Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session R14: Experimental Techniques V |
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Chair: Gary Settles, Pennsylvania State University Room: 317 |
Tuesday, November 22, 2011 12:50PM - 1:03PM |
R14.00001: Velocity measurements in a thermoacoustic refrigerator using Time-Resolved Particle Image Velocimetry Philippe Blanc-Benon, Gaelle Poignand, Arganthael Berson, Emmanuel Jondeau A standing-wave thermoacoustic refrigerator consists of a stack of plates placed in an acoustic resonator with two heat exchangers located at each end of the stack. The full understanding of the heat transfer between the stack and the heat exchangers of thermoacoustic systems is a key issue to improve the global efficiency of such devices. The aim of this work is to investigate the generation of vortices near the ends of the stack, which affects heat transfer. The aerodynamic field in the gap between the stack and the heat exchanger is characterized using a time-resolved particle image velocimetry technique. Measurements are performed in a standing-wave refrigerator operating at a frequency of 200 Hz. Instantaneous velocity fields are recorded at a frequency of 3125 Hz (i.e. 15 velocity fields per acoustic period). Measurements show that vortex shedding occurs at high pressure levels, when the nonlinear acoustic regime prevails and they validate previous experiments [Berson {\&} Blanc-Benon, J. Acoust. Soc. Am., 2007, 122(4), EL122-127]. The increased viscous dissipation generates additional heating and a loss of efficiency. [Preview Abstract] |
Tuesday, November 22, 2011 1:03PM - 1:16PM |
R14.00002: Error Reduction in Molecular Tagging Velocimetry (MTV) Processing Using Image Filtering Michael Caso, Douglas Bohl Prior work has shown that the error level in MTV measurements is closely tied to the image SN level. In practice the SN ratio will depend on experimental conditions such as attenuation, Field of View, laser power, camera, etc.; however, there is a minimum SN level that can be achieved for any given experiment. Experience has shown that MTV images typically have a SN=2-8. It is therefore desirable to be able to lower image noise after the images are acquired to reduce measurement error. In this work post processing MTV images using image filtering schemes such as Gaussian Blur, FFT (band pass), median filtering etc. was investigated using synthetic MTV images with added random noise. The synthetic images were filtered and then processed using a direct correlation technique. The results showed that for very noisy images (i.e. SN$<$4) the all filtering techniques improved the displacement error by 10-40{\%}. As the SN increased filtering because less effective in decreasing error and in some cases increased the measurement error. The FFT band pass filter was most effective and improved measurement error for all SN levels. [Preview Abstract] |
Tuesday, November 22, 2011 1:16PM - 1:29PM |
R14.00003: Determining the Shock Hugoniot of Transparent Materials with Hydrodynamic Pressure Loading Forrest Svingala, Gary Settles The shock Hugoniot is a fundamental relationship between pressure, volume, and energy for a given material. Accurate knowledge of the Hugoniot for a material is critical in order to determine its response to blast waves and ballistic impacts. Traditionally, the shock Hugoniot is measured on a point-by-point basis through an extensive series of high-velocity impact experiments. Observations are confined to pointwise pressure or velocity measurements at the free surfaces of the sample. In this work a new technique is presented, one which allows multiple points of the shock Hugoniot to be determined in a single experiment. A gram-scale explosive charge is detonated to produce an unsteady shock wave in the transparent material sample. Pressure between the charge and sample is initially high, but is rapidly reduced by expansion of the explosive product gases. This loading produces an initially strong shock wave, which attenuates to near the bulk sound speed as it transits the sample. Using a high-speed shadowgraph technique, multiple shock and particle velocity combinations are observed in a single experiment. This allows the measurement of a shock Hugoniot in fewer experiments than by traditional methods. This technique produces data in agreement with published Hugoniot results for polyurethane. It can be easily extended to measure the Hugoniot of any transparent solid, liquid, or gas. [Preview Abstract] |
Tuesday, November 22, 2011 1:29PM - 1:42PM |
R14.00004: Toward an automated background oriented schlieren (BOS) system Michael Hargather, Gary Settles The background oriented schlieren (BOS) technique is a useful method for visualizing refractive disturbances in a wide range of experimental settings. The technique visualizes refractive disturbances via their distortion of a distant background pattern (typically a speckle pattern). A cross-correlation computer algorithm is typically used to identify and measure distortions of the background pattern, thus revealing the refractive disturbance changes between images and producing a schlieren image. The cross-correlation algorithm, however, can be time-consuming and prevents an instantaneous schlieren image from being observed, thus hampering some potential BOS applications. Here a novel background patterning approach is presented which eliminates the need for the cross-correlation algorithm. Results are presented showing the sensitivity of the new background pattern and its potential application for providing instantaneous BOS images. Background pattern characteristics are explored for high- and low-speed fluid-dynamic applications. [Preview Abstract] |
Tuesday, November 22, 2011 1:42PM - 1:55PM |
R14.00005: Molecular candidates of MTV in air Nico Dam, Mehrnoosh Mirzaei, Willem van de Water In molecular tagging velocimetry (MTV), the molecules of a gas are used as flow tracers. These tracers can be produced at will by illumination with a laser which promotes molecules to a long- lived excited state, fuses N$_2$ and N$_2$ to NO, or makes molecules phosphoresce. A while later these tagged molecules can be visualized by laser-induced fluorescence, or by just watching them while they phosphoresce. Candidates for MTV in turbulence research must be arranged in structures narrower than the Kolmogorov scale, which remain narrow as time progresses, and must live longer than the Kolmogorov time. These requirements invalidate many candidates, candidates once deemed successful. They do so in various surprising manners that involve a combination of fluid flow and molecular dynamics. Rather than velocimetry in turbulence, MTV techniques offer a unique view on basic dispersion processes at the smallest scales of turbulence. In this way we have measured the spreading of clouds whose size is a few times the Kolmogorov length and the Batchelor dispersion of objects whose size is inside the inertial range. [Preview Abstract] |
Tuesday, November 22, 2011 1:55PM - 2:08PM |
R14.00006: Constant Current Plasma Anemometer Curtis Marshal, Eric Matlis, Thomas Corke, Sivaram Gogineni An improved design for a plasma anemometer that provides mega-Hertz bandwidth velocity measurements over a range of Mach numbers from subsonic to hypersonic, is presented. The anemometer uses a small volume of ionized air as the sensor. The ionized air is formed between two electrodes that are powered by a high-frequency AC voltage source. Resistance and capacitance elements in the AC power circuit that simulate a dielectric-barrier, have been added to prevent transient filament formation. This resulted in a 300-times reduction in the anemometer EMI of previous designs. In addition, a closed-loop feedback control has been added to maintain a constant current through the sensor. With constant current operation, the voltage drop across the ionized air between the electrodes varies linearly with air velocity (or mass-flux in a compressible flow). This is demonstrated through mean velocity calibrations of the anemometer for a range of velocities and feedback parameters. The dynamic response and other capabilities of the sensor will also be demonstrated. [Preview Abstract] |
Tuesday, November 22, 2011 2:08PM - 2:21PM |
R14.00007: In-situ shear stress indicator using heated strain gages at the flow boundary Chi-An Yeh, Fuling Yang This work borrows the concept of hot-wire anemometry and sketch a technique that uses local heat transfer to infer the flow field and the corresponding stress. Conventional strain gages were mounted at the flow solid boundary as the heat source and acrylic boundary was chosen for its low thermal conductivity ensuring heat accumulation when a gage is energized. The gage would now work in slightly overheated state and its self-heating leads to an additional thermal strain. When exposed to a flow field, heat is brought away by local forced convection, resulting in deviations in gage signal from that developed in quiescent liquid. We have developed a facility to achieve synchronous gage measurements at different locations on a solid boundary. Three steady flow motions were considered: circular Couette flow, rectilinear uniform flow, and rectilinear oscillating flow. Preliminary tests show the gage reading does respond to the imposed flow through thermal effects and greater deviation was measured in flows of higher shear strain rates. The correlation between the gage signals and the imposed flow field is further examined by theoretical analysis. We also introduced a second solid boundary to the vicinity of the gage in the two rectilinear flows. The gage readings demonstrate rises in its magnitudes indicating wall amplification effect on the local shear strain, agreeing to the drag augmentation by a second solid boundary reported in many multiphase flow literatures. [Preview Abstract] |
Tuesday, November 22, 2011 2:21PM - 2:34PM |
R14.00008: Assessment of the Derivative-Moment Transformation method for unsteady-load estimation Ali Mohebbian, David Rival It is often difficult, if not impossible, to measure the aerodynamic or hydrodynamic forces on a moving body. For this reason, a classical control-volume technique is typically applied to extract the unsteady forces instead. However, measuring the acceleration term within the volume of interest using PIV can be limited by optical access, reflections as well as shadows. Therefore in this study an alternative approach, termed the Derivative-Moment Transformation (DMT) method, is introduced and tested on a synthetic data set produced using numerical simulations. The test case involves the unsteady loading of a flat plate in a two-dimensional, laminar periodic gust. The results suggest that the DMT method can accurately predict the acceleration term so long as appropriate spatial and temporal resolutions are maintained. The major deficiency was found to be the determination of pressure in the wake. The effect of control-volume size was investigated suggesting that smaller domains work best by minimizing the associated error with the pressure field. When increasing the control-volume size, the number of calculations necessary for the pressure-gradient integration increases, in turn substantially increasing the error propagation. [Preview Abstract] |
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