# Pressure correction

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 December 23, 2015, 16:40 Pressure correction #1 New Member   Join Date: Dec 2015 Posts: 2 Rep Power: 0 Hi everybody, I am not a cfd expert, I am writing a code that can solve the flow of a lid driven cavity. I am doing this in in-compressible steady-state, using the simple algorithm. When I inverse and multiply my pressure correction matrix with my source terms, I get a near singularity. My equations are: P'=pressure correction. Ap*P'p=Ae*P'e+Aw*P'w+An*P'n+As*P's+Su Su=(density*u_w-density*u_e)*area+(density*v_s-density*v_n)*area I do not know why I keep getting near singularity. Any help would be appreciated.

 December 23, 2015, 16:46 #2 Senior Member   Filippo Maria Denaro Join Date: Jul 2010 Posts: 5,726 Rep Power: 60 The pressure equation Div Grad p = q with proper BC.s leads to an algebric system that has indeed a singular matrix. However, when the BC.s satisfies the compatibility relation, you get a solution apart from a constant value Harr likes this.

December 23, 2015, 17:00
#3
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Quote:
 Originally Posted by FMDenaro However, when the BC.s satisfies the compatibility relation, you get a solution apart from a constant value
Firstly thank you. Sorry I do not know to much about CFD yet, what do you mean by "when the BC.s satisfies the compatibility relation"?

December 23, 2015, 17:15
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Filippo Maria Denaro
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Quote:
 Originally Posted by Harr Firstly thank you. Sorry I do not know to much about CFD yet, what do you mean by "when the BC.s satisfies the compatibility relation"?

Actually, this is not a CFD issue but a general mathematical aspect of linear algebra.
Consider the equation for pressure and integrate both side over the computational domain

Int [V] Div Grad p dV = Int [V] q dV

use Gauss

Int [S] n.Grad p dS = Int[V] q dV

The compatibility relation must be fulfilled by the boundary condition you prescribe on the frontier of the domain

so that you have to fulfill the condition

Int [S] f dS = Int[V] q dV

The above relation must be satisfied in discrete sense after discretization.
Only this way, despite the singular matrix, you get one solution (apart a constant).

For further details you can see in literature, for example

December 25, 2015, 18:49
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Kaya Onur Dag
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Quote:
 Originally Posted by Harr Hi everybody, I am not a cfd expert, I am writing a code that can solve the flow of a lid driven cavity. I am doing this in in-compressible steady-state, using the simple algorithm. When I inverse and multiply my pressure correction matrix with my source terms, I get a near singularity. My equations are: P'=pressure correction. Ap*P'p=Ae*P'e+Aw*P'w+An*P'n+As*P's+Su Su=(density*u_w-density*u_e)*area+(density*v_s-density*v_n)*area I do not know why I keep getting near singularity. Any help would be appreciated.

I am very bad at these finite volume notations to me they all are gibberish so do excuse me

is this poisson equation, is [Ap] the laplace or grad.div. operator?
Quote:
 Ap*P'p=Ae*P'e+Aw*P'w+An*P'n+As*P's+Su Su=(density*u_w-density*u_e)*area+(density*v_s-density*v_n)*area
you may need to set a mean for your correction pressure such as setting 1 single value manually by modifying your matrix and your source (right hand side) otherwise to my knowledge your matrix system will be under determined.

December 26, 2015, 04:13
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Filippo Maria Denaro
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Quote:
 Originally Posted by kaya I am very bad at these finite volume notations to me they all are gibberish so do excuse me is this poisson equation, is [Ap] the laplace or grad.div. operator? you may need to set a mean for your correction pressure such as setting 1 single value manually by modifying your matrix and your source (right hand side) otherwise to my knowledge your matrix system will be under determined.

In a FV formulation, A*p should be the discretization of (1/|V|) Int [S] n.Grad p dS.
However, you could find also several researchers considering Div Grad p. For the pressure equation it is not suitable to discretize the laplacian operator.

The correction to the matrix entry and to the source term is consequent to the proper setting of the BC.s in such a way they satisfy the compatibility constraint