Two-dimensional Particle Tracking Research Articles

Drag reduction utilizing a wall-attached ferrofluid film in turbulent channel flow

This study explores the application of a wall-attached ferrofluid film to decrease skin-friction drag in turbulent channel flow. We conduct experiments using water as a working fluid in a turbulent channel flow set-up, where one wall is coated with a ferrofluid layer held in place by external permanent magnets. Depending on the flow conditions, the interface between the two fluids is observed to form unstable travelling waves. While ferrofluid coating has been previously employed in laminar and moderately turbulent flows (Reynolds number $Re<4000$ ) to reduce drag by creating a slip condition at the fluid interface, its effectiveness in fully developed turbulent conditions, particularly when the interface exhibits instability, remains uncertain. Our primary objective is to assess the effectiveness of ferrofluid coating in reducing turbulent drag with particular focus on scenarios when the ferrofluid layer forms unstable waves. To achieve this, we measure flow velocity using two-dimensional particle tracking velocimetry (2D-PTV), and the interface contour between the fluids is determined using an interface tracking algorithm. Our results reveal the significant potential of ferrofluid coating for drag reduction, even in scenarios where the interface between the surrounding fluid and ferrofluid exhibits instability, with observed drag reduction rates up to 95 %. In particular, waves with an amplitude significantly smaller than a viscous length scale positively contribute to drag reduction, while larger waves are detrimental, because of induced turbulent fluctuations. However, for the latter case, slip outcompetes the extra turbulence so that drag is still reduced.

Mechanics of hypervelocity simultaneously launched distributed particles

Understanding the complex physics of micrometeoroid/orbital debris (MMOD) cluster impacts to spacecraft structures and particulate impacts such as dust, ice, rain, etc. to hypersonic vehicles is essential to ensure safe space travel and hypersonic flight. Hypervelocity impacts resulting from simultaneously launched distributed particles (SLDPs) can be used to realistically replicate such environments. A method to controllably launch and perform in-situ characterization of SLDPs has been developed and implemented. A powder-driven, 12.7 mm smooth-bore two-stage light gas gun was used to launch collections of 0.79 mm and 1.98 mm diameter 2017-T4 aluminum particles to group velocities ranging from 2.4–5.4 km/s. Ultra-high-speed images recorded at 1 MHz were processed using custom-developed two-dimensional digital particle tracking algorithms to characterize the airborne SLDP clusters. In addition, the three-dimensional (3D) features of the SLDP clusters were quantified using impacts to 1.59 mm thick 6061-T6 aluminum plates. Target damage and interactions between adjacent particle impact sites were studied using a combination of numerical simulations and post-impact forensic analyses. Characteristic Elastic Plastic Impact Computation code simulations were used to estimate critical nearest neighbor distances between impact sites below which damage interactions occur and to subsequently probe plastic damage coalescence. For a given particle diameter, the critical nearest neighbor distance was roughly twice the in-plane radial distance from the impact axis to the location of 1% equivalent plastic strain. The plate-thickness-to-particle-diameter ratio was found to affect the plastic strain field more than the crater/perforation size. The predicted crater geometry agreed well with 3D optical profilometry measurements of target impact sites. The approach developed in this work can be readily applied to other particle/target geometries, materials, and impact scenarios, including MMOD cluster impacts and environmental effects on hypersonic vehicles.

Experimental study of velocity and turbulence fields in a square mixer-settler tank: Comparison of shake the box PTV and 2D2C PIV

The characterization of flows in agitated vessels is essential for a good understanding of physical and chemical processes and to validate computational fluid dynamics code. The hydrodynamics of cylindrical vessels have been well studied at various scales, mostly in the turbulent regime, typically using particle image velocimetry (PIV). Studies of square tanks or at moderate Reynolds numbers are much rarer however, and 3D Shake-The-Box (STB) Lagrangian particle tracking has never been used in this context. The objective of the present work was to compare two-dimensional, two-component PIV (2D2C-PIV) and STB particle tracking in characterizing the hydrodynamics of a stirred 20 cm sided (8 L) square tank at Reynolds numbers ranging from 1700 to 4300. The velocity fields, velocity profiles, turbulent kinetic energy fields and turbulent kinetic energy dissipation rates obtained using the two approaches were compared. The velocity fields measured with the two methods were found to be in good agreement, except for a few discrepancies in the velocity profiles of the turbine discharge at 250 rpm. The dimensionless velocity profiles were similar above 100 rpm and indicated that the flow was turbulent at the Reynolds numbers considered. The turbulent kinetic energy fields obtained with the two approaches were globally similar, although with STB, the energy tended to be underestimated, especially in the jet discharge area. The STB method was also found to have lower spatial resolution and precision in measuring turbulent kinetic energy dissipation rates, a consequence of a too large interrogation window. Recommendations are proposed to improve the precision of STB measurements.

Simultaneous measurements of inertial acceleration and flow structures during coarse-grain entrainment under steady currents

An instrumented smart sediment particle (SSP) is used to investigate the entrainment of spherical coarse grains resting on a rough bed comprising semi-spherical elements. The SSP contains electronic sensors facilitating the direct measurement of tri-axial accelerations. The instantaneous flow field is measured simultaneously using two-dimensional particle tracking velocimetry (PTV), enabling joint analysis of accelerations and flow structures during entrainment. Two different initial placement positions of the SSP are tested. Both particle vibration and complete entrainment events are observed in experiments. Results show larger maximum acceleration and longer duration of motion with more significant obstruction by surrounding bed roughness, which entails greater momentum transfer at the initiation of entrainment. During vibration events, acceleration can occasionally reach the threshold value but with no entrainment because of the low persistence of relevant flow features inducing energy transfer. Further analysis on instantaneous flow structures indicates that, at the threshold flow condition, the occurrence and persistence of the “sweep-outward interaction” quadrant motion sequence (i.e. Q4-Q1 sequence) in front the particle, combined with the transient increase of vorticity due to the passage of clockwise vortices above the particle, is the key to energy transfer and complete entrainment. The Q4-Q1 sequence may either be caused by the lower half of anti-clockwise vortices or occur independently. Generally, the joint analysis of acceleration and flow structures provides a new perspective to studying coarse grain entrainment, especially a potential to relate near-bed flow structures to the status of particle motion in the future.

The effect of natural and engineered hydraulic conditions on river-floodplain connectivity using hydrodynamic modeling and particle tracking analysis

Hydraulic conditions and water resources management projects can significantly alter river-floodplain connectivity, which in turn can alter hydrologic and biogeochemical processes in river corridors. In this study, the hydrodynamics of river-floodplain connectivity under different flood conditions and the effect of the Nanchang Water Resources Project Group (NWRPG) in the middle branch of the Ganjang River were investigated using a combination of two-dimensional hydrodynamic simulations and particle tracking. The hydrodynamic model was calibrated and validated using data from several gauging stations and field measurements. Floods in the Ganjang River can be limited to the river itself (“River Flood”, flood with normal lake level) or further extended to the Poyang Lake (“Lake Flood”, flood with high lake level). The results show that compared with “River Flood” scenarios, “Lake Flood” scenarios increased the water level flooding a larger area. The flow velocity decreased and the residence time (RT) of particles increased. The particle travel distance (PTD) of “River Flood” was larger than that of “Lake Flood”. The larger the flood, the greater the transboundary flux between the river and the floodplain, and the shorter the RT and PTD. The effect of NWRPG was the permanent flooding of part of the river floodplain, causing some habitat loss. Due to the increase in discharge, the implementation of the NWRPG results in a shorter RT with a smaller standard deviation, which has little effect on the PTD distribution. These findings can facilitate river connectivity restoration efforts in the Ganjang River and also provide a reference for assessing the impact of barrage projects.

Suppression Of Wall Reflection In PIV Images Over Rough Walls

Wall reflections in particle image velocimetry (PIV) measurements is one of the limiting factors in obtaining velocity information in the vicinity of rough walls. This work tests two different techniques to suppress wall reflections for time-resolved, two-dimensional particle tracking velocimetry (2D-PTV) and stereoscopic-PIV (stereo-PIV). The 2D-PTV was performed for a turbulent boundary layer developed on a rough wall in a water towing tank facility. To measure velocity in the vicinity of the rough wall, the near-wall region on the viewing window was covered with tinted papers to mitigate the wall reflection and maintain a visible reference plane of the towed wall. Using this technique allowed detection of particles at a wall-normal distance of approximately 300 μm (10 wall units) from the roughness valleys for an imaging system with 1 m standoff distance. To suppress the wall refection for the stereo-PIV over a rough wall performed in a wind tunnel, the roughness is coated with fluorescent paint to shift the reflected light to a higher wavelength. A narrow band-pass filter was then used on the imaging lenses, allowing only particle images to be recorded while extinguishing the majority of the reflected laser light from the surface. Reliable velocity measurements with this technique were obtained up to 500 μm (5.0 wall units) from the roughness valleys.

Pulsating Film-Cooling Flow Over Smooth Cutback Surface at Airfoil Trailing Edge Measured by 2D3C-PTV

Abstract The objective of this study is to clarify the effects of the film-cooling flow pulsation and the differences between the Strouhal number ratios of 1.0 and √2. The surface-averaged film cooling effectiveness for the Strouhal number ratios of 1.0 and √2 had decreased and increased, respectively, in comparison with the steady cooling flow in the authors' previous large eddy simulations. Subsequently, clarification on the possible reasons for these changes was sought. Measurements of the instantaneous velocity fields over the smooth cutback surface at two different pulsation frequencies were performed using two-dimensional three-component particle tracking velocimetry (2D3C-PTV). Notably, the power spectrum density of the wall-normal velocity fluctuations showed that the strongest peaks appeared at the pulsation frequencies, and the peak value for the Strouhal number ratio of 1.0 was much higher than those for the steady cooling flow and the Strouhal number ratio of √2. When the absolute Reynolds shear stresses integrated for the mixing-layer region were compared, those for the Strouhal number ratios of 1.0 and √2 were found to be higher and lower, respectively, than those for the steady cooling flow. Remarkably, the suppression of the turbulent mixing for the Strouhal number ratio √2 was caused by the suppressed development of the large-scale alternating vortices shed from the lip edge by imposing the cooling-flow pulsation at the frequency nonresonant with the vortex shedding frequency of the steady cooling flow.

Particle tracking and beam optics analysis on a toroidal gantry for proton therapy

GaToroid is a concept of toroidal gantry for hadron therapy under investigation at CERN It makes use of the toroidal magnetic field between each pair of coils to steer and focus the particle beams down to the patient. This peculiar concept requires detailed studies on particle tracking and beam optics to optimise the winding geometry and explore the properties of the system. The work presented in this manuscript is focused on the features of a GaToroid system for protons, specifically designed to minimise the footprint and weight of the gantry. Firstly, a two-dimensional single particle tracking was developed to optimise the coil geometry and the toroidal magnetic field, aiming to the maximisation of the energy acceptance of the magnet. Particles over the whole spectrum of treatment energy are directed at isocenter within 1 mm of precision. This procedure, restricted to the symmetry plane between each pair of coils, defines different beam orbits, function of the beam energy. Subsequently, a three-dimensional particle tracking was implemented to evaluate the interaction of a beam of finite dimensions with the complete magnetic field map in vacuum. The parameters of the simulated beam at the isocenter are coherent with the clinical requirements. The results of the three-dimensional tracking were then used to calculate the linear transfer matrix associated to each beam orbit. Finally, the option of performing the beam spot scanning at the isocenter by acting on the upstream steering magnet has been investigated, highlighting the potential of the concept, as well as the limitations related to the scanning field dimension and source-to-axis distance. In conclusion, the results described in this paper represent a crucial step toward the understanding of the beam optics properties of a GaToroid gantry.

Direct Lagrangian method to characterize entrainment dynamics using particle residence time: a case study on a laminar separation bubble

This study demonstrates the feasibility of characterizing small-scale flow dynamics using path-specific Particle Residence Time (PRT). PRT, defined as the time a parcel of fluid spends in a region of interest, is a clear indicator of stagnation and recirculation. To test the concept, two-dimensional particle tracking velocimetry is used to measure a laminar separation bubble (LSB) on the suction side of an SD7003 airfoil at $$Re=60,000$$ . Extended pathlines are calculated from the Lagrangian data such that fluid parcels entering the region of interest are tracked continuously. PRT is then directly calculated and used to quantify the entrainment process across the LSB. Using convential Eulerian quantities, the flow field around the LSB is divided into outer (free stream) and inner layers—separated by the local minimum velocity magnitude line. Entrainment, initiated by the formation of vortices near the reattachment point, is quantified according to the trajectories of fluid parcels crossing into the inner flow layer. The PRT values of entrained fluid parcels are significantly increased compared to the bulk flow. Variations of the mean PRT of entrained fluid reveal unsteady features, including the transition and reattachment of the LSB. The path-dependent indicators of separation, transition and reattachment are shown to closely agree with Eulerian-based benchmark measurements.

Effect of Reynolds number on turbulent channel flow over a superhydrophobic surface

The slip boundary and the near-wall statistics of a fully developed turbulent channel flow over a superhydrophobic surface (SHS) was investigated in a low Reynolds number (Re) range. The Re was varied from 6200 to 9400, based on the bulk velocity and the full-channel height. The root-mean-square of the surface roughness, normalized by the inner flow scaling, varied from 0.26 to 0.35 with increasing Re. Time-resolved, two-dimensional particle tracking velocimetry (PTV) was used to obtain the mean velocity profile in the linear viscous sublayer. Furthermore, time-resolved three-dimensional PTV was applied to obtain the Reynolds stresses. The estimated wall shear stress showed that the drag reduction of the SHS increased slightly from 37% to 42% when Re increased. With increasing Re, the slip velocity increased linearly from 0.25 m/s to 0.34 m/s, and the slip length reduced from 97.5 μm to 69.6 µm. When normalized using inner scaling, slip velocity and length remained constant with increasing Re. The mean velocity of the SHS demonstrated a log-law with the universal von Kármán constant but shifted upward by an amount equal to the normalized slip velocity. The SHS increased the dimensional Reynolds stresses in the near-wall region and attenuated them farther away from the wall. With increasing Re, the differences between the dimensional Reynolds stresses of the smooth surface and the SHS increased. However, when Reynolds stresses were normalized using friction velocity, the Reynolds stresses of the SHS overlapped for all the investigated Re and were larger than the normalized Reynolds stresses of the smooth surface.

Particle Residence Time in pulsatile post-stenotic flow

Particle Residence Time (PRT), a measure of a fluid element’s transit time through a region of interest, is a clear indicator of recirculation. The PRT of fluid recirculating downstream of an idealized stenosis geometry is found to vary dramatically under pulsatile flow conditions. Two-dimensional particle tracking velocimetry is used to track particles directly as they exit the stenosis geometry and are entrained into the region of recirculation immediately downstream. A Lagrangian approach permits long pathlines to be drawn, describing each particle’s motion from the instant they enter the domain. PRT along each pathline is compared here for three mean Reynolds numbers; specifically, Rem = 4800, 9600, and 14 400. The pulsatile waveforms are characterized by Strouhal numbers of 0.04, 0.08, and 0.15 and amplitude ratios of 0.50 and 0.95. As the mean Reynolds number is increased, higher fluid velocities are shown to lower PRT. However, the strength of PRT is truly revealed when highlighting the influence pulsatility has on the degree of mixing beyond the stenosis throat. Higher Strouhal numbers correlate with roll-up across the shear layer and increased PRT distribution at all Reynolds numbers in consideration. Similarly, strong temporal velocity gradients generated by a high amplitude ratio carry large volumes of fluid from the jet deep into the recirculation region, contributing to greater PRT.

Arrangement Effects of 30 deg Inclined Teardrop-Shaped Dimples on Film Cooling Flow Over Dimpled Cutback Surface at Airfoil Trailing Edge Investigated by 2D3C-PTV

Abstract The objective of this study is to determine the differences in flow fields between the 30 deg in-line and staggered arrangements of teardrop-shaped dimples, and to explain why the surface-averaged Nusselt number with the 30 deg in-line arrangement was 28.7% higher than that with the 30 deg staggered arrangement in our previous study. Measurements of the instantaneous velocity fields over the dimpled cutback surfaces in the two arrangements were performed at five spanwise cross sections using two-dimensional three-component particle tracking velocimetry (2D3C-PTV). Recirculation flows were observed only inside the dimples in the in-line arrangement, and the region above the recirculation flows exhibited a higher Reynolds shear stress. In this region, turbulent mixing between the high-speed cooling-flow and the low-speed recirculation-flow can be promoted. Streamlines of the time-averaged velocities showed that approximately half the fluid flowing out of a teardrop-shaped dimple in the in-line arrangement hardly flowed into the ones downstream. The remainder of the fluid mostly flowed into the dimple immediately downstream, and the inflow of the fluid into further downstream dimples decreased gradually. From the PTV results, we can deduce that the fluid motions in the in-line arrangement leads to a larger temperature-difference between the dimple wall and the fluid because the inflow of fluid heated inside upstream dimples into the downstream ones is less than in the staggered arrangement. Consequently, the Nusselt number in the in-line arrangement was higher.

Near-wall motion of inertial particles in a drag-reduced non-Newtonian turbulent flow

The kinematics of inertial particles suspended in the near-wall region of Newtonian and non-Newtonian turbulent channel flows was experimentally investigated. The non-Newtonian fluid was a homogeneous solution of 90 part per million of a polyacrylamide polymer in water with 66% drag reduction. All the experiments were performed at the same volumetric flow rate with Reynolds number of 34,300 based on bulk velocity, channel height, and the kinematic viscosity of water. The inertial particles were 250-μm glass beads with St of 35 (in water) at a volumetric concentration of 0.05%. A time-resolved two-dimensional particle tracking velocimetry was used to record particle images at acquisition frequency of 17.6 kHz and detect trajectory of flow tracers and the glass beads. The recorded data were processed using a two-dimensional particle tracking algorithm to obtain the Lagrangian kinematics of the beads. The comparison between laden flows of water and polymer solution showed reduction of number density of the beads and their momentum in the vicinity of the wall in the polymeric flow. The polymer solution remarkably reduced the wall-normal and shear Reynolds stresses of the beads, but had a negligible effect on their streamwise Reynolds stress. The wall-normal fluctuation of the beads reduced in the polymeric flow and their trajectories became parallel with the channel wall. Results also showed that the ejection and sweep motions were not the major mechanism for wall-normal distribution of the beads in the polymeric flow. Outcomes suggest that drag-reducing polymer solutions have the potential of reducing erosive wear in particle-laden pipelines.

Two-Dimensional and Three-Dimensional Single Particle Tracking of Upconverting Nanoparticles in Living Cells.

Lanthanide-doped upconversion nanoparticles (UCNPs) are inorganic nanomaterials in which the lanthanide cations embedded in the host matrix can convert incident near-infrared light to visible or ultraviolet light. These particles are often used for long-term and real-time imaging because they are extremely stable even when subjected to continuous irradiation for a long time. It is now possible to image their movement at the single particle level with a scale of a few nanometers and track their trajectories as a function of time with a scale of a few microseconds. Such UCNP-based single-particle tracking (SPT) technology provides information about the intracellular structures and dynamics in living cells. Thus far, most imaging techniques have been built on fluorescence microscopic techniques (epifluorescence, total internal reflection, etc.). However, two-dimensional (2D) images obtained using these techniques are limited in only being able to visualize those on the focal planes of the objective lens. On the contrary, if three-dimensional (3D) structures and dynamics are known, deeper insights into the biology of the thick cells and tissues can be obtained. In this review, we introduce the status of the fluorescence imaging techniques, discuss the mathematical description of SPT, and outline the past few studies using UCNPs as imaging probes or biologically functionalized carriers.

Modelling transport in fractured media using the fracture continuum approach

Two of the commonly used approaches in modelling flow and transport in fractured geological media are the discrete fracture network (DFN) approach and the stochastic continuum (SC) approach. The former approach is computationally demanding and requires large input parameters, whereas the latter approach is computationally efficient and requires fewer parameters but at the expense of not preserving fracture network details and properties. The fracture continuum (FC) approach combines the advantages of both the DFN and SC approaches. The main objective of this research is to develop a mapping technique utilizing the FC approach to preserve transport characteristics between DFN and the mapped continuum. A two-dimensional particle tracking model is developed to simulate conservative contaminant transport through DFN. The fracture network is randomly generated in a stochastic Monte Carlo framework, and the flow problem is solved using mass conservation at the fracture junctions. The transport problem is then solved via the developed particle tracking model on the DFN. The obtained transport solution is used as a reference solution. The fracture network is mapped onto a finite difference grid at four different grid cell sizes: 1 m × 1 m, 2 m × 2 m, 5 m × 5 m, and 10 m × 10 m, and the flow problem is solved via MODFLOW. The transport problem is solved on the grid using a particle tracking method. Comparisons between the transport characteristics for both approaches (DFN and FC) are performed, and the percentage error in each case is quantified. It is found that a new correction factor is needed to preserve conservative transport characteristics on the grid. The developed correction factor improves the ability of the FC technique to preserve transport on the grid for the case of conservative contaminant transport.

Surface flow profiles for dry and wet granular materials by Particle Tracking Velocimetry; the effect of wall roughness

.Two-dimensional Particle Tracking Velocimetry (PTV) is a promising technique to study the behaviour of granular flows. The aim is to experimentally determine the free surface width and position of the shear band from the velocity profile to validate simulations in a split-bottom shear cell geometry. The position and velocities of scattered tracer particles are tracked as they move with the bulk flow by analyzing images. We then use a new technique to extract the continuum velocity field, applying coarse-graining with the postprocessing toolbox MercuryCG on the discrete experimental PTV data. For intermediate filling heights, the dependence of the shear (or angular) velocity on the radial coordinate at the free surface is well fitted by an error function. From the error function, we get the width and the centre position of the shear band. We investigate the dependence of these shear band properties on filling height and rotation frequencies of the shear cell for dry glass beads for rough and smooth wall surfaces. For rough surfaces, the data agrees with the existing experimental results and theoretical scaling predictions. For smooth surfaces, particle-wall slippage is significant and the data deviates from the predictions. We further study the effect of cohesion on the shear band properties by using small amount of silicon oil and glycerol as interstitial liquids with the glass beads. While silicon oil does not lead to big changes, glycerol changes the shear band properties considerably. The shear band gets wider and is situated further inward with increasing liquid saturation, due to the correspondingly increasing trend of particles to stick together.Graphical abstract

Deterministic particle tracking simulation of pollutant discharges in rivers and estuaries

A two-dimensional deterministic particle tracking model, in which the anisotropic-dispersive process is described by a particle strength exchange scheme, is established for the simulation of pollutant transport in vertically well-mixed rivers and estuaries. By simulating two benchmark problems with analytic solutions, the PSE scheme is shown to be accurate even if the anisotropic ratio of dispersion coefficients is very high. Further simulations of two specific problems concerning the optimal effluent discharge location and procedure are presented. The major conclusion is that in a tidal estuary with a relatively large fresh-water flow, setting the discharge position at the mixing center and making the discharge rate proportional to flow speed may minimize the peaks of concentration.

Simultaneous measurement of size and density of spherical particles using two-dimensional particle tracking analysis method

The motion of spherical particles in liquid phases was investigated using a two-dimensional particle tracking analysis (2D-PTA) method. The analysis of four different materials with this method revealed two different types of particle movements occurring simultaneously: Brownian diffusion and sedimentation. When these two different displacements occurred simultaneously, in the case of particles with a larger size or higher density, the sedimentation displacements were significantly faster than those associated with Brownian diffusion. The occurrence of a faster sedimentation compared to the Brownian diffusion severely affects the accuracy of size determination using PTA methods, as these approaches are based on the Stokes-Einstein relationship, in which the size of a particle is inversely proportional to its Brownian diffusion coefficient. In addition, we successfully demonstrated, for the first time, the potential of the novel PTA method for simultaneously determining the particle size and identifying the constituent material of a limited range of silica and gold particle sizes. The present method may play an important role in the development of new industrial and biological applications of materials.

Concurrent particle diffusion and sedimentation measurements using two-dimensional tracking in a vertical sample arrangement

This letter reports a method for simultaneous tracking of Brownian motion and superimposed sedimentation movement of multiple micro- and nanoparticles in liquid. Simple two-dimensional particle tracking can be employed because the thin liquid sample film is arranged vertically and viewed from the side with a dark field video microscopy setup. Therefore, both diffusion and sedimentation can be used for particle size calculation, allowing analyses over a wide range of sizes and mass densities. To validate the method, size distributions of reference particles with known density and diameters ranging from 100 nm to 6 μm were determined. Brownian motion for size calculation is useful for sufficiently small particles, whereas sedimentation can only be applied if there is significant settling motion superimposed on Brownian motion (which requires large diameters and/or densities). Within a certain range, both principles are suitable for size measurements. As a consequence, this method can be used to determine the size and density of unknown particles in a single measurement step, provided that they exhibit both sedimentation and diffusive motion.

Characterizing Coherent Wind Structures using Large-Scale Particle Tracking Velocimetry: A Proof-of-Principle Study

The following study proposes a two-dimensional large-scale particle tracking velocimetry (LS-PTV) system to characterize coherent wind structures. Seven minutes of LS-PTV data is collected via an apparatus that seeds fog-filled soap bubbles into the wind at a height of 6m from the ground. The LS-PTV data is compared to 20 minutes of data collected concurrently from a wind mast at the same site. The LS-PTV system recorded a mean streamwise velocity of 1.35m/s with a standard deviation of 0.23m/s at a mean height of 2.50m with a standard deviation of 0.7m, which agrees well with the velocity profile measured by the wind mast. Furthermore, the Reynolds stresses measured by the LS-PTV system are found to compare to those measured by the wind mast and by Klebanoff [1] for a canonical turbulent boundary layer. The current study assumes that the centre-of-curvature trajectories of the particle pathlines are representative of the trajectories followed by the spanwise vortices. As a proof-of-principle study, this work has been successful in accurately describing the vortex distribution very near to the ground. However, the trajectories followed by the centres-of- curvat.ure belonging to pathlines concurrently passing through the field-of-view were sporadic and uncorrelated.