Piston Oscillation in Fluid - Resistance Force 1

[vdubinsky]

I would like to estimate the resistance force applied to a piston that oscillates in a fluid.

Let's say we have a piston with the surface area S that oscillates in water with the frequency Fo and the displacement magnitude Ao, i.e. x = Ao*sin(2*pi*Fo*t).

How can I estimate (analytically or numerically) the maximum value of the resistance force as a function of the frequency for a fixed displacement magnitude?

Thanks!

 

[ivymike]

you could approximate the drag based on the frontal area of the piston, a rough Cd from a textbook, the fluid density, and the velocity(t).

 

[ivymike]

is this a homework problem?

 

[vdubinsky]

ivymike,

thanks for your comment... but can you, please, be more specific?
Can you please provide a formula along w/ brief explanatory comments or a reference to a book or a paper that could help my resolve this problem?

It's not a homework, it's a calculation task that needs to be performed to evaluate a design potential.

 

[ivymike]

just about any fluid mechanics text should cover calculation of drag.

this one is an example

http://www.amazon.com/gp/product/00...ref=sr_1_3/103-2933527-8367835?_encoding=UTF8

 

[GregLocock]

That's not really a drag problem, that's a volume source problem. The answer can be found in any worthwhile acoustics textbook. say L. E. Kinsler, A. R. Frey, A. B. Coppens, and J. V. Sanders, Fundamentals of Acoustics, 3 ed: John Wiley & Sons, 1982, or Beranek

The usual case is a piston in a semi-infinite baffle. I have also seen a solution for a rigid body in an unrestricted volume, which is a more complicated case.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

http://www.t-linespeakers.org/projects/tlB/appendix/beranek/index.html

The reason I emphasise that this is an acoustics problem is that the force required to push the piston is heavily frequency dependent. However if you want to persevere with a velocity based approach then don't forgot the drag along the sides of the piston.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

However, thinking about it, in a typical body the acoustice coupling efficiency is very low so most of the input energy must end up as heat, so friction must be the main resistance.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[vdubinsky]

I would like to thank both of you for your valuable input and advice re: solving this problem!