SCREECH TONES FROM RECTANGULAR JETS WITH SPANWISE OBLIQUE SHOCK-CELL STRUCTURES

Understanding screech is especially important for the design of advanc ed aircraft because screech can cause sonic fatigue failure of aircraf t structures. Although the connection between shock-cell spacing and s creech frequency is well understood, the relation between non-uniformi ties in the shock-cell structures and the resulting amplitude, mode, a nd steadiness of screech have remained unexplored. This paper addresse s the above issues by intentionally producing spanwise (larger nozzle dimension) variations in the shock-cell structures and studying the re sulting spanwise screech mode. The spanwise-oblique shock-cell structu res were produced using imperfectly expanded convergent-divergent rect angular nozzles (aspect ratio = 5) with non-uniform exit geometries. T hree geometries were studied: (a) a nozzle with a spanwise uniform edg e, (b) a nozzle with a spanwise oblique (single-bevelled) edge, and (c ) a nozzle that had two spanwise oblique (double-bevelled) cuts to for m an arrowhead-shaped nozzle. For all nozzles considered, the screech mode was antisymmetric in the transverse (smaller nozzle dimension) di rection allowing focus on changes in the spanwise direction. Three typ es 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 s tructure, (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 influe nces the screech source amplitude and determines whether screech is un steady. The results presented here offer hope for a better understandi ng of screech and for tailoring shock-containing jets to minimize fati gue failure of aircraft components.