Instantaneous Velocity Data Research Articles

Reliability and Validity of Maximal Mean and Critical Speed and Metabolic Power in Australian Youth Soccer Players.

The reliability and validity of maximal mean speed (MMS), maximal mean metabolic power (MMPmet), critical speed (CS) and critical metabolic power (CPmet) were examined throughout the 2016-2017 soccer National Youth League competitions. Global positioning system (GPS) data were collected from 20 sub-elite soccer players during a battery of maximal running tests and four home matches. A symmetric moving average algorithm was applied to the instantaneous velocity data using specific time windows (1, 5, 10, 60, 300 and 600 s) and peak values were identified. Additionally, CS and CP¬met values calculated from match data were compared to CS and CPmet values determined from previously validated field tests to assess the validity of match values. Intra-class correlation (one-way random absolute agreement) scores ranged from 0.577 to 0.902 for speed, and from 0.701 to 0.863 for metabolic power values. Coefficients of variation (CV) ranged from good to moderate for speed (4-6%) and metabolic power (4-8%). Only CS and CPmet values were significantly correlated (r = 0.842; 0.700) and not statistically different (p = 0.066; 0.271) to values obtained in a shuttle-running critical test. While the present findings identified match-derived MMS, MMPmet, CS and CPmet to be reliable, only CS and CPmet derived from match play were validated to a CS field test that required changes in speed and direction rather than continuous running. This suggests that both maximal mean and critical speed and metabolic power analyses could be alternatives to absolute distance and speed in the assessment of match running performance during competitive matches.

Velocity distribution characteristics in meandering compound channels with one-sided vegetated floodplains

To discuss the effects of asymmetrically vegetated floodplains on the velocity distribution characteristics and discharge capacity of a meandering main channel, in this paper, continuous river bend models with smooth floodplains and one-sided vegetated floodplains were designed in a generalized way. Three-dimensional instantaneous velocity data were obtained with an Acoustic Doppler Velocimeter (ADV), and the velocity distribution characteristics and discharge capacities of the generalized models were compared. Compared with the smooth floodplain conditions, the one-sided vegetated floodplain conditions are associated with lower cross-sectional average velocities, lower ratios of main channel discharge to total discharge near the bending apexes with minimal extension of vegetated floodplain, but higher ratios near the bending apex with maximal extension of vegetated floodplain. The deflection degrees of depth-averaged velocities in the meandering main channel are related to the cross-sectional position, relative depth and floodplain vegetation. The depth-averaged velocities near the bending apexes are inclined towards the main channel concave sides, those in the upstream crossover area are generally inclined towards the vegetated side, and those in the downstream crossover area are obviously inclined towards the smooth side. For a given relative depth, the deflection degrees near the bending apexes are smaller than those in the crossover areas. The deflection degrees near the bending apexes with minimal extension of vegetated floodplain are smaller than those under smooth floodplain conditions, but the deflection degrees near the bending apex with maximal extension of vegetated floodplain are obviously larger than the corresponding values under smooth floodplain conditions. Near the bending apexes, the longitudinal depth-averaged velocities are higher close to the main channel convex side and lower adjacent to the concave side. When the relative depth is small, one-sided vegetated floodplain conditions feature larger transverse gradients of the cross-sectional longitudinal depth-averaged velocities near the bending apexes with minimal extension of vegetated floodplain. When the relative depth is large, the transverse gradients near the bending apexes with minimal extension of vegetated floodplain are all much smaller than the corresponding values of the smooth floodplain conditions. However, near the bending apex with maximal extension of vegetated floodplain, there are no significant differences concerning the transverse gradients of the cross-sectional longitudinal depth-averaged velocities under the both floodplain conditions. One-sided vegetated floodplain conditions significantly influence the longitudinal velocity distributions in the meandering main channel. For a given relative depth, the longitudinal velocities at the cross-sectional positions under one-sided vegetated floodplain conditions are lower than those under smooth floodplain conditions to varying extents.

Vortex identification in turbulent flows past plates using the Lagrangian method

Vortex identifications in turbulent flows past arrays of tandem plates are performed by employing the velocity field obtained by high-fidelity large eddy simulations. Lagrangian coherent structures (LCSs) are extracted to examine the evolution and the nonlinear interaction of vortices and to characterize the spatial and temporal characteristics of the flow. LCSs’ identification method is based on the finite-time Lyapunov exponent (FTLE), which is evaluated using the instantaneous velocity data. The simulations are performed in three-dimensional geometries to understand the physics of fluid motion and the vortex dynamics in the vicinity of plates and surfaces at Reynolds number of 50 000. The instantaneous vorticity fields, Eulerian Q-criterion, and LCSs are presented to interpret and understand complex turbulent flow structures. The three-dimensional FTLE fields provide valuable information about the vortex generation, spatial location, evolution, shedding, decaying, and dissipation of vortices. It is demonstrated here that FTLE can be used together with Eulerian vortex identifiers to characterize the turbulent flow field effectively.

A Full-Scale Rotor-Wake Investigation of a Free-Flying Helicopter in Ground Effect Using BOS and PIV

Measurements in the wake of a free-flying full-scale helicopter in ground effect were performed for both quasi-steady and unsteady maneuvering flights using stereoscopic particle image velocimetry (PIV), a time-resolved background-oriented schlieren (BOS) setup, and an optical marker tracking technique. The systems were used in a complementary way to both visualize blade tip vortices in a large portion of the rotor wake and to capture spatially resolved wake velocity data close to the ground. The high sensitivity of the BOS system enabled the detection of vortices up to an age of ψ = 630°. Different instability mechanisms as long-wave, short-wave, and pairing instabilities were observed with varying intensity for different flight conditions. A quantitative analysis of vortex locations showed a periodic variation resulting from interactions between consecutive vortices that led to vortex pairing. Characteristics of the wake outwash close to the ground were investigated by means of averaged velocity fields. Different patterns such as wall jet, recirculation, and ground vortex flow were quantitatively analyzed and found to be in good agreement with previous model helicopter experiments. The instantaneous velocity data were used to detect individual blade tip vortices with ages above 450° close to the ground and to extract vortex parameters. For a takeoff maneuver, both concentrated vortices and the formation of larger vortex structures due to bundling of several vortices were observed.

Stereoscopic Particle Image Velocimetry Measurements and Proper Orthogonal Decomposition Analysis of the In-Cylinder Flow of Gasoline Direct Injection Engine

Intake generated flows are known to have a fundamental influence on the combustion both in spark ignition (SI) and compression ignition engines. This study experimentally investigated the tumble flow structures inside a cylinder of gasoline direct injection (GDI) engine utilizing a stereoscopic time-resolved particle image velocimetry (PIV). The experiments were conducted in a GDI engine head for a number of fixed valve lifts and 150 mmH2O pressure difference across the intake valves. A tumble flow analysis was carried out considering different vertical tumble planes. In addition, the proper orthogonal decomposition (POD) identification technique was applied on the PIV data in order to spatially analyze the structures embedded in the instantaneous velocity data sets. The results showed that the flow was dominated by a strong tumble motion in the middle of cylinder at high valve lifts (8–10 mm). Moreover, it is worth pointing out that, because of the complexity of the flow at the high valve lifts, the flow energy was distributed over a higher number of POD modes. This was confirmed by the need of a higher number of POD modes needed to reconstruct the original velocity field to the same level of fidelity.

Properties of a sweeping jet emitted from a fluidic oscillator

This experimental study investigates the flow field and properties of a sweeping jet emitted from a fluidic oscillator into a quiescent environment. The aspect ratio of the outlet throat is 1. Stereoscopic particle image velocimetry is employed to measure the velocity field plane-by-plane. Simultaneously acquired pressure measurements provide a reference for phase correlating the individual planes yielding three-dimensional, time-resolved velocity information. Lagrangian and Eulerian visualization techniques illustrate the phase-averaged flow field. Circular head vortices, similar to the starting vortex of a steady jet, are formed repetitively when the jet is at its maximum deflection. The quantitative jet properties are determined from instantaneous velocity data using a cylindrical coordinate system that takes into account the changing deflection angle of the jet. The jet properties vary throughout the oscillation cycle. The maximum jet velocity decays much faster than that of a comparable steady jet indicating a higher momentum transfer to the environment. The entrainment rate of the spatially oscillating jet is larger than for a steady jet by a factor of 4. Most of the mass flow is entrained from the direction normal to the oscillation plane, which is accompanied by a significant increase in jet depth compared to a steady jet. The high entrainment rate results from the enlarged contact area between jet and ambient fluid due to the spatial oscillation. The jet’s total force exceeds that of an idealized steady jet by up to 30 %. The results are independent of the investigated oscillation frequencies in the range from 5 to 20 Hz.

Measurements of local droplet velocities in horizontal gas-liquid pipe flow with low liquid loading

New data on streamwise droplet velocity profiles for low liquid loading pipe flows are reported. The fluids used in the experiments are SF6 (gas-phase) and Exxsol D60 (oil-phase). Experiments are conducted in a 10 cm pipe diameter high-pressure (∼780 kPa, absolute) flow loop to reproduce gas-condensate field conditions. Instantaneous streamwise velocity data, obtained using the non-intrusive Laser Doppler Anemometry (LDA) technique, are used to calculate mean and root-mean-squared (RMS) local velocities. Asymmetric droplet velocity profiles with respect to the pipe center-line are observed especially for the stratified low atomization flow conditions. However, as the flow momentum increases, the droplet velocity profiles seem to become more symmetric. Also, these data suggest that, irrespective of the conditions studied, the single-phase gas flow characteristics are preserved closer to the top pipe wall. The data from the LDA imply that the bottom pipe half region is highly influenced by the gas-liquid interfacial characteristics. This results in high streamwise turbulence intensity in the region influenced by the interfacial waves (interfacial turbulence). An isokinetic sampling instrument is also used to measure the local instantaneous dynamic pressure. The dynamic pressure and the locally extracted liquid (droplets) volume rate under isokinetic conditions are used to calculate local fluid velocity. Excellent agreement has been obtained when comparing this calculated local velocity from the isokinetic instrument to the LDA data.

Turbulence characteristics of wave-blocking phenomena

This study explores experimentally the turbulent flow in a laboratory flume, interacting with waves propagated against the flow. It focuses a region of ‘wave-blocking’ for which there is a stream-wise location on the water surface, where the wave propagation velocity vanishes. The observations are corroborated by finding a critical wave frequency for a particular discharge above which the waves are effectively blocked; and verified by the dispersion relation of monochromatic wave. The counter-current propagating waves show an evolutionary change in the flow with three segmented regions, viz, flow at the upstream, blocking at the mid-stream and waves in the downstream. The instantaneous velocity data were collected using 3-D Micro-acoustic Doppler velocimeter (ADV) along the flume centerline. This study addresses the changes in the mean flows, Reynolds stresses, eddy viscosity, turbulence kinetic energy fluxes and associated contributions of burst-sweep cycles to the total Reynolds shear stress due to addition of surface waves against a current. The velocity power spectral analysis shows the energy distribution over the whole profile from upstream to downstream. The quadrant analysis is also used to highlight the turbulent event evolutions along the flow; and shows that at the wave-blocking and wave dominated regions, the contributions from ejection and sweep to the total shear stress are dominant. The changes in turbulence key parameters due to wave-blocking may affect the sediment transport in coastal region.

Influence of bridge pier shape on flow field and scour geometry

ABSTRACT The objective of the present paper is to investigate experimentally the flow field and the local scour around bridge pier of different shapes on a sediment bed. Experiments were carried out using individual piers of rectangular, oblong, trapezoidal, triangular and lenticular shapes with a common aspect ratio to compare the flow field and scour geometry among the shapes at identical flow conditions. The novelty of this study is to identify the scour structures due to different pier shapes and associated mean flows and turbulence, which are responsible for generation of scour. A 3D Micro-acoustic Doppler velocimeter (ADV) was used to record the instantaneous velocity data for five different discharges. It is observed that upstream scour depth was maximum at the leading edge of the rectangular pier, and minimum at the same location for the lenticular. Here, the horseshoe vortex is the main agent of the scour development for any kind of pier shapes, even for the sharp edges, though there is an immediate bifurcation of flow in the triangular and lenticular-nosed piers, where the scour formation is less than the others. The changes in mean velocities and Reynolds stresses due to the pier shapes are discussed and compared to that of flat-bed surface. Since the shape of the pier alters the flow and turbulence, one can expect large variations in flow behavior, and hence different scour formations are observed. The results can be used to identify scour structures around the piers of different shapes for designing and suggesting appropriate protection measures.

Investigation of coherent structures in a turbulent channel with built-in longitudinal vortex generators

The present study discusses the results pertaining to eduction of coherent structures and assesses their contribution in promoting mixing between low and high momentum fluid streams to enhance thermal convection. The analysis has been carried out numerically in a periodic parallel-plate-channel containing built-in longitudinal vortex generators on the bottom wall. The Reynolds number of the investigation is 6000 based on the channel height and bulk mean velocity. The computational geometry resembles heat exchanger surfaces on which the vortex generators create strong longitudinal vortices that enhance heat transfer on the plane channel wall. At high Reynolds numbers, the enhancement of convective heat transfer near the wall largely depends on the large scale turbulent structures (coherent structures) that disappear and reappear frequently and exhibit typical correlated scales in time or space. Proper orthogonal decomposition (POD) is an efficient tool to identify these coherent turbulent structures from instantaneous flow field data. Despite geometrical simplicity, the complex swirling motion behind the vortex generators demands highly accurate computational method, like the large eddy simulation (LES) of turbulence for capturing the nuances of flow. The instantaneous velocity data at a cross-flow plane have been assembled over a long time-window and analyzed deploying POD technique to extract the large- and small-scale structures based on the turbulent kinetic energy of the structures. Reconstructions of the velocity field using different combinations of eigenmodes have yielded different patterns of the flow. Large-scale structures have been visualized using first few energetic modes, while remaining less energetic modes have provided small-scale structures. The technique enables us to identify coherent structures hidden in the random flow field by filtering the small scale effects. The analysis yields as large as 15 dominant POD modes to cumulatively capture as low as 25% of the total turbulent kinetic energy of the flow, indicating broad range of energy spectrum that shares a large number of POD modes and thereby concludes highly complex turbulent fields with numerous spatial and temporal turbulent scales. Reconstructed velocity fields considering the leading dominant modes reveal a cut-off of 43% turbulent kinetic energy to clearly expose the coherent structures, which are consistent with published literature on similar turbulent channel flows. Additionally, quadrant analysis on the reconstructed velocity fields has clearly revealed that sweep and ejection modes are the only dominating Reynolds-stress-producing events for the current flow of interest, where the vortices lift the low-momentum fluid away from wall through the ejection mechanism. The high-momentum fluid impinges on the wall in sweep mode.

The metabolic power and energetic demands of elite Gaelic football match play.

Metabolic power has not yet been investigated within elite Gaelic football. The aim of the current investigation was to compare the metabolic power demands between positional groups and examine the temporal profile of elite Gaelic football match play. Global positional satellite system (GPS) data were collected from 50 elite Gaelic football players from 4 inter-county teams during 35 elite competitive matches over a three season period. A total of 351 complete match samples were obtained for final analysis. Players were categorized based on positional groups; full-back, half-back, midfield, half-forward and full-forward. Instantaneous raw velocity data was obtained from the GPS and exported to a customized spreadsheet which provided estimations of both speed based, derived metabolic power and energy expenditure variables (total distance, high speed distance, average metabolic power, high power distance and total energy expenditure). Match mean distance was 9222±1588 m, reflective of an average metabolic power of 9.5-12.5 W·kg-1, with an average energy expenditure of 58-70 Kj·kg-1 depending on position. There were significant differences between positional groups for both speed-based and metabolic power indices. Midfielders covered more total and high-speed distance, as well as greater average and overall energy expenditure compared to other positions (P<0.001). A reduction in total, high-speed, and high-power distance, as well as average metabolic power throughout the match (P<0.001) was observed. Positional differences exist for both metabolic power and traditional running based variables. The middle three positions (midfield, half-back and half-forward) possess greater activity profiles when compared to other positional groups. The reduction in metabolic power and traditional running based variables are comparable across match play. The current study demonstrates that metabolic power may contribute to our understanding of Gaelic football match-play.

Seasonal Hydrodynamics and Salt Exchange of a Shallow Estuary in Northern China

ABSTRACT Zou, T.; Zhang, H.; Meng, Q. J., and Li, J., 2016. Seasonal hydrodynamics and salt exchange of a shallow estuary in Northern China. Estuaries are important components of coastal ecosystem and function as dominant pathways of material exchange at the land-sea interface. The transport of terrestrial input through river inflow is controlled by physical process including tides, waves, and fresh water discharge. This study investigates hydrodynamic characteristic and salt flux within Xiaoqinghe River (XQR) estuary, which is a shallow estuarine system (water depth < 8 m) and exports substantial amount of nutrients and pollutants to the adjacent Laizhou Bay. Profile velocity and salinity are measured using ADCP and CTD through complete tidal cycles (25 hours) in April, July and September 2013. The instantaneous velocity and salinity data are decomposed into time-averaged means and time-varying components based on the improved Kjerfve (1986) method to quantify the contributions of various physical proces...

Physical Demands Of Japan Rugby Top League Match-play Using GPS

Recently, Global Positioning System (GPS) have been used as a method to analyze workload in rugby football games and training sessions. But there are few studies of Japan elite rugby. PURPOSE: To investigate the physical demands of Japan rugby top league match-play using GPS, and to compare these demands between positional groups for a more effective training plan. METHODS: GPS data (sampling rate; 15Hz) were collected from 15 Japan rugby elite players from same team during 14 Japan Rugby Top League competition games over one season. Players were categorized into one of four positional groups, FR (front row forwards, i.e. prop and hooker), 2&3R (2nd and 3rd row forwards, i.e. locks, flanker, andNo.8), SH (scrum-half), and BKs (backs, i.e. fly-half, three-quarters, and full-back). Instantaneous raw velocity data obtained from the GPS units was exported to a customized spreadsheet which provided estimations of speed-based variables (0-6km/h, i.e. Standing & Walking, 6-12km/h, i.e. Jogging, 12-18km/h, i.e. Cruising & Striding, 18-20km/h, i.e. High-intensity running, <20km/h, i.e. Sprinting, and total running distance). Analysis of variance and post hoc test were used to compare differences in physical performance variables between positional groups. RESULTS: Total distances covered during match-play by FR, 2&3R, SH, and BKs were 5603.9±252.4m, 5689.1±284.3m, 7001.0±245.8m, 6054.1±857.6m, respectively. There were significant differences between positional groups, and then SH was significant more covered total distance than other positional groups (p<.05). Standing & Walking distances (and ratio for the total distance, same hereinafter) covered by FR, 2&3R, SH, and BKs were 2209m (39.4%), 2009m (35.3%), 2091m (29.9%), and 2550m (42.1%), respectively. Jogging distances were 2207m (39.8%), 2039m (35.8%), 2031m (28.8%), and 1742m (29.0%). Cruising & Striding distances were 967m (17.3%), 1237m (21.8%), 1930m (27.6%),and 1128m (18.6%). High-intensity running distances were 118m (2.0%), 215m (3.8%), 425m (6.1%), and 230m (3.8%). Sprinting distances were 117m (2.1%), 141m (2.5%), 430m (6.1%), and 371m (6.1%). High-intensity running and sprinting distances covered by SH and BKs were significant greater than them covered by FR and 2&3R (p<.05). CONCLUSIONS: Positional differences exist for running demands as speed-based variables. These results demonstrate the physical demands of elite players in Japan rugby top league.

Fluid flow and sediment entrainment in the Garonne River bore and tidal bore collision

AbstractA detailed field study was carried out on a tidal bore to document the turbulent processes and sediment entrainment which occurred. The measured bore, within the Arcins Channel of the Garonne River (France), was undular in nature and was followed by well‐defined secondary wave motion. Due to the local river geometry a collision between the Arcins channel tidal bore and the bore which formed within the main Garonne River channel was observed about 800 m upstream of the sampling site. This bore collision generated a transient standing wave with a black water mixing zone. Following this collision the bore from the main Garonne River channel propagated ‘backward’ to the downstream end of the Arcins channel. Velocity measurements with a fine temporal resolution were complemented by measurements of the sediment concentration and river level. The instantaneous velocity data indicated large and rapid fluctuations of all velocity components during the tidal bore. Large Reynolds shear stresses were observed during and after the tidal bore passage, including during the 'backward' bore propagation. Large suspended sediment concentration estimates were recorded and the suspended sediment flux data showed some substantial sediment motion, consistent with the murky appearance of the flood tide waters. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Physical modelling of breaking tidal bores: comparison with prototype data

ABSTRACTA tidal bore is a hydraulic jump in translation, propagating upstream as the tide turns to rising and the flood flow advances in a funnel-shaped river mouth under spring tide conditions. This study focused on the unsteady turbulence induced by a breaking tidal bore. Detailed free-surface and velocity measurements were conducted with a high temporal resolution using non-intrusive free-surface measurement probes and acoustic Doppler velocimetry sampled at 200 Hz. The laboratory data were systematically compared with an earlier series of field measurements conducted in the breaking bore of the Sélune River (France). Key findings include the agreement, in terms of dimensionless instantaneous free-surface and velocity data, between laboratory and field observations as well as the existence of a transient recirculation region near the bed.

Developing a High Speed Fiber-optic Endoscopic Technique for Measuring Particle Phase Characteristics in a Spouted Bed

A novel endoscopic photography technique coupling a high speed CCD camera and an intrusive fiber-optic endoscope has been extended to enable the investigation of particle flow behavior in a 3D spouted bed 1500mm in height and 200mm in diameter. A series of algorithms have been specifically optimized to realize the process of image processing and the calculation of the instantaneous particle velocity data. Then particle phase velocity and granular temperature were obtained respectively by statistical analysis and detailed study of the obtained data. The results showed that instantaneous particle velocities exhibited random complexity in both magnitude and direction. Although little information could be extracted from the raw data directly, the profile of particle phase velocity was able to show the mainstream particle flow patters along bed radius. Also, the computed granular temperature could be used to characterize the local particle fluctuation intensity. All experimental results could be useful to validate the CFD models for gas-solid flow systems.

Metabolic power demands of rugby league match play.

To describe the metabolic demands of rugby league match play for positional groups and compare match distances obtained from high-speed-running classifications with those derived from high metabolic power. Global positioning system (GPS) data were collected from 25 players from a team competing in the National Rugby League competition over 39 matches. Players were classified into positional groups (adjustables, outside backs, hit-up forwards, and wide-running forwards). The GPS devices provided instantaneous raw velocity data at 5 Hz, which were exported to a customized spreadsheet. The spreadsheet provided calculations for speed-based distances (eg, total distance; high-speed running, >14.4 km/h; and very-high-speed running, >18.1 km/h) and metabolic-power variables (eg, energy expenditure; average metabolic power; and high-power distance, >20 W/kg). The data show that speed-based distances and metabolic power varied between positional groups, although this was largely related to differences in time spent on field. The distance covered at high running speed was lower than that obtained from high-power thresholds for all positional groups; however, the difference between the 2 methods was greatest for hit-up forwards and adjustables. Positional differences existed for all metabolic parameters, although these are at least partially related to time spent on the field. Higher-speed running may underestimate the demands of match play when compared with high-power distance-although the degree of difference between the measures varied by position. The analysis of metabolic power may complement traditional speed-based classifications and improve our understanding of the demands of rugby league match play.

Turbulent Features in the Coherent Central Region of a Plane Water Jet Issuing Into Quiescent Air

A plane water jet issuing into quiescent air at a Reynolds number of 2.5 × 105 is experimentally studied using phase Doppler anemometry (PDA). The plane water jet contains a coherent central region, which is situated immediately downstream of the nozzle exit. Particular emphasis is placed upon the distinctive attributes of such a region. Both mean flow pattern and turbulent features are obtained statistically based upon instantaneous velocity data. The central region is overwhelmingly dominated by uniformly distributed velocity, and remarkably high velocity gradient is present near the boundary of this region. Evidence shows that self-preservation is not satisfied in the central region. Explicit energy dissipation mechanisms in the central region are appreciated from cross-sectional uniform distributions of Kolmogorov length scales. Turbulent kinetic energy increases as the plane jet progresses, which is opposite to the general tendency associated with self-preservation. Although the central region is filled with near-zero Reynolds shear stress, distributions of skewness and flatness in this region are non-Gaussian and the instability caused by small-scale flow structures is thereby substantiated.

Metabolic power and energetic costs of professional Australian Football match-play

To compare the metabolic power demands between positional groups, and examine temporal changes in these parameters during Australian Football match-play. Longitudinal observational study. Global positioning system data were collected from 39 Australian Football players from the same club during 19 Australian Football League competition games over two seasons. A total of 342 complete match samples were obtained for analysis. Players were categorised into one of six positional groups: tall backs, mobile backs, midfielders, tall forwards, mobile forwards and rucks. Instantaneous raw velocity data obtained from the global positioning system units was exported to a customised spreadsheet which provided estimations of both speed-based (e.g. total and high-speed running distance) and derived metabolic power and energy expenditure variables (e.g. average metabolic power, high-power distance, total energy expenditure). There were significant differences between positional groups for both speed-based and metabolic power indices, with midfielders covering more total and high-speed distance, as well as greater average and overall energy expenditure compared to other positions (all p<0.001). There were reductions in total, high-speed, and high-power distance, as well as average metabolic power throughout the match (all p<0.001). Positional differences exist for both metabolic power and traditional running based variables. Generally, midfielders, followed by mobile forwards and mobile backs had greater activity profiles compared to other position groups. We observed that the reductions in most metabolic power variables during the course of the match are comparable to traditional running based metrics. This study demonstrates that metabolic power data may contribute to our understanding of the physical demands of Australian Football.

Droplet–turbulence interaction in a confined polydispersed spray: effect of droplet size and flow length scales on spatial droplet–gas velocity correlations

AbstractThis paper discusses the interaction between droplets and entrained turbulent air flow in the far-downstream locations of a confined polydispersed isothermal spray. Simultaneous and planar measurements of droplet and gas velocities in the spray along with droplet size are obtained with the application of a novel experimental technique, developed by Hardalupaset al. (Exp. Fluids, vol. 49, 2010, pp. 417–434), which combines interferometric laser imaging for droplet sizing (ILIDS) with particle image velocimetry (PIV). These measurements quantified the spatial correlation coefficients of droplet–gas velocity fluctuations ($R_{dg}$) and droplet–droplet velocity fluctuations ($R_{dd}$) conditional on droplet size classes, for various separation distances, and for axial and cross-stream velocity components. At the measurement location close to the spray edge, with increasing droplet size,$R_{dg}$was found to increase in axial direction and decrease in cross-stream direction. This suggests that as the gas-phase turbulence becomes more anisotropic away from the spray axis, the gravitational influence on droplet–gas correlated motion tends to increase. The effective length scales of the correlated droplet–gas motion were evaluated and compared with that for gas and droplet motion. The role of different turbulent eddies of the gas flow on the droplet–gas interaction was examined. The flow structures were extracted using proper orthogonal decomposition (POD) of the instantaneous gas velocity data, and their contribution on the spatial droplet–gas velocity correlation was evaluated, which quantified the momentum transfer between the two phases at different length scales of the gas flow. The droplets were observed to augment turbulence for the first three POD modes (larger scales) and attenuate it for the rest of the modes (smaller scales). It has been realized that apart from droplet Stokes number and mass loading, the dynamic range of length scales of the gas flow and the relative turbulent kinetic energy content of the flow structures (POD modes) must be considered in order to conclude if the droplets enhance or reduce the carrier-phase turbulence especially at the lower wavenumbers.