Application of a Novel Projection Focusing Schlieren System in NASA Test Facilities

Fagan, A. L.; L’Espérance, Drew; Zaman, K. B. M. Q.

doi:10.2514/6.2014-2522

Cited by 10 publications

(7 citation statements)

References 5 publications

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“…In any case, the data in Figs. [3][4][5][6][7] indicate that the overall effect of the duct on the screech tones and the shock-structure of the jet are rather minimal. However, low frequency noise is generated due to the excited duct modes, and this is further examined in the following with the screech-free jet obtained by using the tabs.…”

Section: Resultsmentioning

confidence: 99%

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Noise from a Supersonic Round Jet Discharging into a Duct

Zaman

Fagan

2014

20th AIAA/CEAS Aeroacoustics Conference

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In an effort to understand an 'unwanted noise' problem occasionally encountered in ground test facilities, the interaction of a jet flow with a duct is studied in a model scale experiment. While the interaction of subsonic jets was studied earlier, that of supersonic jets is considered in this paper. The effect of the presence of a cylindrical duct in the path of the jet is studied through sound pressure level spectral measurements as well as schlieren flow visualization. When the jet involves screech tones the placement of the duct is found to make only minor effects on the tones themselves as well as on the high frequency noise. However, there is increased energy at low frequencies. The increase in low frequency noise becomes clearer when screech is eliminated from the jet by two small tabs placed at the nozzle exit. It is shown that spectral peaks and increased sound pressure levels occur at frequencies corresponding to the axial acoustic resonance modes of the duct. These peaks persist into the supersonic regime, however, their amplitudes diminish relative to increasing spectral amplitudes at other frequencies with increasing jet Mach number. A wire-mesh screen attached to the end of the duct effectively suppresses such unwanted noise at subsonic as well as supersonic conditions.

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Section: Resultsmentioning

confidence: 99%

“…The pictures shown in the following are essentially instantaneous snapshots of the density gradients in the flowfield capturing the shocks and vortical structures of the flow. Further details of the schlieren system can be found in [4].…”

Section: Experimental Facility and Proceduresmentioning

confidence: 99%

Noise from a Supersonic Round Jet Discharging into a Duct

Zaman

Fagan

2014

20th AIAA/CEAS Aeroacoustics Conference

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“…The thickness of the focused field was estimated to be about 8 cm. Further details can be found in Weinstein 25 and Fagan et al 26 Results and discussion Assessment of the flow visualization techniques for measuring shock-spacing (Figure 2(a)) appear sharper and more ''point-like'' than the corresponding tips in the schlieren image (Figure 2(c)). In addition, details are lost between the shock cells due to the dark regions of the time-averaged schlieren image, therefore the time-averaged shadowgraph is deemed more favorable compared to its schlieren counterpart.…”

Section: Experimental Setup and Proceduresmentioning

confidence: 99%

Variation of shock-spacing during screech stage-jumps

Clem

Zaman

Fagan

2016

International Journal of Aeroacoustics

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Flow visualization is used to investigate shock-spacings in a supersonic jet during stage-jumps associated with the screech phenomenon. Conventional schlieren and shadowgraphy techniques as well as a new projection focusing schlieren technique are employed. Both time-averaged and instantaneous snapshots of the flow field are analyzed for a 37.6 mm round convergent nozzle over the jet Mach number range of 1.0 < M j < 1.7. Shock-spacing is studied especially during hysteresis of the stage-jumps where two different screech frequencies could be realized at exactly the same M j depending on whether the latter variable was increased or decreased. The data follow a monotonic trend without any discontinuity at any M j . This establishes the fact that the stage-jumps in frequency are not associated with abrupt changes in the shock-spacing.

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“…The bandwidth primarily dictated the uncertainty in the measurement of the tone frequency. A focusing schlieren system was used for flow visualization The system is based on a lens-and-grid technique where a source grid is projected onto a retro-reflective screen and imaged onto a cut-off grid creating the schlieren effect; an interested reader may find details and further references in Fagan, L'Esperance & Zaman (2014). The light source illuminating the flow field had 1 µs pulse duration.…”

Section: Experimental Facility and Proceduresmentioning

confidence: 99%

An experimental investigation of resonant interaction of a rectangular jet with a flat plate

et al. 2015

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The interaction between an 8:1 aspect ratio rectangular jet and a flat plate, placed parallel to the jet, is addressed in this study. At high subsonic conditions and for certain relative locations of the plate, a resonance takes place with accompanying audible tones. Even when the tone is not audible the sound pressure level spectra are often marked by conspicuous peaks. The frequencies of these peaks, as functions of the plate's length, its location relative to the jet as well as jet Mach number, are studied in an effort to understand the flow mechanism. It is demonstrated that the tones are not due to a simple feedback between the nozzle exit and the plate's trailing edge; the leading edge also comes into play in determining the frequency. With parametric variation, it is found that there is an order in the most energetic spectral peaks; their frequencies cluster in distinct bands. The lowest frequency band is explained by an acoustic feedback involving diffraction at the plate's leading edge. Under the resonant condition, a periodic flapping motion of the jet column is seen when viewed in a direction parallel to the plate. Phase-averaged Mach number data on a cross-stream plane near the plate's trailing edge illustrate that the jet cross-section goes through large contortions within the period of the tone. Farther downstream a clear 'axis switching' takes place for the time-averaged cross-section of the jet that does not occur otherwise for a non-resonant condition.

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Application of a Novel Projection Focusing Schlieren System in NASA Test Facilities

Cited by 10 publications

References 5 publications

Noise from a Supersonic Round Jet Discharging into a Duct

Noise from a Supersonic Round Jet Discharging into a Duct

Variation of shock-spacing during screech stage-jumps

An experimental investigation of resonant interaction of a rectangular jet with a flat plate

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