To improve the performance of particle image velocimetry in measuring instantaneous velocity fields, direct cross-correlation of image fields can be used in place of auto-correlation methods of interrogation of double- or multiple-exposure recordings. With improved speed of photographic recording and increased resolution of video array detectors, cross-correlation methods of interrogation of successive single-exposure frames can be used to measure the separation of pairs of particle images between successive frames. By knowing the extent of image shifting used in a multiple-exposure and by a priori knowledge of the mean flow-field, the cross-correlation of different sized interrogation spots with known separation can be optimized in terms of spatial resolution, detection rate, accuracy and reliability.

Theory of cross-correlation analysis of PIV images : Image analysis as measuring technique in flows

Infrared laser-absorption sensing for combustion gases

Progress in shock wave/boundary layer interactions

Manuscript received April 14, 1983; revised June 24, 1983. N. B. Abraham is with the Department of Physics, BrynMawr College, Bryn Mawr, PA 19010. P. Mandel is with the Service de Chimie Physique 11, Universite Libre de Bruselles, Brussels, Belgium. L. W. Casperson is with the Department of Electrical Engineering and Applied Science: University of California, Los Angeles, CA 90024. Editor’s Note: A similar correction by Dr. F. Holigner and Prof. Dr. H. Weber of the Universitat Kaiserslautern was received on May 4, 1983. tions can be expressed as BR = y + p P (3a j

Principles of lasers

conferencia sobre "Particle Image Velocimetry" de les 13:00-14:00 a la Sala de Juntes de l'FNB impartida pel professor Peter Bueken da l'Antwerp Maritime Acedemy (Belgica)

Particle Image Velocimetry

A constant-area isolator was fabricated and tested in conjunction with a Mach 2 hydrogen-air combustor operating at a simulated Mach 5 flight enthalpy. Predicted isolator performance was validated through pressure measurements obtained via low-frequency pressure taps. The maximum pressure ratio measured in the combustor approached the design limit of 4.5. Scramjet operability, the range of equivalence ratios over which combustion was sustained without shock-inlet interaction, was improved to 0.06-0.32, as opposed to 0.32-0.37 without the isolator. For a given change in fuel equivalence ratio, the location of the shock train was easier to control with the isolator modification. Shock-train location repeatability was found to vary somewhat with equivalence ratio. Small fluctuations in the time-resolved pressure history indicated that the shock train was relatively temporally steady for a given equivalence ratio. High-frequency pressure measurements were within a 95% confidence interval of low-frequency pressure measurements. High-frequency results indicated that an increase in pressure and large pressure fluctuations occurred near the leading edge of the shock train. Power spectral analyses also indicated that there is significant variation in the frequency content of the pressure signal upstream and downstream of the shock-train leading edge. These results suggest that methods of shock-train leading-edge detection may be developed using pressure-time history characteristics other than the pressure magnitude.

Experimental Study of a Dual-Mode Scramjet Isolator

Results of an experimental and numerical study of a dual-mode scramjet combustor are reported The experiment consisted of a direct-connect test of a Mach 2 hydrogen-air combustor with a single unswept-ramp fuel injector The flow stagnation enthalpy simulated a flight Mach number of 5 Measurements were obtained using conventional wall instrumentation and a particle-imaging laser diagnostic technique The particle imaging was enabled through the development of a new apparatus for seeding fine silicon dioxide particles into the combustor fuel stream Numerical simulations of the combustor were performed using the GASP code The modeling, and much of the experimental work, focused on the supersonic combustion mode Reasonable agreement was observed between experimental and numerical wall pressure distributions However, the numerical model was unable to predict accurately the effects of combustion on the fuel plume size, penetration, shape, and axial growth

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Dual-Mode Combustion of Hydrogen in a Mach 5, Continuous-Flow Facility

An experimental study was performed to characterize the effects of vitiation due to combustion-air preheating on dual-mode scramjet combustion.Major combustion vitiation species (H2O andCO2)were added to the freestreamof an electrical-resistance-heated, direct-connect facility simulating Mach 5 flight enthalpy. With clean, dry air, the combustor operated in the supersonic mode at fuel equivalence ratios below 0.22, and in the subsonic mode for equivalence ratios above 0.26. Hysteresis was observed in the dual-mode transition region between 0.22 and 0.26, as the mode of combustion was dependent on whether the fuel rate was increasing or decreasing. Adding increasing amounts of water vapor and carbon dioxide to the freestream decreased combustor pressures by 10 to 30% for the same fuel equivalence ratio. Vitiation also caused transition between supersonic and subsonic combustion to occur at a higher fuel equivalence ratio thanwith clean air. This work represents the first direct evaluation of the effect of testmedium vitiation on dual-mode scramjet combustion atMach 5 enthalpy simulation in the same facility. The results indicate the importance of accounting for test-medium vitiation when extrapolating from ground-testing to flight, particularly in the dual-mode transition region between subsonic and supersonic combustion regimes.

Experimental Study of Test-Medium Vitiation Effects on Dual-Mode Scramjet Performance

Time-resolved pressure measurements in a dual-mode scramjet isolator were examined to investigate the potential for using the measurements for shock train leading-edge detection. Changes in the pressure magnitude, standard deviation levels, and frequency content were observed as the shock train advanced upstream past each pressure measurement station. Three detection criteria were defined and examined: 1) 150% of the normalized pressure magnitude upstream of combustion influences, 2) 150% of the normalized pressure standard deviation level upstream of combustion influences, and 3) the maximum value of the normalized pressure standard deviation. Another method of shock train leading-edge detection involved the examination of the frequency content of the pressure signal using power spectra analysis. Results indicated that the second detection criterion provided the earliest method of shock train detection as the shock train moved upstream, followed by the first and third criteria. Also, the frequency content of the pressure signals significantly changed near the shock train leading edge. However, a comparison of this method to the three criteria first examined showed that it did not provide earlier shock train detection.

Shock Train Leading-Edge Detection in a Dual-Mode Scramjet

A Mach 2, hydrogen-air combustor with an unswept 10-deg ramp fuel injector was experimentally and numerically studied for a simulated flight Mach number near 5. Numerical modeling was performed using the Viscous Upwind Algorithm for Complex Flow Analysis code, and results were compared against experimental wall-pressure distributions, fuel plume images, and fuel plume velocity measurements. The model matched wall-pressure distributions well for the case of fuel-off and fuel-air mixing. For a fuel-air reacting case, pressure was matched well in the upstream third of the duct. Downstream, however, the pressure rise as a result of combustion was underpredicted. Based on the fuel plume imaging and velocity measurements, fuel plume shape was matched well for both the mixing and reacting cases. However, plume size, penetration, and centerplane axial growth were generally underpredicted by the model. The full extent of the velocity reduction caused by thermal choking was also not predicted. Despite these findings, the numerical model performed better than a previous model developed by the investigators. It was proposed that differences between the present numerical model and experiment stemmed from numerical underprediction of fuel-air turbulent mixing, and this resulted in underprediction of heat release.

Christopher P. Goyne

Papers

Experimental Study of a Dual-Mode Scramjet Isolator

Dual-Mode Combustion of Hydrogen in a Mach 5, Continuous-Flow Facility

Experimental Study of Test-Medium Vitiation Effects on Dual-Mode Scramjet Performance

Shock Train Leading-Edge Detection in a Dual-Mode Scramjet

Experimental and numerical study of a dual-mode scramjet combustor