# Calculating forces acting on a body

I’m trying to compare the forces along the X, Y & Z axes of three different configurations of a model. I’m not that experienced with CFD tools, as I’ve only used pretty basic solvers before that were virtually idiot proof! These allowed me to specify as an output numeric values of the three cartesian forces in Newtons.

I’m struggling to do this in SimScale, I’m not sure I’ve got the results set up correctly for a start. Would someone be able to provide a step-by-step guide to calculating these, or key points that I should look for?

Well, @tpickhaver: it is too vague.

Please go first to the documentation: Forces and Moments

• Import a geometry to SimScale Project (your ‘body’)
• Create a ‘Simulation’ (incompressible)
• Create a mesh for external CFD (automatic)

Prepare Boundary Conditions:

• Define Inflow and Outflow (walls of your BMB [Background Mesh Box])
• Define Inflow velocity
• Define other BMB walls as ‘slip’ walls
• Define your body as ‘no-slip’ wall

Define ‘Forces and Moments’ following documentation link.

Start small, do not waste your simulation time on too fine mesh.

Cheers,

Retsam

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Thanks for the reply. I had read the guide you’d linked to, but evidently hadn’t fully understood it. Your post has highlighted a few setup points where it seems my previous CFD applications took things for granted.

I have essentially a vehicle model, so velocity in the ‘x’ axis will be the vehicle driving speed. The base of my bounding volume is the base of the vehicle, so the road surface. Would this therefore make the base of my BMB a ‘no-slip’ wall?

I’ve set these as “Freestream velocity inlet/outlet”, at the Ux speed defined elsewhere in my simulation, in the boundary conditions. Is this correct?

To clarify with setting the faces for the force calculations, is this the entirety of my body, or just its leading faces?

Hi @tpickhaver,

May I suggest you look now at that Simscale tutorial, please? https://www.simscale.com/docs/tutorials/external-aerodynamics-around-a-vehicle/.

You can also find your inspiration in Simscale Public Projects > CFD > CAR | VEHICLE tags. Select small projects, but with at least one simulation, as example. You can just open that project and analyse parameters used or you can make a copy of it and modify simulation parameters or even restart new mesh.

Cheers,

Retsam

2 Likes

I’ve followed the instructions as exactly as possible, given my geometry but don’t appear to be getting any convergence. If I link to my project is there anything obvious that I’m doing wrong?

https://www.simscale.com/projects/tpickhaver/crude_car/

Your simulation converged after ~300 steps. After a long while, something very minor could cause an instability, something linked to numeric methods, mesh itself, rounding errors when approaching very long calculations count. So, you do not need such a huge number of calculations to achieve you goal. Observe convergence plot, follow omega. When omega is not moving lower anymore, just add 50% of steps till that moment (200 steps, stable, 50% ~100 steps, 300 steps will be enough).