Main instabilities of coaxial jets

Two coaxial jets which interact and mix belong to the class of flows which are of great interest from both the research and industrial point of view. They have a simple geometry but still the possibility to easily change various parameters makes them suitable for studies of different phenomena related to stability, mixing and turbulence. The definition of the variables able to control the flow dynamics has high applicative potential because coaxial jets are a prototype of many industrial burners but, on the other hand, they are simple enough to be experimentally characterized in laboratory. Due to the high number of parameters able to influence the flow evolution, a complete characterization of such flow is still missing and, therefore, motivates this paper. An experimental analysis of the dominant instabilities in the near field of two coaxial jets has been performed in order to determine the leading processes which start the transition from the initial laminar state, where the two streams are segregated, to the turbulent state. Different inner/outer jet velocity pairs (Ui,Uo) have been tested in order to investigate the effect of both velocity ratio ru = Uo/Ui and Reynolds number. Three main instabilities have emerged depending on the velocity ratio as can be appreciated from figure 1(a) where isocontours of the dominant frequencies have been plotted. For ru &lt; 0.75, the coaxial jets dynamics are driven by the inner shear layer instability, in agreement with [1]. For ru > 1.6, the outer shear layer dominates the near field vortex dynamics [2, 3] while, for ru nearly unitary (i.e. where 0.75 < ru &lt; 1.6), the vortex shedding behind the separating wall imposes its own dynamics [4]. The latter phenomenon is clearly shown in figure 1(b) where a flow visualization of the near field at ru = 1 is reported. Behind the inner separating wall there is a flow pattern composed by staggered vortices which influence the dynamics of the outer shear layer too. An improved scaling relationship for the Strouhal number StŒ tx is proposed in order to calculate the shedding frequency inside the intermediate velocity ratio region. This relationship takes into account Reynolds number as well as velocity ratio effects and leads to a constant Strouhal number value in the whole region where vortex shedding is observable, as showed in figure 2. The present paper confirms experimentally the theoretical results of [5], who proposed that the wake behind the separation wall between the two streams of two coaxial jets creates the conditions for an absolute instability. Evidences of the dominance of the vortex shedding phenomenon in the whole near field inside a certain range of velocity ratios will be presented in the paper. The characterization of this operating region is of utmost importance because the vortex shedding provides a continuous passive forcing mechanism for the control of the flow field, opposite to conventional strategies which use active methods focused to control the outer shear layer dynamics [3]. Therefore, these results allow many improvements in the industrial geometries where coaxial jets are employed by means of just a proper choice of the exit geometry.