Numerical prediction of unsteady compressible turbulent coaxial jets

Compressible and turbulent air-air coaxial jets with a strong velocity ratio are simulated by using a finite volume scheme and a two-equation turbulence model. For the calculations only the explicit part of the numerical method is used. This allows the simulation of the coherent structures evolving in the flow. The results show a strong instability of the flow near the inlet, this natural unsteadiness decays downstream and vanishes before x/D = 15. In the outer mixing layer the coherent structures evolve as in a simple round jet. Near the inlet, in the inner mixing layer, a recirculation region is found on the jet axis: The presence of this phenomenon in homogeneous coaxial jets is the consequence of the strong velocity ratio between the two streams and explains the lack of the inner potential core. These numerical results agree qualitatively with previous experimental investigations. (C) 1998 Elsevier Science Ltd. All rights reserved.