Calculate the damping coefficient of air

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Discussion Overview

The discussion revolves around calculating the damping coefficient of air in relation to a piston oscillating in a tube. Participants explore theoretical approaches to determine this coefficient, considering fluid dynamics and oscillatory motion.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant outlines the relationship between viscous shear stress and force, questioning how to derive the damping coefficient from theoretical principles.
  • Another suggests attaching the piston to a spring and using the log decrement method to measure displacement and calculate the damping coefficient, emphasizing a practical approach.
  • A different participant states that the damping coefficient can be calculated as the ratio of force to velocity, indicating that the force is already known and only the velocity needs to be measured.
  • One participant expresses a desire to understand the fluid dynamic equations necessary to find the damping coefficient, indicating a focus on the theoretical framework.

Areas of Agreement / Disagreement

Participants present multiple approaches to calculating the damping coefficient, indicating that there is no consensus on a single method. The discussion remains unresolved with various theoretical and practical perspectives offered.

Contextual Notes

Participants reference different methods and assumptions regarding the damping coefficient, but there are no explicit definitions or agreements on the necessary fluid dynamic equations or parameters involved.

a.mlw.walker
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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 don't 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 calculate the damping coefficient?

Alex
 
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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.
 
a.mlw.walker said:
2. Therefore the viscous sheer force = (piston surface area)*mu.du/dy

But this is not the damping coefficient. How do I calculate 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.
 
The damping coefficient defines the velocity, i want to know the fluid dynamic equations to find the damping coefficient..
 

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