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Experiments in Axisymmetric Supersonic Jets.

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Doctoral thesis, 15 Apr 89-30 Sep 95,

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An experimental study of the effects of Mach number and density ratio on the development of axisymmetric jets is described. Values of the ratio of jet to-ambient-density from 0.23 to 5.5 were obtained by using different pairs of helium, nitrogen and argon gases. Jet exit Mach numbers were 1.41, 2.0, and 3.0. These discharge into gases at rest velocity ratio 0, providing density ratios from 0.23 to 5.5. To generate shock-free jets, unique nozzles were designed and constructed for each gas and Mach-number combination. A plating method for the construction of the nozzles was developed to ensure high accuracy and good surface finish at a cost significantly less than direct-machining techniques. The spreading rate of the jet was measured with a pitot probe. Centerline data were used to characterize the length of the potential core of the jet, which correlates well with the relative spreading rates. Some frequency data were obtained by piezo-resistive pressure probes. Spark shadowgraphy was used to visualize both mean and instantaneous flows. Convection velocities of large-scale disturbances were estimated from the visible Mach acoustic waves emanating from the jet. For a wide range of Mach and Reynolds numbers, the convection velocity of these disturbances in the potential core region of the jet is approximately 0.8 times the jet velocity, the approximate velocity of the first helical instability mode of the jet. The convective Mach number concept used in 2-d shear layer research does not collapse the axisymmetric data onto the 2-d curve. However, the density-ratio scaling used for 2-d shear layers appears to work also for the axisymmetric jet. The data suggest that the initial development of the jet is dominated by instability modes of the jet as a whole, rather than the shear layers. An anomaly noted was the long period variations in the centerline total pressur

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  • Fluid Mechanics

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