Numerical simulations of swirling flows in a pipe

Abstract

Axisymmetric swirling flows in a finite-length, straight, circular pipe are conducted. The objective is to replicate the flows studied in the experimental investigation of Liang & Maxworthy (Journal of Fluid Mechanics, (2005), vol. 525, pp. 115-159), where a swirling jet is issued from an inlet nozzle into a large circular pipe with stagnant fluid. The simulations are based on the Navier-Stokes formulation of fluid flow motion with inlet profiles of the axial velocity and circulation function taken from the experimental studies. A zero axial gradient of the radial velocity at the inlet is also imposed. A zero radial velocity is set at the pipe outlet. The no-penetration and no-slip conditions are set along the pipe wall and axisymetric conditions are applied along the pipe centerline. A finite-difference algorithm of the flow equations and boundary conditions is used in discretizing the flow field.

Computed examples of the flow dynamics at Reynolds number Re=500 (based on the inlet nozzle radius) and at various swirl levels demonstrate the existence of a critical swirl level for the first appearance of vortex breakdown. At swirl levels below the critical swirl, the flow evolves to near-columnar states while at above the critical level the flow evolves to near-steady breakdown states. The numerical predictions are compared with the experimental data and exhibit a global agreement with the mean flow properties. In addition, the simulations reveal the existence of a small but finite range of swirl levels where multiple steady states may coexist and form a hysteresis loop between near-columnar and breakdown states. This loop was not found in the experiments due to large steps in the increase of the inlet flow swirl level. The present thesis sheds additional light on the physics of swirling flows and provides guidelines for improving procedures in future experimental studies.

Numerical Simulations of the vortex breakdown process in incompressible, viscous,