Combined Particle Image Velocimetry Research Articles

Time-resolved microrheology of actively remodeling actomyosin networks

Living cells constitute an extraordinary state of matter since they are inherently out of thermal equilibrium due to internal metabolic processes. Indeed, measurements of particle motion in the cytoplasm of animal cells have revealed clear signatures of nonthermal fluctuations superposed on passive thermal motion. However, it has been difficult to pinpoint the exact molecular origin of this activity. Here, we employ time-resolved microrheology based on particle tracking to measure nonequilibrium fluctuations produced by myosin motor proteins in a minimal model system composed of purified actin filaments and myosin motors. We show that the motors generate spatially heterogeneous contractile fluctuations, which become less frequent with time as a consequence of motor-driven network remodeling. We analyze the particle tracking data on different length scales, combining particle image velocimetry, an ensemble analysis of the particle trajectories, and finally a kymograph analysis of individual particle trajectories to quantify the length and time scales associated with active particle displacements. All analyses show clear signatures of nonequilibrium activity: the particles exhibit random motion with an enhanced amplitude compared to passive samples, and they exhibit sporadic contractile fluctuations with ballistic motion over large (up to 30 μm) distances. This nonequilibrium activity diminishes with sample age, even though the adenosine triphosphate level is held constant. We propose that network coarsening concentrates motors in large clusters and depletes them from the network, thus reducing the occurrence of contractile fluctuations. Our data provide valuable insight into the physical processes underlying stress generation within motor-driven actin networks and the analysis framework may prove useful for future microrheology studies in cells and model organisms.

Improved magnetic particle tracking technique in dense gas fluidized beds

Noninvasive monitoring of multiphase flow is rapidly gaining increased interest. More specifically noninvasive particle tracking techniques have received a lot of attention in recent years to study dense granular flow. However, these techniques are usually quite expensive and require strict safety measures. An improved magnetic particle tracking (MPT) technique for dense granular flow will be presented in this article. The improvements of the analysis technique for MPT will be demonstrated and rigorously tested with a three‐dimensional system and two‐dimensional sensor system. The strengths and limitations of the MPT technique will also be reported. Finally, the results of the MPT are compared with data obtained from a combined particle image velocimetry and digital image analysis technique. © 2014 American Institute of Chemical Engineers AIChE J, 60: 3133–3142, 2014

Simultaneous measurement of temperature and velocity fields in convective air flows

Thermal convective air flows are of great relevance in fundamental studies and technical applications such as heat exchangers or indoor ventilation. Since these kinds of flow are driven by temperature gradients, simultaneous measurements of instantaneous velocity and temperature fields are highly desirable. A possible solution is the combination of particle image velocimetry (PIV) and particle image thermography (PIT) using thermochromic liquid crystals (TLCs) as tracer particles. While combined PIV and PIT is already state of the art for measurements in liquids, this is not yet the case for gas flows. In this study we address the adaptation of the measuring technique to gaseous fluids with respect to the generation of the tracer particles, the particle illumination and the image filtering process. Results of the simultaneous PIV/PIT stemming from application to a fluid system with continuous air exchange are presented. The measurements were conducted in a cuboidal convection sample with air in- and outlet at a Rayleigh number Ra ≈ 9.0 × 107. They prove the feasibility of the method by providing absolute and relative temperature accuracies of σT = 0.19 K and σΔT = 0.06 K, respectively. Further open issues that have to be addressed in order to mature the technique are identified.

Combined Stereo-PIV and PLIF Measurements of Transverse Injection in Mach 2 Supersonic Flow

A combined stereoscopic particle image velocimetry (PIV) and acetone planar laser-induced fluorescence (PLIF) measurement system was developed to investigate turbulent mixing of a jet into a supersonic flow. The measurement was performed using a supersonic wind tunnel. Air was injected perpendicular into a Mach 2 freestream with a jet-to-freestream momentum ratio of 2.2. The system successfully measured the molar concentration of the jet using PLIF and the velocity in the flow-field using PIV simultaneously. These data yielded turbulent eddy diffusive flux that is a product of fluctuating velocity (v') and concentration (c'). The eddy flux of the streamwise component (c'u') was negative, and that of the heightwise component (c'v') was positive in the outer jet boundary region. Large-scale structures were periodically generated in the outer jet boundary and convected downstream. This large-scale motion induced negative c'u' and positive c'v' in the outer jet boundary. In the inner region of the jet, the air entrained by the counter-rotating vortices induced a positive v' and negative c' region, resulting in negative c'v'.

Impact Failure of Planetary Materials:

The dynamic fragmentation of a fine grained granitoid material has been examined. Target thicknesses ranged from 7 to 40 mm and impact energies from 12 to 2,500 J. Combined particle image velocimetry and image enhancement techniques are introduced and have been used to measure the size and velocity of material ejected laterally from the rear of the target during impact testing. Non-dimensional groups were formed and fitted with coefficients to predict median values of the distribution of mass and kinetic energy among radial distance, R, from the impact centre, ejecta velocities, v, and ejecta lengths, L. The statistics are well correlated with increasing non-dimensional impact energy (positive correlation for radial distance and velocity, and negative correlation for ejecta length). Median values were used to collapse cumulative distributions and non-centred Gaussian fits were used to describe these curves. Approximately 85 % of the total mass and kinetic energy is captured between R/R50 % = 0.3 to 2, v/v50 % = 0.3 to 2, and L/L50 % = 0.2 and 3. This data facilitates a better comparison among a wide range of test conditions, especially when attempting to extrapolate principal features of impacts into brittle materials at higher velocities. The ejecta tracking techniques and methodologies can be used to improve current impact testing experiments and computer modelling validation.

Laboratory Investigation of the Impact of Lateral Spreading on Buoyancy Flux in a River Plume

AbstractThe relationship between lateral spreading and mixing in stratified gravity currents is investigated by comparing laterally confined and unconfined currents in a series of laboratory experiments. The vertical turbulent buoyancy flux is determined using a control volume approach with velocity and density fields derived from combined particle image velocimetry (PIV) and planar-laser-induced fluorescence (PLIF). Lateral spreading is determined in the unconfined experiments based on plan-view imaging using the optical thickness method (OTM). The authors find that lateral spreading dramatically modifies the plume structure; the spreading plume layer consists of approximately linear density and velocity profiles that extend to the surface, whereas the channelized plume profiles are uniform near the surface. Lateral spreading decreases the average plume density relative to laterally confined currents with similar inflow conditions. However, the local turbulent buoyancy flux in the spreading experiments is approximately equal to that in the confined experiments. This apparent paradox is resolved when the plume areas are taken into account. The total mixing integrated over the horizontal plume area is significantly higher in the spreading experiments. Thus, the experiments suggest that spreading does not appreciably alter the turbulent mixing processes at the base of the plume. However, it significantly increases the area over which this mixing occurs and, through this mechanism, increases the net dilution of river water at a fixed distance from the river mouth. Finally, the authors hypothesize that the spreading does not significantly increase the local turbulent buoyancy flux because spreading occurs preferentially near the surface, whereas buoyancy flux is greatest in the core of the current.

Evaluating Large-Eddy Simulation (LES) and High-Speed Particle Image Velocimetry (PIV) with Phase-Invariant Proper Orthogonal Decomposition (POD)

This study is part of a program to understand the stochastic variations in IC engine flows; in particular, it is a comparison of measured (PIV) and computed (LES) velocity from multiple cycles of the same motored engine. Comparison procedures included traditional RANS (Reynolds Averaged Navier-Stokes) decomposition (ensemble-averaged and RMS (Root Mean Square) velocity), phase-dependent, and phase- invariant POD. Phase-dependent POD was performed on the PIV and LES samples separately and on the combined samples, thus creating separate or a single POD mode sets, respectively. The phase- invariant POD was performed both on normalized snapshots and on snapshots where the original energy was conserved. Initial comparisons of the mass-specific kinetic energies of the ensemble average and RMS velocities revealed that the PIV and LES data sets differed significantly during most of the intake stroke. This discrepancy was quantifiedfirst by comparing the relevance indices calculated between ensemble average velocity fields and, second, using phase-dependent POD, which quantified cycle-to-cycle flow variations of the RANS average and turbulence. Phase-dependent POD was applied separately to the PIV and LES data sets during the intake stroke (76° ATDCE, After Top Dead Center Exhaust), where the intake-valve jet is strong and the PIV and LES data were earlier found to be significantly different. The cyclic variability of the LES ensemble average was estimated to be significantly higher than that of the PIV data set. POD was also applied to the combined sample of LES and PIV snapshots for quantitative comparison creating a single set of modes, so that comparison could be made with POD coefficients alone. Example comparisons were made at again for data at 76° and also 330° ATDCE, which is a viable spark timing in a fired engine. The results at 76° ATDCE were similar to those obtained with the POD analysis of the separate samples. At 330° ATDCE, the PIV ensemble average and RMS velocities showed somewhat more cyclic variability. Phase-invariant POD was applied to the combined PIV and LES velocity data from all crank angles to study the flow evolution over the crank angle range. Conclusions derived from the two different energy transformations are contrasted. When the energy of each velocity field is normalized, the phase-invariant POD results focus on differences in flow structures and their evolution. On the other hand, when the energy of each velocity field is conserved, the phase-invariant POD results also take into account differences in energy between the PIV and LES data.

Experimental investigations of a pseudo-2D spout fluidized bed with draft plates

Spout fluidized beds often prove promising for gas–solid contacting operations involving chemical reactions with changing particle size. However, difficulties may arise during operation, such as unstable spouting, dead zones formation and limited flexibility in adjustment of maximum spoutable height and residence time. This can be improved by inserting draft tubes or plates. In this work, the hydrodynamics of a pseudo-2D spout fluidized bed with draft plates was studied by changing the background velocity from 0 to 1.75m/s with increments of 0.25m/s and the spout velocity from 10 to 50m/s with increments of 5m/s. The distinct flow regimes were identified by capturing high-speed images and Fast Fourier transformation (FFT) of measured pressure signals. In addition, the effects of the entrainment height (in the range of 0.03–0.05m), static bed aspect ratio (in the range of 1–1.5) and particle properties (for glass and γ-aluminum oxide particles) on the hydrodynamics were investigated by determining the time-averaged particle volumetric flux by combined particle image velocimetry (PIV) and digital image analysis (DIA) for two regimes namely, spouting-with-aeration (IV) and fluidized bed-spouting-with-aeration (dispersed spout) (VIII).

2-D Flow and Temperature Measurements in a Multiphase Airlift Crystallizer

A combined particle image velocimetry (PIV) and particle image thermometry (PIT) method was applied to visualize and measure simultaneously the 2-D velocity and temperature fields in a crystallizer. The 2-D supersaturation field can be also determined before nucleation or seeding takes place. The hydrodynamic behavior in an internal loop airlift crystallizer was studied for different air flow rates, sparger types, and crystal holdups to get insight in the optimal process conditions for this crystallizer, i.e. uniform temperatures and particle concentration. The 2-D velocity and temperature gradients showed that in a 2 L crystallizer the mixing is sufficient to ensure uniform supersaturation profiles at an overall superficial air velocity higher than 2.3 mm/s. Suspension tests showed that a superficial air velocity of 7 mm/s was sufficient to avoid settling of the crystals. This air flow rate assured the lifting of the solids from the bottom of the crystallizer for crystals with a mean size of 1.5 mm and h...

The use of high-speed PIV and holographic cinematography in the study of fiber suspension flows

High-speed particle image velocimetry (PIV) and digital holography were applied to dilute, rigid fiber suspensions without floc formation. PIV is suited to study the instantaneous interaction between the flow and the fiber motion down to the scale of the fiber. Based on the size difference between fibers and tracers, fibers were masked in the PIV images. However, this method is not always sufficient, and we propose a more elaborate method based on local cross-correlation between successive cropped images to track and remove fibers from the PIV images. Digital, inline holographic cinematography has the advantage of a relatively easy optical setup. Exploratory experiments were performed using a single-view, inline setup tracking fibers in 3D. Fiber holographic reconstructions were clear and enable accurate determination of fiber length and orientation. This is in contrast to the PIV images, where the fiber optical signature suffers from intensity saturation and reflections. However, to obtain the accurate 3D fiber orientations, a two-orthogonal view holography setup is needed. We propose combined PIV and holography to study the basic fiber-flow interaction mechanisms in turbulent flows.

Measurement of sound source distribution around a circular cylinder in a uniform flow by combined particle image velocimetry and microphone technique

This study examined an experimental method for locating aerodynamic sound sources from a bluff body in a stream. The sound sources are evaluated from cross-correlation analysis of the pressure fluctuations on and around the flow field and the sound pressure fluctuations. Measurements are carried out in the sound field emitted from the circular cylinder at Reynolds number Re=4×104. The pressure fluctuations around the circular cylinder are evaluated by measuring the instantaneous velocity field using particle image velocimetry combined with the pressure Poisson equation, while the sound pressure fluctuations are measured simultaneously by a microphone in the far field. The experimental results showed that sound sources were found near the flow separation on both sides of the cylinder and in the near wake. The experimental results were validated by direct measurement of the pressure fluctuations on the cylinder surface using a pressure transducer. The sound sources obtained from the cross-correlation analysis agreed well with those of the direct measurement of pressure fluctuations on the cylinder surface and those around the cylinder in previous studies.

Flow and Heat Transfer Characteristics of Single Jet Impinging on Dimpled Surface

Based on combined particle image velocimetry (PIV) and numerical simulation, the flow and heat transfer characteristics of a single jet impinging on a dimpled surface for Dj/D = 0.318, 0.5, 1.045; δ/D = 0.1, 0.2, 0.3; Rej = 5000, 10,000, 23,000, were investigated for the first time. The distance between jet nozzle and plate was fixed and equal to H/D = 2. The results show that the flow structures of the single jet impingement with dimpled target surface can be summarized into three typical conceptual flow structures. Particularly, the third flow structure in the form of a large toroidal vortex bound up with the dimple is the result of the centrifugal force of the flow deflection at the stagnation region and spherical centrifugal force of the deep dimple surface. The heat transfer area increases when the dimple relative depth increases. For the cases of Dj/D = 0.318 and 0.5, the area increasing dominate the heat transfer process, and the average Nusselt number increases with the increasing of dimple relative depth. For the cases with Dj/D = 1.045, the local Nusselt number reduction dominate the heat transfer process, the average Nusselt number decreases with the increasing of dimple relative depth. The average Nusselt number of the Dj/D = 0.318 and 0.5 cases is larger than the baseline case, while those of the Dj/D = 1.045 cases are smaller than the baseline case. Furthermore, the correlative expressions of the local Nusselt number, stagnation points Nusselt number and average Nusselt number are obtained.

Experimental test of revised similarity hypotheses without Taylor's hypothesis

The refined similarity hypotheses (RSH) and their extension to passive scalar (RSH-P) are tested experimentally using simultaneous velocity and scalar data in a turbulent jet. Different from the single-point hot-wire probe in a previous experimental test, a combined particle image velocimetry and planar laser induced fluorescence technique enable us to obtain the test data without applying Taylor's hypothesis. RSH is successfully validated with direct examinations of its three hypotheses. RSH-P is partially supported, where the hypothesis of independent behavior of the stochastic variable υ(θ) is not supported. The conditional probability density functions (PDFs) of υ and υ(θ) cannot be described by Gaussian distributions when Re(r)<5 for υ and Re(r)<300 for υ(θ), and their Gaussian peaks converge when Re(r)>100 for υ and Re(r)>1000 for υ(θ). In addition, the bimodal behavior of the PDFs and oscillations at small r in previous experimental tests are not observed in this study.

Experimental study of stratified jet by simultaneous measurements of velocity and density fields

Stratified flows with small density difference commonly exist in geophysical and engineering applications, which often involve interaction of turbulence and buoyancy effect. A combined particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) system is developed to measure the velocity and density fields in a dense jet discharged horizontally into a tank filled with light fluid. The illumination of PIV particles and excitation of PLIF dye are achieved by a dual-head pulsed Nd:YAG laser and two CCD cameras with a set of optical filters. The procedure for matching refractive indexes of two fluids and calibration of the combined system are presented, as well as a quantitative analysis of the measurement uncertainties. The flow structures and mixing dynamics within the central vertical plane are studied by examining the averaged parameters, turbulent kinetic energy budget, and modeling of momentum flux and buoyancy flux. At downstream, profiles of velocity and density display strong asymmetry with respect to its center. This is attributed to the fact that stable stratification reduces mixing and unstable stratification enhances mixing. In stable stratification region, most of turbulence production is consumed by mean-flow convection, whereas in unstable stratification region, turbulence production is nearly balanced by viscous dissipation. Experimental data also indicate that at downstream locations, mixing length model performs better in mixing zone of stable stratification regions, whereas in other regions, eddy viscosity/diffusivity models with static model coefficients represent effectively momentum and buoyancy flux terms. The measured turbulent Prandtl number displays strong spatial variation in the stratified jet.

Combined measurement of concentration distribution and velocity field of two components in a micromixing process

A lot of production processes involve mixing steps. The understanding of fluid flows in mixing processes of liquid components is needed in order to develop appropriate mixers for the chemical and pharmaceutical industry. Especially mixing in microfluidic systems is a challenge due to the diffusion-based processes. A multi-lamination micromixer with chessboard outlet geometry is used to induce the mixing process. To get comprehensive information about the mixing process, the velocity profile of the fluid flow and the species concentration distribution during the mixing process should be measured. Thus, we have combined particle image velocimetry (PIV) and Raman scattering. To enable rapid detection, the Raman imaging mode is used to visualise the concentration distribution. By this setup light sheets along and orthogonal to the outlet of the micromixer are recorded and synchronized with PIV measurement. As a model system we have used water and ethanol/methanol, enabling a selective monitoring of the substances by choosing appropriate spectral areas. The PIV is recorded based on Mie scattering and fluorescence using microsphere tracers. In this study, we present a setup for determination of the velocity profile field and the spatial concentration distribution of water and ethanol/methanol in a micromixer.

Air motion sensing hairs of arthropods detect high frequencies at near-maximal mechanical efficiency

Using measurements based on particle image velocimetry in combination with a novel compact theoretical framework to describe hair mechanics, we found that spider and cricket air motion sensing hairs work close to the physical limit of sensitivity and energy transmission in a broad range of relatively high frequencies. In this range, the hairs closely follow the motion of the incoming flow because a minimum of energy is dissipated by forces acting in their basal articulation. This frequency band is located beyond the frequency at which the angular displacement of the hair is maximum which is between about 40 and 600 Hz, depending on hair length (Barth et al. [1] Phil. Trans. R. Soc. Lond. B 340, 445-461 (doi:10.1098/rstb.1993.0084)). Given that the magnitude of natural airborne signals is known to decrease with frequency, our results point towards the possible existence of spectral signatures in the higher frequency range that may be weak but of biological significance.

Blowoff mechanism of harmonically forced bluff body stabilized turbulent premixed flames

Flame blowoff under harmonic forcing of upstream mixture velocity is analyzed for bluff-body stabilized conical premixed flames in a laboratory burner. It is found that the forced vortex shedding phenomenon within the recirculation zone, accompanies flame blowoff when the convective wavelength of the imposed oscillation is larger than the recirculation zone length in the streamwise direction. The experimental results obtained from combined particle image velocimetry and planar laser induced fluorescence of OH radical provide evidence for this behavior. The differences between forced and unforced flame blowoff are also discussed.

Optical measurement of pore scale velocity field inside microporous media

An experimental technique to quantify velocity field at pore scale with in microporous media, formed by packing of microglass spheres of size 200 μm inside a glass micro-model, is presented. A microparticle image velocimetry (μ-PIV) system is used to study velocity fields at four different spatial regions in the microporous medium. A combined particle image velocimetry (PIV) and particle tracking velocimetry (PTV) scheme is used to quantify velocity within a typical pore size of 10–50 μm. The experiments are conducted at four different flow rates. Two different measurement planes are selected for obtaining the detailed pore scale velocity field—one close to the glass wall and the other inside the porous medium at a distance 100 μm below the glass wall. The image processing technique for dealing with noisy data and sparse vector field has been discussed in detail. Probability density functions of transverse and axial velocity components are compared with available results in literature. The pore scale velocity field obtained can provide insight to flow properties in microporous media and can be a powerful tool to validate existing numerical results for flow through porous media.

Simultaneous particle image velocimetry and chemiluminescence visualization of millisecond-pulsed current–voltage-induced perturbations of a premixed propane/air flame

The effects of millisecond-wide, pulsed current–voltage-induced behavior in premixed laminar flames have been investigated through the simultaneous collection of particle image velocimetry (PIV) and chemiluminescence data with particular attention paid to the onset mechanisms. Disturbances caused by applied voltages of 2 kV over a 30-mm gap to a downward propagating, atmospheric pressure, premixed propane/air flame with a flow speed near 2 m/s and an equivalence ratio of 1.06 are investigated. The combined PIV and chemiluminescence-based experimental data show the observed disturbance originates only in or near the cathode fall region very close to the burner base. The data also suggest that the coupling mechanism responsible for the flame disturbance behavior is fluidic in nature, developing from the radial positive chemi-ion distribution and an ion-drift current-induced net body force that acts along the annular space discharge distribution in the reaction zone in or near the cathode fall. This net body force causes a reduction in flow speed above these near cathodic regions causing the base of the flame to laterally spread. Also, this effect seems to produce a velocity gradient leading to the transition of a laminar flame to turbulent combustion for higher applied current–voltage conditions as shown in previous work (Marcum and Ganguly in Combust Flame 143:27–36, 2005; Schmidt and Ganguly in 48th AIAA aerospace sciences meeting. Orlando, 2010).

Mixing and boundary interactions of 30° and 45° inclined dense jets

60° inclined dense jets had been recommended for brine discharges from desalination plants to achieve a maximum mixing efficiency. However, the terminal rise associated with 60° is relatively high and thus the angle may be too large for disposal in shallow coastal wasters. In this study, we investigate the mixing behavior of dense jets discharging at smaller angles of 30° and 45° in a stationary ambient. Combined Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) were used as the measurement approaches that captured the velocity and concentration fields, respectively. Based on the experimental results, the characteristic geometrical features of the inclined dense jets, including the location of the centerline peak and the return point where the dense jet returns to the source level, etc., are quantified. The mixing and diluting behaviors are also revealed through the analysis of the axial and cross-sectional velocity and concentration profiles. In addition to the free inclined discharges, the present study also examines the effect of the proximity to the bed. Through the comparison of the results between two experimental series with distinct z 0/D but overlapping z 0/L M , the latter is identified as the deciding factor for the boundary influence.