how to define the porous direction vectors like this
 

i want to simualate a problem like flow around cylinder, but the clylinder is covered a porous layer. how to define the vectors in Fluent plane.

please help me! thanks.

The diection vector should be flow direction.

thanks! Eakarach, for your answer.

the flow direction is changing when flow around the object, like the black line in the picture attached.

i like to know whether the direction vectors are also chaged in the flow direction， and if it's right,the direction vectors must be define in the UDF?
or i just use the (1,0) X aix direcion(2D problem) for the direction vector in the FLUENT plane.