The manipulation of large- and small-scale flow structures in single and coaxial jets using synthetic jet actuators

Control of the mixing of fuel and oxidizer in a combustor is necessary for high combustion efficiency, combustor stability, and low emissions. The traditional approach to control of mixing in free shear flows has been indirect, relying on manipulation of large-scale, global instability modes of the base flow upstream of mixing transition. The control influence is transferred to the scales at which molecular mixing occurs by means of the classical cascading mechanism. More efficient control of mixing in fully turbulent shear flows might be achieved by direct control of both the large-scale entrainment and small-scale mixing processes. The present work focuses on mixing control based on concurrent manipulation of both the small- and large-scale dynamical processes via direct long-range couplings between large- and small-scale motions in axisymmetric jets.; Single and coaxial round jets are instrumented with an azimuthal array of nine synthetic jet actuators placed near the jet exit plane around the nozzle circumference. The synthetic jets operate at high frequency (nominally 1200 Hz), resulting in direct excitation of the small-scales of the primary flow. Results are presented for a single jet (Re_D = 1.9 × 104) and coaxial jets (A_i/A_o = 0.5 and U_i/U_o = 0.35, 0.65, and 1.4 for which the Reynolds number based on uniform volume flow rate is Re_DH = 7.6 × 10 3). The present work shows that axisymmetric, high-frequency forcing leads to increases in rms velocity fluctuations and jet spreading in the near field (x/D $\lesssim$ 1). When the excitation waveform is amplitude modulated with all actuators in phase, the small-scale motions are advected within nominally axisymmetric large-scale vortical structures. In the single jet, these structures propagate along the jet axis and lead to temporal modification of the global entrainment of ambient fluid. In the coaxial jet, these structures are strongest when U_i/U_o < 1, leading to significant interaction between the two fluid streams. Manipulation of the geometry of the jet cross section is achieved through non-axisymmetric forcing patterns.