEC with a spatial resolution of 0.15° × 0.15° in longitude and latitude. We have investigated MSTID activity, defined as delta I/overline{I}, where delta I and overline{I} are standard deviation of the detrended TEC and the average vertical TEC within the area of 133.0°–137.0° E and 33.0°–37.0° N for 1 h, respectively. From each 2-h time series of the detrended TEC data within the same area as the MSTID activity, auto-correlation functions (ACFs) of the detrended TEC were calculated to estimate the horizontal propagation velocity and direction of the MSTIDs. Statistical results of the MSTID activity and propagation direction of MSTIDs were consistent with previous studies and support the idea that daytime MSTIDs could be caused by atmospheric gravity waves, and that nighttime MSTIDs were caused by electro-dynamical forces, such as the Perkins instability. From the current long-term observations, we have found that the nighttime MSTID activity and occurrence rate increased with decreasing solar activity. For the daytime MSTID, the occurrence rate increased with decreasing solar activity, whereas the MSTID activity did not show distinct solar activity dependence. These results suggest that the secondary gravity waves generated by dissipation of the primary gravity waves propagating from below increase under low solar activity conditions. The mean horizontal phase velocity of the MSTIDs during nighttime did not show a distinct solar activity dependence, whereas that during daytime showed an anticorrelation with solar activity. The horizontal phase velocity of the daytime MSTIDs was widely distributed from 40 to 180 m/s under high solar activity conditions, whereas it ranged between 80 and 200 m/s, with a maximum occurrence at 130 m/s under low solar activity conditions, suggesting that gravity waves with low phase velocity could be dissipated by high viscosity in the thermosphere under low solar activity conditions.

Fluid–solid Floquet stability analysis of self-propelled heaving foils

Abstract

Swimming performance of subarctic Calanus spp. facing downward currents

Calanoid copepods dominate mesozooplankton communities in temperate and Nordic seas. The ability of copepods to remain and feed in productive surface waters depends on their ability to overcome downward flows. In this study, we assessed the swimming performance of subarctic Calanus spp. and tested how the copepods can retain their vertical position in a representative range of downward currents (between 0 and 5.4 cm s-1) simulated in a downwelling flume. Mean vertical and horizontal copepod swimming velocities and accelerations, movement periodicity and trajectory complexity were obtained by tracking individual trajectories in the field of view of 2 cameras. Copepod swimming velocity increased with increasing downward flow and matched downward flows up to 2 cm s-1. Beyond 2 cm s-1, animals were still able to significantly reduce their sinking rates, but their motions changed. Trajectories became simpler, swimming velocities changed on shorter time scales and instantaneous acceleration increased. These results are consistent with predictions of balancing depth retention against encounter rates with food and predators. Frequency distributions of vertical swimming speeds were mostly unimodal, with entire experimental populations responding in the same way. Coordination of movements and the ability to resist moderate downwelling flows can result in the accumulation of copepods in large surface swarms as observed in the field.