I used a simple geometry to make a test of a “Fixed-Fixed, Uniform Distributed Load”:
Round Tube: r1=79mm, r2=80mm, L=1200mm, F= 10’000N
(E = 205 GPa; I = 1.578.586,69mm^4)

I used the formula [(FL^4) / (384E*I)] and by that the deflection is : 1,67E-01.
BUT
according to the Simulation in Simscale its: 6,712E-02.

Am I simply to tiered , am i missing something big? If so sorry to disturb.
Thank in advance for helping me understand the difference in results.

I assume you doubled checked your hands calculation, including the units of the force and everything, and that it is consistent with the model implemented in the platform. If you haven’t, please do so first.

First thing that I would review is the force model, is it consistent between your ideal model and the BC used in SimScale? Did you check the total force and the reactions in your model?

Then, you really need to refine this mesh. For thin walled parts, you should have at least two elements across the thickness and preferable use 2nd order mesh. A mesh convergence analysis is always a must.

Changing the mesh settings helped much: I set the fineness to highest setting and checked the second order elements box.

Using the Fixed-Fixed , Center Load Formula deflection is = 2,781E-04 ; Simscale 2,132E-04 ; so its much better! (maybe??? see below)
BUT I was looking for Fixed-Fixed , Uniform Distributed Load. Here the values are less good: 1,67E-04 (Simscale 2,132E-04)

Then i tried (as suggested) a different model - without thin walls:
r1=10mm;r2=20mm;L=1000mm; Highest Settings for Mesh. And results are Perfect!
Uniform Distributed Load by Formula - deflection is = 1,725E-02 ; Simscale 1,729E-02. Great ,

BUT What is it now? Uniform Distributed Load or Center Load?

Because when using the thin wall tube (first model) Simscale results for deflection are almost equal to Center Load Formula
and when using a “normal / not thin wall” tube (second model) Simscale results for deflection are perfect to Uniform Distributed Load Formula.