The effect of the diameter ratio on the absolute and convective instability of free coflowing jets
Abstract
The stability of two coflowing streams with finite cross stream extent has been studied in both axisymmetric as well as plane geometries. Transition curves from convective to absolute instability have been obtained in terms of the control parameters, namely, the density ratio, S, the velocity ratio, Lambda, and, more importantly, the diameter ratio, a. A new mode of instability caused by the external shear layer included in the velocity profile has been identified. It has also been found that this mode is responsible for triggering the absolute instability of configurations considered convectively unstable in the limit a[right arrow]infinity. Moreover, in the present study it has been shown that, when Lambda=1, the critical density ratio necessary to sustain absolute instability in axisymmetric jets decreases with the diameter ratio as S_{c}[is proportional to]a^{4}, while it decreases as S_{c}[is proportional to]a^{6/5} in plane jets. In more general situations, where Lambda[not equal]1, two modes of instability associated, respectively, with the inner and the outer shear layers coexist. The analysis reveals that the outer shear layer has a significant effect on the stability of the internal interface, especially for light jets, corresponding to S<1, when a is sufficiently close to unity. Furthermore, a new region of absolute instability has been described in the aLambdaS parameter space.
 Publication:

Physics of Fluids
 Pub Date:
 September 2002
 Bibcode:
 2002PhFl...14Q3028S
 Keywords:

 Convection;
 Flow Stability;
 Free Jets;
 Jet Flow;
 Jets;
 Fluid Mechanics and Thermodynamics