Symmetry breaking and vortex precession in low-swirling annular jets

Abstract

In this paper, the flow dynamics in the wake of a turbulent annular jet is studied using Time-Resolved Stereoscopic Particle Image Velocimetry and Proper Orthogonal Decomposition (POD). In this wake, a central recirculation zone is present which, under certain conditions, shows a low-frequency precessing motion. POD analysis of the measured velocity data shows that at zero swirl, an asymmetry is present in the wake, which motion is random in time. This asymmetry originates from a bifurcation of the flow once a threshold Reynolds number is exceeded. For low-swirl numbers, ranging from 0 < S < 0.12, the asymmetry is still present and its motion becomes structured into a well defined precession. For S > 0.12, the precession is gone and the motion of the asymmetric wake is again random in time, similar like the non-swirling jet. In this paper, a model is developed to describe the influence of swirl on the wake dynamics. The model assumes that perturbations in the inner shear layer near the bluff body wall are convected towards the stagnation point. These perturbations cause a shift in the stagnation points position. This shift is convected back to the inner shear layer through convection in the recirculating flow. The dynamics of this feedback mechanism can be modeled by the nonlinear delayed saturation model. In this paper, the model is adapted for swirling flow and simulations show that good agreement is found with the experiments.

Analysis of the pressure fields in a swirling annular jet flow

“…The first distinct region is the CRZ zone (y∕D 0 < 0.4) which is characterized by a peak Strouhal number (St = f × D h ∕U 0 ) of approximately 0.094 (corresponding to a frequency of 9.8 Hz). This corresponds to the precession frequency of the CRZ as also reported by Vanierschot et al (2014). The second region is downstream of the vortex breakdown location (y∕D 0 > 0.8) with a clear peak St of 0.27, corresponding to a frequency of 28.2 Hz.…”

“…Although there have been a number of numerical studies focusing on the recirculation zones of annular swirling jet flows Wegner et al 2004), the first experimental studies to obtain flow field information resolved in both time and space were performed by (Vanierschot and Van den Bulck 2008a) by conducting timeresolved stereoscopic particle image velocimetry (Stereo-PIV) measurements in the central plane of the annular jet. In subsequent works, they investigated the dynamics of the precessing vortex core (Vanierschot and Van den Bulck 2011;Vanierschot et al 2014) and calculated the mean pressure field in the initial merging zone of the swirling jet (Vanierschot and Van den Bulck 2008b). However, these studies are limited due to the planar measurement of the three-dimensional swirling jet flow fields.…”

“…In addition to the aforementioned CRZ, annular jet flows feature different complex flow characteristics despite their simple geometry: an outer (between the jet and the environment) and inner (between the jet and the central recirculation region) shear layer, which are both characterized by strong anisotropic turbulence (Vanierschot et al 2014). The complexity of the flow is further enhanced when introducing swirl which leads to the formation of large zones of recirculation and large-scale instabilities at certain swirl numbers, such as vortex breakdown or a precessing vortex core (PVC) (Vanierschot and Van den Bulck 2008a; Lucca-Negro and O'Doherty Abstract In this paper, we investigate the flow structures and pressure fields of a free annular swirling jet flow undergoing vortex breakdown.…”

Formation–breakdown cycle of turbulent jets in a rotating fluid

“…The total number of averaged frames used for the POD analysis was approximately 450, for each experiment, containing several formation-breakdown cycles. This number is sufficiently larger than the minimum of approximately 400 frames required to capture the statistics of the first three POD modes for these types of flows (Patte-Rouland et al 2001;Vanierschot et al 2014). Figure 10(a-c) displays typical results obtained for the velocity vector field, with superposed associated vorticity field, of the first three POD modes for a jet at Re 0 = 2300 with Ω = 0.21 rad s −1 .…”

“…The method is described in detail in Joliffe (2002) and it represents a tool to identify coherent structures in turbulent flow (see e.g. Patte-Rouland et al 2001;Vanierschot, Dyck & den Bulck 2014). If there exists a coherent, periodic formation-breakdown scenario for the jet, then one expects that the associated breakdown frequency of the jets is revealed through the time characteristics of the time coefficient, c 1 , of the first POD mode.…”