Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session A14: Experimental Techniques I: PIV Algorithms |
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Chair: Kenneth Christensen, University of Illinois at Urbana-Champaign Room: 302 |
Sunday, November 24, 2013 8:00AM - 8:13AM |
A14.00001: Direct calculation of the weighting function and depth of correlation in Micro-Particle Image Velocimetry (Micro-PIV) from particle images Michael Hein, Bernhard Wieneke, Ralf Seemann Micro-PIV has become the most popular tool to measure flow profiles in microfluidics. When measuring in-plane velocities in a three dimensional flow the measured velocity depends on all particles in the images, even on defocused particles, and is given by a weighted average of the true velocity dx(z) with a weighting function W(z). W(z) depends on the optical setup as well as on the particle diameter and gradients of the flow-profile. The width of W(z) determines the height-extension of the plane in which particles can influence the measurement (Depth of Correlation, DOC). Thus the knowledge of the system dependent W(z) is crucial and can be used to reduce the errors introduced by depth-averaging the velocity field. We determine W(z) and thus the DOC using artificial double images for any given flow profile generated from particle images taken with the same optical setup as used for the PIV measurements. Experimental results for objectives with different numerical apertures (NA), different particle sizes and various out-of-plane gradients will be discussed. The resulting weighting function turns out to be quite asymmetric for air-objectives with high NAs, differing significantly in shape and width (DOC) from existing theoretical predictions. [Preview Abstract] |
Sunday, November 24, 2013 8:13AM - 8:26AM |
A14.00002: Direct Measurement of Rotation and Scaling in Particle Image Velocimetry using the Fourier-Mellin Transform Matthew Giarra, John Charonko, Pavlos Vlachos Traditional particle image velocimetry (PIV) can fail in the presence of spatial velocity gradients because the shearing, stretching, and rotation of particle image patterns can corrupt Cartesian cross correlations. We propose a novel algorithm that measures the rotation and isotropic scaling of individual subregions of PIV images. Our algorithm adopts the Fourier-Mellin (FM) image transformation, which decouples rotation from isotropic scaling and is invariant to translation. Rotation and scaling in the original image manifest as orthogonal translations in the FM-domain, which can then be measured by standard cross correlation. These properties allow for the direct measurement of vorticity (rotation) within a region of interest without relying on the spatial differencing of adjacent velocity vectors. Our algorithm also improves velocity estimates in regions of large rotation (like vortex cores) by applying the inverse rotation and stretching of the particle pattern prior to performing Cartesian correlations that estimate displacements. In this work, we apply our algorithm to synthetic and experimental PIV images and show significant improvement to the vorticity and velocity estimates compared to traditional PIV in regions where rotation is significant. [Preview Abstract] |
Sunday, November 24, 2013 8:26AM - 8:39AM |
A14.00003: A general approach for time-supersampling of 3D-PIV data by the vortex-in-cell method Fulvio Scarano, Jan Schneiders, Richard Dwight Advancements of tomographic PIV [1] have led into 3D time-resolved experiments to study the dynamical evolution of 3D turbulent flows [2]. The known bottleneck of Tomo-PIV is the high laser power required to illuminate large volumes in airflows, which becomes critical beyond 10kHz. Time-super-sampling is an approach to reduce the sampling rate, proven for frozen turbulence where the advection model yields a significant increase of temporal resolution [3]. Instead, in separated flows, the advection principle yields unacceptable distortions. The use of Navier-Stokes numerical calculations with the vortex-in-cell (VIC) method is proposed herein. The assumption is made of inviscid incompressible flow [4]. The spatial-resolution of the data is exploited to increase the temporal resolution. The dynamical evolution of the vorticity and velocity field between subsequent snapshots in the 3D domain is numerically evaluated. The verification with fully time resolved data of a circular jet indicates a substantial increase of temporal resolution. Interestingly, data sampled below the Nyquist limit could be reconstructed faithfully, indicating the potential of VIC in alleviating requirements on PIV measurement rate. [Preview Abstract] |
Sunday, November 24, 2013 8:39AM - 8:52AM |
A14.00004: Divergence-free filtering and pressure determination from 3D velocimetry: applications to flows of industrial and biomedical relevance Daniele Schiavazzi, Filippo Coletti, Julien Bodart, John K. Eaton Methodologies to acquire three-dimensional velocity fields are becoming increasingly available. However unavoidable experimental errors limit the possibility of exploiting the data to extract further information. We recently introduced a noise reduction algorithm which eliminates spurious divergence in incompressible flow measurements, removing about fifty percent of the Gaussian noise. Here we apply the algorithm to the mean velocity field in an inclined jet in crossflow measured by Magnetic Resonance Velocimetry. The de-noised field is used to calculate the mean pressure distribution by integrating the Reynolds-averaged momentum equation. A simple eddy-viscosity model is used for the estimation of the Reynolds stresses. The results are compared with a highly resolved Large Eddy Simulation of the same configuration. It is argued that filtering of the spurious noise can be critical to obtain a correct evaluation of the pressure field. Applications to biomedical flows are also discussed. Results are presented for in vivo cardiac flow measurements as well as in vitro velocimetry in a model of human airways. [Preview Abstract] |
Sunday, November 24, 2013 8:52AM - 9:05AM |
A14.00005: A Comparison of 3D3C Velocity Measurement Techniques Roderick La Foy, Pavlos Vlachos The velocity measurement fidelity of several 3D3C PIV measurement techniques including tomographic PIV, synthetic aperture PIV, plenoptic PIV, defocusing PIV, and 3D PTV are compared in simulations. A physically realistic ray-tracing algorithm is used to generate synthetic images of a standard calibration grid and of illuminated particle fields advected by homogeneous isotropic turbulence. The simulated images for the tomographic, synthetic aperture, and plenoptic PIV cases are then used to create three-dimensional reconstructions upon which cross-correlations are performed to yield the measured velocity field. Particle tracking algorithms are applied to the images for the defocusing PIV and 3D PTV to directly yield the three-dimensional velocity field. In all cases the measured velocity fields are compared to one-another and to the true velocity field using several metrics. [Preview Abstract] |
Sunday, November 24, 2013 9:05AM - 9:18AM |
A14.00006: Quantitative PIV measurement in narrow channels Dana Ehyaei, Ken Kiger This work focuses on making quantitative velocity measurements with a large depth-of-focus within a thin-gap channel, typical of Hele-Shaw cells. The inherent difficulty in such flows is due to the large velocity gradient across the gap and effects due to particle migration. In the simplest case of no particle migration, the PIV correlation peak is broadened due to the parabolic velocity profile, with an expected peak value at the maximum centerline velocity. However, there is an inevitable under-estimation that is typically up to 33{\%} of the centerline velocity for all but the smallest particle images and largest displacements, due to particle image size effects. In addition, inertial particle migration within the channel results in a second correlation peak as the particles rapidly move away from the wall. In later times, as the particles reach their equilibrium position, the particles sample only a single velocity value, and present conditions similar to traditional PIV interrogation. A practical procedure is proposed to make PIV quantitative by manipulating the particles to their equilibrium position prior to performing measurements and a reliable PIV measurement under appropriate working conditions is discussed for diffusive Rayleigh-B\'{e}nard convection in a Hele Shaw cell. [Preview Abstract] |
Sunday, November 24, 2013 9:18AM - 9:31AM |
A14.00007: Quantifying large-scale flow structures in the wake of a 2.5 MW wind turbine using natural snowfall Jiarong Hong, Mostafa Toloui, Sean Riley, Michele Guala, Kevin Howard, Leonardo Chamorro, James Tucker, Fotis Sotiropoulos The atmospheric inflow conditions around utility-scale turbines and multi-turbine arrayed wind farms remain poorly known, despite ongoing research, resulting in considerable wind plant power loss and increased annual operating costs. Gaining detailed full-scale flow information is constrained by low resolution spatial characterization of the flow field around turbines due to a lack of utility-scale research facilities and a number of technical challenges associated with obtaining measurements. Taking advantage of natural snowfall, we now achieve velocity field measurements in the wake of a 2.5 MW wind turbine at a scale of 36x36 m$^{\mathrm{2}}$. The spatial and temporal resolutions of the measurements are sufficiently high to quantify the evolution of blade-generated coherent motions, such as the tip and trailing sheet vortices, identify their instability mechanisms, and correlate them with turbine operations, control, and performance. This technique has been further validated by comparing the obtained mean velocity and Reynolds stress profiles, up to 60 m above the ground with sonic anemometer measurements at specific elevations, where less than a 3{\%} and 10{\%} difference were observed, respectively. [Preview Abstract] |
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