This shows a cylinder in cross flow in a water tunnel. The Reynolds number is about 500. In water this corresponds to a velocity of 2 cm/sec over a 3 cm dia cylinder. The flow is visualized with dilute food coloring injected at the top and bottom
This shows the same cylinder in cross flow in a water tunnel. The Reynolds number is about 5000. The flow pattern is very unsteady and oscillates up and down. The frequency of oscillation, when nondimensionalized by the diameter and the free-stream velocity is know as the Strouhal number: St=(f*d)/V It is about .2 over a wide range of Reynolds numbers for a circular cylinder.
In this case a second smaller cylinder has been added downstream. The second cylinder damps out the up and down oscillations. The Reynolds number is about 5000.
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