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 L14: Experimental Techniques III |
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Chair: John Foss, Michigan State University Room: 317 |
Monday, November 21, 2011 3:35PM - 3:48PM |
L14.00001: An optimal arrangement of a three or four hot-wire sensor array to simultaneously measure velocity component statistics in turbulent wall flows James Wallace, Petar Vukoslavcevic' A highly resolved turbulent channel flow DNS with $Re_\tau =200$ has been used to investigate the ability of probes made up of arrays of three or four hot-wire sensors to simultaneously and accurately measure statistics of all three velocity components in turbulent wall flows. Such arrays have also been combined in probes to measure, in addition, velocity gradient based statistics. Various virtual sensor arrangements have been tested in order to study the effects of position, number of sensors and spatial resolution on the measurements. First, the effective cooling velocity was determined for each sensor of an idealized probe, where the influence of the velocity component tangential to the sensors and flow blockage by the presence of the probe are neglected. Then, simulating the response of the virtual probes to obtain the effective velocities cooling the sensors, velocity component statistics have been calculated neglecting the velocity gradients over the probe sensing area. A strong influence of both mean and fluctuation velocity gradients on measurement accuracy was found. A new three-sensor array configuration designed to minimize the influence of the velocity gradients is proposed, and its accuracy is compared to two-sensor X- and V-array configurations. [Preview Abstract] |
Monday, November 21, 2011 3:48PM - 4:01PM |
L14.00002: An in-situ calibration technique for four-wire hot-wire probe in conjunction for atmospheric studies Reza Sadr, Arindam Singha There is an increasing need to resolve the small-scales of atmospheric turbulence in order to estimate the higher order statistics of the turbulent flow. Sonic anemometers are commonly used in atmospheric research; however their application can only provide data with low special and temporal resolution. Hot-wire (HW) probes are still the best tool to obtain turbulent statistics with high temporal and spatial resolution. But HW probes are rarely used for atmospheric measurement due to the intricacy and logistical difficulties associated with the calibration and applications of the required probes for this flow field. In the present study, an in-situ method of calibration of a four-wire hot-wire anemometer is proposed, which bypasses the need for prior calibration. A proper data reduction algorithm has been developed to be used in conjunction with the four-wire probe. The proposed methodology enables one to use the hot-wire anemometer for atmospheric measurement to obtain three dimensional velocity information, at high spatial and temporal resolution, without the necessity of going through extensive calibration procedure. The feasibility of this method has been tested in laboratory and Monte Carlo simulation has been used to establish the stability and sensitivity of the data reduction algorithm. [Preview Abstract] |
Monday, November 21, 2011 4:01PM - 4:14PM |
L14.00003: New measurement technique for turbulent flow as a replacement for hot-wire anemometry Jaroslaw Puczylowski, Michael H\"olling, Joachim Peinke We present latest developments of the 2d-Laser Cantilever Anemometer (2d-LCA), a sensor, which has been developed for highly resolved measurements of turbulent flows. Its measuring principle allows for high temporal resolutions of beyond 50kHz at spatial scales in sub millimeter range. This performance is achieved by measuring the deformation of a tiny cantilever via laser pointer, which experiences drag forces caused by the flow. The cantilever features two deformation modes, i.e. bending and twisting, whereas the latter occurs only for oblique inflow. Thus the sensor is capable of measuring two velocity components. Latest developments focus on the design of the cantilever. For example, an additional structure for a better sensitivity towards cross winds and an improved reflection pad were realized. Further improvements concern the laser beam guiding within the sensor. Beside this we are in the process of setting up advanced electronics and new types of PSD-elements with the goal of increasing the sensitivity. Comparison measurements between the re-designed 2d-LCA and older versions were performed and showed improvements relating signal quality and reliability. Further measurements in turbulent flow with an x-wire as a reference confirmed the ability of the new sensor to carry out measurements at comparable high resolutions. [Preview Abstract] |
Monday, November 21, 2011 4:14PM - 4:27PM |
L14.00004: Multi-color particle shadow accelerometry (cPSA) Grant Dowell, McPhail Michael, Arnold Fontaine, Michael Krane, Larry Goss, James Crafton We present an extension of multi-color particle shadow velocimetry (cPSV) to unsteady acceleration measurement. cPSV uses a multi-color, pulsed LED light source for illumination. Particle shadow images recorded by a digital color camera are color separated and inverted. Standard DPIV processing methods are then used to estimate 2-D displacement vector fields. Acceleration estimates are facilitated by acquisition of three sequential images (one per color) in each camera exposure. Here, we prove the technique by measuring the tangential acceleration of a moving solid body, and compare the results to accelerometer measurements. We also present preliminary acceleration measurements performed in a near-wall turbulent pipe flow. [Preview Abstract] |
Monday, November 21, 2011 4:27PM - 4:40PM |
L14.00005: Development of an Acoustic Localization Method for Cavitation Experiments in Reverberant Environments Minna Ranjeva, Lee Thompson, Daniel Perlitz, William Bonness, Dean Capone, Brian Elbing Cavitation is a major concern for the US Navy since it can cause ship damage and produce unwanted noise. The ability to precisely locate cavitation onset in laboratory scale experiments is essential for proper design that will minimize this undesired phenomenon. Measuring the cavitation onset is more accurately determined acoustically than visually. However, if other parts of the model begin to cavitate prior to the component of interest the acoustic data is contaminated with spurious noise. Consequently, cavitation onset is widely determined by optically locating the event of interest. The current research effort aims at developing an acoustic localization scheme for reverberant environments such as water tunnels. Currently cavitation bubbles are being induced in a static water tank with a laser, allowing the localization techniques to be refined with the bubble at a known location. The source is located with the use of acoustic data collected with hydrophones and analyzed using signal processing techniques. To verify the accuracy of the acoustic scheme, the events are simultaneously monitored visually with the use of a high speed camera. Once refined testing will be conducted in a water tunnel. This research was sponsored by the Naval Engineering Education Center (NEEC). [Preview Abstract] |
Monday, November 21, 2011 4:40PM - 4:53PM |
L14.00006: MLM: Dust Explosion Potential Warning System John Foss, Alan Lawrenz A quite large range of materials, when dispersed as a dust cloud in air, can support an explosion. Empirically derived minimum explosive concentration (MEC) values are typically in the range: 30-80 grams/m$^{3}$; that is, nominally 2.5-8.3{\%} of STP density. Currently, there is no field-deployable measurement system to determine the mass loading (grams/m$^{3})$ of airborne dust. Proof-of-concept measurements for the MSU Mass Loading Monitor (MLM) are reported. A charge of dusty air, ingested into a cylinder, is accelerated (a$_{p}$=ct) by a driving piston and partially ($\approx $8{\%}) discharged from the open end of the cylinder. The deformable control volume momentum equation can be integrated with respect to time to yield $\alpha _{( )}-\beta _{( )}=\gamma \rho _{( )}$ where ( ) will indicate with (w) or without (w/o) dust. The pressure integral ($\alpha )$ and the shear integral ($\beta )$ balance the momentum within the cylinder at the end of the smoke plus the integral of the momentum flux. The kinematic attributes of these terms are represented by $\gamma $. It will be shown how the mass loading ($\rho _{w}-\rho _{w/o})$ can be determined. A full length paper (The Mass Loading Monitor Fundamental Principles And Proof Of Concept) will be published in \textit{Meas. Sci. and Tech.} [Preview Abstract] |
Monday, November 21, 2011 4:53PM - 5:06PM |
L14.00007: Comparing the sphere anemometer to standard sensors for 2D wind measurements Hendrik Heisselmann, Michael Hoelling, Joachim Peinke The cup anemometers commonly used for wind energy applications are fairly robust, but suffer from several drawbacks like their limited temporal resolution, a systematic overestimation of the wind speed in turbulent flows and the inability to measure the wind direction. While sonic anemometers can measure the wind vector at a higher temporal resolution, they are more fragile and significantly more expensive. Therefore, we propose the sphere anemometer as a robust and highly-resolving alternative to standard anemometers. Designed without wearing parts, the sphere anemometer provides simultaneous wind speed and direction measurements as needed for wind turbine operation especially under challenging conditions such as offshore installation. In our contribution, we introduce the setup of the sphere anemometer which is based on the velocity-dependent deflection of a flexible tube with a sphere mounted atop. The deflection is measured in two dimensions using a light pointer, which allows for the simultaneous determination of wind speed and direction via calibration. Experimental results from wind tunnel measurements with sonic anemometer and sphere anemometer are presented, as well as first comparative measurements from the operation on the nacelle of a near-shore wind turbine. [Preview Abstract] |
Monday, November 21, 2011 5:06PM - 5:19PM |
L14.00008: Geometrical Optimization of a Cylindrical Plasma Lens Brian Neiswander, Eric Matlis, Thomas Corke Previous work by the authors have demonstrated the concept of an AC ``plasma lens'' for optical path difference (OPD) control of laser wavefronts. Plasma lenses feature no moving parts and a high frequency response, both of which are highly favorable for adaptive optics. This work investigates the geometrical constraints the plasma lens design, particularly the influence of the diameter of electrodes and gap distance between electrodes. A simplified electrostatic model for the plasma lens is developed and compared with experimental results. A good agreement is found between the experiments and theory. In regard to plasma lens design, the findings indicate (1) that there exists a critical gap-distance-to-diameter ratio, and (2) that the spatial efficiency of the device depends highly on the gap-distance-to-diameter ratio. [Preview Abstract] |
Monday, November 21, 2011 5:19PM - 5:32PM |
L14.00009: Time-resolved tomographic PIV measurements in swirling jets Andrea Ianiro, Daniele Violato, Gennaro Cardone, Fulvio Scarano The vortex dynamics of swirling jets at a Reynolds number of 1,000 is investigated by time-resolved tomographic Particle Image Velocimetry. Experiments are conducted at several values of the swirl number S from 0 to 0.8, therefore spanning the transition between weak and strong swirl regimes. Time resolved measurements are performed with the intention to shed more insight into the relation between shear layer instability at the outer side of the jet and the unsteady behaviour of the core region characterized by tumbling/precession motion. The average flow topology is addressed first in order to highlight the topological differences between weak and strong swirl flows. The three-dimensional topology of vortex structures is then visualized with Q criterion. Finally, the measured velocity and vorticity field is employed to estimate the terms of the vorticity equation, which enable to discuss on a more quantitative basis the effect on vortex dynamics and on the overall phenomenology of swirling jets. [Preview Abstract] |
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