weird air flow

Hi folks,

I want to simulate an air flow. The real model is as follows:
The air is sucked in by a fan and flows through 2 lattices which lie one on top of each other and have a distance of 4 mm. The sides are covered with walls.

That brings me to the 3D Model with the size:

4 x 111 x 67 mm

I defined the top and the ground to be lattices, the front as outlet, the sides as walls (no slip).

When I now run simulation I get a weird air speed distribution, I can't understand.
It starts to get faster and faster and reaches 4m/s at the outlet, what is pretty logic.
But short before the outlet it is close to zero, what I can not at all understand.

Am I doing something wrong?

Please feel free to correct my work or my model as I do not have any experience with comsol.

thanks in advance,
ben