Proper Orthogonal Decomposition Analysis of Coherent Structures in Simulated Reacting Buoyant Jets

The spatial evolution of a circular reacting buoyant jet at moderate Reynolds number (Re = 10 3 ) has been investigated using large-eddy simulation. Infinitely fast chemistry of a Burke―Schumann formulation is employed to model the combustion process in a reacting buoyant jet. Dynamic puffing phenomena is observed and corresponds to the formation of large-scale vortex structures near the plume base. These toroidal vortical structures break down into smaller, disorganized eddies with increasing distance downstream. Two-point correlation variances of temperature and velocities generated from large-eddy simulation are analyzed using the proper orthogonal decomposition method. The energy of the flow is found to be well represented by a finite number of eigenmodes. Further, the corresponding eigenfunctions accurately capture the large-scale coherent structures: namely, the vortex rings in the laminar region and a large-scale strong helical motion in the turbulent region. In the near-field region the dominant vortex-shedding frequency obtained from Fourier analysis agrees well with experimental data. Proper orthogonal decomposition analysis in the transitional and turbulent regions reveals the dominance of subharmonic frequencies, indicating vortex merging in the downstream flow that cannot be detected from Fourier analysis of raw large-eddy simulation data.