The photograph on the left shows a slice of a columnar vortex after it is chopped by a thin blade that is traversed from left to right in the image. The image is obtained using laser-induced fluorescence with the yellow dye (Rhodamine chloride 590) released into the blade boundary layer from small holes along the blade leading edge and the red dye (Sulfurhodamine 640) released at the top of the columnar vortex and carried downward by the vortex axial flow. Light illumination is provided by a vertical sheet from the green line of a continuous argon ion laser with wavelength in the range 457-514 nm. The vortex core radius increases on the side where the axial flow is oriented toward the blade and the core radius decreases on the side where the ambient axial flow is oriented away from the blade. The blade boundary-layer fluid (yellow) is entrained into the vortex on both sides of the blade due to the low pressure within the vortex core. The photograph on the right shows a bubble-type vortex breakdown, followed by a double-helix breakdown, which is observed on the upper part of the vortex following cutting by the blade and which propagates upstream relative to the vortex core axial flow.

References:

Marshall, J.S. and Krishnamoorthy, S., "On the instantaneous cutting of a columnar vortex with non-zero axial flow," Journal of Fluid Mechanics, Vol. 351, 1997, pp. 41-74.

Krishnamoorthy, S. and Marshall, J.S., "An experimental investigation of 'vortex shocks'," Physics of Fluids, Vol. 6, No. 11, 1994, pp. 3737-3741.