Learning Forum

[Luca Andrea Mifsud]

Hi all, I am a student performing a basic 2D analysis for practice, involving fluid flow onto a round object. The outline of the round object is a wall, on which I am finding the drag. What is strange is that I put three report definitions, consisting of drag in x-component, drag in y-component, and drag in both components (i.e. I set x and y components both as 1 in the definition). However, the values obtained from the report in both components is not coming equal to the resultant of the two other values by Pythagoras' theorem. Any idea what causes this? If you need any further info just let me know, but I am hoping it is just a fundamental error on my part. Thank you!

[Federico]

Hello Luca, 

The drag force on a wall zone is computed by summing the dot product of the pressure and viscous forces on each face with the specified force vector. The terms in this summation represent the pressure and viscous force components in the direction of the vector. They are not the force components.

For example, consider the drag of an infinite cylinder subjected to flow around its surface. Say the drag is equal to 1N in the axial direction. Depending on the drag-force vector direction that you define, taking the dot product of this force with that direction you will find:

Fd [1,0] = 1N

Fd [0,1] = 0N (for steady-flow, there will be no force normal to the direction of the flow. For example, a cylinder will not produce lift)

Fd [1,1] = 0.707N

[Luca Andrea Mifsud]

Thank you for the extremely detailed response! :)