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Screech Tones from Rectangular Jets with Spanwise Oblique Shock-cell Structures
Title
Screech Tones from Rectangular Jets with Spanwise Oblique Shock-cell Structures
Creator
Raman, G.
Date
1997-01-10
Publisher
Cambridge Univ Press
Description
Understanding screech is especially important for the design of advanced aircraft because screech can cause sonic fatigue failure of aircraft...
Show moreUnderstanding screech is especially important for the design of advanced aircraft because screech can cause sonic fatigue failure of aircraft structures. Although the connection between shock-cell spacing and screech frequency is well understood, the relation between non-uniformities in the shock-cell structures and the resulting amplitude, mode, and steadiness of screech have remained unexplored. This paper addresses the above issues by intentionally producing spanwise (larger nozzle dimension) variations in the shock-cell structures and studying the resulting spanwise screech mode. The spanwise-oblique shock-cell structures were produced using imperfectly expanded convergent-divergent rectangular nozzles (aspect ratio = 5) with non-uniform exit geometries. Three geometries were studied: (a) a nozzle with a spanwise uniform edge, (b) a nozzle with a spanwise oblique (single-bevelled) edge, and (c) a nozzle that had two spanwise oblique (double-bevelled) cuts to form an arrowhead-shaped nozzle. For all nozzles considered, the screech mode was antisymmetric in the transverse (smaller nozzle dimension) direction allowing focus on changes in the spanwise direction. Three types of spanwise modes were observed: symmetric (I), antisymmetric (II), and oblique (III), The following significant results emerged: (i) for all cases the screech mode corresponds with the spanwise shock-cell structure, (ii) when multiple screech modes are present, the technique presented here makes it possible to distinguish between coexisting and mutually exclusive modes, (iii) the strength of shocks 3 and 4 influences the screech source amplitude and determines whether screech is unsteady. The results presented here offer hope for a better understanding of screech and for tailoring shock-containing jets to minimize fatigue failure of aircraft components.
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Cessation of Screech in Underexpanded Jets
Title
Cessation of Screech in Underexpanded Jets
Creator
Raman, G.
Date
1997-04-10
Publisher
Cambridge Univ Press
Description
In significantly underexpanded jets, screech inherently ceases to exist. This paper studies screech cessation in a supersonic rectangular jet...
Show moreIn significantly underexpanded jets, screech inherently ceases to exist. This paper studies screech cessation in a supersonic rectangular jet and provides an explanation for its occurrence. Experimental data are presented for fully expanded Mach numbers, M-j, ranging from 1.1 to 1.9. Screech becomes unsteady beyond M-j = 1.65 and ceases to exist beyond M-j = 1.75. The reason for this cessation has remained a mystery, and this paper examines three suspects: (i) the theory of a frequency mismatch between screech tones and the band of the most-amplified jet instability waves, (ii) the notion that Mach disk formation disrupts the shock-cell structure and weakens the screech-producing shocks, and (iii) the idea that acoustic feedback and receptivity diminish at high levels of underexpansion. A thorough interrogation of experimental data shows that (i) is not the main cause of screech cessation here, (ii) plays an insignificant role, and (iii) appears to have been largely responsible for screech cessation. Cessation occurs because feedback to the jet lip is diminished due to excessive expansion of the jet boundary. Further, since the jet lip now reflects and scatters low intensity sound, the end result is poor receptivity at the initial shear layer. This theory is substantiated by the re-activation of screech when the nozzle lip thickness is made larger than the expanded jet boundary. Finally, increasing lip thickness is seen to produce a systematic shift (to higher M-j) of the onset of cessation. The results of this study are of direct relevance to the sonic fatigue problem in aircraft structures, because understanding screech helps prevent such damage.
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