MECHANISM OF VORTEX GENERATION AND ITS CONTRIBUTION TO MIXING ENHANCEMENT IN A CONTROLLED COAXIAL JET

The mechanism of vortex generation and its contribution to mixing enhancement in a coaxial jet controlled by miniature flap actuators are investigated experimentally and numerically. First, flow visualization and PIV measurement are carried out in order to study the velocity field induced by the flapping motion of actuators and the resultant vortex ring generation in the initial inner and outer jet shear layers. Then, we numerically simulate such near-field evolution and associated mixing process by assuming the periodic velocity profile of different frequencies at the outer nozzle exit. In addition, the three-dimensional vortical structures and mixing property are studied under two different control conditions, i.e., axisymmetric and helical flapping modes. As a result, it is found that the helical forcing mode generates more streamwise vortices in the shear layers than the axisymmetric mode. This enhancement is due to the increased production of rib vortex structures through tilting and subsequent stretching of vortices generated by flapping. INTRODUCTION Coaxial turbulent jets are widely used in industrial applications, so fundamental flow structures and their possible control have been intensively studied (Kasagi, 2006). Various control methods for turbulent jets have been investigated by previous workers. For instance, passive control methods such as a tab (Zaman et al., 1994) and a robe nozzle (Hu et al., 2002) are proposed to modify the vortical structure in the initial shear layer. On the other hand, active control which excites the inherent instability of jet shear layer has also been investigated. Hussain and Zaman (1981) excited the jet column mode, which is defined by the frequency of vortex ring generation (Crow and Champagne, 1971), by implementing a loudspeaker and drastically changed the vortical structure of a round jet. Hilgers (1999) carried out direct numerical simulation for the optimum control combining the axisymmetric and helical forcing modes. In our previous studies (Kurimoto et al., 2004, 2005), we tried to control a coaxial methane/air jet by using miniature flap actuators arranged on the inner surface of an annular nozzle (Fig. 1) in order to achieve optimal mixing and combustion. In this method, large-scale vortex rings can be induced at the inner and outer jet shear layers perfectly in phase with the flapping motion. The size and spacing of vortices can be controlled by changing the flapping frequency as shown in Fig. 2. Large-scale vortices are shed continuously in the inner and outer shear layers when Sta(= faDo/Um,o) = 1.0, where fa, Do, and Um,o are the flapping frequency, the outer nozzle diameter, and the bulk mean velocity of the annular air jet, respectively. Under this condition, methane/air mixing is most enhanced with the length of inner potential core most reduced. The large-scale vortical structure at the inner shear layer plays a primary role in the initial mixing process of coaxial jets, although the interaction between the structures in the inner and outer shear layers must also be important. The latter has not been explored fully. Angele et al. (2006) showed that the number and strength of streamwise vortices become maximized at Sta = 1.0 under the axisymmetric forcing. They also showed that the helical forcing generates more streamwise vortices than axisymmetric or alternate forcing. Mituishi et al. (2007) carried out direct numerical simulation (DNS) of a controlled coaxial jet and demonstrated the mixing can be much enhanced with a modelled control input at the nozzle exit. They analyzed the development of vortex rings and streamwise vortices with axisymmetric forcing. Despite these efforts, the relationship between mixing enhancement and streamwise vortices has not been fully explored, while the detailed vortical structure in each excitation mode remains to be further studied. Inner nozzle = 20 mm