Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session D8: Particles: Experimental |
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Chair: Greg Voth, Wesleyan University Room: 25A |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D8.00001: In situ calibration of volume concentration measurements using PTV correlation for particle-laden flows Rahul Mulinti, Kyle Corfman, Ken Kiger Determination of the effective measurement volume is essential for making quantitative concentration measurements of a dispersed phase when using particle-imaging technique for two-phase flows. While nominally determined by the local light sheet thickness, the actual value depends also on the dispersed phase identification characteristics used to detect the particles (relative brightness, size, etc.), and stray illumination such as scattering by tracer particles and wall reflections, necessitating use of local calibration techniques. In the current work, a novel \textit{in situ} method is proposed where the effective light sheet thickness is estimated using particle image correlation information of free falling dispersed-phase particles settling through a tilted light sheet. Increasing the delay time between the image pairs results in in-plane loss of correlation as the particle images move out of the light sheet. By employing a threshold on the height of the normalized mean correlation peak, and relating this to the actual particle image identification characteristics, the effective light sheet thickness is estimated. The effects of tracer particles and presence of a strong wall reflection on the effective light sheet thickness is also reported. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D8.00002: Using direct numerical simulation to improve experimental measurements of inertial particle radial relative velocities Peter J. Ireland, Lance R. Collins Turbulence-induced collision of inertial particles may contribute to the rapid onset of precipitation in warm cumulus clouds. The particle collision frequency is determined from two parameters: the radial distribution function $g(r)$ and the mean inward radial relative velocity $\langle w_r ^{(-)} \rangle$. These quantities have been measured in three dimensions computationally, using direct numerical simulation (DNS), and experimentally, using digital holographic particle image velocimetry (DHPIV). While good quantitative agreement has been attained between computational and experimental measures of $g(r)$ (Salazar et al. 2008), measures of $w_r$ have not reached that stage (de Jong et al. 2010). We apply DNS to mimic the experimental image analysis used in the relative velocity measurement. To account for experimental errors, we add noise to the particle positions and `measure' the velocity from these positions. Our DNS shows that the experimental errors are inherent to the DHPIV setup, and so we explore an alternate approach, in which velocities are measured along thin two-dimensional planes using standard PIV. We show that this technique better recovers the correct radial relative velocity PDFs and suggest optimal parameter ranges for the experimental measurements. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D8.00003: Rotation in turbulence of aquatic organisms modeled as particles Evan Variano, Margaret Byron, Gabriele Bellani We investigate which length and time scales are relevant for determining the rotation of aquatic organisms and their gametes. We are interested in parameter space beyond the Stokes regime, and also the effect of particle shape on rotation. We report experimental measurements that use custom--manufactured particles to model aquatic organisms, which are designed with the necessary optical properties so that we can measure their rotation, simultaneously with the vorticity statistics of the surrounding fluid. Lagrangian timeseries of particles' angular velocity allows investigation of rotational diffusion. [Preview Abstract] |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D8.00004: Turbulence modulation by Taylor-scale particles: in search of a universal parameter Gabriele Bellani, Jeff Semigran, Margaret Byron, Evan Variano In this work we investigate turbulence modulation effects by Taylor microscale-sized particles in homogeneous isotropic turbulence. We present a novel experimental technique that allows us to explore a wide range of parameters in terms of particle shape and concentration. In lab experiments we perform Particle Image Velocimetry on both the fluid phase and cross-sections of suspended particles. These measurements yield the fluid-phase velocity, linear and angular velocities of the particles, as well as the detail of the flow near the particle interface. We report turbulence modulation effects as function of particle shape and concentration. These include change in turbulent kinetic energy, mean dissipation rates, and spectral analysis. The results are normalized using various scaling factors, in an attempt to find a universal parameter that describes turbulence modulation for particles of any shape and size. The details of the interphase-coupling mechanisms are also investigated by comparing statistics of particle translation and rotation rates to statistics of the fluid-phase velocity field at various length scales. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D8.00005: Particle behavior in linear shear flow: an experimental and numerical study Nima Fathi, Marc Ingber, Peter Vorobieff We study particle behavior in low Reynolds number flows. Our experimental setup can produce both Couette flow and Pouseuille flow at low Reynolds numbers. Spherical particles are suspended in gravity-stratified Newtonian fluid. Their predominantly two-dimensional motion is driven by moving belts (and/or piston) that produce shear in the fluids. Particle migration and translational velocity have been studied. The irreversibility of particle motion has been investigated. The experimental results are compared to the numerical simulations performed with discrete phase element method (DPM). Particle trajectories with the same boundary conditions in viscous fluids have been studied. The irreversibility in numerical simulation has been modeled for different cases. Results show the particle migration is a function of shear rate, particle size, degree of symmetry of the fluid domain, and also of the initial starting position, the latter playing an important role in the irreversibility of particle motion. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D8.00006: Measuring the effects of finite length of rods on rotation rate in turbulent flow Shima Parsa, Greg Voth We study the rotational dynamics of single rod-like particles ranging from tracer rods to long rods and quantify the effects of length of rod on it's rotation rate in turbulent flow. The position and orientation of rods are measured experimentally using Lagrangian particle tracking with multiple cameras in a flow between oscillating grids. \newline The rotation rate of rods is determined by the velocity gradient of the flow. As rods are transported in the flow they develop alignment with the directions of the vorticity and eigenvectors of the strain-rate. This alignment results in a smaller rotation rate from randomly oriented rods. Tracer rods rotate due the velocity gradient of the smallest eddies that produce the largest shear rate while longer rods average over length-scales ranging from eddies smaller than their size to eddies order of their own length-scale. As the length of the rods increases the rotation rate variance gets smaller. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D8.00007: Lagrangian Measurements of Vorticity and the Rotational Dyanmics of Anisotropic Particles in Turbulence Guy Geyer, Shima Parsa, Stefan Kramel, Greg Voth We measure the Lagrangian rotational dynamics of anisotropic particles in turbulent flow using stereoscopic video imaging. Using 3D printing technology, we fabricate rods, crosses (two perpendicular rods), and jacks (three mutually perpendicular rods). The three dimensional position and orientation of these objects can be reconstructed using a combination of stereomatching and optical tomography. We apply these techniques to measurements in a $R_\lambda \approx 200$ flow, where turbulence is generated by two grids oscillating in phase. Since the advected particles have a largest dimension less than 10 times the Kolmogorov length, they are good approximations of tracer particles. Using resistive force theory, we demonstrate that tracer jacks and crosses have the same rotational dynamics as spheres and disks, respectively. Thus, we can measure the rotation rates of ellipsoidal particles at aspect ratios, $\alpha$, that span the entire range: $\alpha \approx 0$ (disks) $\alpha=1$ (spheres), and $\alpha \approx \infty$ (rods). Furthermore, measurements of the rotation rate of jacks constitute a novel method for obtaining Lagrangian measurements of the vorticity in turbulent flows. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D8.00008: ABSTRACT WITHDRAWN |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D8.00009: Simultaneous 3D measurement of the translation and rotation of finite size particles and the flow field in a fully developed turbulent water flow Mathieu Gibert, Simon Klein, Eberhard Bodenschatz We report a novel experimental technique that measures simultaneously in three dimensions the trajectories, the translation, and the rotation of finite size inertial particles together with the turbulent flow. The flow field is analyzed by tracking the temporal evolution of small fluorescent tracer particles. The inertial particles consist of a super-absorbent polymer that renders them index and density matched with water and thus invisible. The particles are marked by inserting at various locations tracer particles into the polymer. Translation and rotation, as well as the flow field around the particle are recovered dynamically from the analysis of the marker and tracer particle trajectories. We apply this technique to study the dynamics of inertial particles much larger in size ($Rp/{\eta} \approx 100$) than the Kolmogorov length scale $\eta$ in a von K\'arm\'an swirling water flow ($R_{\lambda} \approx 400$). We show, using the mixed (particle/fluid) Eulerian second order velocity structure function, that the interaction zone between the particle and the flow develops in a spherical shell of width 2Rp around the particle of radius Rp. This we interpret as an indication of a wake induced by the particle. (http://arxiv.org/abs/1205.2181) [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D8.00010: Hydrodynamic alignment of Nano-Fibrillated Cellulose in extensional flow Karl H{\aa}kansson, Fredrik Lundell, Lisa Prahl Wittberg, Lars W{\aa}gberg, Daniel S\"oderberg The aim of this work is to manipulate the orientation of cellulose fibrils in order to enable control of material properties. Cellulose fibrils are the load bearing component of wood and separation of the fibrils from the cell wall is possible through enzymatic and mechanical treatment. The resulting product is called Nano-Fibrillated Cellulose, NFC, consisting of elongated particles with diameters of 40 nm and lengths of a few $\mu$m. Films and fibers made by NFC show great potential in terms of material properties. This work includes experiments, computations and simulations in order to determine the alignment of NFC in a laminar extensional flow. A flow focusing setup is used where water is accelerating a semi-dilute NFC-dispersion, this particular design minimizes the shear on the NFC-dispersion. The relative mean orientation is found through 2D birefringence measurements. The Smoluchowski equation including an orientational diffusion term and a flow forcing term is solved numerically in 1D. Flow field simulations are made in order to find the local acceleration, and also to confirm the shape of the suspension thread formed. The computations predict the same trend as is seen in the experiments; at higher accelerations the NFC-fibrils become more aligned. [Preview Abstract] |
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