[Ahmed Khattab]

Hi,

i'm using groovy boundary condition to express velocity as a relation of pressure difference of pressure of two regions. the problem is that velocity is vector field while pressure is scalar so, there is a problem to express vector value as a relation of scalar value. i think that if i use gradp instead of p it will be helpful but openfoam can't understand gradp since it is not stored in file.

any one can help me figuring out a solution for this problem.

[gschaider]

Originally Posted by rebel ahmed

Hi,

i'm using groovy boundary condition to express velocity as a relation of pressure difference of pressure of two regions. the problem is that velocity is vector field while pressure is scalar so, there is a problem to express vector value as a relation of scalar value. i think that if i use gradp instead of p it will be helpful but openfoam can't understand gradp since it is not stored in file.

any one can help me figuring out a solution for this problem.

If you look at http://openfoamwiki.net/index.php/Co...her_field_are: then you'll find that snGrad(p) gives you the gradient of p in the normal()-direction

[Ahmed Khattab]

Dear gschaider,

thanks for your reply, i tried this Boundary condition for velocity

               type            groovyBC;
               
               valueExpression "(pUpperFlow-pMiddleFlow)/(mu)";
               variables "pUpperFlow{patch'upperFlow_to_upperFilter}=snGrad(p);pMiddleFlow{patch'middleFlow_to_upperFilter/middleFlow}=snGrad(p);"; 
               value uniform (0 0 0);

and this is the error message:

[2] --> FOAM FATAL ERROR: [3] 
[3] 
[3] --> FOAM FATAL ERROR: 
[3] The expected return type vector is different from the stored result type "scalar"

--> FOAM FATAL ERROR: 
[1] The expected return type vector is different from the stored result type "scalar"

[1] 
[1] 
[1]     From function tmp<Field<Type> > ExpressionResult::getResult()
[1] 
[2] The expected return type vector is different from the stored result type "scalar"

[gschaider]

Originally Posted by rebel ahmed

Dear gschaider,

thanks for your reply, i tried this Boundary condition for velocity

               type            groovyBC;
               
               valueExpression "(pUpperFlow-pMiddleFlow)/(mu)";
               variables "pUpperFlow{patch'upperFlow_to_upperFilter}=snGrad(p);pMiddleFlow{patch'middleFlow_to_upperFilter/middleFlow}=snGrad(p);"; 
               value uniform (0 0 0);

and this is the error message:

[2] --> FOAM FATAL ERROR: [3] 
[3] 
[3] --> FOAM FATAL ERROR: 
[3] The expected return type vector is different from the stored result type "scalar"

--> FOAM FATAL ERROR: 
[1] The expected return type vector is different from the stored result type "scalar"

[1] 
[1] 
[1]     From function tmp<Field<Type> > ExpressionResult::getResult()
[1] 
[2] The expected return type vector is different from the stored result type "scalar"

As I said below: snGrad is the gradient IN THE DIRECTION NORMAL TO THE BOUNDARY. So it is a scalar. To get the actual vector multiply it with normal()

[STutexas]

Hi
I am a new Openfoam user and I have a question about the snGrad operator. In the post you describe the use of operator on a scalar field. But if the operator is applied to a vector field would it give a tensor?