Name: methods for measuring the orientation and rotation rate of 3d-printed particles in turbulence
Uploaded: 2016-06-24T13:00:00.000+00:00
Duration: 12 min 34 s
Description: Watch this Scientific Journal Video about Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence at JoVE.com

We thank Susantha Wijesinghe who designed and constructed the image compression system we use. We acknowledge support from the NSF grant DMR-1208990.

<ol>
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The authors have no competing financial interests to disclose.

Measurements of the vorticity and rotation of particles in turbulent fluid flow have long been recognized as important goals in experimental fluid mechanics. The solid-body rotation of small spheres in turbulence is equal to half the fluid vorticity, but the rotational symmetry of spheres has made direct measurement of their solid-body rotation difficult. Traditionally, the fluid vorticity has been measured using complex, multi-sensor, hot-wire probes14. But these sensors only get single-point vorticity measurements in airflows that have large mean velocity. Other vorticity measurement methods have been developed. For example, Su and Dahm used flow field velocimetry based on scalar images15 and L&#252;thi, Tsinober, and Kinzelbach used 3D particle tracking velocimetry16. Measurements of vorticity in turbulence by tracking rotations of single particles were pioneered by Frish and Webb, who measured the rotations of solid spherical particles using a vorticity optical probe17. This probe uses small particles with planar crystals embedded that act as mirrors to create a beam whose direction changes as the particle rotates. Recently, methods have been developed for measuring the rotational motion of large spherical particles using imaging of patterns painted on the particles18,19 or fluorescent particles embedded in transparent hydrogel particles20. To track anisotropic particles, Bellani et al. have used custom-molded hydrogel particles21. Parsa et al. have tracked the rotations of segments of nylon threads5,6,12. The methods for measuring vorticity and particle rotations presented in this paper have advantages over these alternative methods. 3D-printed anisotropic particles can be small, with arm thicknesses down to 0.3 mm in diameter, and their rotations can still be resolved very accurately. Other methods traditionally require larger particles because they involve the resolution of structures on or within the particles themselves. In addition, the use of image compression systems allows for many more particle trajectories to be recorded and measured than would otherwise be reasonable. Having more measurements makes it possible to study rare events like those with very high rotation rates in Figure 5, which reveal intermittency phenomena of great interest to researchers.
Particle concentrations in these experiments were about 5 x 10-3 cm-3, which meant that typically only about 20% of images from the cameras had a particle. To study rare events, thousands of particle trajectories are typically required, which meant that hundreds of thousands of images of particles were needed. With these low concentrations, therefore, millions of images needed to be recorded to obtain an adequate volume of data. If real-time image compression systems were not used to facilitate data acquisition, this would require hundreds of TB of data storage and the analysis would be much more computationally intensive. Image compression systems decrease this load by factors of several hundred10. However, standard video recording would be adequate for higher particle densities and if data storage space is not an issue. If 100,000 particles of each type were ordered instead of 10,000, fewer images would, in principle, be needed to capture the same statistics. However, at higher particle densities particles begin to shadow one another more often. That is, there will be more times when there are particles between the laser and the particle in view, or between the particle in view and the camera. These shadowing events make measuring orientations throughout a track across the viewing volume more difficult and less reliable. For these reasons, lower particle concentrations were chosen for these experiments and image compression systems were therefore necessary.
There may be times when arm shadowing will affect the results of the nonlinear search algorithm. For certain orientations of the jack, arm shadowing causes there to be multiple minima in Euler angle space, which lead to indeterminacies in the measured orientations. This reduces the accuracy of orientation measurements for these particular orientations and occasionally leads to erroneously high measurements of the solid-body rotation rate, which pushes additional probability density towards the tail of the PDF in Figure 5. For jacks, whose arms are perpendicular to each other, this issue could be decreased by changing the angles of the cameras with respect to one another to be farther away from 90&#176;. If the configuration of the apparatus makes this change difficult to implement, one alternative is to change the geometry of the particles to decrease shadowing. This was the reason tetrads were chosen for experiments after those with jacks had been completed, and recent tetrad measurements have shown significantly improved orientation accuracy when compared to jacks.
The methods of 3D particle tracking presented here are not confined to this particular flow or the particle sizes and shapes we use. We have already begun experiments tracking tetrads and triads with much larger sizes using similar techniques. The use of high-speed cameras to measure particle orientations and rotations can be extended to a wide array of shapes and can be used for inertial particles as well as in the neutrally buoyant case presented here. Using more cameras would allow for an even wider array of potential particle shapes, as the primary limitations to this method are the resolution of the cameras and particles&#39; self-shadowing, as discussed in the previous paragraph.
In step 5.1.6 of the Protocol, we smooth Euler angles measurements by assuming that a particle would not rotate by more than half of an angle between arms over the course of two frames &#8212; that is, we assume that the accurate orientation measurement at frame i+1 retains the chosen symmetric orientation found for frame i. If the particle had rotated by more than half of one of these interior angles, then smoothing in this way would result in a sudden and incorrect reversal of the direction of rotation. In Ref. 5 we show that an upper limit on particle tumbling rate is:
<img alt="Equation 5" src="/files/ftp_upload/53599/53599eq5.jpg" />
So the largest tumbling rate (<img alt="Equation 6" src="/files/ftp_upload/53599/53599eq6.jpg" />) is <img alt="Equation 7" src="/files/ftp_upload/53599/53599eq7.jpg" /> which for <img alt="Equation 8" src="/files/ftp_upload/53599/53599eq8.jpg" /> sec is 16.2 sec-2. This is a root mean square (RMS) tumbling rate of 4.0 sec-1. Since we record images at 450 frames per second, particles would then typically rotate 0.009 radians between frames. The smallest interior angle of any of the particles in these experiments was <img alt="Figure 5" src="/files/ftp_upload/53599/53599divident.jpg" />, so this smoothing method would fail if particles tumble more than <img alt="Figure 5" src="/files/ftp_upload/53599/53599equal.jpg" /> radians between frames. Thus, we can accurately track particles with tumbling rates of more than 80 times the RMS, which is much faster than the&#160;<img alt="Equation 6" src="/files/ftp_upload/53599/53599squareroot.jpg" /> times the RMS that we actually observe in Figure 5.

In a recent publication, we introduced the use of particles made from multiple slender arms for measuring rotational motion of particles in turbulence1. These particles can be fabricated using 3D printers, and it is possible to accurately measure their position, orientation, and rotation rate using multiple cameras. Using tools from slender body theory, it can be shown that these particles have corresponding effective ellipsoids2, and the rotational motions of these particles are identical to those of their respective effective ellipsoids. Particles with symmetric arms of equal length rotate like spheres. One such particle is a jack, which has three mutually perpendicular arms attached at its center. Adjusting the relative lengths of the arms of a jack can form a particle equivalent to any tri-axial ellipsoid. If the length of one arm is set equal to zero, this creates a cross, whose equivalent ellipsoid is a disk. Particles made of slender arms take up a small fraction of the solid volume of their solid ellipsoidal counterparts. As a result, they sediment more slowly, making them easier to density match. This allows the study of much larger particles than is convenient with solid ellipsoidal particles. Additionally, imaging can be performed at much higher particle concentrations because the particles block a smaller fraction of the light from other particles.
In this paper, methods for fabrication and tracking of 3D-printed particles are documented. Tools for tracking the translational motion of spherical particles from particle positions as seen by multiple cameras have been developed by several groups3,4. Parsa et al.5 extended this approach to track rods using the position and orientation of the rods seen by multiple cameras. Here, we present methods for fabricating particles of a wide variety of shapes and reconstructing their 3D orientations. This offers the possibility to extend 3D tracking of particles with complex shapes to a wide range of new applications.
This technique has great potential for further development because of the wide range of particle shapes that can be designed. Many of these shapes have direct applications in environmental flows, where plankton, seeds, and ice crystals come in a vast array of shapes. Connections between particle rotations and fundamental small-scale properties of turbulent flows6 suggest that study of rotations of these particles provides new ways to look at the turbulent cascade process.

<table><tbody><tr><td>Condor Nd:YAG 50 W laser</td><td>Quantronics</td><td>532-30-M</td><td></td></tr><tr><td>High speed camera</td><td>Basler</td><td>A504k</td><td></td></tr><tr><td>High speed camera</td><td>Mikrotron</td><td>EoSens Mc1362</td><td></td></tr><tr><td>Rhodamine-B</td><td>ScienceLab.com</td><td>SLR1465</td><td></td></tr><tr><td>Sodium Hydroxide</td><td>Macron</td><td>7708</td><td>Pellets.</td></tr><tr><td>500 Connex 3D printer</td><td>Objet</td><td></td><td>Used to make smaller particles. Particles ordered from RP+M (rapid prototyping plus manufacturing).</td></tr><tr><td>VeroClear</td><td>Stratasys</td><td>RGD810</td><td>Objet build material.</td></tr><tr><td>Clear Form 1 Photopolymer Resin</td><td>Formlabs</td><td></td><td></td></tr><tr><td>Cylindrical and spherical lenses</td><td></td><td></td><td></td></tr><tr><td>200 mm macro camera lenses</td><td></td><td></td><td>F-mount.</td></tr><tr><td>Ultrasonic bath</td><td>Sonicator</td><td></td><td></td></tr><tr><td>Calcium Chloride</td><td>Spectrum Chemical Mfg. Corp.</td><td>CAS 10043-52-2</td><td>Pellets.</td></tr><tr><td>LabVIEW System Design Software</td><td>National Instruments</td><td></td><td>Used to trigger cameras, control grid, and trigger laser.</td></tr><tr><td>XCAP Software</td><td>EPIX</td><td></td><td>Used with LabVIEW to trigger cameras.</td></tr><tr><td>MATLAB</td><td>Mathworks</td><td></td><td>Used for all image and data analysis. Programs for extracting 3D orientations from multiple images are included with this publication.</td></tr><tr><td>OpenPTV: Open Source Particle Tracking Velocimetry</td><td>OpenPTV Consortium</td><td></td><td></td></tr><tr><td>ParaView</td><td>Kitware</td><td></td><td></td></tr><tr><td>AutoCAD</td><td>AutoDesk</td><td></td><td>Used to design all particles. Screenshots of particle designs are all of AutoCAD.</td></tr><tr><td>Mesh with 0.040 x 0.053 inch holes</td><td>Industrial Netting</td><td>XN5170&amp;ndash;43.5</td><td></td></tr><tr><td>Camera filters</td><td>Schneider Optics</td><td>B+W 040M</td><td></td></tr></tbody></table>

methods for measuring the orientation and rotation rate of 3d-printed particles in turbulence

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing technology is used to fabricate particles with slender arms connected at a common center. Shapes explored are crosses (two perpendicular rods), jacks (three perpendicular rods), triads (three rods in triangular planar symmetry), and tetrads (four arms in tetrahedral symmetry). Methods for producing on the order of 10,000 fluorescently dyed particles are described. Time-resolved measurements of their orientation and solid-body rotation rate are obtained from four synchronized videos of their motion in a turbulent flow between oscillating grids with R&#955; = 91. In this relatively low-Reynolds number flow, the advected particles are small enough that they approximate ellipsoidal tracer particles. We present results of time-resolved 3D trajectories of position and orientation of the particles as well as measurements of their rotation rates.

Figure 3a shows an image of a tetrad from one of our cameras above a plot of the Euler angles obtained from a section of its trajectory (Figure 3c). In Figure 3b, the results of the orientation-finding algorithm, described in Protocol 5 - 5.3, are superimposed on the tetrad image. The arms of the tetrad in Figure 3a do not follow the simple intensity distributions that are used to create the model (Protocol 5.1.3.1). This is true for all of the particles. The observed intensity furthermore has a non-trivial dependence on the angles between the arms, the illumination, and the viewing direction12. The models do not include any of these factors but nonetheless produce very accurate measurements of particle orientations.
Once an orientation is found with a least-squares fit, the 3D coordinates of the particle center and the three Euler angles,&#160;(&#966;,&#160;&#952;,&#160;&#968;) , that specify its orientation matrix11 are saved. This is done for every frame where the particle is in view of all four cameras. These data enable the reconstruction of the complete trajectory of the particle across the viewing volume, as are shown in Figure 4 for a cross and a jack. Figure 4 was made using the Paraview open source visualization package and is based on measurements made with images from the experiments.
<img alt="Figure 3" src="/files/ftp_upload/53599/53599fig3.jpg" /> 
Figure 3. Reconstructed particle orientations from measured images. a) A sample image from one of the four cameras. The object shown is a tetrad, which has four arms at 109.5&#176; interior angles to one another. b) The same tetrad shown with the results of our orientation-finding algorithm. c) Measured Euler angles plotted as a function of time for a single trajectory. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53599/53599fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/53599/53599fig4.jpg" /> 
Figure 4. Reconstructed trajectories of a cross (a) and a jack (b) in three-dimensional turbulence. (a) The two different color sheets trace the path of the two arms of the particle through space over time. The length of the track is 336 frames, or 5.7&#160;&#964;&#951;, and a cross is shown every 15 frames. (b) The blue, orange, and blue-green paths trace the paths of the three arms of the jack as the particle rotates and moves through the fluid. The dark green line denotes the path of the jack&#39;s center. The length of the particle track is 1,025 frames, or 17.5&#160;&#964;&#951;, and a jack is shown every 50 frames. (Note: Neither the crosses nor the jacks above are drawn to scale.) Figure from 1, where it is Figure 3. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53599/53599fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Two different but related quantities based on particle orientations are calculated over the entire trajectory: tumbling rate and solid-body rotation rate. Tumbling rate, <img alt="Equation 1" src="/files/ftp_upload/53599/53599pdot.jpg" /> , is the rate of change of the unit vector defining the orientation of the particle. In previous measurements of rods, <img alt="Equation 1" src="/files/ftp_upload/53599/53599p.jpg" /> was defined as the axis of symmetry along the rod; for crosses and triads,&#160;<img alt="Equation 1" src="/files/ftp_upload/53599/53599p.jpg" /> is normal to the plane of the arms; for jacks and tetrads,&#160;<img alt="Equation 1" src="/files/ftp_upload/53599/53599p.jpg" /> is along one of the arms. Because rotation along the axis of rods cannot be directly measured, studies of the rotations of rods in turbulence have largely been limited to measuring the tumbling rate. This is not an issue for any of the particles in these experiments. All rotations of these particles can be measured and, with orientation measurements smoothed along a particle&#39;s trajectory, the full solid-body rotation rate vector,&#160;<img alt="Equation 1" src="/files/ftp_upload/53599/53599ws.jpg" /> , can be found.
To extract the solid-body rotation rate from measured particles orientations, smoothing needs to be done over several time steps. The problem is to find the rotation matrix <img alt="Equation 1" src="/files/ftp_upload/53599/53599R.jpg" /> that relates an initial orientation&#160;<img alt="Figure 5" height="19" src="/files/ftp_upload/53599/53599oti.jpg" width="37" />&#160; to the measured orientations <img alt="Figure 5" src="/files/ftp_upload/53599/53599ot.jpg" /> at a sequence of time steps:
<img alt="Equation 1" src="/files/ftp_upload/53599/53599eq1.jpg" />
where&#160; <img alt="Figure 5" src="/files/ftp_upload/53599/53599tf.jpg" /> is the period between images and&#160;<img alt="Figure 5" src="/files/ftp_upload/53599/53599ti.jpg" /> is the time of the initial frame. In Marcus et al.1, we used a nonlinear least-squares fit to determine the six Euler angles defining the initial orientation matrix, <img alt="Figure 5" height="19" src="/files/ftp_upload/53599/53599oti.jpg" width="37" />, and the rotation matrix over a single time step,&#160;<img alt="Equation 1" src="/files/ftp_upload/53599/53599R.jpg" />, that best match the measured orientation matrices as a function of time. More recent work has shown that this algorithm sometimes has difficulty when the rotation rate is small because the nonlinear search is exploring the region where the Euler angles are approximately equal to zero and are degenerate. In the case where the rotation in a time step is sufficiently small, <img alt="Equation 1" src="/files/ftp_upload/53599/53599R.jpg" /> can be linearized using&#160;<img alt="Equation 2" src="/files/ftp_upload/53599/53599eq2.jpg" />, where &#937; is a rotation rate matrix. As described in the Discussion below, these experiments are in this low rotation limit, so &#937; can be found from the measured <img alt="Figure 5" src="/files/ftp_upload/53599/53599ot.jpg" /> using a linear least squares fit.
From the measured rotation matrix over a time step, <img alt="Equation 1" src="/files/ftp_upload/53599/53599R.jpg" />, we can extract the solid-body rotation rate and the tumbling rate. By Euler&#39;s theorem11 <img alt="Equation 1" src="/files/ftp_upload/53599/53599R.jpg" /> can be decomposed as a rotation by an angle&#160;&#934; about the solid-body rotation axis,&#160;<img alt="Figure 5" src="/files/ftp_upload/53599/53599wns.jpg" />. The magnitude of the solid-body rotation rate is&#160;<img alt="Equation 3" src="/files/ftp_upload/53599/53599eq3.jpg" />. The tumbling rate is the component of the solid-body rotation rate perpendicular to the orientation of the particle, and so it can be calculated as&#160;<img alt="Equation 4" src="/files/ftp_upload/53599/53599eq4.jpg" />. Figure 5 compares PDFs of the measured mean square tumbling rate for crosses and jacks to direct numerical simulations of spheres. Small jacks rotate just like spheres in fluid flows1, so the fact that the PDF for jacks agrees with the simulated PDF for spheres demonstrates that the experiments are able to capture the rare high rotation events that occur in turbulent flows.
<img alt="Figure 5" src="/files/ftp_upload/53599/53599fig5.jpg" /> 
Figure 5. PDF of mean-square tumbling rate. The probability density function of the measured mean-square tumbling rate for our crosses (red squares) and jacks (blue circles) as well as direct numerical simulations of spheres (solid line). Error bars include the random error due to limited statistical sampling estimated by dividing the data set into subsets, as well as the systematic error that results from the fit length dependence of the tumbling rate, which is estimated by performing the analysis at a range of fit lengths. Figure from 1 where it is Figure 5. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/53599/53599fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

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Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence

We use 3D printing to fabricate anisotropic particles in the shapes of jacks, crosses, tetrads, and triads, whose alignments and rotations in turbulent fluid flow can be measured from multiple simultaneous video images.

Experimental methods are presented for measuring the rotational and translational motion of anisotropic particles in turbulent fluid flows. 3D printing ...

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10.0K Views.  Wesleyan University. We use 3D printing to fabricate anisotropic particles in the shapes of jacks, crosses, tetrads, and triads, whose alignments and rotations in turbulent fluid flow can be measured from multiple simultaneous video images.

Video: Methods for Measuring the Orientation and Rotation Rate of 3D-printed Particles in Turbulence

Encapsulation and Permeability Characteristics of Plasma Polymerized Hollow Particles

In this protocol, core-shell nanostructures are synthesized by plasma enhanced chemical vapor deposition. We produce an amorphous barrier by plasma polymerization of isopropanol on various solid substrates, including silica and potassium chloride. This versatile technique is used to treat nanoparticles and nanopowders with sizes ranging from 37 nm to 1 micron, by depositing films whose thickness can be anywhere from 1 nm to upwards of 100 nm. Dissolution of the core allows us to study the rate of permeation through the film. In these experiments, we determine the diffusion coefficient of KCl through the barrier film by coating KCL nanocrystals and subsequently monitoring the ionic conductivity of the coated particles suspended in water. The primary interest in this process is the encapsulation and delayed release of solutes. The thickness of the shell is one of the independent variables by which we control the rate of release. It has a strong effect on the rate of release, which increases from a six-hour release (shell thickness is 20 nm) to a long-term release over 30 days (shell thickness is 95 nm). The release profile shows a characteristic behavior: a fast release (35% of the final materials) during the first five minutes after the beginning of the dissolution, and a slower release till all of the core materials come out.

We have used plasma enhanced chemical vapor deposition to deposit thin films ranging from a few nm to several 100 nm on nano-sized particles of various materials. We subsequently etch the core material to produce hollow nanoshells whose permeability is controlled by the thickness of the shell. We characterize the permeability of these coatings to small solutes and demonstrate that these barriers can provide sustained release of the core material over several days.

In this protocol, core-shell nanostructures are synthesized by plasma enhanced chemical vapor deposition. We produce an amorphous barrier by plasma ...

Experimental Measurement of Settling Velocity of Spherical Particles in Unconfined and Confined Surfactant-based Shear Thinning Viscoelastic Fluids

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic (VES) fluids. The measurements are made for particles settling in unbounded fluids&#160;and fluids between parallel walls. VES fluids over a wide range of rheological properties are prepared and rheologically characterized. The rheological characterization involves steady shear-viscosity and dynamic oscillatory-shear measurements to quantify the viscous and elastic properties respectively. The settling velocities under unbounded conditions are measured in beakers having diameters at least 25x the diameter of particles. For measuring settling velocities between parallel walls, two experimental cells with different wall spacing are constructed. Spherical particles of varying sizes are gently dropped in the fluids and allowed to settle. The process is recorded with a high resolution video camera and the trajectory of the particle is recorded using image analysis software. Terminal settling velocities are calculated from the data.
The impact of elasticity on settling velocity in unbounded fluids is quantified by comparing the experimental settling velocity to the settling velocity calculated by the inelastic drag predictions of Renaud et al.1&#160;Results show that elasticity of fluids can increase or decrease the settling velocity. The magnitude of reduction/increase is a function of the rheological properties of the fluids and properties of particles. Confining walls are observed to cause a retardation effect on settling and the retardation is measured in terms of wall factors.

This paper demonstrates the experimental procedure to measure terminal settling velocities of spherical particles in surfactant-based shear thinning viscoelastic fluids. Fluids over a wide range of rheological properties are prepared and settling velocities are measured for a range of particle sizes in unbounded fluids and fluids between parallel walls.

An experimental study is performed to measure the terminal settling velocities of spherical particles in surfactant based shear thinning viscoelastic ...

Automation of Mode Locking in a Nonlinear Polarization Rotation Fiber Laser through Output Polarization Measurements

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a new and inexpensive procedure to force mode locking in a pre-adjusted nonlinear polarization rotation fiber laser. This procedure is based on the detection of a sudden change in the output polarization state when mode locking occurs. This change is used to command the alignment of the intra-cavity polarization controller in order to find mode-locking conditions. More specifically, the value of the first Stokes parameter varies when the angle of the polarization controller is swept and, moreover, it undergoes an abrupt variation when the laser enters the mode-locked state. Monitoring this abrupt variation provides a practical easy-to-detect signal that can be used to command the alignment of the polarization controller and drive the laser towards mode locking. This monitoring is achieved by feeding a small portion of the signal to a polarization analyzer measuring the first Stokes parameter. A sudden change in the read out of this parameter from the analyzer will occur when the laser enters the mode-locked state. At this moment, the required angle of the polarization controller is kept fixed. The alignment is completed. This procedure provides an alternate way to existing automating procedures that use equipment such as an optical spectrum analyzer, an RF spectrum analyzer, a photodiode connected to an electronic pulse-counter or a nonlinear detecting scheme based on two-photon absorption or second harmonic generation. It is suitable for lasers mode locked by nonlinear polarization rotation. It is relatively easy to implement, it requires inexpensive means, especially at a wavelength of 1550 nm, and it lowers the production and operation costs incurred in comparison to the above-mentioned techniques.

A protocol to detect and automate mode locking in a pre-adjusted nonlinear polarization rotation fiber laser is presented. The detection of a sudden change in the output polarization state when mode locking occurs is used to command the alignment of an intra-cavity polarization controller in order to find mode-locking conditions.

When a laser is mode-locked, it emits a train of ultra-short pulses at a repetition rate determined by the laser cavity length. This article outlines a ...

Three-dimensional Particle Tracking Velocimetry for Turbulence Applications: Case of a Jet Flow

3D-PTV is a quantitative flow measurement technique that aims to track the Lagrangian paths of a set of particles in three dimensions using stereoscopic recording of image sequences. The basic components, features, constraints and optimization tips of a 3D-PTV topology consisting of a high-speed camera with a four-view splitter are described and discussed in this article. The technique is applied to the intermediate flow field (5 &#60;x/d &#60;25) of a circular jet at Re &#8776; 7,000. Lagrangian flow features and turbulence quantities in an Eulerian frame are estimated around ten diameters downstream of the jet origin and at various radial distances from the jet core. Lagrangian properties include trajectory, velocity and acceleration of selected particles as well as curvature of the flow path, which are obtained from the Frenet-Serret equation. Estimation of the 3D velocity and turbulence fields around the jet core axis at a cross-plane located at ten diameters downstream of the jet is compared with literature, and the power spectrum of the large-scale streamwise velocity motions is obtained at various radial distances from the jet core.

A three-dimensional particle tracking velocimetry (3D-PTV) system based on a high-speed camera with a four-view splitter is described here. The technique is applied to a jet flow from a circular pipe in the vicinity of ten diameters downstream at Reynolds number Re &#8776; 7,000.

3D-PTV is a quantitative flow measurement technique that aims to track the Lagrangian paths of a set of particles in three dimensions using stereoscopic ...

Visually Based Characterization of the Incipient Particle Motion in Regular Substrates: From Laminar to Turbulent Conditions

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to turbulent flow conditions are presented. For that purpose, the incipient motion of a single bead is studied on regular substrates that consist of a monolayer of fixed spheres of uniform size that are regularly arranged in triangular and quadratic symmetries. The threshold is characterized by the critical Shields number. The criterion for the onset of motion is defined as the displacement from the original equilibrium position to the neighboring one. The displacement and the mode of motion are identified with an imaging system. The laminar flow is induced using a rotational rheometer with a parallel disk configuration. The shear Reynolds number remains below 1. The turbulent flow is induced in a low-speed wind tunnel with open jet test section. The air velocity is regulated with a frequency converter on the blower fan. The velocity profile is measured with a hot wire probe connected to a hot film anemometer. The shear Reynolds number ranges between 40 and 150. The logarithmic velocity law and the modified wall law presented by Rotta are used to infer the shear velocity from the experimental data. The latter is of special interest when the mobile bead is partially exposed to the turbulent flow in the so-called hydraulically transitional flow regime. The shear stress is estimated at onset of motion. Some illustrative results showing the strong impact of the angle of repose, and the exposure of the bead to shear flow are represented in both regimes.

Two different methods for characterizing the incipient particle motion of a single bead as a function of the sediment bed geometry from laminar to turbulent flow are presented.

Two different experimental methods for determining the threshold of particle motion as a function of geometrical properties of the bed from laminar to ...

Simultaneous Measurement of Turbulence and Particle Kinematics Using Flow Imaging Techniques

Numerous problems in scientific and engineering fields involve understanding the kinematics of particles in turbulent flows, such as contaminants, marine micro-organisms, and/or sediments in the ocean, or fluidized bed reactors and combustion processes in engineered systems. In order to study the effect of turbulence on the kinematics of particles in such flows, simultaneous measurements of both the flow and particle kinematics are required. Non-intrusive, optical flow measurement techniques for measuring turbulence, or for tracking particles, exist but measuring both simultaneously can be challenging due to interference between the techniques. The method presented herein provides a low cost and relatively simple method to make simultaneous measurements of the flow and particle kinematics. A cross section of the flow is measured using a particle image velocimetry (PIV) technique, which provides two components of velocity in the measurement plane. This technique utilizes a pulsed-laser for illumination of the seeded flow field that is imaged by a digital camera. The particle kinematics are simultaneously imaged using a light emitting diode (LED) line light that illuminates a planar cross section of the flow that overlaps with the PIV field-of-view (FOV). The line light is of low enough power that it does not affect the PIV measurements, but powerful enough to illuminate the larger particles of interest imaged using the high-speed camera. High-speed images that contain the laser pulses from the PIV technique are easily filtered by examining the summed intensity level of each high-speed image. By making the frame rate of the high-speed camera incommensurate with that of the PIV camera frame rate, the number of contaminated frames in the high-speed time series can be minimized. The technique is suitable for mean flows that are predominantly two-dimensional, contain particles that are at least 5 times the mean diameter of the PIV seeding tracers, and are low in concentration.

The technique described herein offers a low cost and relatively simple method to simultaneously measure particle kinematics and turbulence in flows with low particle concentrations. The turbulence is measured using particle image velocimetry (PIV), and particle kinematics are calculated from images obtained with a high-speed camera in an overlapping field-of-view.

Numerous problems in scientific and engineering fields involve understanding the kinematics of particles in turbulent flows, such as contaminants, marine ...

3D Printing - Evaluating Particle Emissions of a 3D Printing Pen

Three-dimensional (3D) printing as a type of additive manufacturing shows continuing increase in application and consumer popularity. The fused filament fabrication (FFF) is an inexpensive method used most frequently by consumers. Studies with 3D printers have shown that during the printing process particulate and volatile substances are released. Handheld 3D printing pens also use the FFF method but the consumer&#8217;s proximity to the 3D pens gives reason to higher exposure compared to a 3D printer. At the same time, 3D printing pens are often marketed for children who could be more sensitive to the printing emission. The aim of this study was to implement a low cost method to analyze the emissions of 3D printing pens. Polylactide (PLA) and acrylonitrile butadiene styrene (ABS) filaments of different colors were tested. In addition, filaments containing metal and carbon nanotubes (CNTs) were analyzed. An 18.5 L chamber and sampling close to the emission source was used to characterize emissions and concentrations near the breathing zone of the user.
Particle emissions and particle size distributions were measured and the potential release of metal particles and CNTs investigated. Particle number concentrations were found in a range of 105 - 106 particles/cm3, which is comparable to previous reports from 3D printers. Transmission electron microscopy (TEM) analysis showed nanoparticles of the different thermoplastic materials as well as of metal particles and CNTs. High contents of metal were observed by inductively coupled plasma mass spectrometry (ICP-MS).
These results call for a cautious use of 3D pens due to potential risk to the consumers.

This protocol presents a method to analyze the emission of 3D printing pens. Particle concentration and particle size distribution of the released particle is measured. Released particles are further analyzed with transmission electron microscopy (TEM). Metal content in filaments is quantified by inductively coupled plasma mass spectrometry (ICP-MS).

Three-dimensional (3D) printing as a type of additive manufacturing shows continuing increase in application and consumer popularity. The fused filament ...

Frugal Imaging Technique of Capillary Flow Through Three-Dimensional Polymeric Printing Powders

The development of novel imaging techniques of molecular and colloidal transport, including nanoparticles, is an area of active investigation in microfluidic and millifluidic studies. With the advent of three-dimensional (3D) printing, a new domain of materials has emerged, thereby increasing the demand for novel polymers. Specifically, polymeric powders, with average particle sizes on the order of a micron, are experiencing a growing interest from academic and industrial communities. Controlling material tunability at the mesoscopic to microscopic length scales creates opportunities to develop innovative materials, such as gradient materials. Recently, a need for micron-sized polymeric powders has been growing, as clear applications for the material are developing. Three-dimensional printing provides a high-throughput process with a direct link to new applications, driving investigations into the physio-chemical and transport interactions on a mesoscale. The protocol that is discussed in this article provides a non-invasive technique to image fluid flow in packed powder beds, providing high temporal and spatial resolution while leveraging mobile technology that is readily available from mobile devices, such as smartphones. By utilizing a common mobile device, the imaging costs that would normally be associated with an optical microscope are eliminated, resulting in a frugal-science approach. The proposed protocol has successfully characterized a variety of combinations of fluids and powders, creating a diagnostic platform for quickly imaging and identifying an optimal combination of fluid and powder.

The proposed technique will provide a novel, efficient, frugal, and non-invasive approach for imaging fluidic flow through a packed powder bed, yielding high spatial and temporal resolution.

The development of novel imaging techniques of molecular and colloidal transport, including nanoparticles, is an area of active investigation in ...

Visualizing Polymer Dynamics for Optimizing 3D Printing Quality

Real-Time Imaging of Bonding in 3D-Printed Layers

In recent times, 3D printing technology has revolutionized our ability to design and produce products, but optimizing the print quality can be challenging. The process of extrusion 3D printing involves pressuring molten material through a thin nozzle and depositing it onto previously extruded material. This method relies on bonding between the consecutive layers to create a strong and visually appealing final product. This is no easy task, as many parameters, such as the nozzle temperature, layer thickness, and printing speed, must be fine-tuned to achieve optimal results. In this study, a method for visualizing the polymer dynamics during extrusion is presented, giving insight into the layer bonding process. Using laser speckle imaging, the plastic flow and fusion can be resolved non-invasively, internally, and with high spatiotemporal resolution. This measurement, which is easy to perform, provides an in-depth understanding of the underlying mechanics influencing the final print quality. This methodology was tested with a range of cooling fan speeds, and the results showed increased polymer motion with lower fan speeds and, thus, explained the poor printing quality when the cooling fan was turned off. These findings show that this methodology allows for optimizing the printing settings and understanding the material behavior. This information can be used for the development and testing of novel printing materials or advanced slicing procedures. With this approach, a deeper understanding of extrusion can be built to take 3D printing to the next level.

Author Spotlight: Real-Time Imaging of Bonding in 3D-Printed Layers  

With a non-invasive and real-time technique, nanoscopic polymer motion inside a polymer filament is imaged during 3D printing. Fine-tuning this motion is crucial for producing constructs with optimal performance and appearance. This method reaches the core of plastic layer fusion, thus offering insights into optimal printing conditions and material design criteria.

In recent times, 3D printing technology has revolutionized our ability to design and produce products, but optimizing the print quality can be ...

Measuring Turbulent Flows

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
Turbulent flows exhibit very high frequency fluctuations that require instruments with high time-resolution for their appropriate characterization. Hot-wire anemometers have a short enough time-response to fulfill this requirement. The purpose of this experiment is to demonstrate the use of hot-wire anemometry to characterize a turbulent jet.
In this experiment, a previously calibrated hot-wire probe will be used to obtain velocity measurements at different positions within the jet. Finally, we will demonstrate a basic statistical analysis of the data to characterize the turbulent field.

Measuring Turbulent Flows Using Hot-Wire Anemometers

Source: Ricardo Mejia-Alvarez and Hussam Hikmat Jabbar, Department of Mechanical Engineering, Michigan State University, East Lansing, MI
Turbulent flows ...