# Dynamic modeling of a system and transients of the system

## Main Question or Discussion Point

Consider, I have a dynamic system model for air compressor. Which means I have modelled the system by including its physics. Does this means I also included the transients of the system?
What I think is, when i modelled any system with equations, I think that includes the transients. Is it correct?

I want to know how can I model a pneumatic tank with equations in matlab?

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anorlunda
Staff Emeritus
What I think is, when i modelled any system with equations, I think that includes the transients. Is it correct?
Not necessarily. You could model it by saying mass in equals mass out. That is simply conservation of mass, a very important equation. But that model is too simple to include transients.

The key question you need is "are the equations differential equations?"

Not necessarily. You could model it by saying mass in equals mass out. That is simply conservation of mass, a very important equation. But that model is too simple to include transients.

The key question you need is "are the equations differential equations?"
That means, if the system modelled with differential equations will includes the transients. Isn't it?

anorlunda
Staff Emeritus
That means, if the system modelled with differential equations will includes the transients. Isn't it?
Yes, but to be careful with words, only if the differentials are with respect to time. It is possible to have partial differential equations that do not include time. We don't use the word transients to describe their solutions.

Then you need one more thing for a transient simulation. You need the time behavior of the boundary conditions.

It makes me nervous to provide these answers because it sounds like you don't really understand what transient simulation means.

Simply having time differential equations does not necessarily mean that you have correctly included the transient. As a simple example, if you describe the dynamics of a simple spring-mass system but do not include the damping, you will get an undamped solution; this does not decay and thus has no transition into steady state. It is only when you get the complete system model that you have all of the intransigent behavior.

Yes, but to be careful with words, only if the differentials are with respect to time. It is possible to have partial differential equations that do not include time. We don't use the word transients to describe their solutions.

Then you need one more thing for a transient simulation. You need the time behavior of the boundary conditions.

It makes me nervous to provide these answers because it sounds like you don't really understand what transient simulation means.
I mean differential equation w.r.t time only. I have an idea, what is transient behaviour is.

Can anybody help to derive a mathematical model of an air storage tank for an air compressor system?

I have a scew type compressor(fixed speed type), which will deliver 1000cfm to a storage tank. The output for end use require 80-90psi. But the load(air flow output from tank) is varying w.r.t time, I need to include these all cases in consideration in the model. I think I may have to add a valve too after the storage tank. Anybody has any idea how to do this?

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anorlunda
Staff Emeritus
Anybody has any idea how to do this?
Of course, you start with the first principles: conservation of mass, energy, volume, and the perfect gas law. But we can't do your work for you. Post your equations and a diagram and we can offer comments on it.