An experimental investigation of the flow dynamics of streamwise vortices of various strengths interacting with a supersonic jet

Abstract

The results of an experimental study involving the introduction of streamwise vorticity into a supersonic jet are presented. Both streamwise and cross stream PIV measurements of a baseline jet and three vortex generator configurations of varied penetration were acquired in the overexpanded, ideally expanded, and underexpanded regimes. Streamwise vortex pairs were shown to persist no further than two diameters downstream independent of initial magnitude. Integration of the modulus of streamwise vorticity was shown to better correlate with secondary flow field modifications than maximum values. Reductions in shock cell spacing and downstream turbulence, and increases in initial spread rate and upstream integrated turbulence were correlated with increases in streamwise vorticity for all operating conditions. Streamwise vorticity was shown to achieve opposite effects on shock strength in the over/under expanded regimes due to introduction of secondary shock structures in the underexpanded case. Additionally, limited effect on potential core length was quantified.

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

“…Due to the increased radial spacing between the chevrons, a reduction in negative vortex interaction occurred, allowing the cross-sectional modification to persist further downstream than the standard configuration, as seen at z=D d ¼ 3:0. As evident by the cross-sectional modifications, the typical under performance of chevrons in the highly overexpanded regime [5,8,24] was not an issue in the current study due to the use of relatively high penetration chevrons.…”

“…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.…”

“…Shock strength is defined here following the manner of Heeb et al [24] as the difference between the upstream velocity maximum and downstream minimum velocity divided by the fully expanded velocity ðu 1 Àu 2 Þu À 1 j . Following the reasoning of Tam [50], which indicated that the upstream Mach disk was uncorrelated with shock noise levels, the most upstream shock strength was excluded from the average calculation.…”

“…The chevrons employed in this study had a length, penetration, and width of 0:26D d , 0:12D d , and 30°, respectively. This relatively aggressive design, in terms of penetration (see Heeb et al [24] for example), was chosen for the current study to ensure that an appreciable effect would occur in a highly overexpanded flow field.…”