CFD - request for independant review on airodynamical subjects

Project Support
1

Hi All,
I’m stepping in this simulations of wind, in particular for wind on structures. I have particular intrest in drag forces on welded boxed girder constructions composed with steel sheets.

Aim of the simulation: Compare the simulation results against existing methods of wind load calculation used in different available standards. The aim is to get have complete & correct calculated drag force.

So far I have managed to create succesful simulation but I need expert review on the aerodynamical side and the results. My simulation you can find:

INPUT & to know:

  • CFD
  • incompressable fluid: air / air temp 10°C / density 1,246 kg/m3
  • windspeed 22ms
  • wind laminair air flow / direction cross or perpendicular to the length of the object (Xplane)
  • sim done with half girder (10m) / symmetry line
  • meshing refined around the girder using cartesian box and 50mm size

first question:
What wall model should be used slip or no slip, the information I have found is for me unclear:

  • In the tutorial I found no slip
    Wall | Boundary Conditions | SimScale
    In some other references in this forum I have found not to use the slip configuration?
    Results for forces and pressure in symmetry case - #2 by Get_Barried
    In my actual sim the viscous forces are way low compared to the pressure forces but I still believe this ratio is depending to the type of construction.
    Second question:
    The part is welded steel construction parts painted. Why is there no surface definition to set up about the finishing quality / roughness?? I have chosen wall setup as

Third quaestion:
Is the simulation set up correct? tunnel size, object positioning, meshing, size of the tunnel, boundaries…and the outputs correct?

Fourth question:
IN case I have to run the simulation for air speeds up to 142ms. In my opinion close to 0,3 Mach, so what to take compressible… turbulent

Thanks for the support.
Danny F

2

Hello @dannyfocquaert,

From what I can see, both links contain the same message. Just to remove some ambiguity, the solid walls of the crane should receive a wall/no-slip boundary condition, since the velocity at the interface will be zero (this is what a no-slip wall represents).

For the sides of the domain, the approach is a wall/slip boundary condition, since we are interested in allowing velocities in the tangent direction. A “Symmetry” boundary condition would also work here. Maybe it will be clearer in this tutorial.

On your second question: this article contains the workflow to define a roughness value to your walls.

About your third pointer: all of these things take tests, and it’s not possible to tell you that everything is perfect just from looking at it. To me, the enclosure is currently way too small. Again, it takes tests, but I’d have at least 3x the height of the building to the sides and upstream, and perhaps 8-10x the height of the building downstream.

On the accuracy: do you have any experimental data to compare with? Otherwise, mesh sensitivity studies are the way to go.

About your 4th question: the 0.3 Mach limit is for velocities anywhere in the domain (not necessarily at the inlet). So yes, you will need to take compressibility effects into account.

Cheers