On stationary and travelling vortex breakdowns

Abstract

This paper describes some experiments in swirling flows in a diverging cylindrical tube in which various types of vortex breakdowns were observed.In one set of experiments, the position of the breakdown, axial component of the velocity of the vortex core, swirl angle distribution ahead of the breakdown, and the pressure distribution along the tube were determined for various flow rates and for various values of circulation imparted to the fluid (water). Basically, three types of vortex breakdown were observed, viz. mild (double helix) breakdown, spiral breakdown (followed by turbulent mixing), and axisymmetric breakdown (followed by a thicker vortex core, then a spiral breakdown, and finally by turbulent mixing). The type and the location of the stationary breakdowns were found to be dependent, for the particular vortex tube used, upon the Reynolds and circulation numbers of the flow. In a spiral breakdown, the vortex core filament maintained the same sense of rotation as the upstream fluid elements. In an axisymmetric breakdown, the bubble included an inclined vortex-ring whose axis gyrated about the axis of the tube.In a second set of experiments, the response of the abrupt structural change along the axis of flow to gradual and abrupt changes in the upstream and downstream flow conditions was examined. The axisymmetric breakdown responded in a manner analogous to the hydraulic jump in open-channel flow before if reached a new stationary position along the axis of the tube.The observations reported and the evidence presented herein revealed that the axisymmetric breakdown is a finite transition between two sequent states of flow as proposed by Benjamin (1962, 1965, 1967) on theoretical grounds.

Solitary waves with recirculation zones in axisymmetric rotating flows

“…These transient processes are beyond the scope of this present work, where we have examined the steady-state structure of such recirculation zones, or 'bubbles'. We note that many experimental observations show that the development of a 'bubble' is associated with the formation of a stagnation point (Sarpkaya 1971), an initial expansion of the recirculation zone, followed by a contraction, with the occurrence of a second stagnation point. This tendency towards a steady-state structure is quite consistent with the results obtained here.…”

“…However, a local contraction is not the only way to initiate vortex breakdown; Brown & Lopez (1990) observed a downstream widening of the vortex owing to viscous dissipation that eventually induces vortex breakdown. Further, Darmofal (1996) and Snyder & Spall (2000) claimed a reasonably good agreement with the relevant experiments of Faler & Leibovich (1978) and Sarpkaya (1971). They did not use a local contraction to induce the breakdown, but carefully modelled both the flow field in the approaching vortex (Snyder & Spall started with the vortex shedding from the vane generator).…”

“…Such a formulation does not exactly correspond to the experimental set-ups described in Sarpkaya (1971), Leibovich (1984) and Escudier (1988), or to the numerical simulations by Beran & Culick (1992), Lopez (1994) and Hanazaki (1996). In all of these numerical studies, breakdown of the approaching vortex was induced by a local contraction of the tube, so that the separation zone was observed in the lee of that contraction.…”

A numerical investigation of the aerodynamics of a furnace with a movable block burner

“…One possible explanation for the numerical divergence found is that it was due to the physical phenomenon itself, as a consequence of the lower swirl intensity resulting from the smaller aperture of the tangential openings in the burner. This could cause an unstable or even a multistable flow with intermittent relocations involving two or more distinct stable flows, as described by Sarpkaya in 1971 andFaler andLeibovich in 1977. The iterative calculation was not able to manage these fast flow instabilities.…”

“…On the other hand, Sarpkaya, 1971 andFaler andLeibovich, 1977, obtained experimentally diverse patterns of flow reversal in swirling flows for the same operational condition, using water as fluid. In the case of multiple steady-state patterns, the predicted and the experimental patterns should be equivalent.…”