[aylalisa]

Hi All,

I've a question about expressionField (,already discussed in other threads. discussion, example from above)

Version 1:

functions
{             
yPlusField             
{type expressionField;                
outputControl timeStep;                 
outputInterval 1;                 
fieldName yPlus;                 
expression "pow(0.09,0.25)*sqr(k)*nearDist()/mu";                 
autowrite true;             
}            

yPlusWall             
{                 
type patchExpression;                 
expression "pow(0.09,0.25)*sqr(k)*dist()/mu";                 
accumulations 
(                     
min                     
max                     
average                 
);                 
patches (wall);                 
verbose true;             
}         
}

I've tried to extract the desired value on a patch with following variation but only receive result 0, whereas the upper version works. Does this mean that expressionField computes an internal field?

Version 2

functions
{             
yPlusField            
{                 
type expressionField;                 
outputControl timeStep;                 
outputInterval 1;                 
fieldName yPlus;                 
expression "pow(0.09,0.25)*sqr(k)*nearDist()/mu";                 
autowrite true;            
}             
yPlusWall             
{                 
type patchExpression;                 
expression "yPlus";                 
accumulations 
(                     
min                     
max                     
average                
);                 
patches (wall);                 
verbose true;             
}         
}

I've tried as well to add accumulations( average ) to the expressionField object to condense the field to one scalar but don't get a result either.

Creating expression field yPlus... type:volScalarField (but no scalar) and...
Expression compute_yPlus on bottom: average=0 sum=0
Is Version 1 the only possibility to write the result in the log-File?

Aylalisa

[gschaider]

Originally Posted by aylalisa

Hi All,

I've a question about expressionField (,already discussed in other threads. discussion, example from above)

Version 1:

functions
{             
yPlusField             
{type expressionField;                
outputControl timeStep;                 
outputInterval 1;                 
fieldName yPlus;                 
expression "pow(0.09,0.25)*sqr(k)*nearDist()/mu";                 
autowrite true;             
}            

yPlusWall             
{                 
type patchExpression;                 
expression "pow(0.09,0.25)*sqr(k)*dist()/mu";                 
accumulations 
(                     
min                     
max                     
average                 
);                 
patches (wall);                 
verbose true;             
}         
}

I've tried to extract the desired value on a patch with following variation but only receive result 0, whereas the upper version works. Does this mean that expressionField computes an internal field?

Version 2

functions
{             
yPlusField            
{                 
type expressionField;                 
outputControl timeStep;                 
outputInterval 1;                 
fieldName yPlus;                 
expression "pow(0.09,0.25)*sqr(k)*nearDist()/mu";                 
autowrite true;            
}             
yPlusWall             
{                 
type patchExpression;                 
expression "yPlus";                 
accumulations 
(                     
min                     
max                     
average                
);                 
patches (wall);                 
verbose true;             
}         
}

I've tried as well to add accumulations( average ) to the expressionField object to condense the field to one scalar but don't get a result either.

Creating expression field yPlus... type:volScalarField (but no scalar) and...
Expression compute_yPlus on bottom: average=0 sum=0
Is Version 1 the only possibility to write the result in the log-File?

Aylalisa

The usual behaviour of the internalField-parser (which is used by the expressionField-function object) is to only calculate the internal field (hence the name) and calculate a zeroGradient for the patches (which basically means that a face gets the value of the neighbour cell). Why that is 0 in your case ... I don't know (unless nearDist is 0 in the boundary cells)

Anyway: variation 1 is better anyway.
a) because it only has to be computed on the boundary. Not on the whole field
b) you get yPlus for the face. Not the value of the cell projected onto the face. Which might differ (think of a stretched cell in a corner: all three boundary faces of that cell will get the same value. Although they don't have the same yPlus)

There is another way to calculate that. You could use the readAndUpdateField-functionObject to load a field myYPlus. In that field you implement the expression from version 1 as a groovyBC. Then you can use the patchExpression from version 2. The only advantage of this is that afterwards you have the whole field on disc and can visualize the distribution of yPlus in paraView