Slosh - John Dunn, Consultant, Ambertec, P.E., P.C.

On a couple of different occasions, I needed to move a plastic tray of roughly 12 by 18 inches, filled with a couple of inches of water, from one part of the building to the other. At first, I tried to simply carry the thing, but it was too much of a challenge not to spill it. Slosh, slosh, round and about, oh boy!

So the next time, I thought I'd smarten up. I got this really slick rolling cart with great big smooth-riding wheels. That card would glide across a tile floor like Michelle Kwan on ice.

I put the filled tray on the cart and started off down the hall, but it wasn't long before the water started to really swing around in there, slosh, slosh, round and about, good grief!!

What I hadn't considered was that as smooth as the cart's ride was, it still had some roughness and that the roughness' noise was operating on the water like a broadband, mechanical noise stimulus. The water in the tray had some mechanical resonant frequency and would extract energy at that frequency from the broadband spectral content of the floor noise, thus building up a resonant back and forth motion until something got wet, namely me.

It was quite a physics lesson.

The other day, I was standing on the second floor of the Roosevelt Field shopping mall where I found myself thinking about how all the feet and baby strollers of people passing by would probably make a pretty good broadband, mechanical noise source.

Then, I felt the floor under me oscillate, just a little bit.

Hmmmmmm.

Comments

[Carl Meshenberg]

Had the same problem many times - isn't it just the momentum of the induced motion on the water molecules that causes the sloshing. Also, I wonder if there is any way to harvest the background motion and/or noise to power small devices.

[Andrew]

The roman armies had the same problem crossing bridges, hence the development of the command 'Change Step' to vary the oscillations of the soldier's feet on the bridge surface and thus avoid a 'Tacoma Narrows' type situation.