Application of spectral proper orthogonal decomposition to velocity and passive scalar fields in a swirling coaxial jet

Abstract

Direct numerical simulation is used to study unconfined coaxial jets under the influence of strong swirl imparted to the outer jet. Spectral proper orthogonal decomposition is employed to elucidate the physically important structures or modes in the flow. The analysis is extended to the transport of passive scalars injected through each jet. A partially penetrated vortex breakdown bubble is formed as a result of the strong swirl. In the region upstream of the central stagnation point, the first two (most energetic) spatial modes of the velocity field at the cross-stream section reveal three pairs of counter-rotating vortical structures, while the succeeding two modes reveal four pairs of such structures. The centers of these vortical structures are found to lie in the inner mixing layer present between the two jets. The corresponding spatial modes of the scalars also exhibit organized lobelike structures in this region. These organized structures are subsequently disrupted in the downstream region. The significance of these pairs of counter-rotating vortical structures is demonstrated by reconstruction of various turbulence statistics, namely, the root mean square (rms) velocities, the rms scalar fluctuations, the covariance between the two scalars, and the radial turbulent fluxes of the scalars. The results show that the first four modes make a greater contribution to these statistics except for the covariance between two scalars, particularly in the inner mixing layer.