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Name: Tim A.
Status: Educator
Age: 50s
Location: N/A
Country: N/A
Date: April 2004


Question:
We have found that when a steel ball bearing is released from an immersed position in an oil column it initially speeds up to a maximum value and then slows down, apparently exponentially to a much lower value before reaching the bottom. Has anyone got any ideas why this may happen?



Replies:
Most likely it is eddy currents produced in the wake of the ball bearing. Eddy currents are complex currents behind the ball that will cause extra drag when a maximum velocity is reached. Try looking into eddy currents on the web or at your local library.

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A good thing to notice; maybe we are all wrong about everything...

fishing:

- oil is a bit thin, so not always laminar-flow? laminar flow later gives way to turbulent flow, or viceversa, increasing drag in later state. Make dye-streaks to detect turbulence.

-surface freedom?

When you start dropping the ball, part of the fluid which must move in the viscous- deformation response is at the surface, and not feeling drag there. (start farther from the surface than 2 x tube diameter, with tube diameter 3-6x ball diameter. or put marker on surface, see how far it moves.)

- oil in your container is stratified in composition or temperature or ?. (stir it)
- minority oil thickeners shredded near the surface? (same as above)
-thixotropic fluid?

If the oil has long chain molecules, and the drag might be low until motion shears them into aligned orientations, then higher. ( do not know if this is reasonable.)
- your release causes ball to spin, dies out later?
- you are in a tapered tube and have not noticed?
- the place where you drop the ball over and over has an invisible embedded trail of watery sweat? (last 3 facetious)

I wonder if there is a functional conflict in finding truly precise high-viscosity oils. If it has homogenous molecular weight, then usually it is either not too viscous or it is close to freezing. If it has low weight relatively liquid oil, thickened by higher-weight minority additives, then they are vulnerable to effects like alignment or shredding or chemical cutting?

Jim Swenson



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