Primitive variables in Fluid Mechanics

  • Thread starter hanson
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  • #1
hanson
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Hello!
I know that the primitive variables in fluid mechanics are velocity and pressure. But why?
I don't see how "primitive" these variables are...What does it mean by a "primitive variable"? Is that all other variables in a fluid flow can be derived from velocity and pressure? Is there any other set of primitive variable? Lastly, how do you know velocity and pressure are primituve variables?
 

Answers and Replies

  • #2
hanson
319
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can anyone kindly address this?
 
  • #3
quark
231
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I am not aware of such terminology but they are the main variables. You can manage any fluid flow problem if you know these two things.
 
  • #4
hanson
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quark, thanks! I was afarid that no one is going to help me out. :frown:

Why can we manage all flow problems if we know these 2 things?
 
  • #5
Rocketa
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hanson said:
Hello!
I know that the primitive variables in fluid mechanics are velocity and pressure. But why?
I don't see how "primitive" these variables are...What does it mean by a "primitive variable"? Is that all other variables in a fluid flow can be derived from velocity and pressure? Is there any other set of primitive variable? Lastly, how do you know velocity and pressure are primituve variables?

Try this ...
When you try to solve a fluid field without energy equation (heat transfer)? Conservation of mass and Conservation of momentum (Derived from Newton's Second law) are sufficient. You will find the main variables are pressure and velocity. With pressure and velocity information you can find the fluid field. BESIDES all the properties.
 
  • #6
abhaythermax
2
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QUOTE=hanson;752113]Hello!
I know that the primitive variables in fluid mechanics are velocity and pressure. But why?
I don't see how "primitive" these variables are...What does it mean by a "primitive variable"? Is that all other variables in a fluid flow can be derived from velocity and pressure? Is there any other set of primitive variable? Lastly, how do you know velocity and pressure are primituve variables?[/QUOTE][/FONT][/SIZE]
 
  • #7
abhaythermax
2
0
what is primitive variable? i know it is velocity and pressure,but why it is? Is that all other variables in a fluid flow can be derived from velocity and pressure?
 
  • #8
minger
Science Advisor
1,496
2
It depends on what form the governing equations are written in, and how we interpret them.

Typically, when we write the Navier-Stokes in strongly conservative form, they are in terms of the conserved variables. That is, mass, momentum and energy. However, one doesn't typically think of flows in terms of momentum and energy, we would prefer to use velocity and pressure. So, the primitive variables are mass, velocity and pressure.

You are opposite in thinking that everything can be derived from velocity and pressure. It is in fact that everything can be derived from momentum and energy (with some other variables of course).
 

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