fluid compressibility - spring effect 1

[nado59]

Dear All,

I have found a brief statement that the spring effect of compressed fluid in hydraulic devices depends on the jerk (first derivative of acceleration) of the piston movement.

I can clearly see from an experiment that this is true but I would like to know the detailed explanation. My first thought would be that it depends on piston velocity but obviously I am wrong.

Many thanks in advance!

Nado

 

[BigInch]

The first derivative of acceleration with respect to time is the rate of change of acceleration. Since F = m a, force associated with jerking can be seen to equal dF/dt = m da/dt, or change in force over time. When a changing force is applied to a fluid, it creates a fluid pressure varying in time, which in turn would create variations in the volume of a compressible fluid that is being compressed by those varying pressures. That relationship of pressure to fluid volume change is described by the fluid's bulk modulus and is how a compressible fluid can store the energy of compression. Release of pressure will cause the fluid's volume to "spring back" to its original volume before compression.

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"The problem isn't finding the solution, its trying to get to the real question." BigInch

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[nado59]

BigInch,

Thank you! So it was good old Mr Newton again Very clear explanation from you, many thanks again!

 

[BigInch]

As you can see by looking at some typical bulk modulus values, fluid compressibility probably has a minor effect on jerk when compared to deformation of the system frame in bending. When forces are applied to a boom, the boom frame itself bends under the applied load and I would presume that much more relative energy is stored in the bending displacements of the frame than in the fluid itself. Hence a good portion of the resulting spring-back effect when pressure is reduced is from the return of any framing members back to their unloaded positions.

**********************
"The problem isn't finding the solution, its trying to get to the real question." BigInch

http://virtualpipeline.spaces.live.com/