Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session D31: Experimental Techniques - 3D Particle Velocimetry |
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Chair: Alex Techet, MIT Room: F152 |
Sunday, November 20, 2016 2:57PM - 3:10PM |
D31.00001: Robust 4 Camera 3D Synthetic Aperture PIV Abhishek Bajpayee, Alexandra Techet We present novel processing techniques which allow for robust 4 camera 3D synthetic aperture (SA) PIV. These pre and post processing techniques, applied to raw images and reconstructed volumes, significantly improve SA reconstruction SNR values and consequently allow for accurate SAPIV velocity fields. SA, or light field, PIV has typically required 8 or 9 cameras in order to achieve high reconstruction quality and velocity field reconstruction quality values, $Q$ and $Q_v$ respectively. This is primarily because the effective signal to noise ratio (SNR) of refocused images, when using traditional multiplicative or additive refocusing techniques, increases with the number of cameras being used. However, tomographic reconstruction (used with TomoPIV), is able to achieve relatively high SNR reconstructions using 4 or 5 cameras owing to its iterative but significantly more computationally expensive algorithm. Our processing techniques facilitate better recovery of relevant information in SA reconstructions using only 4 views. As a result, we no longer have to trade setup cost and complexity (number of cameras) for computational speed of the reconstruction algorithm. [Preview Abstract] |
Sunday, November 20, 2016 3:10PM - 3:23PM |
D31.00002: Tomographic Aperture-Encoded Particle Tracking Velocimetry: A New Approach to Volumetric PIV Dan Troolin, Aaron Boomsma, Wing Lai, Stamatios Pothos Volumetric velocity fields are useful in a wide variety of fluid mechanics applications. Several types of three-dimensional imaging methods have been used in the past to varying degrees of success, for example, 3D PTV (Maas et al., 1993), DDPIV (Peireira et al., 2006), Tomographic PIV (Elsinga, 2006), and V3V (Troolin and Longmire, 2009), among others. Each of these techniques has shown advantages and disadvantages in different areas. With the advent of higher resolution and lower noise cameras with higher stability levels, new techniques are emerging that combine the advantages of the existing techniques. This talk describes a new technique called Tomographic Aperture-Encoded Particle Tracking Velocimetry (TAPTV), in which segmented triangulation and diameter tolerance are used to achieve three-dimensional particle tracking with extremely high particle densities (on the order of ppp $=$ 0.2 or higher) without the drawbacks normally associated with ghost particles (for example in TomoPIV). The results are highly spatially-resolved data with very fast processing times. A detailed explanation of the technique as well as plots, movies, and experimental considerations will be discussed. [Preview Abstract] |
Sunday, November 20, 2016 3:23PM - 3:36PM |
D31.00003: High fidelity digital inline holographic PTV for 3D flow measurements: from microfluidics to wall-bounded turbulence Jiarong Hong, Mostafa Toloui, Kevin Mallery Three-dimensional PIV and PTV provides the most comprehensive flow information for unraveling the physical phenomena in a wide range of fluid problems, from microfluidics to wall-bounded turbulent flows. Compared with other commercialized 3D PIV techniques, such as tomographic PIV and defocusing PIV, the digital inline holographic PTV (namely DIH-PTV) provides 3D flow measurement solution with high spatial resolution, low cost optical setup, and easy alignment and calibration. Despite these advantages, DIH-PTV suffers from major limitations including poor longitudinal resolution, human intervention (i.e. requirement for manually determined tuning parameters during tracer field reconstruction and extraction), limited tracer concentration, small sampling volume and expensive computations, limiting its broad use for 3D flow measurements. Here we will report our latest work on improving DIH-PTV method through an integration of deconvolution algorithm, iterative removal method and GPU computation to overcome some of abovementioned limitations. We will also present the application of our DIH-PTV for measurements in the following sample cases: (i) flows in bio-filmed microchannel with 50-60 $\mu $m vector spacing within sampling volumes of 1 mm (streamwise) x 1 mm (wall-normal) x 1 mm (spanwise); (ii) turbulent flows over smooth and rough surfaces (1.1 mm vector spacing within 15 mm x 50 mm x 15 mm); (iii) 3D distribution and kinematics of inertial particles in turbulent air duct flow. [Preview Abstract] |
Sunday, November 20, 2016 3:36PM - 3:49PM |
D31.00004: Smartphone based Tomographic PIV using colored shadows Andres A. Aguirre-Pablo, Meshal K. Alarfaj, Er Qiang Li, Sigurdur T. Thoroddsen We use low-cost smartphones and Tomo-PIV, to reconstruct the 3D-3C velocity field of a vortex ring. The experiment is carried out in an octagonal tank of water with a vortex ring generator consisting of a flexible membrane enclosed by a cylindrical chamber. This chamber is pre-seeded with black polyethylene microparticles. The membrane is driven by an adjustable impulsive air-pressure to produce the vortex ring. Four synchronized smartphone cameras, of 40 Mpx each, are used to capture the location of particles from different viewing angles. We use red, green and blue LED's as backlighting sources, to capture particle locations at different times. The exposure time on the smartphone cameras are set to 2 seconds, while exposing each LED color for about 80 $\mu $s with different time steps that can go below 300 $\mu $s. The timing of these light pulses is controlled with a digital delay generator. The backlight is blocked by the instantaneous location of the particles in motion, leaving a shadow of the corresponding color for each time step. The image then is preprocessed to separate the 3 different color fields, before using the MART reconstruction and cross-correlation of the time steps to obtain the 3D-3C velocity field. This proof of concept experiment represents a possible low-cost Tomo-PIV setup. [Preview Abstract] |
Sunday, November 20, 2016 3:49PM - 4:02PM |
D31.00005: 3D-PTV around Operational Wind Turbines Ian Brownstein, John Dabiri Laboratory studies and numerical simulations of wind turbines are typically constrained in how they can inform operational turbine behavior. Laboratory experiments are usually unable to match both pertinent parameters of full-scale wind turbines, the Reynolds number (Re) and tip speed ratio, using scaled-down models. Additionally, numerical simulations of the flow around wind turbines are constrained by the large domain size and high Re that need to be simulated. When these simulations are preformed, turbine geometry is typically simplified resulting in flow structures near the rotor not being well resolved. In order to bypass these limitations, a quantitative flow visualization method was developed to take in situ measurements of the flow around wind turbines at the Field Laboratory for Optimized Wind Energy (FLOWE) in Lancaster, CA. The apparatus constructed was able to seed an approximately 9m x 9m x 5m volume in the wake of the turbine using artificial snow. Quantitative measurements were obtained by tracking the evolution of the artificial snow using a four camera setup. The methodology for calibrating and collecting data, as well as preliminary results detailing the flow around a 2kW vertical-axis wind turbine (VAWT), will be presented. [Preview Abstract] |
Sunday, November 20, 2016 4:02PM - 4:15PM |
D31.00006: Uncertainty quantification in volumetric Particle Image Velocimetry. Sayantan Bhattacharya, John Charonko, Pavlos Vlachos Particle Image Velocimetry (PIV) uncertainty quantification is challenging due to coupled sources of elemental uncertainty and complex data reduction procedures in the measurement chain. Recent developments in this field have led to uncertainty estimation methods for planar PIV. However, no framework exists for three-dimensional volumetric PIV. In volumetric PIV the measurement uncertainty is a function of reconstructed three-dimensional particle location that in turn is very sensitive to the accuracy of the calibration mapping function. Furthermore, the iterative correction to the camera mapping function using triangulated particle locations in space (volumetric self-calibration) has its own associated uncertainty due to image noise and ghost particle reconstructions. Here we first quantify the uncertainty in the triangulated particle position which is a function of particle detection and mapping function uncertainty. The location uncertainty is then combined with the three-dimensional cross-correlation uncertainty that is estimated as an extension of the 2D PIV uncertainty framework. Finally the overall measurement uncertainty is quantified using an uncertainty propagation equation. The framework is tested with both simulated and experimental cases. For the simulated cases the variation of estimated uncertainty with the elemental volumetric PIV error sources are also evaluated. The results show reasonable prediction of standard uncertainty with good coverage. [Preview Abstract] |
Sunday, November 20, 2016 4:15PM - 4:28PM |
D31.00007: Studying Vortex Dynamics of Rotating Convection with High-resolution PIV Measurement Hao Fu, Shiwei Sun, Yu Wang, Bowen Zhou, Yuan Wang A novel experimental setup for studying vortex dynamics in rotating Rayleigh-Benard convection has been made in School of Atmospheric Sciences, Nanjing University. With water as the working fluid, three lasers with different frequencies and the corresponding three CCDs have been placed to complete 2D2C (two dimensions, two components) PIV measurement. The lasers are fixed on two crossing guiding ways and can move up and down to scan the flow field. An algorithm has been made to reconstruct 3D velocity field based on multiple 2D2C PIV data. This time, we are going to present the details of this new machine and algorithm, as well as some scientific understanding of vortex dynamics owing to this high-resolution velocity measurement system. [Preview Abstract] |
Sunday, November 20, 2016 4:28PM - 4:41PM |
D31.00008: A Laser Sheet Self-Calibration Method for Scanning PIV Anna N. Knutsen, James R. Dawson, John M. Lawson, Nicholas A. Worth A laser sheet self-calibration method for scanning PIV has been developed to replace the current laser sheet calibration, which is complex, time consuming and very sensitive to misalignment of the optics or cameras during experiments. The new calibration method is simpler, faster and crucially more robust. The concept behind the method is to traverse a laser sheet through the measurement volume, take a series of images from two different views, and calculate the global 3D particle locations. This information is used to find the real space coordinates of the measurement volume and the orientation and width of the laser sheets. The spatial location of the particles is found by object matching and triangulation. The light intensity in the laser sheet has an approximately Gaussian shape, and the illumination of one particle which will be illuminated multiple times during the scan will thus vary as the sheet is scanned across the measurement volume. The thickness of the laser sheet is calculated by identifying the variation of illumination of the particles during a scan and fitting this to a Gaussian shaped curve, while the orientation is found using a least square fit. The accuracy of the new method will be presented with respect to both synthetic and experimental data. [Preview Abstract] |
Sunday, November 20, 2016 4:41PM - 4:54PM |
D31.00009: Tracer Particle Response in High-Gradient Flow Joshua Herzog, David Rothamer Many laser-based fluid velocity measurements depend on the motion of tracer particles seeded into the flow. In most cases, the tracers are assumed to follow the flow exactly. However, this is not always the case. The actual motion of a tracer particle is dependent on the properties of both the particle and the fluid surrounding it. Previous analysis for spherical particles in the Stokes regime (assumes Re $\ll 1$) shows that the absolute difference between the particle and fluid velocity exponentially decays in time, with the relaxation time constant dependent on particle diameter, free stream velocity, Reynolds number, and both particle and fluid mass density. For all cases, it is necessary to accurately describe the physics of the tracer particle motion to perform rigorous quantitative studies with particle-based techniques. This study aims to measure and describe particle response to a step change in velocity in a uniform flow. Velocity profiles of solid tracer particles ranging from 300 to 3800 nm in diameter, with initial particle Reynolds numbers up to 100, were measured in a shock tube using particle image velocimetry. The goal of this study is to assess velocity relaxation estimates and assumptions for particle-based velocimetry techniques. [Preview Abstract] |
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