Flow Structure of a Coaxial Circular Jet with Axisymmetric and Helical Instability Modes

Abstract

The flow structures in a coaxial jet with axisymmetric and helical instability modes for a comparatively low Reynolds number were investigated. The flow visualization and the measurements of velocities were carried out in an open water tank. In addition, three-dimensional numerical simulation of a coaxial jet was also performed using the commercial CFD software FLUENT 6.3. It was confirmed that the helical vortex was shed for the range of ratios of velocity of the inner to that of the outer jet from 0.5 to 1.0. Two characteristic flow regimes, i.e., axisymmetric and helical instability modes, were simulated in the flow field. For the coaxial jet with helical instability, the axial velocity along the centerline of the jet decreased more than that of the coaxial jet with axisymmetric instability. The axial velocity fluctuation at the centerline of the jet was small near the nozzle exit. However, the radial velocity fluctuation at that location increased. The convection velocity of vortices in the inner shear layer was larger than that for the outer shear layer. The convection velocity of vortices with the helical instability was slightly larger than that of the vortices with the axisymmetric instability. Consequently, the variation of velocity fluctuations and the convection velocity was associated with the arrangement of the vortex street in the shear layer.

Experiment and Large-Eddy Simulation on Free Air Jets Issuing from a Rectangular Nozzle with Deflectors

“…The jet spread at x/H ≥ 3 is smaller than that of the other three jets. It can be seen that velocity profiles similar to that of a coaxial jet [12] are obtained by installing deflectors inside the rectangular nozzle. Some interesting features can be observed in the x-z plane.…”

“…The Courant number is less than 0.1.A uniform velocity profile corresponding to the nozzle exit velocity was specified at the inlet boundary. The velocity on the upper and lower inlet surfaces, which were outside of the rectangular nozzle inlet surface, was adjusted to 0.03U 0 to maintain a weak straight flow aligned toward the entrainment flow outlet[12]. The nonslip boundary condition was applied to the walls of the rectangular nozzle.…”