Role of buoyancy-driven vortices in inducing different modes of coupled behaviour in candle-flame oscillators

Abstract

We investigate the coupled behaviour of two oscillatory flames produced by separate bundles of candles, referred to as candle-flame oscillators, as the distance between them is varied. Each bundle consists of four candles whose individual flames are fused so that the resultant flame produces self-sustained limit cycle oscillations. The recent study by Manoj et al. [Scientific Reports 8, 11626 (2018)] has reported the occurrence of four different modes of coupled behaviour, which include in-phase synchronization, amplitude death, anti-phase synchronization, and desynchronization by observing the flame dynamics of such coupled candle-flame oscillators. Here, we investigate the physical mechanism behind the occurrence of these different dynamical modes. Towards this purpose, we perform simultaneous measurements of the flow field around the candle flames using high-speed shadowgraph and of the reaction zone of each flame using high-speed CH* chemiluminescence imaging. We notice that these modes are distinguished by the distinct features of the flame dynamics and the corresponding buoyancy-induced flows surrounding the flames. We observe that the difference in the interaction of vortices, formed due to the instability of buoyancy-induced flows around each flame at various distances, plays a significant role in inducing different modes of coupled dynamics between the oscillators. Furthermore, we find that the change in the length scales of vortices shed around the flames is a contributing factor in increasing the frequency of the oscillators during the transition from in-phase to anti-phase mode of synchronization.

Vortex-dynamical interpretation of anti-phase and in-phase flickering of dual buoyant diffusion flames

“…A more reasonable conjecture was given by Nakamura et al [26] that the viscosity suppresses the hydrodynamic disturbance and leads to the mode transition of double flickering jet flames. It is worth mentioning that Dange et al [56] recently provided a direct experimental support to our hypothesis [57] that the mode transition is determined by the interactions between the buoyancyinduced vortices. In this section, we shall focus on quantifying the interaction between the inner-side shear layers of the two different flames to reveal the nature of the transition between the two flickering modes.…”

“…Another interesting observation is the flame interaction mode of Fig. 11(e), which is termed the "amplitude death" mode [56] meaning no flickering.…”

Experimental investigation on the susceptibility of minimal networks to a change in topology and number of oscillators

“…When the link distance between the oscillators is d = 3, the neighbouring oscillators have a tendency to exhibit anti-phase synchrony. According to Dange et al [34], anti-phase synchrony is observed between the oscillators at d = 3 due to the alternate shedding of vortices from neighboring oscillators. If the number of oscillators in a network is even, there exists a global synchrony, in the form of clustering, between the oscillators, where the neighbouring oscillators are locked at 180 degrees of phase shift and alternate oscillators are locked at 0 degrees of phase shift.…”

Oscillation and collective behavior in convective flows

“…If two diffusion flames are placed nearby each other collective behaviour in form of synchronization of the flickering can appear. [5,7,9,[19][20][21][22][23]. Similarly with the studies performed on candle flames, first we will examine the collective oscillation frequency and the synchronization order parameter z for two Helium columns with the same flow parameters (yield and nozzle diameter) as a function of the distance between the nozzles.…”