# Neumann Boundary condition (outflow bc)

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 December 18, 2009, 04:48 Neumann Boundary condition (outflow bc) #1 New Member   Thiyagarajan dhayalan Join Date: Jun 2009 Posts: 7 Rep Power: 10 Hi all, iam solving flow in a 2D duct using SIMPLE algorithm.at the inlet iam specifying velocity inlet(u_in) and at the outlet iam specifying outflow bc i.e du/dx=0.iam just extrapolating 'u' values to the boundary from immediate neighbour cells inside the domain.similarly for 'v' velocity .but for pressure iam not specifying any boundary condition.i just initialised it to zero everywhere.should i specify pressure?.if i specify wouldn't it become pressure outlet i.e Dirichlet bc?.iam getting correct velocity values but pressure values increase with number of iterations.I think it is something to do with pressure bc. can pressure be extrapolated like velocity? please clarify me? thanks for ur time. reg Thiyaga. Last edited by Thiyaga; December 18, 2009 at 06:27.

 December 18, 2009, 20:53 #2 Senior Member     p ding Join Date: Mar 2009 Posts: 330 Rep Power: 11 tha anwser is yes

 December 19, 2009, 01:51 #3 New Member   Thiyagarajan dhayalan Join Date: Jun 2009 Posts: 7 Rep Power: 10 Hi Mr.Ztdep, thanks for ur response.from ur answer i suppose that i should extrapolate pressure at the outlet.in that case what is the pressure bc at inlet. please clarify me in detail. again thanks for ur time. reg Thiyaga.

 December 21, 2009, 08:03 #4 Senior Member   Prapanch Nair Join Date: Mar 2009 Location: Bangalore, India Posts: 105 Rep Power: 10 Hi Thyagarajan, You should probably think of giving a fixed value for total pressure at the inlet (alternatively to the outlet). It is important that pressure has to have a ref value somewhere in the domain or you will see pressure values increasing. I assume your flow problem does not have a shock occuring in it. In that case you should consider given restraints on the total temperature and thereby having a value for pressure. Hope this helped. Prapanj

December 21, 2009, 23:16
#5
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p ding
Join Date: Mar 2009
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Quote:
 Originally Posted by Thiyaga Hi Mr.Ztdep, thanks for ur response.from ur answer i suppose that i should extrapolate pressure at the outlet.in that case what is the pressure bc at inlet. please clarify me in detail. again thanks for ur time. reg Thiyaga.
just extrpolate it . no matter what kind of bc it is

 January 2, 2010, 07:32 #6 New Member   sandip Join Date: Dec 2009 Location: Jamshedpur Posts: 4 Rep Power: 9 Hi, I too had solved the problem by using SIMPLE algorithm. There I used neumann boundary condition for pressure as dp/dn=0 at all planes except at outlet, there I took p=0.(Assuming atmospheric pressure). That worked well and got right solutions! You may try with the same if your equations are non dimensional. Regards Sandip

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