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-   -   Absolute or Gauge pressure? NS and Equation of state (http://www.cfd-online.com/Forums/main/107963-absolute-gauge-pressure-ns-equation-state.html)

cici October 10, 2012 10:59

Absolute or Gauge pressure? NS and Equation of state
 
Hi All,

Are those pressure used in NS equations the same as in equation of state? My understanding has always been that the former gauge pressure, the latter absolute one. But how come most of the references use the same symbol to refer to these two different variables. Or is ot just a convention??

Thanks.

cdegroot October 10, 2012 12:47

The Navier-Stokes equations include just the pressure gradient, therefore it doesn't matter which one you use since their gradients are the same.

cici October 10, 2012 13:23

Thanks! But I knew this. I should use governing or conservation equations instead of NS equations. In the conservation of engergy, there are u(dp/dx) and p(du/dx), if we use total enthalpy as the conservative, which rise with the relation between energy and enthalpy:h=e+p/rho. What is the pressure here then?

cdegroot October 10, 2012 13:34

If you are using pressure to define enthalpy you should use absolute pressure since the use of gauge pressure would make the definition of enthalpy and terms like p(du/dx) arbitrary (and possibly negative!).

cici October 11, 2012 09:52

Thanks.
Some references ignore the acoustic pressure Pa, and decompose the pressure into the thermodynamic pressure Pt and hydrodynamic pressure Ph. Pt is used for equation of state, not the total pressure, and Ph for momentum. In this case, what is the absolute pressure? If acoustic pressure is included, which part of pressure does it fit into? On other words, how is pressure decomposed?


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