Journal of Fluid Mechanics


Global modes and transient response of a cold supersonic jet


a1 Center for Turbulence Research, Stanford University, Stanford, CA 94305-3035, USA

a2 Department of Aeronautics and Astronautics, Stanford University, Stanford, CA 94305-4035, USA

a3 Department of Mechanical Engineering, Stanford, CA 94305-3030, USA


Global-mode analysis is applied to a cold, M = 2.5 laminar jet. Global modes of the non-parallel jet capture directly both near-field dynamics and far-field acoustics which, in this case, are coupled by Mach wave radiation. In addition to type (a) modes corresponding to Kelvin–Helmholtz instability, it is found that the jet also supports upstream-propagating type (b) modes which could not be resolved by previous analyses of the parabolized stability equations. The locally neutrally propagating part of a type (a) mode consists of the growth and decay of an aerodynamic wavepacket attached to the jet, coupled with a beam of acoustic radiation at a low angle to the jet downstream axis. Type (b) modes are shown to be related to the subsonic family of modes predicted by Tam & Hu (1989). Finally, significant transient growth is recovered by superposing damped, but non-normal, global modes, leading to a novel interpretation of jet noise production. The mechanism of optimal transient growth is identified with a propagating aerodynamic wavepacket which emits an acoustic wavepacket to the far field at an axial location consistent with the peaks of the locally neutrally propagating parts of type (a) modes.

(Received April 15 2010)

(Revised October 07 2010)

(Accepted October 08 2010)

(Online publication January 31 2011)

Key words:

  • absolute/convective instability;
  • jet noise;
  • jets


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