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August 8, 2013, 06:25 
Total Pressure

#1 
Senior Member
James
Join Date: May 2013
Posts: 102
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Hi everyone,
I have a very stupid question (sorry I am new in CFD). I have to specify a Total pressure BC on my inlet. I know that Gauge pressure is 0 Pa on inlet and 20 Pa on outlet, but I have no information about velocity (pressure driven flow), so how can I compute 1/2*rho*v^2 for dynamic pressure? Thanks in advance 

August 9, 2013, 08:26 

#2 
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Marco
Join Date: Jul 2013
Location: Italy
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Plase, explain better what is your domain and your bonduary condition....


August 9, 2013, 09:43 

#3 
Senior Member
James
Join Date: May 2013
Posts: 102
Rep Power: 5 
Hi Marco,
First of all, thakns for your reply. My domain is a little complicated, but imagine a channel with two inlets and an outlet. I need to specify a TotalPressure BC at inlets and a static pressure at outlet. So my question is how can I compute dynamic pressure on inlet and outlet if I donīt have any information about velocitites (P_dynamic=0.5*rho*v^2)? In other words, if I understanding well the theory: P_absolute=P_atmospheric+P_gauge P_total=P_static+P_dynamic Then P_total=P_gauge, so at inlets is enough to specify 0 Pa, but what happens with outlet? I need to obtain from P_total=20 the value of P_static and P_dynamic, but how if I have no idea about v values at outlet? I hope I am explaining well (and I am understanding well too...) Thanks 

August 10, 2013, 04:23 

#4 
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Marco
Join Date: Jul 2013
Location: Italy
Posts: 36
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why do you have information only about the pressure ?
are you immage to obtain information only with a pressure gauge installed on your channel ? The total pressure keep the contribution of the dynamic term only if you have the inlet of the gauge normal to the velocity vector if you can post the wbpj file I try to understand better.... 

August 10, 2013, 05:42 

#5 
Senior Member
James
Join Date: May 2013
Posts: 102
Rep Power: 5 
Marco,
I have only info about pressure at boundaries because itīs a pressure driven flow, a typical problem in fluid dynamics. Sometimes is physically more suitable to specify this than a flow rate or a velocity inlet (because for example velocity profile is not uniform at my inlet, so if I suppose there a velocity inlet condition I am changing the physics, and for example the computing of volumetric flow rate will be altered). I am trying to do the analysis in OpenFOAM, but I have seen that some solvers specifies a TotalPressure BC at inlet (and a pIOV condition for velocity) and static pressure (fixedValue)at oultet (and a zeroGRadient condition for velocity). So if I want to setup the problem correctly I should be able to compute static pressure at outlet, and I think I have: P_Total=20, P_Total=P_static+0.5*rho*v^2 then: 20=P_static+0.5*1.225*v^2. There are 2 unknowns, and about velocity the only info I have is du/dn=0. How can I obtain static pressure then? Thanks 

August 12, 2013, 02:14 

#6 
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Marco
Join Date: Jul 2013
Location: Italy
Posts: 36
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in these situations I use to fix some values of first attempt,
after the solution you can verify other bonduary condition you know: for example fix the value of static pressure and control the value of another variable in another poit of your bonduary. In every case you need an additional information about the problem. Sorry, but I haven't a better idea... 

August 12, 2013, 03:56 

#7 
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Lefteris
Join Date: Oct 2011
Location: UK
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So, at inlet Ptotal=0 and at outlet Ptotal=20?
__________________
Lefteris 

August 12, 2013, 06:40 

#8 
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Liam
Join Date: Aug 2013
Posts: 38
Rep Power: 4 
Thanks Marco for your suggestions, I īll take in count.
Lefteris, the problem is about computing static pressure from total pressure when the velocity at that boundary is unknown. Of course if the solver only needs total pressure at boundaries, these values are given (as you said), but I have read something like OF needs one TotalPressure condition in a boundary and a static pressure in another. Thanks! 

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