
[Sponsors] 
December 31, 2012, 05:44 
Pressure boundary condition for unsteady velocity inlet

#1 
New Member
Dan M
Join Date: Sep 2010
Location: Munich, Germany
Posts: 20
Rep Power: 7 
Hello everyone,
I have a fairly general question: When solving a flow problem with an unsteady velocity inlet, how should the pressure boundary condition at the inlet be defined? For steady flow a zero gradient condition is adequate and ensures consistency at the inlet boundary. However, for an unsteady velocity boundary I believe the pressure should also fluctuate in time and space. Zero gradient is still often used for the unsteady case, but doesn't that introduce an error? I would appreciate your comments! Thanks a lot, Dan 

January 2, 2013, 16:27 

#2 
New Member
Martin Lubej
Join Date: Jul 2012
Posts: 5
Rep Power: 5 
Hello Dan,
I do not see a problem with zeroGradient boundary condition for pressure at unsteady inlet velocity. The zeroGradient means that the normal gradient of pressure is zero, so if your inlet is normal to x, then dp/dx=0, but it doesnt mean that dp/dt=0, the value of pressure at inlet can still change based on the velocity, if the outlet pressure is constant. Regards 

January 3, 2013, 06:07 

#3 
New Member
Dan M
Join Date: Sep 2010
Location: Munich, Germany
Posts: 20
Rep Power: 7 
Dear Martin,
Thank you very much for your reply. The problem I see is that in certain cases the pressure field associated with a time varying velocity field does not satisfy dp/dx=0 (assuming the inlet normal points in the x direction). Take for example the simulation of an ocean wave. Here, the pressure gradient in xdirection is not zero, yet when solving these kind of problems, a zeroGradient condition for the pressure is typically used. See for example the pressure files in the following tutorial: https://github.com/ogoe/waves2Foam/t...am/waveFlume/0 When defining a velocity field at a boundary a zeroGradient pressure condition implies that there are no pressure forces introduced that might be inconsistent with the defined velocity field. However, I would think that in cases where the normal pressure gradient is not zero, an error in the p field is introduced. I would appreciate your thoughts! Thank you, Dan 

July 9, 2013, 08:05 

#4 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,205
Rep Power: 18 
Hi Dan
did you find out your answer? and what in case of outlet boundaries in unsteady cases?does a fixed pressure have any sense?and setting velocity and temperature are zeroGradient in your opinion?
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

July 9, 2013, 08:21 

#5 
Member
Join Date: Sep 2012
Posts: 60
Rep Power: 5 
Hi,
this seems to be a nice discussion. ZeroGradient is generally suited for unsteady cases also as already pointed out. In cases where normal pressure gradient at inlet is expected to be nonzero, one can try to extrapolate pressure based on internal field values. This will better capture the wavy distribution of pressure. But again, something must be known about the flow field prior to the computation. At outlet for an u/s case, one can use convective b.c (solving an ode for that field) for incompressible flow. achyutan 

July 9, 2013, 08:36 

#6 
Senior Member
Ehsan
Join Date: Oct 2012
Location: Iran
Posts: 2,205
Rep Power: 18 
Hi achyutan
my case is compressible.how can I use convective BC? and how can I write it by groovyBC?whats the formulas of it to simulate? thanks.
__________________
Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked. 

July 5, 2015, 09:21 

#7 
New Member
kazem
Join Date: Mar 2015
Posts: 1
Rep Power: 0 
Hi Dan
did you get your answer? I have the same problem, If anyone can help me 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Pressure boundary condition  CH Kuo  Main CFD Forum  11  September 17, 2015 11:41 
Boundary Conditions: Pressure Inlet, Velocity Outlet  BigPapi34  OpenFOAM Running, Solving & CFD  4  August 2, 2014 11:39 
Pressure Inlet Boundary Condition Profile  arapha  FLUENT  0  April 4, 2011 16:23 
RPM in Wind Turbine  Pankaj  CFX  9  November 23, 2009 05:05 
Pressure Inlet Boundary Condition  Trinity_psu  FLUENT  0  June 1, 2005 14:15 