Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session M28: Experiments: Visualization, Tagging and Tracking |
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Chair: James Gregory, The Ohio State University Room: 309 |
Tuesday, November 24, 2015 8:00AM - 8:13AM |
M28.00001: Time-Resolved Visualization of G\"{o}rtler Vortices in a Pulsed Convex Wall Jet using Fast Pressure-Sensitive Paint James Gregory, Ron Danon, David Greenblatt The time-resolved formation and structure of G\"{o}rtler vortices in a pulsed convex wall jet are studied in this work. While the presence of G\"{o}rtler vortices in laminar boundary layers on concave surfaces can be clearly observed, their presence in wall jets flowing over convex surfaces is difficult to discern due to transition to turbulence in the outer part of the jet. This work employed fast-response pressure-sensitive paint (PSP), which has a documented flat frequency response greater than 5 kHz, to visualize the time-resolved formation of the wall jet and the details of the G\"{o}rtler vortices. The radius of curvature of the wall jet was 8 cm, and the Reynolds number (based on slot height and jet exit velocity) was varied between $5\times 10^{2}$ and $4\times 10^{4}$. The characteristic spanwise wavelength of the vortices was studied as a function of jet Reynolds number. Furthermore, as the Reynolds number was increased, various secondary instabilities were observed that led to laminar-turbulent transition. [Preview Abstract] |
Tuesday, November 24, 2015 8:13AM - 8:26AM |
M28.00002: Spectral Measurements from the Optical Emission of the A.C. Plasma Anemometer Eric Matlis, Curtis Marshall, Thomas Corke, Sivaram Gogineni The optical emission properties of a new class of AC-driven flow sensors based on a glow discharge (plasma) is presented. These results extend the utility of the plasma sensor that has recently been developed for measurements in high-enthalpy flows. The plasma sensor utilizes a high frequency (1MHz) AC discharge between two electrodes as the main sensing element. The voltage drop across the discharge correlates to changes in the external flow which can be calibrated for mass-flux $(\rho U)$ or pressure depending on the design of the electrodes and orientation relative to the free-stream flow direction. Recent experiments examine the potential for spectral analysis of the optical emission of the discharge to provide additional insight to the flow field. These experiments compare the optical emission of the plasma to emission from breakdown due to an ND:YAG laser. The oxygen 777.3 nm band in particular is a focus of interest as a marker for the determination of gas density. [Preview Abstract] |
Tuesday, November 24, 2015 8:26AM - 8:39AM |
M28.00003: High-Speed OH* Chemiluminescence Imaging of Shock Tube End-Wall V.A. Troutman, V.A. Miller, C.S. Strand, A.M. Tulgestke, M.F. Campbell, D.F. Davidson, R.K. Hanson We have developed a high-speed OH* chemiluminesence imaging diagnostic and a transparent end-wall for the Stanford Aerosol Shock Tube to better understand the structure and homogeneity of the combustion event behind a reflected shock wave. We use an intensified high repetition rate imaging system to acquire images of OH* chemiluminescence (near 308 nm) at 10-33 kHz from n-heptane combustion. Case studies are presented to illustrate the power of this novel imaging diagnostic: first, we infer the temperature homogeneity of the ignition event; then we image the effect of surface imperfections in the wall of the shock tube; lastly, we visualize the effect of particulates in the shock tube and verify the importance of shock tube cleaning routines. [Preview Abstract] |
Tuesday, November 24, 2015 8:39AM - 8:52AM |
M28.00004: The 3D flow structures generated by a pair of cubic roughness elements in a turbulent channel flow resolved using holographic microscopy Jian Gao, Joseph Katz In studies of turbulent flows over rough walls, considerable efforts have been put on the overall effects of roughness parameters such as roughness height and spatial arrangement on the mean profiles and turbulence statistics. However there is very little experimental data on the generation, evolution, and interaction among roughness-initiated turbulent structures, which are essential for elucidating the near-wall turbulence production. As a first step, we approach this problem experimentally by applying digital holographic microscopy (DHM) to measure the flow and turbulence around a pair of cubic roughness elements embedded in the inner part of a high Reynolds number turbulent channel flow (Re$_{\mathrm{\tau }}=$2000 -- 5000). The ratio of half-channel height ($h)$ to cube height ($a)$ is 25, and the cubes are aligned in the spanwise direction, and separated by 1.5$a$. DHM provides high-resolution three-dimensional (3D) three-component (3C) velocity distributions. The presentation discusses methods to improve the data accuracy, both during the hologram acquisition and particle tracking phases. First, we compare and mutually validate velocity fields obtained from a two-view DHM system. Subsequently, during data processing, the seven criteria used for particle tracking is validated and augmented by planar tracking of particle image projections. Sample results reveal instantaneous 3D velocity fields and vortical structures resolved in fine details of several wall units. [Preview Abstract] |
Tuesday, November 24, 2015 8:52AM - 9:05AM |
M28.00005: Color gradient background oriented schlieren imaging Frank Austin Mier, Michael Hargather Background oriented schlieren (BOS) imaging is a method of visualizing refractive disturbances through the comparison of digital images. By comparing images with and without a refractive disturbance visualizations can be achieved via a range of image processing methods. Traditionally, backgrounds consist of random distributions of high contrast speckle patterns. To image a refractive disturbance, a digital image correlation algorithm is used to identify the location and magnitude of apparent pixel shifts in the background pattern. Here a novel method of using color gradient backgrounds is explored as an alternative. The gradient background eliminates the need to perform an image correlation between the two digital images, as simple image subtraction can be used to identify the location, magnitude, and direction of the image distortions. This allows for quicker processing. Two-dimensional gradient backgrounds using multiple colors are shown. The gradient backgrounds are demonstrated to provide quantitative data limited only by the camera's pixel resolution, whereas speckle backgrounds limit resolution to the size of the random pattern features and image correlation window size. Additional results include the use of a computer screen as a background. [Preview Abstract] |
Tuesday, November 24, 2015 9:05AM - 9:18AM |
M28.00006: Femtosecond laser flow tagging in non-air flows Yibin Zhang, Nathan Calvert The Femtosecond Laser Electronic Excitation Tagging (FLEET) [Michael, J. B et. al. \textit{Applied optics},~\textit{50}(26), 2011] method is studied in nitrogen-containing gaseous flows. The underlying mechanism behind the FLEET process is the dissociation of molecular nitrogen into atomic nitrogen, which produces long-lived florescence as the nitrogen atoms recombine. Spectra and images of the resulting tagged line provide insight into the effects of different atmospheric gases on the FLEET process. The ionization cross-section, conductivity and energy states of the gaseous particles are each brought into consideration. These experiments demonstrate the feasibility for long-lived flow tagging on the order of hundreds of microseconds in non-air environments. Of particular interest are the enhancement of the FLEET signal with the addition of argon gas, and the non-monotonic quenching effect of oxygen on the length, duration and intensity of the resulting signal and spectra. FLEET is characterized in number of different atmospheric gases, including that simulating Mar's atmospheric composition. [Preview Abstract] |
Tuesday, November 24, 2015 9:18AM - 9:31AM |
M28.00007: A method for extracting the turbulence intensity and integral length scale form single-component molecular tagging velocimetry David Olson, Ahmed Naguib, Manoochehr Koochesfahani This study demonstrates a method to extract the turbulence intensity and integral length scale from single-component molecular tagging velocimetry (1c-MTV) measurements of freestream turbulence. These measurements are challenging because of the very small magnitude of the fluctuating velocities compared to the freestream velocity, and the presence of low-frequency facility unsteadiness as well as measurement white noise. The approach takes advantage of the inherent capabilities of 1c-MTV to measure the streamwise velocity at a very high spatial resolution of 52~$\mu $m over a line that extends 5.3~cm in the cross-stream direction. The resulting data set is equivalent to that which would result from a linear sensor array of 1024 tightly-spaced hot wires, enabling computation of the cross-stream autocorrelation function efficiently. The high-spatial resolution of the measurements allows removal of the white noise contribution to the autocorrelation function, whereas the extended domain of the measurements facilitates rejection of the influence of the low-frequency facility unsteadiness. The ``noise-removed'' autocorrelation function is used to compute the intensity and integral length scale of turbulence. The procedure is applied to grid-generated freestream turbulence. [Preview Abstract] |
Tuesday, November 24, 2015 9:31AM - 9:44AM |
M28.00008: Study of gas-liquid flow in model porous media for heterogeneous catalysis Marie Francois, Hugues Bodiguel, Pierre Guillot Heterogeneous catalysis of chemical reactions involving a gas and a liquid phase is usually achieved in fixed bed reactors. Four hydrodynamic regimes have been observed. They depend on the total flow rate and the ratio between liquid and gas flow rate. Flow properties in these regimes influence transfer rates. Rather few attempts to access local characterization have been proposed yet, though these seem to be necessary to better describe the physical mechanisms involved. In this work, we propose to mimic slices of reactor by using two-dimensional porous media. We have developed a two-dimensional system that is transparent to allow the direct observation of the flow and the phase distribution. While varying the total flow rate and the gas/liquid flow rate ratio, we observe two hydrodynamic regimes: at low flow rate, the gaseous phase is continuous (trickle flow), while it is discontinuous at higher flow rate (pulsed flow). Thanks to some image analysis techniques, we are able to quantify the local apparent liquid saturation in the system. Its fluctuations in time are characteristic of the transition between the two regimes: at low liquid flow rates, they are negligible since the liquid/gas interface is fixed, whereas at higher flow rates we observe an alternation between liquid and gas. This transition between trickle to pulsed flow is in relative good agreement with the existing state of art. However, we report in the pulsed regime important flow heterogeneities at the scale of a few pores. These heterogeneities are likely to have a strong influence on mass transfers. [Preview Abstract] |
Tuesday, November 24, 2015 9:44AM - 9:57AM |
M28.00009: Measuring turbulent fluid dispersion using laser induced phosphorescence Dennis van der Voort, Nico Dam, Willem van de Water, Rudie Kunnen, Herman Clercx, GertJan van Heijst Fluid dispersion due to turbulence is an important subject in both natural and engineering processes, from cloud formation to turbulent mixing and liquid spray combustion. The combination of small scales and often high velocities results in few experimental techniques that can follow the course of events. We introduce a novel technique, which measures the dispersion of ``tagged'' fluid particles by means of laser-induced phosphorescence, using a solution containing a europium-based molecular complex with a relatively long phosphorescence half-life. This technique is used to measure transport processes in both the dispersion of droplets in homogeneous isotropic turbulence and the dispersion of fluid of near-nozzle spray breakup processes. By tagging a small amount of droplets/fluid via laser excitation, the tagged droplets can be tracked in a Lagrangian way. The absolute dispersion of the droplets can be measured in a variety of turbulent flows. Using this technique it is shows that droplets around ${\rm{St}}=\tau_p/\tau_{\eta}\approx 1$ (Stokes number) disperse faster than true fluid tracers in homogeneous isotropic turbulence, as well as differences between longitudinal and radial dispersion in turbulent sprays. [Preview Abstract] |
Tuesday, November 24, 2015 9:57AM - 10:10AM |
M28.00010: Improvements of a nano-scale crossed hot-wire for high Reynolds number measurements Yuyang Fan, Marcus Hultmark Hot-wire anemometry, despite its limited spatial and temporal resolution, is still the preferred tool for high Reynolds number flow measurements, mainly due to the continuous signal. To address the resolution issues, the Nano-Scale Thermal Anemometry Probe (NSTAP) was developed at Princeton University. The NSTAP has a sensing volume more than one order of magnitude smaller than conventional hot-wires, and it has displayed superior performance. However, the NSTAP can only measure a single component of the velocity. Using a novel combining method, a probe that enables two-component velocity measurements has been created (the x-NSTAP). The measurement volume is approximately $50\times50\times50\mu$m, more than one order of magnitude smaller in all directions compared to conventional crossed hot-wires. The x-NSTAP has been further improved to allow more accurate measurements with the help of flow visualization using a scaled model but matching Reynolds number. Results from turbulent flow measurements with the new x-NSTAP are also presented. [Preview Abstract] |
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