Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session AT: Experimental Techniques I |
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Chair: Michael Hargather, The Pennsylvania State University Room: 204A |
Sunday, November 23, 2008 8:00AM - 8:13AM |
AT.00001: Nano-PIV for flows near nano-structured surfaces Gea Parikesit, Ralph Lindken, Jerry Westerweel Previous studies have shown that nano-structured surfaces can exhibit different wetting characteristics and higher slip-length values compared to smooth (i.e. non-structured) surfaces. In order to quantitatively measure the flows near such nano-structured surfaces, a Nano-PIV method with high spatial and temporal resolution is required. The TIRF-based PIV is a good candidate because it has been successfully applied for 3D nano-velocimetry near smooth surfaces, but it cannot be applied in a simple and direct manner since the nano-structures optically complicates the measurements: (i) they spatially influence and modulate the TIRF illumination, and (ii) they increase the probability of obtaining errors caused by the tracers' own emitted evanescent-waves. For fabricated periodic nano-structures with known dimensions and geometry, however, the spatially modulated TIRF illumination can be very useful for (i) a simple estimation of the illumination depth directly inside the microfluidic channels, and (ii) detection and measurement of the thin layer of air bubbles trapped at the nano-structures in the `Cassie-Baxter' wetting mode. [Preview Abstract] |
Sunday, November 23, 2008 8:13AM - 8:26AM |
AT.00002: Improving the Accuracy of the Laser Doppler Accelerometer Technique Holger Nobach, Matthias Kinzel, Eberhard Bodenschatz The measurement of particle accelerations gives insights into the fundamental properties of fluid flows. The basic principles of the Laser Doppler Accelerometer (LDA) technique follows closely those introduced in Lehmann et al. (2002). Recently it was successfully applied to a commercial of-the-shelf laser Doppler system by Kinzel et al. (2006). Since then we implemented numerous improvements in the signal processing and increased the reliability. In order to reach acceptable resolution of the measurement system, both, the optical setup and the signal processing system must realize the highest possible accuracies. The main contribution of this study is the assessment of the accuracy of the method, and the quantification of the errors due to optical fringe divergence in the detection volume and due to signal processing using a falling wire as a reference. --- Lehmann B, Nobach H, Tropea C (2002): Measurement of acceleration using the laser Doppler technique. Meas. Sci. Technol. 13(9):1367-1381 --- Kinzel M, Nobach H, Tropea C., Bodenschatz E (2006): Measurement of Lagrangian acceleration using the laser Doppler technique. Proc. 13th Int. Symp. on Appl. of Laser Techn. to Fluid Mech., Lisbon, Portugal [Preview Abstract] |
Sunday, November 23, 2008 8:26AM - 8:39AM |
AT.00003: Accuracy of Velocity Estimation Using Global Variational Methods Doug Bohl, Naratip Santitissadeekorn, Erik Bollt In this work a method of processing digital images, such as those from PIV, MTV, or LIF, for flow velocities using Global Variational Method (GVM) is investigated. This technique is based on principles of Frobenius-Perron (FP) operator theory in which image sequences can be related to the infinitesimal generator of the FP operator to motivate a flow-recovery constraint. A regularization method is then used to minimize this constraint along with an additional constraint required to stabilize a solution. Synthetic images, with typical MTV tagging patterns, and variable noise levels were first created and then displaced using analytically derived flow fields. Displacements were calculated from pairs of images and the error was determined by comparing the measured displacements to those of the analytical flow field. A direct correlation technique (DCT) was also used to process the synthetically derived images for comparison. Results show that the GVM error levels are nominally 5-10 times higher than for the DCT. While the error is higher for GVM compared to DCT the results show potential for using this technique to provide quantitative flow measurements in cases where DCT cannot be applied. [Preview Abstract] |
Sunday, November 23, 2008 8:39AM - 8:52AM |
AT.00004: The Development of a Feature Comparison Based Technique to Analyze PTV Results Joey Duncan, Dana Dabiri, Morteza Gharib, Jay Hove A new post processing technique based on feature comparison is developed for analysis of Particle Tracking Velocimetry (PTV) results. Similar to methods employed in Finite Element Modeling, fluid properties such as shear strain rate and rotation are calculated for the centers of triangular features whose vertices are particle locations. These features are created using Delaunay Tessellation. In addition, no interpolation step is required, meaning that derivative data can be obtained directly from nonuniform velocity data, increasing computational efficiency. Analysis of the validity of this novel technique is given, showing that a least squares fit is required to find two directional derivatives from three data points (vertices) in each feature. Comparisons to traditional differentiation methods for various examples of fluid flow are given. [Preview Abstract] |
Sunday, November 23, 2008 8:52AM - 9:05AM |
AT.00005: Nano-scale thermal anemometry probe Sean Bailey, Marcus Hultmark, Karl Meyer, Jeff Hill, 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, two sizes of probe have been manufactured. Each probe consists of a platinum sensing wire of length 60$\times $1$\times $0.1 $\mu $m or 20$\times $0.1$\times $0.1 $\mu $m suspended between two contact pads. Preliminary measurements have been made comparing the nano-scale probe to a conventional hot-wire probe in both a zero pressure gradient turbulent boundary layer and in turbulent pipe flow using constant current anemometry and constant temperature anemometry. Results indicated that the nano-scale probe exhibits typical hot-wire behavior, but with a frequency response of at least three-times that of a conventional probe. [Preview Abstract] |
Sunday, November 23, 2008 9:05AM - 9:18AM |
AT.00006: A Comparison of Condensation Fog and Wide-Field PIV Measurements in a Mach 5 Turbulent Boundary Layer Bulent Yuceil, Noel Clemens, David Dolling Condensation fog planar laser scattering has been used to visualize the structure of supersonic turbulent shear flows. Some previous work at Princeton in a supersonic boundary layer has suggested that the scattering from CO$_{2}$ clusters closely reflects the fluctuating density field. However, the relationship between the fog scattering and the velocity field has not been established. In the current study wide-field, side-view PIV measurements of a turbulent boundary layer that develops naturally on the floor of a Mach 5 wind tunnel is performed using three 1k$\times $1k resolution cameras. For the PIV, TiO$_{2}$ particles are seeded into the flow, but the particle images also reveal a fog of ice crystals that is formed due to isentropic cooling in the nozzle from the water vapor already present in the flow. This natural fog in the freestream evaporates and disappears in the boundary layer forming a clear demarcation of the boundary layer edge. The interface between the fog and the boundary layer seems to correlate with the instantaneous velocity contours. The measurements suggest that the structures observed with the fog technique are dynamically significant and not just artifacts of an advecting passive scalar. A similar comparison is made in the plan view to visualize the very large-scale coherent structures that have been observed in previous studies at lower Mach number. [Preview Abstract] |
Sunday, November 23, 2008 9:18AM - 9:31AM |
AT.00007: The microPIVOT: An Integrated Micron Resolution Particle Image Velocimeter and Optical Tweezers Instrument for Microscale Studies Derek Tretheway, Nathalie Neve, Jeremiah Zimmerman, Sean Kohles An instrument to manipulate and characterize the mechanical environment around microscale objects has been developed by integrating two laser-based techniques: micron-resolution particle image velocimetry (microPIV) and optical tweezers (OT) [Meas. Sci. Technol. 19 (2008) 095403]. The integrated device, the microPIVOT, was validated by comparing computational flow predictions to the measured velocity profile around a trapped particle in either a uniform flow or an imposed, gravity driven microchannel flow. Interaction between both techniques is shown to be negligible for 15 to 35 micron diameter trapped particles subjected to fluid velocities from 50 to 500 microns/s even at the highest laser power. The integrated techniques will provide insight into microscale phenomena including single-cell biomechanics, non-Newtonian fluid mechanics, and single particle or particle-particle hydrodynamics. [Preview Abstract] |
Sunday, November 23, 2008 9:31AM - 9:44AM |
AT.00008: Micro-PIT/V --- Simultaneous temperature and velocity fields in microfluidic devices Tait Pottebaum The use of encapsulated thermochromic liquid crystals (TLC) for the simultaneous measurement of temperature and velocity fields in microfluidic devices has been demonstrated. Implementation of TLC thermometry at the micro-scale is significantly different than at the macro-scale due to the constraints on imaging and illumination configurations and the proximity of the measurements to interfaces and surfaces from which light will scatter. Unlike in micro-PIV, wavelength filtering (such as with fluorescent particles) cannot be used to remove undesired reflections, because the temperature information is carried by the particle color. Therefore, circular polarization filtering is used, exploiting the circular dichroism of TLC. Micro-PIT/V will enable new investigations into the physics of microfluidic devices involving temperature gradients, such as thermocapillary actuated devices and many ``lab-on-a-chip'' applications involving temperature sensitive chemical and biological processes. In addition, the design of operational devices can be improved by applying micro-PIT/V to the characterization of prototypes. [Preview Abstract] |
Sunday, November 23, 2008 9:44AM - 9:57AM |
AT.00009: Influence of the accuracy in index of refraction matching on fluid flow measurements Vibhav Durgesh, Manoochehr Koochesfahani, Yiying Tong Fluid flow imaging through curved surfaces suffers from optical distortion caused by the mismatch in the refractive indices of the fluid and the solid surface, leading to errors in velocity measurements and the resulting estimates of wall shear stress. Examples include imaging of flow through porous media and in-vitro studies of biological flow problems. A common approach for minimizing optical distortions is to adjust the refractive index of the fluid to closely match that of the solid. In this study we investigate how the accuracy in the index of refractive matching influences the image distortion and, in particular, the measurement accuracy of wall shear stress, a critical property in biofluid dynamics. A ray-tracing approach is used to simulate the optical distortion and is validated against experiments in a simple geometry, i.e. imaging of a liquid flow inside a cylindrical tube. Ray-tracing is implemented through an in-house code for simple geometries and the results are compared with simulations using a more complete (free) software that is already available online and can deal with complex geometries. Results show that a slight mismatch in the indices of refraction, as small as one part in a thousand, can lead to a significant error in the estimate of wall shear stress. [Preview Abstract] |
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