I wonder how to choose a specific region of the boundaries say number 4 to calculate the drag force from the definition. I know that it is included in the engineering data but I want to calculate it using posdat. I'm using star-cd 4.02.

I'm going to sum the pressure*Ax*isens for all the cells.

Thanks /Hamodi

Have a look into suplementary notes in the documentation. There is an example for forces calculation on specific regions.