Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session AN: Experimental Techniques I |
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Chair: Lester Su, Johns Hopkins University Room: 200C |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AN.00001: Evaluation of burst-mode LDA spectra with implications Clara Velte, William George Burst-mode LDA spectra, as described in [1], are compared to spectra obtained from corresponding HWA measurements using the FFT in a round jet and cylinder wake experiment. The phrase ``burst-mode LDA'' refers to an LDA which operates with at most one particle present in the measuring volume at a time. Due to the random sampling and velocity bias of the LDA signal, the Direct Fourier Transform with accompanying weighting by the measured residence times was applied to obtain a correct interpretation of the spectral estimate. Further, the self-noise was removed as described in [2]. In addition, resulting spectra from common interpolation and uniform resampling techniques are compared to the above mentioned estimates. The burst-mode LDA spectra are seen to concur well with the HWA spectra up to the emergence of the noise floor, caused mainly by the intermittency of the LDA signal. The interpolated and resampled counterparts yield unphysical spectra, which are buried in frequency dependent noise and step noise, except at very high LDA data rates where they perform well up to a limited frequency.\\[4pt] [1] Buchhave, P. {\bf PhD Thesis}, SUNY/Buffalo, 1979.\\[0pt] [2] Velte, C.M. {\bf PhD Thesis}, DTU/Copenhagen, 2009. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AN.00002: Two-dimensional velocity measurements using Laser-Cantilever-Anemometry in comparison to x-wire anemometry Michael H\"olling, Jaroslaw Puczylowski, Joachim Peinke We present an improved 2D Laser-Cantilever-Anemometer (2D LCA) which allows for measurements in two dimensions. The two velocity components are resolved by detecting the bending and the torsion of a tiny cantilever using the laser pointer principle. Thereby a two dimensional position sensitive detector measures the movement of the reflected laser light coming from the cantilever. Measurements carried out with this 2D LCA in comparison to x-wire data acquired in the wake of a cylinder are presented. [Preview Abstract] |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AN.00003: ABSTRACT WITHDRAWN |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AN.00004: Design and Characterization of an Optical Feedback-Controlled Microphone for Aeroacoustics Research Eliott Radcliffe, Ahmed Naguib, William Humphreys, Jr. An optical feedback-controlled microphone was designed and tested for potential use in phased ``beam-forming'' arrays used in aeroacoustics research. Optical sensing was employed as a means for measuring center displacement of a stretched thin membrane due to incident acoustic pressure. The membrane was constructed of PVDF which exhibits piezoelectric properties allowing actuation of the membrane in a feedback system. The latter was used to actively modify sensor parameters, most notably membrane stiffness, resonant frequency, and damping. Testing of a prototype microphone was performed using a plane wave tube calibrator. The results demonstrate that feedback control is an effective method for improving the microphone's transient response, as well as for ``self-tuning'' and matching of microphone parameters in sensing arrays. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AN.00005: Measurements of instantaneous temperature in oscillating flows Philippe Blanc-Benon, Arganthael Berson, Gaelle Poignand, Genevieve Comte-Bellot Temperature fluctuations in turbulent flows are usually investigated using cold wires operated by a constant-current anemometer. However, the output voltage of such anemometers is not hardware compensated for the thermal inertia of the wire. A correction is applied only during the post-processing of the data and requires the knowledge of the time lag of the wire, which depends both on the wire properties and on the instantaneous incident flow velocity. Here, a simple procedure for the instantaneous correction of the thermal inertia of cold wires is proposed. The method relies on the splitting of the time lag of cold wires operated in a constant-current mode into two factors: one depending on the wire properties and the other depending on flow velocity. These two factors are obtained from the operation of the wire by a constant-voltage anemometer in the heated-mode. The uncompensated signal delivered by the constant-current anemometer operating the cold wire is then processed to restore the signal that would be delivered by an ideal cold wire. Validation experiments are conducted in an acoustic standing-wave resonator where large-amplitude oscillatory flows take place. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AN.00006: Use of grid generated turbulence to assess hot-wire spatial resolution and Pitot probe turbulence corrections Anand Ashok, Mohammad Javed, Sean Bailey, Alexander Smits The objective of the present study is to use grid generated homogeneous isotropic turbulence as a benchmark flow to test the effect of turbulence on different measurement techniques. The grid turbulence is generated in a low speed 2 foot by 3 foot closed circuit wind tunnel using a 1 inch square mesh grid placed at the test section inlet. Measurements of the turbulence using a variety of hot-wire sensor lengths, at a series of streamwise distances downstream of the grid, will be used to investigate the impact of spatial filtering on different turbulence statistics and fully characterize the grid turbulence. Points of comparison will include basic mean and turbulent kinetic energy profiles as well as higher order statistics, the turbulent dissipation rate and finally turbulent spectra. These statistics will then be combined with additional measurements to assess the performance of turbulence corrections for Pitot probe velocity measurements. Finally, it is hoped that this flow can be used to assess the performance of a new nano-scale hot wire probe currently under development. [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AN.00007: The accuracy of cross-stream velocity gradients measured by multi-sensor hot-wire probes Milan \v{S}ekularac, Petar Vukoslav\v{c}evi\'{c}, James Wallace, Elias Balaras, Nikolaos Beratlis A highly resolved turbulent minimum channel flow DNS with $Re_{\tau} =180$ was used to investigate the effects on the accuracy of simultaneous measurements of velocity gradient components resulting from the spatial resolution and sensor arrangement of twelve-sensor hot-wire probes. The sensors were represented as points on the simulation grid, the effective velocity cooling each sensor was determined and sensor equations were then solved in response to the DNS field to obtain velocity and velocity gradient components. It has been found that all cross-stream velocity gradients except $\partial v/\partial y$ and $\partial w/\partial z$ can be measured with reasonable accuracy. Depending on the arrangement of the sensors and the array and probe sizes, either one or the other of these two gradients is subject to high measurement error in the near wall region of this bounded flow. As a consequence, the estimation of $\partial u/\partial x$ from the direct measurement of $\partial v/\partial y$ and $\partial w/\partial z$ by applying the continuity equation for incompressible flow is questionable. It appears that this is the likely explanation for the weak correlation in the near wall region of $\partial u/\partial x$, estimated by applying Taylor's hypothesis, with its value estimated using the continuity equation as has been found in several investigations using twelve-sensor probes. [Preview Abstract] |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AN.00008: Effect of Wind Tunnel Wall Adaptation on Flow over a Circular Cylinder Serhiy Yarusevych, Michael Bishop The presence of test section walls in many experimental facilities give rise to blockage effects, which detrimentally influence experimental data. A unique method to eliminate such blockage effects is to adapt the walls of a test section so as to mimic the conditions of an unbounded flow. The effect of such wall adaptation on flow development over a circular cylinder was the focus of this investigation. Velocity and surface pressure measurements were made in three test section wall configurations: geometrically straight walls (GSW), aerodynamically straight walls (ASW), and streamlined walls (SLW). In all the wall configurations investigated, tests were conducted for Re$_{d}$ = 58,000 and model blockage ratios of up to 17{\%}. The results show that, in GSW and ASW, blockage effects significantly alter flow development, affecting separated shear layer instability frequency, vortex shedding frequency, and limiting wake growth. Streamlining the walls successfully mitigates these adverse effects, with the relevant flow parameters shown to match those obtained in previous investigations conducted at low blockage ratios. Although the blockage effects produce an increase of both the separated shear layer instability and the wake vortex shedding frequency in GSW and ASW, the results suggest the ratio of these frequencies is invariant with the wall configuration. A comparative analysis of experimental data is performed to explain the observed trends. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AN.00009: Design of a High Viscosity Couette Flow Facility for Patterned Surface Drag Measurements Tyler Johnson, Amy Lang Direct drag measurements can be difficult to obtain with low viscosity fluids such as air or water. In this facility, mineral oil is used as the working fluid to increase the shear stress across the surface of experimental models. A mounted conveyor creates a flow within a plexiglass tank. The experimental model of a flat or patterned surface is suspended above a moving belt. Within the gap between the model and moving belt a Couette flow with a linear velocity profile is created. PIV measurements are used to determine the exact velocities and the Reynolds numbers for each experiment. The model is suspended by bars that connect to the pillow block housing of each bearing. Drag is measured by a force gauge connected to linear roller bearings that slide along steel rods. The patterned surfaces, initially consisting of 2-D cavities, are embedded in a plexiglass plate so as to keep the total surface area constant for each experiment. First, the drag across a flat plate is measured and compared to theoretical values for laminar Couette flow. The drag for patterned surfaces is then measured and compared to a flat plate. [Preview Abstract] |
Sunday, November 22, 2009 9:57AM - 10:10AM |
AN.00010: Redesign of contraction, test section and diffuser for a six-inch high speed water tunnel Ivaylo Nedyalkov, Martin Wosnik The six-inch high speed water tunnel was recently moved from St. Anthony Falls Laboratory to the University of New Hampshire, where it is being restored. This water tunnel was a 1:6 scale model for the 36-inch Variable Pressure Cavitation Tunnel at David Taylor Model Basin and was used in many fundamental cavitation studies in the past, including the development of Schiebe bodies. It originally had a 6-inch circular test section and was later retrofitted with a 7-inch octagonal test section. In order to increase the maximum achievable velocity in the test section and improve the flow quality a new 6-inch square test section with diminishing 1-inch fillets was designed, which also required the design of a new contraction and diffuser. Contraction, test section and diffuser configurations were studied parametrically using CFD. The numerical predictions are compared to results in the literature and measurements in the tunnel. Further improvements include a new motor and control system. The renovated six-inch tunnel will be used for research on control of cavitating flows, hydrofoil development and general cavitation studies. [Preview Abstract] |
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