The Onset of Vortex Breakdown in Swirling, Turbulent Jets

Open Access
Author:
Clees, Sean Michael
Area of Honors:
Mechanical Engineering
Degree:
Bachelor of Science
Document Type:
Thesis
Thesis Supervisors:
  • Jacqueline Antonia O'Connor, Thesis Supervisor
  • Jacqueline Antonia O'Connor, Honors Advisor
  • Rui Ni, Faculty Reader
Keywords:
  • vortex
  • breakdown
  • turbulent
  • swirling
  • jet
  • swirl
  • unsteady
  • DMD
  • dynamic
  • mode
  • decomposition
  • combustion
  • fluid
  • mechanics
  • turbine
  • PVC
Abstract:
Swirling jets are commonly used in combustion applications to stabilize flames and improve emissions. Thus, their dynamics play an important role in combustor design. Despite the prevalence of swirling flows in industrial applications that involve highly turbulent flow fields, the majority of experimental and numerical studies consider only laminar conditions. In this study, the dynamics of the vortex core are investigated in a swirling, turbulent jet at swirl numbers in the range of the critical swirl number for vortex breakdown. Vortex breakdown, a bifurcation in the structure of a swirling jet, results in the establishment of a stagnation point and recirculation region along the centerline of the jet. To study these dynamics, dynamic mode decomposition, an order-reduction technique used to extract coherent structures from flow data, is implemented. Investigation of time-averaged velocity fields and profiles leads to the identification of three flow regimes: pre-breakdown, near-breakdown, and post-breakdown. Velocity fields in these regimes are further analyzed using dynamic mode decomposition, Rankine-vortex fitting, and proper orthogonal decomposition to characterize jet dynamics with a particular focus on the development of the recirculation region characteristic of vortex breakdown. A precessing vortex core is also identified in the post-breakdown regime and its behavior is discussed.