Flow dynamics in a variable-spacing, three bluff-body flowfield

Abstract

This work explores the wake dynamics of systems with three bluff bodies with variable spacing. Studies of single-wake systems have shown that coherent wake vortices have a regular, and predictable, periodicity. A growing literature of dual-wake studies has shown that multi-wake systems are more stochastic than single-wake systems, and their dynamics are highly dependent on the spacing between the wakes. Here, we expand on this literature by investigating three-wake systems, and find that the coherent dynamics of the wakes are highly intermittent. We use proper orthogonal decomposition to extract the most energetic modes of the three-wake system at six bluff-body spacings that span a range of dynamical "regimes." After describing the time-dependent behavior of the interacting wakes in these regimes, we use a statistical approach to describe the relative phase between oscillations in each of the wakes, identifying regimes where oscillations are more or less random. Interestingly, the wake oscillations are less random when the bluff bodies are positioned very close and relatively far from each other. In between these two extremes, an intermediary regime is identified where the wake oscillations are almost completely random; this finding parallels data from the dual-wake literature. Finally, we discuss the implications of the observed behaviors and possible future directions for this work. I. INTRODUCTION The goal of this paper is to characterize transitions in flow development and stability characteristics with variations in bluff-body spacing in a three bluff-body flow field. The applications of this work are far reaching, from flame holders in jet engine augmentors, to air moving though power transmission lines, to flows through heat exchangers, to piers supporting trusses in unsteady waters. In each of these applications, adjacent flow fields, or a series of individual wakes, experience a significant level of interaction. The interaction can change both the time-averaged and dynamical characteristics of

Cluster-based Reduced-Order Modeling to Capture Intermittent Dynamics of Interacting Wakes

“…The RMS levels of the three-wake cases are lower, even in the regions downstream of the bluff bodies, as a result of the reduction in vortex-shedding coherence. Previous work by Meehan et al [17] details the statistics of this vortex shedding in these cases, clearly indicating the statistical nature of the vortex shedding. The CROM analysis will also show that the vortex shedding in these regions is more intermittent, which results in a lower RMS velocity, and will more clearly highlight the dominant flow structures that drive this intermittency.…”

“…In a case where the dynamics are strongly oscillatory and repeatable, the number of clusters case is shown in Figure 8. The POD results from this case, discussed in Meehan et al [17], show pairs of modes with outer wake oscillations (modes 1 and 2) and center wake oscillations (modes 3 and 4), and a large number of low-energy modes with oscillations in the shear layers shed from the edge of each bluff body. In the CROM results, there are three categories of centroids represented.…”

“…It should be noted that many previous multi-bluff-body experiments in the literature look at systems in wind or water tunnels, which creates a very different boundary condition on the flow than the current open configuration does. We have discussed the significant implications for this difference in our previous work [17]. The velocity RMS (comprised of two components of velocity from the 2D PIV) is shown in Figure 4.…”

“…Futhermore, the facility is highly modular such that it can be operated with a single bluff-body or multiple bluff-bodies with variable spacings A schematic is shown in Figure 1. More details about the facility can be found in recent work by Meehan et al [17]. In this work, the Reynolds number, defined in Equation 1, is held fixed at 4000:…”

“…In working to interpret these complex flow physics, techniques that reduce the complexity of the problem are beneficial. Recent work by Meehan et al [17] used statistical analysis of the data discussed in this paper to describe the intermittency. In this work, some more traditional analysis methods (POD and phase-resolved analysis) were used to investigate this three bluff-body flow.…”