Discussion Forum

Hello,

I am trying to perform a CFD flow field simulation of a 3D parachute to find its lift and drag. The geometry model is imported from a CAD software as surfaces only (no thickness). I attached one picture of how it looks like. In order to make the simulation, I created the computational domain (box) big enough to neglect the boundary effects (I saw in a post from an airfoil SST model that a recommended value of 100 chord-lengths toward all sides is enough). For the physics, I am using the SST turbulence model with inlet (velocity input), outlet (pressure output) and symmetry (box lateral sides) boundary conditions. I used an interior wall boundary condition on the surfaces of the parachute with no slip condition. For the mesh, I have tetahedrals for the domains and prisms for the boundary layer over the parachute surfaces. After running the simulation, I visualized clearly the vortices created in the interior of the parachute. However, when calculating the surface integrals to determine lift and drag (I saw in a comsol blog to use spf.T_stress to calculate lift and drag at the correspondent axis direction), I am getting nonsense values (my drag is on the opposite direction resulting in a propulsive force). For sanity checks, I was also trying to see how the pressure differs on the parachute surfaces from the inside and outside but I only see the same values on both sides.
My questions would be:
1. Is this approach valid for this type of scenario?
2. How would it be possible to find the pressure difference on the inside and outside surfaces of the parachute? (I am not sure if this has something to do on how the interior wall boundary condition works). From my understanding, I thought this internal wall boundary condition would act as a baffle (very thin surface) for the parachute surfaces (which will therefore have both inside and outside surfaces). Please correct me if I am wrong on this.

I would really appreciate some help with this issues. Thanks.