“The Acoustic Mode and its Sources in a Supersonic Jet”
A universal definition based on momentum density is adopted to distinguish between the acoustic, hydrodynamic and thermal modes of a Mach 1.3 supersonic cold jet. The extracted hydrodynamic mode highlights the shear layer breakdown and intense mixing of the jet. Particular emphasis is placed in this paper on the acoustic mode which has a coherent 3-D wavepacket form in the core of the jet and is characterized by one component of the irrotational field of momentum density, the other being thermal. The acoustic wavepacket naturally yields the nearfield radiative pattern and propagates at local sonic velocity. An azimuthal mode analysis of the acoustic wavepacket shows the prominence of the axisymmetric mode in regions upstream of the core collapse location and its predominant role in transmitting acoustic energy into the nearfield. The higher azimuthal modes are limited to the vicinity of nozzle exit, resulting from the destabilization of the shear layer. The total fluctuating enthalpy (TFE) equation is then examined to identify the most significant acoustic sources of the jet. Statistical analysis of the hydrodynamic mode inside the core indicates that vortices intruding into the core generate TFE, setting up a necessary condition for transfer of energy from the highly disorganized hydrodynamic mode to the well-defined and coherent wavepacket, resulting in sound radiation. On the other hand, shearing of fluid entrained into the core of the jet leads to dissipation of TFE.
55th AIAA Aerospace Sciences Meeting