Precession Effects on the Relationship Between Time-Averaged and Instantaneous Swirl Flow and Flame Characteristics

Abstract

Swirling flows exhibit a variety of unsteady fluid mechanic features, including large scale vortical structures and precessing recirculation zones. This paper considers the specific influence of precession on the relationship between time-averaged and instantaneous flow and flame features. The objective of this study is to aid in developing insight into high fidelity computations or experimental results. In particular, we describe how certain topological features in the time-averaged flow, such as centerline axial jets, centerline stagnation points, and symmetry of the flow about the centerline are influenced by precession. Insight is built by presenting results from a simplified model of a two-zone flow, consisting of a precessing reverse flow region embedded in a positive axial flow. A particularly significant result of this work is in regards to aerodynamically stabilized flames, which rely on the low velocity interior stagnation points in the vortex breakdown region for flame stabilization. We show how precession causes systematic differences between the location of the stagnation point of the time-averaged velocity and the time-averaged position of the instantaneous stagnation point. Indeed, an important implication of this point is that a perfect prediction of the time-averaged flow field could still lead to a completely erroneous time-averaged flame position prediction. Finally, we discuss the influence of precession and coherent motion on convergence of estimated averaged quantities.