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 D14: General Experiments II |
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Chair: Rinaldo Miorini, The Johns Hopkins University Room: 27B |
Sunday, November 18, 2012 2:15PM - 2:28PM |
D14.00001: Novel Method to Characterize Superhydrophobic Coatings Gary C. Tepper, Mohamed A. Samaha, Hooman Vahedi Tafreshi, Mohamed Gad-el-Hak Superhydrophobic surfaces possess strong water-repellent characteristic, which, among several other potential applications, enhances the mobility of water droplets over the coatings. The surface entraps air within its micropores. When a coated body is submerged and in relative motion with water, shear-free and no-slip regions alternate over, respectively, the air pockets and the solid surface. The coating maintains its hydrophobicity as long as the air remains entrapped. It is therefore of great interest to precisely measure the amount of trapped air, which is particularly difficult to estimate for coatings with disordered microstructures. A novel method to measure the gas volume fraction of superhydrophobic coatings with either ordered or random microroughness is advanced. The technique is applied to both aerogel and electrospun-fibrous coatings. The experiments utilize a very sensitive weighing scale (down to $10^{-4}$ gm) and height gauge (down to 10 micron) to determine the buoyancy force on an immersed, coated glass-slide substrate. The measured force is used to calculate the volume fraction of entrapped air. {\em Effective} coating's thickness also follows from the same calculations. [Preview Abstract] |
Sunday, November 18, 2012 2:28PM - 2:41PM |
D14.00002: Measuring Cavitation with Synchrotron X-Rays Daniel Duke, Alan Kastengren, Chris Powell Cavitation plays an important role in the formation of sprays from small nozzles such as those found in fuel injection systems. A sharp-edged inlet from the sac into the nozzle of a diesel fuel injector is shown to inititate a strong sheet-like cavitation along the boundary layer of the nozzle throat, which is difficult to measure and can lead to acoustic damage. To investigate this phenomenon, a diagnostic technique capable of mapping the density field of the nozzle through regions of intense cavitation is required. Available visible-light techniques are limited to qualitative observations of the outer extent of cavitation zones. However, brilliant X-rays from a synchrotron source have negligible refraction and are capable of penetrating the full extent of cavitation zones. We present the early results of a novel application of line-of-sight, time-resolved X-ray radiography on a cavitating model nozzle. Experiments were conducted at Sector 7-BM of the Advanced Photon Source. Density and vapor distribution are measured from the quantitative absorption of monochromatic X-rays. The density field can then be tomographically reconstructed from the projections. The density is then validated against a range of compressible and incompressible numerical simulations. [Preview Abstract] |
Sunday, November 18, 2012 2:41PM - 2:54PM |
D14.00003: ABSTRACT WITHDRAWN |
Sunday, November 18, 2012 2:54PM - 3:07PM |
D14.00004: 3D effects in axisymmetric reconstructions Hanna Makaruk, Christopher Tomkins In a variety of penetrating diagnostics in fluid mechanics and elsewhere, quantities of interest are reconstructed from a single-view measurement under an assumption of axisymmetry. Here we employ theory, simulation and experiment to explore the effects of 3-dimensionality on these reconstructions. A key finding is that 3D effects may cause local negative densities, which are clearly unphysical, to appear in Abel inverse-type reconstructions. Analytical solutions are derived for violations of the axisymmetric assumption in the form of simple geometric shapes, and numerical Abel inversions are also performed on similar geometric problems. Both theory and numerics predict significant regions of inferred negative density under these conditions. These predictions are tested against an experimental measurement of known, idealized objects (spheres) using quantitative penetrating radiography. Results are compared for various values of the characteristic parameter, $D/r$ (where $D$ is the distance from reconstruction axis, $r$ is the sphere radius). The results suggest that negative density values in Inverse-Abel reconstructions should not be ignored; instead, they provide potentially important insights into the 3D nature of the underlying phenomena. [Preview Abstract] |
Sunday, November 18, 2012 3:07PM - 3:20PM |
D14.00005: Performance of hot-wire probes designed to simultaneously measure three velocity components Rachel Ebner, Caleb Morrill-Winter, Rio Baidya, Petar Vukoslavcevic, Joseph Klewicki, James Wallace, Nicholas Hutchins Vukoslavcevic (\textit{Exp. in Fluids} \textbf{53}, 2012) recently used highly resolved channel flow DNS at low Reynolds number to investigate a number of hot-wire probe configurations for obtaining simultaneous measurements of all three velocity components. A focus of his effort was to minimize errors due to velocity gradients across the sensor array. A physical realization of Vukoslavcevic's XP (and XL) probe configuration was designed and fabricated. New fabrication techniques were implemented to minimize flow blockage and ensure uniform prong taper. In-situ pitch and yaw calibrations of the sensor are realized using a compact articulating jet that employs a novel flow-speed controller as the angle of the jet is varied. We present measurements derived from three facilities: the UNH 8m boundary layer wind tunnel, the University of Melbourne High Reynolds Number Boundary Layer Wind Tunnel (HRNBLWT), and the UNH Flow Physics Facility (FPF). The performance of the XL and XP probe configurations are assessed over a range of Reynolds numbers, as is the effect of pitching the probe at a fixed angle during calibration. [Preview Abstract] |
Sunday, November 18, 2012 3:20PM - 3:33PM |
D14.00006: Reconstructing dominant three-dimensional flow structures in the wakes of cylindrical bodies using planar velocity measurements Chris Morton, Serhiy Yarusevych The flow over cylindrical bodies typically involves the periodic formation of spanwise vortices. Numerous experimental techniques have been employed in the past several decades for investigating the physical characteristics of wake vortices, e.g., surface pressure measurement with microphone arrays, velocity measurement with Laser Doppler Velocimetry (LDV) and Particle Image Velocimetry (PIV). More recently, direct numerical simulations and the advancements in volumetric measurement techniques have enabled quantitative investigation of the three-dimensional wake development. However, in the domain of physical experiments, planar velocity measurement systems still remain to be the mainstream tool. Thus, dominant three-dimensional flow features are commonly reconstructed by using phase-averaged planar measurements conducted at several planes, which requires a periodic reference signal related to flow development. The present investigation utilizes a novel approach for phase averaging 2D PIV data by extracting the reference signal via Proper Orthogonal Decomposition (POD) of the PIV data. The method is applied to investigate three-dimensional development of coherent structures in the wake of complex cylindrical geometries. [Preview Abstract] |
Sunday, November 18, 2012 3:33PM - 3:46PM |
D14.00007: In measurements of spatial distribution of surface deformation in studies of flow induced vibration Cao Zhang, Rinaldo Miorini, Joseph Katz In studies involving flow-induced vibrations, it is necessary to measure the spatial distribution of surface motion simultaneously with the unsteady flow causing it. To achieve this goal, we developed a method for measuring the time resolved spatial distribution of surface deformation. Different approaches can be used for transparent and opaque surfaces. For a transparent wall, e.g. a compliant PDMS coating, high speed, digital Mach-Zehnder Interferometry maps the time-resolved surface shape. Calibrations demonstrate that a sub-micron resolution can be readily achieved, and that the sensitivity of the method can be adjusted to the expected range of surface deformations by varying the refractive index of the fluid. This technique is integrated with a time-resolved tomographic PIV system by bleeding/sampling 0.1{\%} of the thick laser sheet energy illuminating the wall, and does not require an additional light source. Combined, it allows simultaneous flow and deformation measurements. For opaque surface, time resolved electronic speckle pattern interferometry (ESPI) could be used to measure the surface deformation by interfering light reflected from the surface with an external reference beam. Calibrations demonstrate that this method also achieves sub micron resolution. [Preview Abstract] |
Sunday, November 18, 2012 3:46PM - 3:59PM |
D14.00008: Development of Luminescent Imaging for Capturing Cavitation in Water on Solid Surface Akihisa Aikawa, Jun Ando, Hirotaka Sakaue Two-color pressure-sensitive paint (PSP) is applied to a solid surface to capture the cavitation acting on the surface in water. It is found that the luminescent signal increases under a cavitation region. The luminescence change of a PSP can be related to the oxygen quenching. Based on these, we discuss the luminescence increase at the cavitation region related to the oxygen concentration in water and oxygen pressure of a cavitation bubble. To extract the cavitation from an acquired luminescent image, the motion-capturing PSP method is applied. It eliminates the variation in illumination caused by the bubble creations between the PSP-coated surface and the imaging-acquisition instruments. The time-resolved cavitation images on the PSP-coated surface are captured inside an ultrasonicate bath. [Preview Abstract] |
Sunday, November 18, 2012 3:59PM - 4:12PM |
D14.00009: Global Temperature Measurement of Supercooled Water under Icing Conditions using Two-Color Luminescent Images and Multi-Band Filter Mio Tanaka, Katsuaki Morita, Shigeo Kimura, Hirotaka Sakaue Icing occurs by a collision of a supercooled-water droplet on a surface. It can be seen in any cold area. A great attention is paid in an aircraft icing. To understand the icing process on an aircraft, it is necessary to give the temperature information of the supercooled water. A conventional technique, such as a thermocouple, is not valid, because it becomes a collision surface that accumulates ice. We introduce a dual-luminescent imaging to capture a global temperature distribution of supercooled water under the icing conditions. It consists of two-color luminescent probes and a multi-band filter. One of the probes is sensitive to the temperature and the other is independent of the temperature. The latter is used to cancel the temperature-independent luminescence of a temperature-dependent image caused by an uneven illumination and a camera location. The multi-band filter only selects the luminescent peaks of the probes to enhance the temperature sensitivity of the imaging system. By applying the system, the time-resolved temperature information of a supercooled-water droplet is captured. [Preview Abstract] |
Sunday, November 18, 2012 4:12PM - 4:25PM |
D14.00010: Lifetime Characterization of Electro-Luminescence Based Pressure-Sensitive Paint System for Unsteady Flow Field Measurements Yoshimi Iijima, Hirotaka Sakaue Electro-luminescence based pressure-sensitive paint (EL-PSP) system uses an EL as an illumination source for a PSP measurement. EL can be directly applied onto a PSP model to eliminate a remote illumination. This gives a uniform illumination on a PSP model without moving/re-directing the illumination. The temperature dependency can be reduced by the opposite temperature dependency of the EL and PSP. At present, the system is demonstrated in a steady flow field. To extend the system for capturing an unsteady flow field, a fast responding PSP and the lifetime characterization of the system are required. The former can be achieved by using a porous PSP. The latter is discussed in the present presentation. The EL-PSP system needs an AC input to illuminate the EL, which gives a pulsed/periodic excitation to a PSP. This limits the acquisition timing of the flow field; a frequent timing can resolve a fast unsteady flow field. The lifetime of the PSP emission can be related to the pressure. The lifetime decays of the EL and PSP are measured to discuss the lifetime characterization of the system. [Preview Abstract] |
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