IngentaConnect Some Observations of Vortex Breakdown in a Confined Flow with Sol...

Abstract:

The occurrence of breakdown in slender vortex flows as a ``bubble'' or ``spiral'' pattern depends on the degree of radial deflection of the vortex core from its original axis as shown in [1]. A smooth transition from a bubble to a spiral-type ``mode'' can be forced by inducing a small asymmetric disturbance which led to the conclusion, that the patterns do not represent different fundamental modes of breakdown. The subject presented herein addresses the following question: how does breakdown evolve in a swirling flow in which the vortex core is forced on a straight axis? In addition, what is the effect of turbulent inflow conditions? This type of vortex conditions is achieved in a spinning tube flow. The swirl is introduced at the entrance of the rotating tube with a honeycomb package and maintained by the viscous action in the boundary layer of the spinning tube. A diffuser at the end induces an adverse pressure gradient to force the breakdown. Flow visualization experiments are carried out to characterize the nature of breakdown over a range of different flow conditions. For some selected characteristic stages, detailed velocity fields were obtained using the method of Digital Particle-Image-Velocimetry (DPIV). The results show, that for the range of parameters investigated, breakdown is initiated at Rossby-numbers below a critical value of Ro 0.6 similar to those observed in other experiments. The bursted part of the vortex has a near axi-symmetric slender conical shape containing approximately stagnant flow. Its downstream end is characterized by a jump-like contraction where the flow evolves into a jet with enhanced swirl on the axis. It is only in this region downstream of the jump-like contraction that asymmetric instabilities and wavy flow patterns could be observed. Perturbations caused by them travel upstream but do not change the near-axisymmetric shape of the bursted part of the vortex.

Keywords: confined vortex; conical bubble; DPIV; spinning tube; vortex breakdown

Language: English

Document Type: Research article

Affiliations: 1: Aerodynamic Institute of the RWTH Aachen, Wüllnerstr. 5–7, D-52062 Aachen, Germany; E-mail: bruecker@aia.rwth-aachen.de

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Some Observations of Vortex Breakdown in a Confined Flow with Solid Body Rotation