Controlled mixing enhancement in turbulent rectangular jets responding to periodically forced inflow conditions

We present numerical studies of active flow control applied to jet flow. We focus on rectangular jets, which are more unstable than their circular counterparts. The higher level of instability is expressed mainly by an increased intensity of mixing of the main flow with its surroundings. We analyse jets with aspect ratio A_r = 1, A_r = 2 and A_r = 3 at Re = 10,000. It is shown that the application of control with a suitable excitation (forcing) at the jet nozzle can amplify the mixing and qualitatively alter the character of the flow. This can result in an increased spreading rate of the jet or even splitting into nearly separate streams. The excitations studied are obtained from a superposition of axial and flapping forcing terms. We consider the effect of varying parameters such as the frequency of the excitations and phase shift between forcing components. The amplitude of the forcing is 10% of the inlet centreline jet velocity and the forcing frequencies correspond to Strouhal numbers in a range St = 0.3–0.7. It is shown that qualitatively different flow regimes and a rich variety of possible flow behaviours can be achieved simply by changing aspect ratio and forcing parameters. The numerical results are obtained applying large eddy simulation in combination with a high-order compact difference code for incompressible flows. The solutions are validated based on experimental data from literature for non-excited jets for A_r = 1 at Re = 1.84 × 10⁵ and A_r = 2 at Re = 1.28 × 10⁵. Both the mean velocities as well as their fluctuations are predicted with good accuracy.