- Author
- Battaglia, F.
- Title
- Computational Study of Characteristic Instabilities That Arise in Flows With and Without Buoyancy. BFRL Fire Research Seminar. VHS Video.
- Coporate
- National Institute of Standards and Technology, Gaithersburg, MD
- Report
- Video, February 22, 1999,
- Keywords
- buoyant plumes
- Abstract
- The seminar will focus on the use of numerical simulations to investigate instabilities that affect symmetric and time-dependent characteristics of (non-buoyant) jet flows in channels, and buoyant plumes. Jet flows are important for applications in aircraft propulsion, combustors, boiler furnaces, and fluidics. However, the presence of flow instabilities can reduce the efficiency of these devices. An example of an unstable buoyant flow in the environment is a fire whirl, which has been responsible for forest devastation and countless deaths. Suddenly-expanding jet flows in symmetric planar and cylindrical channels transition from a symmetric to an asymmetric jet due to instabilities with increasing Reynolds number. Planar flows can bifurcate, producing both asymmetric and transient flows (referred to as the flapping jet). The three-dimensional analogy is the fluid mechanical phenomenon of a precessing jet, which can be described as an asymmetric gyration of the flow in a cylindrical channel. Effective control of jet flows can increase the performance of devices as well as optimize the natural motion of the flow. A density-based, time-marching algorithm was formulated for the incompressible Navier-Stokes equations, employing an artificial compressibility method and a dual-time stepping technique. For the numerical simulations, the precession was induced with the addition of swirl to the jet flow. Fire whirls are a rare but potentially catastrophic form of fire known to arise naturally. The swirling buoyant plume essentially combines the dynamic effects of vorticity and buoyancy. The buoyancy-driven plume entrains ambient fluid as heated gases rise. Vorticity associated with a mechanism such as wind shear can be concentrated by the fire, creating a vortex core along the axis of the plume. The whirling fire constricts radially and stretches the plume vertically, dramatically changing the entrainment and mixing which control combustion. Understanding how swirl alters the buoyant plume dynamics is important for modeling efforts to predict forest fire scenarios. An approximate form of the Navier-Stokes equations was formulated to directly simulate large scale eddies, and a Smagorinsky model represented the sub-grid scale motion. The fire was prescribed in a manner consistent with a mixture-fraction approach to combustion.