Eddies

Eddies appear in all sorts of flows where there is no mechanical constraint on the  flow. I am thinking of the flow that occurs in wakes. in a tidal river that flows through a rocky estuary bed one might see deep quickly rotating eddies in the wake produced by water flowing over rocky outcrops. One can see the same where a river flows under a bridge as shown in figure 15-20

 

Figure 15-20 shows the river Stour flowing under a bridge at Sandwich in England. If you look carefully there are two eddies that rotate in the same direction and a third about to be shed at the sharp corner of the abutment. Those eddies did not last long. They are formed between the solid surface of the bridge pier that exerts a friction drag on the water tending to slow it down and the main flow that has separated from the wake and exerts a friction drag on the water in the wake tending to speed it up. In this case the result is that eddies are shed one after another into the wake and this is very common. The eddies do not persist for very long and the wake develops into a wide confused flow. These eddies clearly come under the heading of swirling flow.

 

Another form of eddy is very common. Figure 15-21 shows a paint picture of the flow over a stalled aerofoil. It is due to Prandtl. I have seen a virtually identical photograph of a model sail in a water channel. What is different is first that these eddies are position-fixed relative to the aerofoil for a given wind speed and for a given angle of attack and secondly that the eddies are not circular. Eddies have to fit into a space between the solid boundary and the separated free stream.

 

For another article I converted Prandtl’s picture to a negative and drew lines to show other details of the flow. There are eddies rotating in both directions and in fact a flow path between the main one and the rest of the wake. These eddied are not random even if they do wobble about somewhat; they are a proper flow pattern.