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 L14: Experiments: PIV II |
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Chair: David Frakes, Arizona State University Room: 27B |
Monday, November 19, 2012 3:35PM - 3:48PM |
L14.00001: PIV Measurement of velocity and acceleration fields using multi-pulse technology Liuyang Ding, Ronald Adrian, Sivaram Gogineni PIV is extended to the measurement of simultaneous acceleration and velocity fields by the addition of a small number of light pulses. We begin by considering triple-pulse and quadruple-pulse systems, and discuss the optimization of the pulse locations for various measurements. Third-order correlation techniques are applied to the triple-pulse images, yielding significantly improved correlation detectability. Results from experiments in steady flows of know acceleration are used to assess the accuracy and performance. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L14.00002: A Further PIV Validation: The Topological Rule John Foss, Douglas Neal Once fully processed, a PIV image provides the velocity vectors (at discrete locations) that are projected onto the plane of the laser light sheet. These discrete vectors are to represent the continuous vector field whose validity can be negated if the vector field does not satisfy the Topological Rule (Foss 2004 and 2007). All, or a portion, of the image can be treated as a collapsed sphere with holes (if appropriate) at the perimeter and handles (if appropriate) in the interior. The Euler characteristic (X) of the selected surface is X = 2 --Sum(holes) -- 2Sum(handles) (Eq.1) and X = 2 Sum(nodes) + Sum(1/2 nodes) -- 2 Sum(saddles) -- Sum(1/2 saddles) (Eq.2) where the ``full'' singular points are in the interior and the ``1/2'' singular points are at the perimeter. X from (Eq.1) is known ``by construction''. X from (Eq.2) is known from an analysis of the resolved vector field of the PIV image. (Note that a given image can be resolved into different regions for different X(1) values.) The Rule is satisfied if X(1) = X(2). Examples of how these concepts have been utilized will be presented. [J. Foss, \textit{Springer Handbook of Experimental Fluid Mechanics}, Chapter C.13, Springer-Verlag, Berlin, 2007.] [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L14.00003: Multi-planar velocimetry for 3D reconstruction of the flow Ahmad Falahatpisheh, Gianni Pedrizzetti, Arash Kheradvar Several extensions of PIV have been proposed for measurements of 3D fields which are restricted for full-volume quantification. We have introduced a fundamentally different solution for experimentally characterizing the incompressible and time-periodic flows in 3D, such as those found in the cardiovascular system. 2D velocity data, acquired by 2C-PIV in multiple planes, is reconstructed to a 3D velocity field taking advantage of the incompressibility of the flow. Using 2D samples instead of scanning the entire 3D domain leads to higher temporal/spatial resolutions since each slice is acquired in a 2D fashion. Hence, there is the possibility of extension to other (medical) imaging modalities that cannot employ advanced 3D optical techniques. 2C-velocimetry on two perpendicular stacks is used for 3D interpolation. The interpolated velocity field is then corrected to satisfy the incompressibility constraint by adding an irrotational velocity field that projects the velocity into a divergence-free vector field space. The method has been validated by exemplary flows having both compact and non-compact structures and different levels of noise. The results show improvements in the reliability of the reconstructed vector field. Application to cardiac flow is also verified. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L14.00004: Tomographic PIV using pulsed, high power LED illumination Christian Willert, Nicolas Buchmann, Julio Soria High-power light emitting diode (LED) illumination is investigated as an alternative to traditional laser-based Nd:YAG illumination for Tomographic Particle Image Velocimetry (TPIV). Light pulses of significant intensity (1-10 mJ) are obtained by briefly operating the LED at high drive currents and short pulse durations ($I_f = 24$A, $\tau_p = 150 \mu$s). Two LEDs of different wavelengths (green, 525nm; red, 623nm) are investigated with both LEDs providing sufficient pulse energy and image quality to perform reliable TPIV measurements. Volumetric illumination is achieved by direct projection of the LED into the flow, which yields a measurement volume of approximately 8 x 8 x 14 mm$^{3}$. The feasibility of this illumination approach is assessed by performing TPIV measurements in homogenous, grid-generated turbulence. in comparison to Nd:YAG laser illumination the absence of laser speckle and excellent pulse-to-pulse stability of the LEDs yield particle image data of high quality with a 3-D displacement measurement uncertainty on the order of 0.2-0.3 pixel. Using an array of LEDs the illuminated volume can be further increased. For instance six high power LEDs are sufficient to illuminate a volume of about 50 x 50 x 10 mm$^{3}$. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L14.00005: Hands-On Particle Image Velocimetry Experience for Bioengineering Students Using the Interactive Flowcoach System to Understand Aneurysm Hemodynamics Breigh N. Roszelle, Murat Okcay, B. Uygar Oztekin, David H. Frakes The Flowcoach system is a flow visualization and analysis platform from Interactive Flow Studies that uses particle image velocimetry (PIV) and computational fluid dynamics to provide interactive fluid dynamics education. In the spring of 2012, Flowcoach was used at Arizona State University to help teach bioengineering students about biofluid mechanics. A custom insert was made for Flowcoach to model an anatomical aneurysm that could be treated with a high-porosity flow diverting stent. Students performed PIV on the treated aneurysm model in small lab groups using Flowcoach and then wrote reports comparing their results to those from an untreated aneurysm model. The students were surveyed before and after the project and asked to rate their understanding of general biofluid mechanics, as well as experimental fluid mechanics and aneurysmal hemodynamics. Of the 76 students surveyed, 86\% indicated an increase in their understanding of biofluid mechanics, and 90\% indicated an increase in their understanding of both PIV and cerebral aneurysm hemodynamics. Students' written feedback showed that they felt Flowcoach and the interactive learning experience it provided were both interesting and beneficial to their future careers as engineers. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L14.00006: Development of refractively matched hydrogels for PIV applications Margaret Byron, Evan Variano We present a technique for fabricating models whose refractive indices are close to that of water, using two hydrogel polymers. The models' transparency and matched refractive index makes them useful for experiments in Refractive-Index-Matched Particle Image Velocimetry (RIM-PIV). The materials used --polyacrylamide and agarose hydrogel-- are inexpensive and can be cast into a variety of desired shapes using injection molding. The models' utility is demonstrated with sets of vector fields, calculated with standard PIV algorithms; vectors can be obtained from the surrounding flowfield and from interior points within the model. Using these data, we calculate solid-body rotation and translation in combination with fluid-phase velocities, and investigate coupling between the two. [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L14.00007: Devolpment and Evaluation of an Echo Particle Image Velocimetry (EPIV) System Nicholas DeMarchi, Christopher White An echo particle image velocimetry (EPIV) system capable of acquiring planar fields of velocity in optically opaque fluids or through optically opaque geometries is described, and validation measurements in Hagen-Poiseiulle (pipe) flow are reported. The accuracy limitation and measurement error of the EPIV measurements are assessed by comparing them to theoretically expected flow fields and optical PIV measurements acquired in the same facility. Lastly, the practical issues associated with building a EPIV system are described. [Preview Abstract] |
Monday, November 19, 2012 5:06PM - 5:19PM |
L14.00008: On the extraction of pressure fields from PIV velocity measurements in turbines Arturo Villegas, Fancisco J. Diez In this study, the pressure field for a water turbine is derived from particle image velocimetry (PIV) measurements. Measurements are performed in a recirculating water channel facility. The PIV measurements include calculating the tangential and axial forces applied to the turbine by solving the integral momentum equation around the airfoil. The results are compared with the forces obtained from the Blade Element Momentum theory (BEMT). Forces are calculated by using three different methods. In the first method, the pressure fields are obtained from PIV velocity fields by solving the Poisson equation. The boundary conditions are obtained from the Navier-Stokes momentum equations. In the second method, the pressure at the boundaries is determined by spatial integration of the pressure gradients along the boundaries. In the third method, applicable only to incompressible, inviscid, irrotational, and steady flow, the pressure is calculated using the Bernoulli equation. This approximated pressure is known to be accurate far from the airfoil and outside of the wake for steady flows. Additionally, the pressure is used to solve for the force from the integral momentum equation on the blade. From the three methods proposed to solve for pressure and forces from PIV measurements, the first one, which is solved by using the Poisson equation, provides the best match to the BEM theory calculations. [Preview Abstract] |
Monday, November 19, 2012 5:19PM - 5:32PM |
L14.00009: Coherent structure evolution in a turbulent round-jet using scanning tomographic particle image velocimetry Tiernan Casey, Jun Sakakibara, Sigurdur Thoroddsen In order to overcome the inherent spatial resolution limitations and increased noise associated with tomographic PIV when applied to large depth domains, we present a high-speed light-volume scanning technique, using up to 9 adjacent volume slices. This reduces the number of ghost particles while allowing for a large number of total depthwise reconstructed planes, up to 1500. The approach is demonstrated for a turbulent round-jet with Re = 2,500-10,000, using 4 high-speed video cameras to acquire images at up to 1,279 fps, giving over 1 million time-resolved velocity vectors with up to 520 time-steps in sequence. The evolution of tube-like coherent vortical structures are identified and tracked in time across the entire width of the jet - the dynamics of which are compared to existing experimental data using pointwise analysis of velocity gradients. [Preview Abstract] |
Monday, November 19, 2012 5:32PM - 5:45PM |
L14.00010: Tomographic PIV measurement in complex geometries of nasal cavity Sunghyuk Im, Hyung Jin Sung, Sung Kyun Kim Flow inside a scaled model of nasal cavity was measured by tomographic PIV. The model was constructed with transparent silicon and a refractive index of working fluid was matched to the model index by mixing glycerol and water. Four cameras and double pulse laser system were used for tomographic PIV. To obtain a high SNR, red fluorescence particles and longpass glass filters were used. Three-dimensional (3D) surface geometry of nasal cavity model from the stereolithography file (.stl) was converted to volume data by adopting the morphological closing and flood-filling algorithm. Coordinates and scaling of the model were adjusted by comparing time series stacking of 3D particle position from volume self-calibration. The geometry information was used to distinguish the fluid and solid region in the tomographic reconstruction procedure. Flow velocity field was calculated from 3D cross-correlation of reconstructed voxel intensities. [Preview Abstract] |
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