A laminar vortex ring with swirl, which has the meridional velocity component inside the vortex core, was experimentally generated by the brief fluid ejection from a rotating outlet. The evolution of the vortex ring was investigated with flow visualizations and particle image velocimetry measurements in order to find the influence of swirling flow in particular upon the transition to turbulence. Immediately after the formation of a vortex ring with swirl, a columnar strong vortex along the symmetric axis is observed in all cases of the present experiment. Then the characteristic fluid discharging from a vortex ring with swirl referred to as “peeling off” appears. The amount of discharging fluid due to the “peeling off” increases with the angular velocity of the rotating outlet. We conjectured that the mechanism generating the “peeling off” is related to the columnar strong vortex by close observations of the spatio-temporal development of the vorticity distribution and the cutting 3D images constructed from the successive cross sections of a vortex ring. While a laminar vortex ring without swirl may develop azimuthal waves around its circumference at some later time and the ring structure subsequently breaks, the swirling flow in a vortex ring core reduces the amplification rate of the azimuthal wavy deformation and preserved its ring structure. Then the traveling distance of a vortex ring can be extended using the swirl flow under certain conditions.

A novel experiment was conducted to investigate the aerodynamic forces acting on a train traveling through a tornado, in which we developed a moving model rig with a tornado simulator. The flow field generated by the tornado simulator was validated by comparison with those of real tornadoes and the Rankine vortex model. Using this setup, we measured unsteady surface pressures on a model train as it passed through the vortex center. The side force, lift force, and yawing moment were estimated from the pressure data. The results were as follows: 1) the side force acting on the train changed its direction from negative to positive while passing through the tornado-like swirling flow; 2) the lift force increased as the train approached the flow and became maximum around the vortex center; 3) the yawing moment first decreased slightly and then reached its maximum around the vortex center. Asymmetric wave forms of the forces and moment at the center of the tornado simulator suggested that the train itself may have affected the vortex structure of the flow.

No abstract

Behavior of the series of laminar vortex rings with circumferential flow, so-called swirl, were investigated using flow visualization, to evaluate the transport efficiency of the ejected fluid as vortex rings. In this study, the interval time of the vortex ring ejection, the formation number of vortex ring L O /D O (the normalized length of the ejected slug of fluid), and the angular velocity of the ejected fluid  are changed, while the mean ejection velocity is fixed. When vortex rings were generated at a short time interval, independent of L O /D O and , they were broken, and most of the fluid included in them was diffused near the orifice. When the vortex rings with little mutual interference were generated at an appropriate interval time, the breakdown of vortex ring structure is suppressed with moderate swirling flow. In those cases, each vortex ring moves separately for a long distance and the distribution area becomes wider as L O /D O increases.

Basic experiments were performed to investigate the use of vortex rings as a means to transport a small amount of fluid. The compact and unchanging nature of vortex rings is suitable to obtain high-density distribution of the transported fluid within a specific confined region. A certain amount of fluid in a cylinder was discharged into a quiescent fluid in the water tank by a piston through an orifice 20.0 mm in diameter. The fluid distributions transported by a set of 10 vortex rings, by 20 vortex rings, and by a single, short puff-like jet were quantitatively analyzed and compared. For all ejection conditions, the total volume of discharged fluid and the average ejection velocity through the orifice are the same (78.5×10 4 mm 3 and 125 mm/s, respectively). The particle count method was used to quantitatively measure the distribution of the transported fluid. As a result, the vortex rings were found to be superior to the single short discharge for transporting a fluid to obtain a high density distribution in a narrow region.