Flow structure and acoustics of supersonic jets from conical convergent-divergent nozzles

Abstract

Conical convergent-divergent (CCD) nozzles represent an important category of supersonic jet-engine nozzles which require variable throat areas and variable exit areas to adapt to a range of operating conditions. CCD nozzles with design Mach numbers of 1.3, 1.5, and 1.65 are examined experimentally over a range of fully expanded Mach numbers from 1.22 to 1.71. The characteristics of the flow and acoustic fields from these nozzles are explored. Shadowgraph, Particle Image Velocimetry, far-field and near-field acoustic surveys are presented. Results of a Monotonically Integrated Large Eddy Simulation are presented for the Mach 1.5 nozzle at an underexpanded condition. The agreement between simulations and measurements is excellent. It is shown that these nozzles differ from traditional smoothly contoured method-of-characteristics nozzles in that they never achieve a shock free condition. Furthermore it is shown that these nozzles produce a "double diamond" pattern in which two sets of shock diamonds are generated with an axial displacement between them. The cause of this phenomenon is explored. It is further shown that as a consequence they are never free from shock-associated noise even when operated at perfect expansion. In spite of this difference, it is found that CCD nozzles behave like traditional convergent-divergent nozzles in that they produce the same shock-cell size, broadband shock-associated noise peak frequency, and screech frequency as traditional convergent-divergent nozzles. The apparent source regions for mixing noise, broadband shock associated noise and screech are all similar to those from traditional convergent-divergent nozzles. V

Impact of chevron spacing and asymmetric distribution on supersonic jet acoustics and flow

“…18a. Additionally, though the baseline jet exhausts from a conical nozzle, the typical double diamond shock cell structure was not present at M j ¼1.22, similar to the results shown by Munday et al [25]. The standard configuration reduced the shock cell spacing and the centerline Mach disk strength, while the initial spread rate and slip line radius was increased.…”

“…(6) was for moderately imperfectly expanded jets from smooth nozzles and is consequently only valid for the jet Mach number range which just encompasses the maximum BSAN in the over and under expanded regimes. The results of Munday et al [25] indicate that this valid Mach number range for the nozzle under investigation here is 1:36 r M j Z1:80. Therefore, the shock strength at the M j ¼1.22 condition was expected to be grossly over predicted by Eq.…”

“…The baseline jet flow field possess the large Mach disk and corresponding slip lines near the jet axis typical of overexpanded jets from conical CD nozzles [24,25,58], which are particularly apparent in Fig. 18a.…”

“…Drastic modification of the jet cross-section was achieved by the investigated configurations, with both elliptic and triangular shapes attained downstream. Consequently, screech was nearly eliminated with reductions in the range of [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] dB depending on the operating condition. Analysis of the streamwise velocity indicated that both the mean shock spacing and strength were reduced resulting in an increase in the broadband shock associated noise spectral peak frequency and a reduction in the amplitude, respectively.…”

“…Two overexpanded, the design, and an underexpanded condition were investigated following the work of Munday et al [25]. Table 1 outlines the chosen conditions along with the fully expanded parameters of Mach number, M j , jet velocity, u j , Reynolds number, Re j , and the fully expanded diameter to design diameter ratio, D j =D d .…”