# Calculate the damping coefficient of air

#### a.mlw.walker

So I am trying to calculate the damping coefficient of air passing around a piston that is oscillating in a tube. The idea is completely theoretical. I dont have any real data.

As far as i understand:

1. The viscous sheer stress = mu.du/dy
2. Therefore the viscous sheer force = (piston surface area)*mu.du/dy

But this is not the damping coefficient. How do I calcualte the damping coefficient?

Alex

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#### minger

So long as it's completely theoretical....

You could attach the piston to a spring. In the absence of the air, the spring would oscillate forever given an initial displacement. However, with the air, there will be damping. You could measure the displacement of the spring and use something like the log decrement method to calculate the damping coefficient of the system.

#### jack action

Gold Member
2. Therefore the viscous sheer force = (piston surface area)*mu.du/dy

But this is not the damping coefficient. How do I calcualte the damping coefficient?
The damping coefficient is the ratio of the force over the velocity (F = C * v). So you already have the force, all you need is the velocity of the piston and divide one by the other to get the damping coefficient.

#### a.mlw.walker

The damping coefficient defines the velocity, i want to know the fluid dynamic equations to find the damping coefficient..

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