Understanding the Effects of Stirring vs. Rotating Fluids: A Physics Discussion

[Denise Wetli]

Hello,
An item in Jearl Walker's "The Flying Circus of Physics" discusses stirring tea. (4.63) I understand that stirring causes the fluid to move faster toward the center thus producing lower pressure near the center. Higher pressure exists at the wall and this pressure difference causes the centripetal force. Also, the stirring creates a vortex and the fluid is deeper at the wall than at the center.

My question is: Do you get the same effect if, instead of stirring, you rotate the container on a turntable? Where is the speed of the fluid now greater? Is the pressure difference still from the outside to inside? Will the fluid "rise" up the sides of the container while being spun? I have tried spinning a bucket of water on a turntable but am not convinced that what I am observing is just due to the sloppy way in which I'm operating the turntable.

Thanks

 

[LightHero]

for your help.Yes, you can get the same effect if you rotate the container on a turntable. The speed of the fluid will be greater at the edges and sides, where the force from the turntable is strongest. The pressure difference will still be from the outside to inside, due to the centripetal force. The fluid may "rise" up the sides of the container while being spun, although this depends on the size and shape of the container. If you are not convinced of the effect you are seeing, it is possible that the turntable is not spinning quickly enough or that the container is too large and has too much inertia to be affected by the turntable.

 

[charng]

for your question and interest in understanding the effects of stirring versus rotating fluids. I can provide some insight into this topic.

First, let's discuss the basic principles at play here. Stirring and rotating both cause the fluid to move in a circular motion, creating a vortex. This vortex causes the fluid to move faster towards the center and slower towards the outer edges. This difference in speed creates a pressure difference, with higher pressure at the outer edges and lower pressure at the center.

In terms of the specific question about rotating the container on a turntable, the effect would be similar to stirring. The fluid would still experience a circular motion and create a vortex, resulting in a pressure difference and centripetal force. However, the speed of the fluid would be greater at the outer edges of the container, as the turntable would be rotating the entire container rather than just stirring the fluid within it.

To answer your other questions, the pressure difference would still be from the outside to the inside, and the fluid may still "rise" up the sides of the container while being spun. This can depend on factors such as the speed of rotation and the shape of the container.

In terms of your observation with a bucket of water on a turntable, it is possible that your results are affected by the way in which you are operating the turntable. It is important to ensure that the rotation is smooth and consistent, and that the container is properly centered on the turntable. You may also want to consider using a more precise measurement tool, such as a flow meter, to accurately measure the fluid's movement.

In conclusion, both stirring and rotating can have similar effects on fluids, creating a vortex and pressure difference. However, the specifics of the effect may differ depending on the method used. I hope this helps to clarify your understanding of this topic.