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 BN: Experimental Techniques II: Near Wall Flows |
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Chair: John Foss, Michigan State University Room: 200C |
Sunday, November 22, 2009 10:30AM - 10:43AM |
BN.00001: Wall shear measurements of turbulent flow over backward facing step Nguyen Thien Duy, Wells John Craig, Nguyen Chuong Vinh This paper is aim to generalize our recent developments of an extension of particle image velocimetry (PIV) measurement technique, named as Interfacial PIV (noted as IPIV). It enables us to deal with near-wall flows over curved walls by means of conformal transformation. In addition, if the displacement of tracer in wall-normal direction is less than its diameter, IPIV could instantaneously and precisely measure the wall shear gradient as well as draw out the tangential velocity profile. Our integration of measured velocity gradients upward from wall could provide a continuous profile with single pixel resolution. In this communication, we extend our wall shear measurement technique to stereo-PIV setup. Validation of IPIV wall shear measurement to 2-dimensional and 3-dimensional synthetic images of turbulent flows over a wavy bed is performed. In practice, experiment with backward-facing step (BFS) at a low Reynolds number of 2800 is examined. A system of two two-component PIV (2CPIV) coupled with a stereo-PIV (SPIV) is conducted to inquire the flow structure. Applications of IPIV to our 2CPIV and SPIV experimental images are also described. [Preview Abstract] |
Sunday, November 22, 2009 10:43AM - 10:56AM |
BN.00002: Measurements of mechanical torque acting on a model wind turbine Hyung-Suk Kang, Raul B. Cal, Jose Lebron-Bosques, Luciano Castillo, Charles Meneveau A high precision torque converter is developed and calibrated to measure torques on model wind-turbine generators (small DC motor). By multiplying the measured torque and rotor angular velocity, a direct measurement of the extracted power from the wind turbine can be obtained. This direct method is more advantageous compared to the electrical power measured from the model generator, since it avoids contamination by internal friction, and electric and magnetic losses. The torque converter is mounted on a model wind turbine in a 3 by 3 array (3 rows by 3 columns) of wind turbines in the Corrsin wind tunnel. The power coefficients are obtained as a function of the tip speed ratio. Significant difference between the electrical and mechanical powers is observed, which emphasizes on the importance in using the direct mechanical power measurement. Also, the extracted power by the model turbine is found to be comparable with the power estimated from the kinetic energy flux measured using SPIV. [Preview Abstract] |
Sunday, November 22, 2009 10:56AM - 11:09AM |
BN.00003: Measurement of the Wall Shear Stress Field Created by a Supersonic Impinging Microjet Bridget Schabron, Jonathan Naughton, Farrukh Alvi The study of wall shear stress beneath a supersonic impinging microjet was carried out using oil film interferometry. The measurement of wall shear stress in a microjet is an interesting flow both for its practical applications as well as the challenges the surface flow poses. The wall shear stress measurement challenges for this flow included the radially symmetric wall shear stress field, the very high gradients of wall shear stress, optical access issues, and the significant temperature variations on the surface that affect the oil's viscosity. Measurements were made for various jet height above the plate $h$ to jet diameter $D$ ratios and for various pressure ratios. Analysis of the resulting interferograms and the challenges posed by this particular geometry are discussed. The results demonstrate the capability of oil film interferometry, particularly its dynamic range, for measuring wall shear stress in complex flow. [Preview Abstract] |
Sunday, November 22, 2009 11:09AM - 11:22AM |
BN.00004: The diagnostic plot - a new way to appraise turbulent boundary layer data P.H. Alfredsson, R. Oerlue Most turbulent boundary layer data are obtained with hot-wire anemometry which gives access both to the mean ($U$) and turbulence intensity ($u^{\prime}$) distributions of the streamwise velocity. Comparisons between different measurements strongly depend on the accuracy of the determination of the friction velocity ($u_{\tau}$) and for comparisons in the near wall region the determination of the wall position is crucial. If $u^{\prime}$ is plotted as function of $U$, where both quantities are normalized by the free stream velocity ($U_{\infty}$) (hereafter called the {\it diagnostic plot}), any uncertainties in the wall position and $u_{\tau}$ are avoided when comparing different cases. For a given $Re$ all such distributions should fall on top of each other if $u^{\prime}$ and $U$ are accurately measured and the measurements are made in standard zero pressure gradient turbulent boundary layers. Close to the wall ($y^+<10$) the distribution is nearly self similar and independent of $Re$. Moreover the distribution should be linear with a constant slope ($\sim 0.40$) up to, at least, $y^+=3$ (corresponding to $U/U_{\infty} > 0.1$ for typical laboratory experiments) and thereafter the slope should decrease. The diagnostic plot clearly indicates at what position the measured values show a wall interference effect. Also in the outer region the distributions at different Reynolds numbers overlap, and the diagnostic plot has the interesting property that both the inner ($y^+<10$) and outer regions can be made to collapse in the same plot. [Preview Abstract] |
Sunday, November 22, 2009 11:22AM - 11:35AM |
BN.00005: Simultaneous drag and flow measurements of Olympic skeleton athletes Yae Eun Moon, David Digiulio, Steve Peters, Timothy Wei The Olympic sport of skeleton involves an athlete riding a small sled face first down a bobsled track at speeds up to 130 km/hr. In these races, the difference between gold and missing the medal stand altogether can be hundredths of a second per run. As such, reducing aerodynamic drag through proper body positioning is of first order importance. To better study the flow behavior and to improve the performance of the athletes, we constructed a static force balance system on a mock section of a bobsled track. Athlete and the sled are placed on the force balance system which is positioned at the exit of an open loop wind tunnel. Simultaneous drag force and DPIV velocity field measurements were made along with video recordings of body position to aid the athletes in determining their optimal aerodynamic body position. [Preview Abstract] |
Sunday, November 22, 2009 11:35AM - 11:48AM |
BN.00006: Near Wall measurement in Turbulent Flow over Rough Wall using microscopic HPIV Siddharth Talapatra, Jiarong Hong, Joseph Katz Using holographic PIV, 3D velocity measurements are being performed in a turbulent rough wall channel flow. Our objective is to examine the contribution of coherent structures to the flow dynamics, momentum and energy fluxes in the roughness sublayer. The 0.45mm high, pyramid-shaped roughness is uniformly distributed on the top and bottom surfaces of a 5X20cm rectangular channel flow, where the Re$_{\tau }$ is 3400. To facilitate recording of holograms through a rough plate, the working fluid is a concentrated solution of NaI in water, whose optical refractive index is matched with that of the acrylic rough plates. The test section is illuminated by a collimated laser beam from the top, and the sample volume extends from the bottom wall up to 7 roughness heights. After passing through the sample volume, the in-line hologram is magnified and recorded on a 4864X3248 pixels camera at a resolution of 0.74$\mu $m/pixel. The flow is locally seeded with 2$\mu $m particles. Reconstruction, spatial filtering and particle tracking provide the 3D velocity field. This approach has been successfully implemented recently, as preliminary data demonstrate. [Preview Abstract] |
Sunday, November 22, 2009 11:48AM - 12:01PM |
BN.00007: Performance of a Photonic Wall Shear Stress Sensor Ulas Ayaz, Tindaro Ioppolo, Volkan Otugen The performance of a photonic wall shear stress sensor prototype based on the so-called whispering gallery modes (WGM) of polymeric microspheres is investigated in steady and unsteady flows. In this sensor, the shear force due to the fluid flow is transmitted to a Polydimethylsyloxane sphere of several hundred microns in diameter which serves as the sensor. The corresponding optical resonance (WGM) shifts are monitored to determine the wall shear stress. Sensor performance for dynamic range, resolution and bandwidth are studied analytically, and validated experimentally. The validation experiments for the prototype sensor with measurement area of 1mm$^{2}$ are made in a two-dimensional channel flow and in an acoustic plane wave tube. These measurements indicate a shear stress resolution of $\sim $10$^{-3 }$Pa and a dynamic range of $\sim $100dB for the prototype. The PDMS sphere used in the prototype has a base-curing-agent ratio of 40:1. Different sensitivities and measurement ranges can be obtained using different PDMS mixing ratios. [Preview Abstract] |
Sunday, November 22, 2009 12:01PM - 12:14PM |
BN.00008: Simultaneous Reference- and Signal-Imaging for Cancellation of Unsteady Motion in Pressure-Sensitive Paint Measurement Kensuke Miyamoto, Takeshi Miyazaki, Hirotaka Sakaue Simultaneous reference- and signal-image acquisition for pressure-sensitive paint (PSP) measurement is presented in this paper. The system to acquire these images consists of a two-color PSP and a high-speed CCD camera. The two-color PSP provides a reference and a signal luminescence separated by their luminescent peak wavelengths of 520 nm and 620 nm, respectively. The reference luminescent image is acquired through a green filtered CCD, while the signal image through a red filtered CCD. This system can cancel a non-uniform illumination for exciting two-color PSP as well as a non-uniform image acquisition of a CCD camera due to the location between the camera and the testing object. This system can be applied to a PSP measurement, which includes translation, vibration, and deformation of the testing object. Our measurement system is thus advantage from a conventional PSP measurement, which requires stationary location among the testing object, camera, and illumination source. Unsteady motion of a plate coated with our two-color PSP is included as a demonstration. The developed system captured the unsteady motion as well as gas impingement on the coated plate with camera frame rate of 300 Hz. [Preview Abstract] |
Sunday, November 22, 2009 12:14PM - 12:27PM |
BN.00009: Global Visualization in Water using AnodizedAluminum PressureSensitive Paint and Dissolved Oxygen as Tracer Tatsuya Ozaki, Hitoshi Ishikawa, Hirotaka Sakaue We have developed anodized-aluminum pressuresensitive paint (AA-PSP) for flow visualization in water using dissolved oxygen as a tracer. Developed AA-PSP is characterized using water calibration setup by controlling a dissolved oxygen concentration. It is shown that the developed AA-PSP gives 4.0 percent change in luminescence per 1 mg/l of oxygen concentration. This AA-PSP is applied to visualize flows in a water tunnel. Oxygen concentrations of the water tunnel and the dissolved oxygen are 9.5 mg/l and 20 mg/l, respectively. We can capture horseshoe vortices over the base of 10 mm cylinder by using this technique at Reynolds number of 1000 and a water speed of 100 mm/s, respectively. Unlike conventional tracers such as ink, milk, and fluorescent dyes, this visualization technique gives flow information on the AA-PSP coated surface without integrating flows between the AA-PSP and an optical detector. Because of using dissolved oxygen as a tracer, it holds the material properties of testing water except for the amount of oxygen. The tracer does not interfere with optical measurements and it does not contaminate the testing water. A conventional visualization technique using milk as a tracer is also employed for comparison. [Preview Abstract] |
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