Vacuum flow simulation

Dear all
I am doing a project which let the robot climb the tree with the adhesion force generated by vacuum motors.
Now, I have some problems with the flow simulation.
Here is the flow simulation process: Draw a tree and the vaccum

The goal of the simulation to shows how much adhesion force can be generated under different rotation speeds, different fan heights and different distances between the tree and the fan's surface.

First, I intend to do the simulation with internal. However I do not know how to choose the speed air of the inlet with different rotation speeds, distance and so on.

Then I try do the external way.
I can adjust the all the parameters( distance, fan height, speeds), but the simulation result is not good, The pressure drop is almost zero!
I attached my files. The asm file is flow.
Thanks.

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