# Physics laboratory setup for fluid dynamics experiments

I'm beginning research on how the Earth's Coriolis Effect affects solids suspended in moving liquids. I have run into a challenge that lifehackers may be able to help solve.

I would like to compare two similar liquids—one turning clockwise and the other turning counter clockwise at the same rate. This has presented two challenges to either solve or adequately counteract/compensate.

1. How can I stir/spin/swirl two similar liquids in different containers at the same rate in diametrically opposite directions?
2. How can I prove/verify that both are turning at the same rate.

I can't figure out how to "mark" a liquid to track its movement. I can't figure out how to affect the liquid (move it) without the interference of some object. For example: I have used magnetic mixers that use encapsulated stir-bars to mix chemicals in my lab but they (and all others I've found) all spin in the same direction sitting on the bench. I found it might be possible to change the direction of the rotation of one of two magnetic mixers; but, that introduces other distractions (the stir bar) that I'd prefer to avoid if possible.

A fair number of laboratories use toys like Fischer Technik or Lego in simple experimental setups. You could use them or similar, modular equipment to build a frame that connects 2 stirrers with an even number of same size gearwheels.

(I'm certain I don't have to explain to you, but maybe some other users might not know immedieately)
The even number of gears means that the gears at opposite ends of the setup rotate in the opposite direction. Using gears of the same size means that all gears rotate with the same speed.

Attach a stirrer on each end and let them get your fluid up to the desired speed. Then move the whole frame up and away from the containers. This exposes both liquids to the same influences (like drag from the stirrers). To make results as comparable as possible, I would build some kind of guide rail to move the stirrers out of the containers evenly. Maybe you can keep the stirrers rotating while moving them up, so the motion of the liquids is disturbed as little as possible.

• Your suggestion to use gears suggests designing a 'clutch' (gear-box) to drive the apparatus. As you have observed, a rotating shaft—when touching another freely mounted one—spins it opposite to the first one's direction. Regarding the number of gears, the fewer the better. : ) Good stuff.
– Stan
Commented Oct 13, 2020 at 15:27
1. if you don't want to insert a stirring implement of some sort, you could spin the entire container, it'd just take a long time for the liquid to get up to speed.

2. Measuring the speed of the stirrer is easy: stick a revcounter on it. These usually consist of an object stuck to the moving part of the stirrer, and a sensor that detects that object (a vane interrupting a beam of light, a magnet moving past a magnetic sensor.

Measuring the speed of the liquid is a bit more difficult. You might be able to do a magnetic revcounter with a small magnet stuck to a bit of cork that will float on the liquid. Or aim a video camera at the liquid, drop a floating object in and measure the spinning speed on the video. Make sure the framerate is high enough though: 24 fps video means you shouldn't spin the liquid at more than ~12 revs/s.

• Thank you. A stir bar is not symmetrical so it can be used as the 'vane' to interrupt a beam of light. Good idea in concept. The floating cork might not be the best way to measure the rotation of the liquid. In fact, I don't care what the speed so long as both sites are the same. Spinning the containers is an interesting thought - Ideally, I wanted liquid-filled pipes to move the solids to conform to practice; but, maybe there's a way.
– Stan
Commented Oct 13, 2020 at 15:19
• Here's the question I must be able to prove: How do I know both liquids ARE up to speed? "Appears to be" ≠ "is." How do I quantify the margin of error?
– Stan
Commented Oct 18, 2020 at 15:01
• Elmy's idea is better for this, I think. Use cylindrical containers. Use a stirrer that consists of a blade that fills the entire container (minus just enough of a gap to allow the blade to rotate in the container). This ensures there's no mismatch between the speed of the liquid and the speed of the blade. With Elmy's gearing, you know the speeds in the two containers match. Pull the blades out once the liquids are up to speed. Commented Oct 19, 2020 at 8:44

Certain popular second hand selling sites appear to have a reasonable proliferation of vinyl turntables for not very much money. I'd suggest that a belt driven one would work better for your purposes because even if you cannot get the motor to reverse by swapping the wiring over, you can reverse the rotation of the deck by having one belt be looped onto its motor with a single twist (figure 8 rather than figure 0). If you mount the motor at an angle it will help the twisted belt avoid coming into contact with itself

You can also be reasonably sure that your centrally placed container is rotating at a particular speed, given that these things are calibrated for audio playback and you'll get a choice of speeds; 33, 45 or 78 rpm

If speed matching is very important to you, perhaps seek out some intended for DJ mixing. Not only will they have a control that will vary the speed up/down by some reasonable percentage, but they will have a sequence of dots embedded into the edge of the table and a strobe light that shines on said dots. The dots are arranged in a few bands at different densities and appear to stay stationary when the table RPM hits an exact value. What value isn't so important; even if you configured the table so the speed was X and the dots were "crawling past at a rate of 3 dots per second" on both decks you could be certain they were rotating at the same RPM. One table would be in reverse but the strobe system wouldn't care; its just an on/off light and doesn't rely on a particular direction to indicate rotation rate

• This set-up ensures that the mechanism is performing according to spec. The liquid contents are the stumbling block for me. I have access to a strobotac but it is not effective for suspended freely moving particles in the liquid. Even doing that is an indirect method of indication. I have access to virtually any piece of equipment. I have had precision parts machined. The problem is the configuration—the set-up—the technique.
– Stan
Commented Oct 18, 2020 at 14:55