Drag Force Sign Problem

Hi guys,

I could use some help. I got a problem with my coupled simulation.
I got a box as domain and particles in it. The top side of the box is moving with a certain velocity in X-Direction, the bottem side of the wall isn't moving (like the case from evansmuts). The other remaining sides of the box have periodic boundaries. If I have a look at the dragforces in X-Direction the Drag Forces of the particles have different signs (see picture 1).
In my physical understanding of the problem the majority of the particle's Dragforce should be positive and some could be negativ (like seen in the Ergun test case, picture 2). But right now it's around 50/50.
I already tried different coupling intervals(1-100) and timesteps (50 times smaller than rayleigh), I changed young modulus, coefficient of restitution and friction, possion ratio, domain sizes. I also tried explicit and implicit solvers, but nothing seems to work.

Do you have any idea what could have gone wrong?

Regards Mark