Did you ever notice how when certain ferries dock, they stop, already parallel to the dock, a couple of meters away from the dock and then just move sideways towards the dock? Usually they don’t even move passenger ferries any more, just use thrusters to keep them steady while people get on and off.
Currents caused by thrusters of a harbor ferry in the port of Hamburg
But why this weird sideward motion?
One reason is the Coanda effect – the effect that jets are attracted to nearby surfaces and follow those surfaces even when they curve away. You might know it from putting something close to a stream of water and watching how the stream gets pulled towards that object, or from a fast air stream that can lift ping pong balls. So if the ship was moving while using the thrusters, the jets from the thrusters might just attach themselves to the hull of the ship and hence not act perpendicularly to the ship as intended.
But I think there is a secret second reason: Because it just looks awesome :-)
Do you use a tide chart to find the best time for your Saturday walk, too?
I showed you a vortex street on a plate formed by pulling a paint brush through sugary water as an example. Now today I want to show you the real thing: Instead of stagnant water and a moving object, I bring to you the flowing Elbe river and a bollard!
Watch how vortices with alternating spin are shed every three or four seconds!
Kelvin-Helmholtz instabilities in a shear flow in Elbe river.
Last week I talked about how I wanted to use the “Elbe” model in teaching. Here is another idea for an exercise:
On the picture below you see Kelvin-Helmholtz instabilities. They might be kinda hard to make out from the picture, but there is a movie below where they are a bit easier to spot.
Kelvin-Helmholtz instabilities the boundary layer of Elbe river
Anyway, this is what they look like: Kind of like the ones we saw off Jan Mayen in 2012.
Kelvin-Helmholtz instability off Jan Mayen
Kelvin-Helmholtz instabilities occur in shear flows under certain conditions. And those conditions could be explored by using a tool like Elbe. And once students get a feel for the kind of shear that is needed, why not try to reproduce a flow field that causes something similar to the instabilities seen in the movie below?