Bulletin of the American Physical Society
60th Annual Meeting of the Divison of Fluid Dynamics
Volume 52, Number 12
Sunday–Tuesday, November 18–20, 2007; Salt Lake City, Utah
Session GI: Experimental Techniques III |
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Chair: Scott Morris, University of Notre Dame Room: Salt Palace Convention Center 250 C |
Monday, November 19, 2007 10:30AM - 10:43AM |
GI.00001: Design of a 3D Digital Liquid Crystal Particle Thermometry and Velocimetry (3DDLCPT/V) System Rob Grothe, Greg Rixon, Dana Dabiri A novel 3D Digital Liquid Crystal Particle Thermometry and Velocimetry (3DDLCPT/V) system has been designed and fabricated. By combining 3D Defocusing Particle Image Velocimetry (3DDPIV) and Digital Particle Image Thermometry (DPIT) into one system, this technique provides simultaneous temperature and velocity data using temperature-sensitive liquid crystal particles (LCP) as flow sensors. A custom water-filled prism corrects for astigmatism caused by off-axis imaging. New optics equations are derived to account for multi-surface refractions. This redesign also maximizes the use of the CCD area to more efficiently image the volume of interest. Six CCD cameras comprise the imaging system, with three allocated for velocity measurements and three for temperature measurements. The cameras are optically aligned to sub-pixel accuracy using a precision grid and high-resolution translation stages. Two high-intensity custom-designed xenon flashlamps provide illumination. Temperature calibration of the LCP is then performed. These results and proof-of-concept experiments will be discussed in detail. [Preview Abstract] |
Monday, November 19, 2007 10:43AM - 10:56AM |
GI.00002: Simultaneous measurement of velocity and temperature fields in micro-scale flow and its application to electrokinetic flow Beomjoon Lee, Songwan Jin, Young Won Kim, Jung Yul Yoo In this research, a technique of simultaneously measuring the velocity and the temperature in micro-scale flow is proposed. This method uses particle tracking velocimetry (PTV) for measuring the velocity and laser induced fluorescence (LIF) for measuring the temperature. To measure the accurate velocity and temperature, images for PTV and for LIF are separated by using two light sources and a shutter which is synchronized with a camera. By using only one camera, measurement system can be simplified and the error from complicate optical system can be minimized. Error analyses regarding the concentrations of fluorescent dye and particle and the light source fluctuation are also conducted. It is found that the error of the temperature and the velocity highly depends on the concentration of fluorescent particles which are used for PTV. This technique is applied to the simultaneous measurement of the velocity and the temperature in the electrokinetic flow. It is found that the velocity and temperature vary with the electric field strength and the concentration of electrolyte. Also, it is found that the effect of Joule heating increases the electrokinetic velocity. [Preview Abstract] |
Monday, November 19, 2007 10:56AM - 11:09AM |
GI.00003: Nano-scale thermal anemometry probe Marcus Hultmark, Jeff Hill, Sean Bailey, Gary Kunkel, Craig Arnold, Alexander Smits A nano-scale thermal anemometry probe is being developed with high spatial and temporal resolution to measure small-scale turbulence in high Reynolds number flows. Manufactured using a combination of semiconductor and micro-electromechanical manufacturing processes, current models of the probe consist of a platinum sensing wire of length of 60$\times $1$\times $0.1 $\mu $m suspended between two contact pads. Future versions of the probe will have their sensing length reduced, through the use of focused ion beam milling, to 20$\times $0.1$\times $0.1 $\mu $m. Preliminary comparison has been made in a low turbulence free-stream between a conventional hot-wire probe and a nano-scale probe before reshaping of the supporting silicon substrate. Despite $\sim $100 $\mu $m of silicon protruding alongside of the free-standing wire, results showed that the two probes had similar static response with qualitatively higher frequency response observed for the nano-scale probe. [Preview Abstract] |
Monday, November 19, 2007 11:09AM - 11:22AM |
GI.00004: Multilayer Nano-Particle Image Velocimetry in Microscale Poiseuille Flows Haifeng Li, Minami Yoda In multilayer nano-particle image velocimetry (mnPIV), fluorescent colloidal tracers are illuminated by evanescent waves with an intensity that decays exponentially along $z$, or the direction normal to the wall. Multilayer nPIV exploits the non-uniform nature of this illumination, binning the tracers in ``standard'' evanescent wave PIV images into a few sub-images at different $z$ based upon tracer image intensities. These sub-images are then processed to extract the velocity components parallel to the wall at distinct $z$-locations within about 400 nm of the wall. Although the feasibility of this technique has already been demonstrated using synthetic images of plane Couette flow [Li \textit{et al.} (2006) \textit{Exp Fluids}, \textbf{41}, 185], we present here results from experimental images. Velocity profiles obtained from three sub-images in Poiseuille flow through rectangular 40 $\mu $m $\times $ 300 $\mu $m microchannels will be presented for pressure gradients up to about 1 Bar/m. The two mnPIV points farthest from the wall are used to estimate velocity gradients (and hence wall shear stresses). The accuracy of the mnPIV velocity gradient results is discussed. [Preview Abstract] |
Monday, November 19, 2007 11:22AM - 11:35AM |
GI.00005: A Cross-Correlation Based Accuracy Assessment of Tomographic PIV Nicholas Worth, Timothy Nickels Tomographic Particle Image Velocimetry (Tomo-PIV) is a promising new PIV technique capable of producing high-frequency time-resolved full 3D velocity fields. Advantages include: higher possible resolution than Particle Tracking Velocimetry (PTV), a simpler fully digital set-up in comparison with standard Holographic-PIV, and instantaneous flow field capture as opposed to quasi-instantaneous in Scanning-PIV. However, based on previous investigations (Elsinga \textit{et al}. 2005) measurement resolution appears to be somewhat limited, and although higher than PTV is still lower than other 3D techniques. However, this conclusion appears to be largely based on a correlation coefficient based accuracy assessment of artificial and reconstructed objects. Although useful, this measure may not adequately represent the accuracy of the PIV system, which is based on the cross-correlation of reconstructed objects. Therefore, in order to determine setup parameter effects more realistically, the current computational study details a cross-correlation based accuracy analysis of a simple vortical flow-field. A more direct understanding of how setup choices affect PIV results will allow the accuracy and resolution of results to be maximized, through simple parameter selection. [Preview Abstract] |
Monday, November 19, 2007 11:35AM - 11:48AM |
GI.00006: A Color-Coded Single Camera Three-Dimensional Defocusing Particle Image Velocimetry System Wei-Hsin Tien, Dana Dabiri A color-coded 3-D Defocusing Particle Image Velocimetry (3DDPIV) is a new modification of the 3-D measurement system originally developed by Willert {\&} Gharib (1992). It uses a single lens with 3 color-coded pinholes to overcome limitations of image saturation due to multiple exposures of each particle, and a 3-CCD color camera for image acquisition. The spectrum difference between the color filters and the CCD sensors is solved by a color space linear transformation, separating each pinhole's exposure. The requirement for a high intensity light source prevalent in conventional lighting setups is solved by backlighting the field-of-view and seeding the flow with black particles. An effective pinhole separation, $d'$, is proposed for use with multi-element lenses, and a multi-surface refraction correction to $d'$ is also proposed. Calibration results of the system with and without fluid are performed and compared. The technique is successfully applied to a buoyancy-driven flow, and a three-dimensional velocity field is extracted. The image volume is 3.25mm$\times $2.45mm$\times $1.5mm. [Preview Abstract] |
Monday, November 19, 2007 11:48AM - 12:01PM |
GI.00007: Bootstrapping Dip Test for PIV Outlier Identification and Correction Andree Susanto, Chan-Seng Pun, Dana Dabiri A PIV outlier detection and correction method is proposed that does not directly rely on local statistics. A bootstrapping method uses interpolation to generate a distribution of points for each vector component. Statistics are obtained by applying the Hartigan's dip test for bimodality on the distribution points to estimate the mode for each vector component. Significant difference between the estimated modes and outliers is observed; non-spurious vector components are close to the estimated modes. The bootstrapping dip test outlier detection scheme is then repeated until no more vector components are rejected. Two approaches to replace the detected vector components are considered. First, the corresponding mode values are replaced with the detected components, second the remaining vector components are used to re-interpolate the field. Validation includes parametric studies based on the number of undetections and overdetections on simulated fields to determine the optimal sets of parameters. Applications of the optimum parameters to a turbulent jet flow and a synthetic Rankine Vortex flow, both obtained from the PIV Challenge website are shown and details of this methodology is discussed. [Preview Abstract] |
Monday, November 19, 2007 12:01PM - 12:14PM |
GI.00008: Distributed Processing of PIV images with a low power cluster supercomputer Barton Smith, Kyle Horne, Thomas Hauser Recent advances in digital photography and solid-state lasers make it possible to acquire images at up to 3000 frames per second. However, as the ability to acquire large samples very quickly has been realized, processing speed has not kept pace. A 2-D Particle Image Velocimetry (PIV) acquisition computer would require over five hours to process the data that can be acquired in one second with a Time-resolved Stereo PIV (TRSPIV) system. To decrease the computational time, parallel processing using a Beowulf cluster has been applied. At USU we have developed a low-power Beowulf cluster integrated with the data acquisition system of a TRSPIV system. This approach of integrating the PIV system and the Beowulf cluster eliminates the communication time, thus speeding up the process. In addition to improving the practicality of TRSPIV, this system will also be useful to researchers performing any PIV measurement where a large number of samples are required. Our presentation will describe the hardware and software implementation of our approach. [Preview Abstract] |
Monday, November 19, 2007 12:14PM - 12:27PM |
GI.00009: PIV Driven Computational Flow Simulation John Charonko, Pavlos Vlachos PIV is a well-accepted non-invasive technique capable of time-resolved velocity measurements. However, in order to obtain good temporal or spatial resolution in a particular region, more global measurement of the flow must often be sacrificed. A new method has been developed to integrate planar PIV measurements of incompressible flows into a 2D CFD solver so that the computational results remain synchronized with and guided by the experimental data. This allows extrapolation of the flow outside the measured area. Simultaneously, this method utilizes established methods for deriving pressure field data from velocity measurements in a new way to obtain time-resolved pressures without the need for a known reference within the experimental domain. The procedure has been tested with various analytical and experimental flow fields and has shown good agreement with expected results. It is believed that this method may prove useful in reconstructing the velocity distribution in regions of the flow obstructed from view. Finally, it suggests the possibility of designing an experiment so that regions of the flow which are challenging to simulate numerically (such as separation and transition) can be measured experimentally, while the remainder can be simulated using CFD techniques. [Preview Abstract] |
Monday, November 19, 2007 12:27PM - 12:40PM |
GI.00010: On the accuracy of nPIV Velocity Measurements in Poiseuille Flow inside Micro Channels: Effect of non uniform out-of-plane illumination Reza Sadr Nano-particle image velocimetry (nPIV) uses evanescent-wave illumination to measure two tangential velocity components $U$ and $V$ averaged in a region next to the wall with submicron thickness. The illumination intensity decays exponentially with $z$, or normal to the wall, in this region and hence tracers at smaller $z$ have images that are ``brighter'' and ``bigger'' than those at larger $z$. In a flow field where the velocity profile varies with $z$ the tracers at different distance from the wall move at different speeds. The variation in the displacement of particle images in this region with different brightness and sizes can bias the near-wall velocities measured by nPIV. Artificial PIV images of plane Couette flow were used in this work to investigate the impact of these issues upon the accuracy of nPIV data. Results were obtained for various experimental parameters incorporating different illumination profiles and hindered Brownian diffusion. The results demonstrate that non uniform illumination can lead to a bias in the estimated tracer velocities if it is not taken into considerations appropriately. The uncertainties associated with the estimated velocity is then identified and discussed. The artificial images are then compared with the experimental images. [Preview Abstract] |
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