Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session LH: Experimental Techniques: General II |
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Chair: Ahmed Naguib, Michigan State University Room: Hilton Chicago Williford B |
Tuesday, November 22, 2005 8:00AM - 8:13AM |
LH.00001: An indirect pressure-gradient technique for measuring instantaneous flow rate in unsteady duct flows G.J. Brereton, H.J. Schock, M.A.A. Rahi A technique is presented for making measurements of the instantaneous flow rate in unsteady laminar pipe flows. It utilizes a relationship expressing flow rate as a functional of pressure-gradient history that is an exact solution to the Navier-Stokes equations for parallel, developed flow of constant-property Newtonian fluids undergoing {\it arbitrary\/} unsteadiness from an initially (or to a finally) steady or stationary state. The technique applies instantaneously during momentary reversals of the flow, and requires that the velocity field only remain laminar and symmetric about its axis. Experimental comparisons between direct measurements of the cumulative flow and the results of this technique indicate it is capable of providing measurements of cumulative flow and flow rate that are accurate to within a few percent at any instant during a flow transient, provided the instantaneous pressure gradient can be resolved with this accuracy. [Preview Abstract] |
Tuesday, November 22, 2005 8:13AM - 8:26AM |
LH.00002: In-Situ Acoustic Measurement of Aircraft Trailing Vortices William Durgin, Rebecca Rodenhiser, Hamid Johari Closed path acoustic measurement of circulation, often used in the laboratory environment, has been extended to the airport runway environment. The difference in travel time of sound pulses propagating in opposite directions around a closed path is directly proportional to the circulation enclosed by that path. Apparatus was developed to produce, direct, and measure high frequency acoustic pulses traveling around a triangular closed path which enclosed trailing vortices of light aircraft. The experimental apparatus was installed alongside a runway to measure the strength of trailing vortices as would be produced by aircraft taking off or landing. The measurements clearly show the strength of trailing vortices and their movement out of the measurement zone by mutual induction. [Preview Abstract] |
Tuesday, November 22, 2005 8:26AM - 8:39AM |
LH.00003: Enhancement of Conditionally Sampled Signals for Coherent Structure Identification Stavros Tavoularis, M. Sadok Guellouz A new iterative method for enhancing phase-averaged statistics of conditionally sampled signals was developed. The first estimate of the time-dependent phase average is correlated with the signal of each detected event, following which the time axis of each event is shifted and adjusted in scale to produce a waveform that best matches that of the ensemble average. This also provides an opportunity to reject events whose maximum cross-correlation coefficient with the average is lower than a certain value. A new phase average of the modified events is then computed and the process is repeated until convergence has been achieved. This approach has been applied to three-dimensional velocity measurements in axial flow in a rectangular channel containing a cylindrical rod, in which large-scale, quasi-periodic, vortical coherent structures are known to appear. The phase average of the local velocity converged to essentially the same shape following a single iteration. The enhancement method yielded better defined ensemble averages and revealed details of the coherent structures that would otherwise be missed by the conditional sampling technique. When applied to the Variable Interval Time Average (VITA) conditional sampling technique, the present approach removes the dependence of the results on the threshold value, thus eliminating the subjectivity of the VITA technique. *Supported by the Natural Sciences and Engineering Research Council of Canada. [Preview Abstract] |
Tuesday, November 22, 2005 8:39AM - 8:52AM |
LH.00004: Using laser-cantilever anemometry under various flow conditions Michael H\"olling, Stephan Barth, Joachim Peinke, Jean-Daniel R\"uedi We present measurements executed with the new laser-cantilever anemometer (LCA) under various flow conditions. Previously, the basic principles and characteristics of the LCA were investigated. Measurements led to results comparable to common measurement techniques for turbulent flows, such as hot-wire anemometry for air and hot-film anemometry for water. Here we present further experiments under various flow conditions. The LCA was used in a snow wind tunnel to investigate the behavior of the cantilever under particle impact. In comparison to data collected with a hot-film anemometer under same conditions the times series of the LCA showed less pronounced impact characteristics than that of the hot-film, namely a shorter and easier to identify recovery time. In addition the behavior of the LCA at low velocities in air was investigated to determine the threshold velocity for measurements. [Preview Abstract] |
Tuesday, November 22, 2005 8:52AM - 9:05AM |
LH.00005: Tomographic Laser-Doppler-Velocimetry - a new method to increase spatial resolution Dietmar Kuhn, Gerald Fast, Andreas G. Class Laser-Doppler-Velocimetry, LDV is a common measurement principle to obtain local velocities. In most applications the finite size of the probe volume is neglected. Using a 400mm lens leads to a probe volume depth of 4mm that restricts the spatial resolution. To minimize the dimension either the intersection angle of the laser beam is increased or the laser beam is expanded. These methods often fail because of geometrical constraints, i.e. in combustion chambers, pressure chambers. In the context of theoretical considerations a tomographic algorithm is presented to increase the spatial resolution of the LDV based on the Inverse Radon Transformation. In this new approach integral information on velocity and particle density within the probe volume is analyzed to obtain sub-volume resolution. The probe volume is traversed from different viewing angles and evaluated with the presented algorithm. This leads to the two-dimensional velocity field. The new spatial resolution is mainly limited by the traverse increment. By means of an axis symmetric artificial velocity field the algorithm is verified We acknowledge the DFG for supporting this work by grant SFB- 606. [Preview Abstract] |
Tuesday, November 22, 2005 9:05AM - 9:18AM |
LH.00006: Multilayer Nano-Particle Image Velocimetry Minami Yoda, Haifeng Li, Reza Sadr Nano-particle image velocimetry (nPIV) uses evanescent-wave illumination of fluorescent colloidal tracers to measure the two tangential velocity components $u$ and $v$ averaged over the first 300 nm next to the wall. The evanescent-wave intensity decays exponentially with $z$, or the distance normal to the wall. Illuminated tracers at smaller $z$ therefore have images that are ``brighter'' than those at larger $z$. This variation in tracer intensity suggests the possibility of ``multilayer nPIV,'' where $u$ and $v$ are obtained at different $z$-locations \underline {within} the first 300 nm next to the wall. The variation of tracer image intensity with distance from the wall is modeled using a basic diffraction optics approach. The tracer images in artificial nPIV images of plane Couette flow for various experimental parameters incorporating hindered Brownian diffusion and image noise were divided into three sub-images, or ``layers,'' based on tracer image intensity. Standard techniques were used to extract average velocities at three different $z$-locations, with velocity data in the first layer obtained well within 100 nm of the wall. The results demonstrate that multilayer nPIV is feasible if appropriate classification techniques can be determined and used to separate tracer images into different layers. [Preview Abstract] |
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