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  • 1
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    Characterization of Measurement Error Sources in Doppler Global Velocimetry (2001)
    In:  Other Sources
    Details
    Publication Date: 2011-08-23
    Description: Doppler global velocimetry uses the absorption characteristics of iodine vapor to provide instantaneous three-component measurements of flow velocity within a plane defined by a laser light sheet. Although the technology is straightforward, its utilization as a flow diagnostics tool requires hardening of the optical system and careful attention to detail during data acquisition and processing if routine use in wind tunnel applications is to be achieved. A development program that reaches these goals is presented. Theoretical and experimental investigations were conducted on each technology element to determine methods that increase measurement accuracy and repeatability. Enhancements resulting from these investigations included methods to ensure iodine vapor calibration stability, single frequency operation of the laser and image alignment to sub-pixel accuracies. Methods were also developed to improve system calibration, and eliminate spatial variations of optical frequency in the laser output, spatial variations in optical transmissivity and perspective and optical distortions in the data images. Each of these enhancements is described and experimental examples given to illustrate the improved measurement performance obtained by the enhancement. The culmination of this investigation was the measured velocity profile of a rotating wheel resulting in a 1.75% error in the mean with a standard deviation of 0.5 m/s. Comparing measurements of a jet flow with corresponding Pitot measurements validated the use of these methods for flow field applications.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: Measurement Science and Technology (ISSN 0957-0233); Volume 12; 357-368
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  • 2
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    Doppler Global Velocimeter Measurements of the Vortical Flow Above a Thin Delta Wing (1991)
    In:  CASI
    Details
    Publication Date: 2018-06-05
    Description: A new measurement technique is being developed by NASA to measure off-surface flow fields. This method, Doppler global velocimetry, will allow quantification of complex three-dimensional flow fields at video camera rates. The entire flow field structure within a selected plane is measured simultaneously rather than by scanned, point-by-point measurements using conventional laser velocimetry. Data obtained using this technique will be used to correlate with other data sets for verification, and following verification, provide a quantified, highly detailed definition of the flow field. This will help to improve the understanding of fluid physics, supplement and broaden the database required to validate and refine computational fluid dynamics (CFD) models, and improve aircraft design methodology. To assess the capability of the technique, velocity measurements of the vortical flow field above a thin 75-degree delta wing were made in the NASA - Langley Basic Aerodynamics Research Tunnel. Preliminary comparisons of the results were made with similar measurements obtained using a three component laser velocimeter indicate that this technique is capable of describing the entire three - component velocity flow field simultaneously within a measurement plane in real time.
    Keywords: Fluid Mechanics and Thermodynamics
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  • 3
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    Assessing the Capability of Doppler Global Velocimetry To Measure Vortical Flow Fields (1994)
    In:  CASI
    Details
    Publication Date: 2018-06-02
    Description: A new nonintrusive flow diagnostics instrumentation system, Doppler global velocimetry, is presented. The system is capable of making simultaneous, three-component velocity measurements within a selected measurement plane at video camera rates. These velocity images can provide the researcher with spatial and temporal information about the flow field in a global sense. The investigation of a vortical flow above a 75-degree delta wing comparing standard three-component, fringe-type laser velocimetry measurements with Doppler global velocimetry measurements is presented.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: Journal of Aerospace Engineering; Volume 208; Part G; 99-105
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  • 4
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    Application of Doppler Global Velocimetry to Supersonic Flows (1996)
    In:  CASI
    Details
    Publication Date: 2018-06-02
    Description: The design and implementation of Doppler Global Velocimetry (DGV) for testing in the Langley Unitary Plan Wind Tunnel is presented. The discussion begins by outlining the characteristics of the tunnel and the test environment, with potential problem areas highlighted. Modifications to the optical system design to implement solutions for these problems are described. Since this tunnel entry was the first ever use of DGV in a supersonic wind tunnel, the test series was divided into three phases, each with its own goal. Phase I determined if condensation provided sufficient scattered light for DGV applications. Phase II studied particle lag by measuring the flow about an oblique shock above an inclined flat plate. Phase III investigated the supersonic vortical flow field above a 75-degree delta wing at 24-degrees angle of attack. Example results from these tests are presented.
    Keywords: Fluid Mechanics and Thermodynamics
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  • 5
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    Assessing the Capability of Doppler Global Velocimetry to Measure Vortical Flow Fields (1992)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A new measurement technique, Doppler global velocimetry, has been described along with results from an experimental investigation of a vortical flow field. The results of this investigation, when compared to conventional laser velocimeter measurements, indicate that this technique is capable of describing the global velocity field within a measurement plane in real time. Further investigations are being conducted to correct the deficiencies described and to refine the measurement technique for flight applications.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: Seminar on Optical Methods and Data Processing in Heat and Fluid Flow; Apr 02, 1992 - Apr 03, 1992; London; United Kingdom
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  • 6
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    Investigation of Particle Sampling Bias in the Shear Flow Field Downstream of a Backward Facing Step (1990)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The flow field about a backward facing step was investigated to determine the characteristics of particle sampling bias in the various flow phenomena. The investigation used the calculation of the velocity:data rate correlation coefficient as a measure of statistical dependence and thus the degree of velocity bias. While the investigation found negligible dependence within the free stream region, increased dependence was found within the boundary and shear layers. Full classic correction techniques over-compensated the data since the dependence was weak, even in the boundary layer and shear regions. The paper emphasizes the necessity to determine the degree of particle sampling bias for each measurement ensemble and not use generalized assumptions to correct the data. Further, it recommends the calculation of the velocity:data rate correlation coefficient become a standard statistical calculation in the analysis of all laser velocimeter data.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: Fifth International Symposium on Applications of Laser Techniques to Fluid Mechanics; Jul 09, 1990 - Jul 12, 1990; Lisbon; Portugal
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  • 7
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    Investigation of Point Doppler Velocimetry (PDV) for Transition Detection in Boundary Layers (1999)
    In:  CASI
    Details
    Publication Date: 2019-07-10
    Description: A two-component Point Doppler Velocimeter (PDV) system has been improved through the use of vapor-limited iodine cells that have responses that are insensitive to temperature variations. Two-component PDV velocity measurements have been obtained for a 1 inch diameter uniform circular jet flow at a nominal exit velocity of 60 m/sec, corresponding to a Reynolds number of 100,000. Similar data have also been obtained for an annular jet and a swirling jet, These PDV data runs have been duplicated to judge the repeatability of these measurements, and also have been compared with hot wire anemometer data for the same flow conditions. PDV mean velocity results are repeatable to within approximately 1-2 meters per second; the PDV RMS velocity results are also quite repeatable. Exit profiles of PDV mean axial velocity data generally agree with hot wire anemometer results to within about 2 meters per second as well. However, the PDV RMS velocity results are consistently lower than the hot wire results everywhere but at the exit of the standard jet, where they are too high relative to the hot wire data. This is believed to at least be partially due to the method used to compute the RMS (Root Mean Square).
    Keywords: Fluid Mechanics and Thermodynamics
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  • 8
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    Shock-Strength Determination With Seeded and Seedless Laser Methods (2008)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Two nonintrusive laser diagnostics were independently used to demonstrate the measurement of time-averaged and spatially-resolved pressure change across a twodimensional (2-D) shock wave. The first method is Doppler global velocimetry (DGV) which uses water seeding and generates 2-D maps of 3-orthogonal components of velocity. A DGV-measured change in flow direction behind an oblique shock provides an indirect determination of pressure jump across the shock, when used with the known incoming Mach number and ideal shock relations (or Prandtl-Meyer flow equations for an expansion fan). This approach was demonstrated at Mach 2 on 2-D shocks and expansions generated from a flat plate at angles-of-attack approx. equals -2.4deg and +0.6deg, respectively. This technique also works for temperature jump (as well as pressure) and for normal shocks (as well as oblique). The second method, laser-induced thermal acoustics (LITA), is a seedless approach that was used to generate 1-D spatial profiles of streamwise Mach number, sound speed, pressure, and temperature across the same shock waves. Excellent agreement was obtained between the DGV and LITA methods, suggesting that either technique is viable for noninvasive shock-strength measurements.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: LF99-7418
    Format: application/pdf
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  • 9
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    Velocity Vector Analysis of a Juncture Flow Using a Three-Component Laser Velocimeter (1984)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A specialized single-axis, five-beam three-component laser velocimeter was constructed and used to study the flow field in a juncture. The juncture was defined by a blunt leading, edged vertical splitter plate and a sharp leading edged horizontal plate. The investigations were conducted in the Low Turbulence Pressure Tunnel at a Mach number of 0.1 and a Reynolds number of 2.2 x 10(exp 6) per meter over the model. The three-component velocity flow field in the juncture was measured, Reynolds stresses calculated and the velocity vector analysis performed.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: Second International Symposium on Applications of Laser Anemometry to Fluid Mechanics; Jul 02, 1984 - Jul 04, 1984; Lisbon; Portugal
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  • 10
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    The Elusive Third Component (2004)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The historical development of techniques for measuring three velocity components using laser velocimetry is presented. The techniques are described and their relative merits presented. Many of the approaches currently in use based on the fringe laser velocimeter have yielded inaccurate measurements of turbulence intensity in the on-axis component. A possible explanation for these inaccuracies is presented along with simulation results.
    Keywords: Fluid Mechanics and Thermodynamics
    Type: Symposium on Laser Anemometry ASME 1985 Winter Annual Meeting; Nov 17, 1985 - Nov 21, 1985; Miami, FL; United States
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