Boundary layers play an important role in many fluid mechanics applications including drag, lift, and flow in conduits. This demonstration illustrates the role of boundary layers as part of the classic spin-up problem. The demonstration is a cheap and easy version of one written up by Nicholas Rott in the book ‘Experiments in fluid mechanics’ (out of print but worth getting a second hand copy of). The demonstration uses tea leaves to visualize the secondary vortex that forms during spin-up. See here for more on tea cup fluid dynamics.
- Turntable (Lazy Susan)
- Bag of tea
- Half-filled glass of water
- Tape (to secure the glass to the turntable)
- Place the tea bag in a glass of hot water to wet the tea leaves.
- Fasten the half-filled glass of water to the turntable with tape.
- Using the scissors, cut open the used tea bag and dump roughly half of the tea leaves into the glass of water fastened to the turntable.
- Spin the turntable quickly, so that the tea leaves move to the outer edge of the glass. Keep spinning until the water is fully spun up (at least thirty seconds for the glass we used you will need to test this out prior to using the demonstration).
- After the elapsed thirty seconds, stop the turntable abruptly.
- The tea leaves should move from the outer edge and settle in a heap in the center of the bottom of the glass.
- Alternatively, if you do not allow the water in the cup to fully spin up, when you stop it the tea leaves will form a circle at the edge of the secondary vortex (see analysis below).
CAUTION: If not attached well, the glass of water can slide off of the turntable when rotated.
The images below show the tea leaves location when, from left to right, the cup is being spun up, the cup is stopped having been fully spun up, and the cup has been stopped after partial spin up.
When, starting from rest, the cup is initially spun, a boundary layer forms along the base of the cup. This drives the fluid in a circumferential direction. However, in the absence of any force to balance the resulting normal acceleration, the water in the boundary layer is driven radially outward. This drives the tea leaves to the edge of the cup. The radial outflow is then forced up the side of the cup, though the tea leaves stay in the corner at the base as they are denser than the water.
The vertical flow then turns back in toward the cup center and then down when it reaches the water surface. This creates a cylindrical vortex around the edge of the cup (see figure below). Inside the cylindrical vortex is a non-rotating core with a flat water surface.
Over time, the cylindrical vortex grows toward the center of the cup until there is no longer a non-rotating core and the water surface is curved all the way across (see figure below). At this point the flow is fully spun up and the tea leaves should still be at the corner of the cup.
When the cup is abruptly stopped, the water in contact with the base also stops moving. There is, therefore, no longer anything driving the flow radially outward. Instead, there is a hydrostatic pressure gradient toward the center of the cup due to the curved water surface (the water surface remains curved as all the fluid outside the boundary layer does not know the cup has stopped and is still rotating). Therefore, the flow in the bottom boundary layer reverses and the tea leaves are driven into the center of the cup (see figure below).
In the event that the cup is not fully spun up (step 7 in the procedure section), the hydrostatic pressure gradient only extends from the side of the cup to the edge of the cylindrical vortex (recall that the water surface in the non-rotating core is horizontal). Therefore, the lower boundary layer only flows radially inward to the edge of the cylindrical vortex. The tea leaves thus accumulate at the inner edge of the cylindrical vortex (see figure below).
This is a remarkably robust experiment. It is almost impossible for it not to work (provided that the cup is secured to the center of the turntable). Thanks to Alex, and Meredith for putting together this write up and demonstration. Videos to follow soon.
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 (email@example.com). I also welcome comments (through the comments section or via email) on improving the demonstrations.