So have a(adsbygoogle = window.adsbygoogle || []).push({}); loosefitting piston attached to a conrod, attached to a flywheel. The chamber the piston is in is sealed reasonably well around the controd, and is full of a fluid with a viscosity.

Lets say the flywheel is spinning.

I want to work out from the fluid dynamics occuring, what the damping coefficient is around the piston due to fluid moving over it.

What I think is true.

1. There is shear stress occuring, due to the shearing layers of the fluid. I have included that.

2. The model can be assumed to be a pipe, and therefore there is friction occuring due to surface roughness, and the head loss (pressure) can be calculated using a moody chart. I am including this.

3. A pressure gradient builds up across the piston, due to the piston accelerating towards the middle of the stroke, fluid in front of it increases in pressure, and fluid behind it decreases in pressure.

This pressure gradient forces fluid over and around the piston.

This is acting against the motion, and is another source of damping.

If the first two are agreed with, it is the third one I think has the most effect, because my matlab code runs nicely but far too fast, i.e for a specified torque applied to the flywheel, the real model runs slower than the simulation (in matlab) - like a tenth of the speed.

I think I need to consider fluid compressibility (currently the fluid is air, but could be changed - so compressibility would change...?)

But I also don't know how to calculate the coefficient that is mulitplied by the velocity of the piston to give the damping term. Does anyone have any (prefferably justified) equation that can help me calculate this coefficient. Assume other variables are obtainable. I believe it could be NOT a constant, although I may be wrong.

I also am not sure whether (using the same equation) I can calculate any leakage past the conrod of the piston, as it moves linearly in and out of the piston chamber - I think there would be some around it

If anyone has any commments about my approach to this they would be very welcome.

Alex

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# Dashpot Damping Coefficient

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