Modification of vortex dynamics and transport properties of transitional axisymmetric jets using zero-net-mass-flux actuation

Abstract

We study the near field of a zero-net-mass-flux (ZNMF) actuated round jet using direct numerical simulations. The Reynolds number of the jet ReD = 2000 and three ZNMF actuators are used, evenly distributed over a circle, and directed towards the main jet. The actuators are triggered in phase, and have a relatively low momentum coefficient of Cμ = 0.0049 each. We study four different control frequencies with Strouhal numbers ranging from StD = 0.165 to StD = 1.32; next to that, also two uncontrolled baseline cases are included in the study. We find that this type of ZNMF actuation leads to strong deformations of the near-field jet region that are very similar to those observed for non-circular jets. At the end of the jet's potential core (x/D = 5), the jet-column cross section is deformed into a hexagram-like geometry that results from strong modifications of the vortex structures. Two mechanisms lead to these modifications, i.e., (i) self-deformation of the jet's primary vortex rings started by distortions in their azimuthal curvature by the actuation, and (ii) production of side jets by the development and subsequent detachment of secondary streamwise vortex pairs. Further downstream (x/D = 10), the jet transforms into a triangular pattern, as the sharp corner regions of the hexagram entrain fluid and spread. We further investigate the global characteristics of the actuated jets. In particular when using the jet preferred frequency, i.e., StD = 0.33, parameters such as entrainment, centerline decay rate, and mean turbulent kinetic energy are significantly increased. Furthermore, high frequency actuation, i.e., StD = 1.32, is found to suppress the mechanisms leading to large scale structure growth and turbulent kinetic energy production. The simulations further include a passive scalar equation, and passive scalar mixing is also quantified and visualized.

Coherent structures in interacting vortex rings

“…The dynamics of a vortex ring has been found to be dominated by large-scale structures, for instance in the breakdown of a vortex ring [7], and in turbulent transport and subsequently the efficient and environmentally-clean operation of a combustor [8]. The manipulation, either excitation or suppression, of such large-scale coherent structures by means of passive or active control has been found to enhance mass entrainment rate [9][10][11][12] or reduce noise [13]. In Ref.…”

“…In Ref. [11],Önder & Meyers applied three actuators near the nozzle exit and observed a hexagram deformation of the vortex ring, as well as finer secondary structures further downstream.…”

Optimal control of a transitional jet using a continuous adjoint method

“…If unsteady actuators are driven in a regime close to the jet preferred mode, large-scale structures are significantly promoted and the jet spreading is increased [20,21]. This yields dramatic changes in global mixing characteristics such as the entrainment rate, the decay rate of centreline velocity and passive scalar [22]. However, enhanced coherence in the flow increases the strength and stability of large scale structures and therefore delays their breakdown into smaller scales [23].…”

“…Some similarities may exist, e.g., with flap actuators [25], but we selected this type of control mainly for computational reasons (as discussed in detail in Section 2.2). Moreover, radial perturbations are effective in promoting large-scale structures, as the development of primary and secondary vortices in the transition region of a round jet is initiated by the distortions in the azimuthal symmetry of the jet [26,22]. The signal of controls is designed as a finite Fourier series with 10 frequency components varying between 0.5St pm and 5St pm where St pm is the considered preferred mode frequency with a value of St D = 0.33.…”

Lifted and reattached behaviour of laminar premixed flame under external acoustic excitation

“…The stabilization mechanism of the vortices has been reported by many researches [33,34]. The 261 researchers noticed that, for the lifted flame, at the hysteresis region, the vortices play an important 262 role for the flame stabilization and reattachment.…”