This photograph shows a slice of the boundary-layer fluid shed by a cylinder (yellow) as wraps around a columnar vortex (red). The cylinder is traversed toward the columnar vortex from left to right. The image is obtained using laser-induced fluorescence with the yellow dye (Rhodamine chloride 590) released into the cylinder boundary layer from small holes along the leading edge of the cylinder and the red dye (Sulfurhodamine 640) released at the top of the 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 sketch on the right gives an interpretation of the loop-like structure of the secondary vorticity field ejected from the cylinder boundary layer.

References:

Krishnamoorthy, S., Gossler, A.A. and Marshall, J.S., "Normal vortex interaction with a circular cylinder," AIAA Journal, Vol. 37, No. 1, 1999, pp. 50-57.

Gossler, A.A. and Marshall, J.S., "Simulation of normal vortex-cylinder interaction in a viscous fluid," Journal of Fluid Mechanics, Vol. 431, 2001, pp. 371-405.