Illustrating hydraulic jumps is often done in teaching labs with water flumes that are often too large to move into a classroom for an in class demonstration. Here is a simple way to illustrate hydraulic jumps and flow control on a much smaller scale.

Equipment

1. Syringe (at least 10 ml, ideally larger)
3. Clear glass tray (I use a square pyrex baking dish)
4. Shallow, transparent round dish (I use a lid from a quart tub of yogurt)

Demonstration

1. Place the glass tray on the OHP and the round dish in the tray.
2. Fill the round dish with water. It is OK if it overflows a bit, but the water level in the outer container should be below the lip of the smaller container. The outer container is just for catching the overflow.
3. Fill the syringe with water
4. Inject the water from the syringe as rapidly as possible vertically (roughly) down in the middle (roughly) of the small round dish. A radial (approximately) hydraulic jump should form.
5. Repeat as needed. The demonstration can be run multiple times, just re-fill the syringe and re-inject. If the water level of the outer container gets too large just pour it out.

Discussion

If you inject the water rapidly enough then the radial outflow from where the jet impinges will be fast, shallow, and super critical. However, the lip of the small dish (yogurt lid) is a weir and, therefore, the flow must be downstream controlled at that point. As such, the flow must undergo a hydraulic jump to adjust from upstream to downstream control. This can be clearly seen in the dish provided the water is injected fast enough. Unfortunately there is not a whole lot of direct analysis that can be done here. There is just no enough quantitative input data. The experimental setup was based on much more precise work by John Bush at MIT (http://math.mit.edu/~bush/?p=843).

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.