# Why is conservative_1 in incompressible flow the same as pressure?!

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 June 7, 2017, 20:07 Why is conservative_1 in incompressible flow the same as pressure?! #1 Member   Join Date: May 2017 Posts: 39 Rep Power: 8 Hello, I know the conservative_1 is supposed to be the density of the flow. However, in my 3d incompressible simulation with SST model, my conservative_1 field is the same as pressure (also includes negative values). I am a little confused about this since pressure is not a conservative field. Can someone please explain what is going on? Thanks

 June 8, 2017, 09:44 #2 Member   Ole Burghardt Join Date: Mar 2016 Location: Kiel, Germany Posts: 60 Rep Power: 9 Hi, solving the incompressible equations is a bit different: "Find the fields that fulfill the conservation of mass/momentum/energy" already sounds more natural than "Find the fields that fullfill the conservation of momentum under the assumption that the velocity is divergence free". The "under the assumption" part is both in physical intuition and theory translated into finding a pressure field that effectuates this, in SU2 a method called "artificial compressibility" is used. So you are right, there is no "pressure conservation", but SU2 has to solve for the pressure and it's also not so odd to just put it in the Conservative_1's place as you might think: For compressible flows at low Mach numbers, you can simply translate pressure into density (or the other way around). So it's "nearly" such a mass conservation field if you think of it from the physical meaning's side. Hope that makes sense to you Regards, Ole

June 8, 2017, 10:55
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 Originally Posted by Sprotte Hi, solving the incompressible equations is a bit different: "Find the fields that fulfill the conservation of mass/momentum/energy" already sounds more natural than "Find the fields that fullfill the conservation of momentum under the assumption that the velocity is divergence free". The "under the assumption" part is both in physical intuition and theory translated into finding a pressure field that effectuates this, in SU2 a method called "artificial compressibility" is used. So you are right, there is no "pressure conservation", but SU2 has to solve for the pressure and it's also not so odd to just put it in the Conservative_1's place as you might think: For compressible flows at low Mach numbers, you can simply translate pressure into density (or the other way around). So it's "nearly" such a mass conservation field if you think of it from the physical meaning's side. Hope that makes sense to you Regards, Ole
Thanks a lot for the detailed explanations.

 Tags conservative variables, conservative1, incompatible, sst k-omega, su2