Compute fluid forces on boundaries

[Florian660]

Hello,

I am working on a 2D FSI problem involving two cylinders immersed in a viscous stagnant fluid. One cylinder is fixed while the other one is oscillating around its initial position. My goal is to estimate the hydrodynamic forces (pressure and viscous force) applied onto the two cylinders.

I managed to run simulations but I have some concerns :

• How to access to the forces ? I tried converting the bin files to vtk files to calculate the forces with Paraview but it is quite challenging. In fact, I would like to get at the end of the simulations a file that contains all the informations needed (pressure, velocity, normal vectors) at the nodes of the cylinders' boundaries.

• At each time iteration, i have this Warning : - ResidualBasedBlockBuilderAndSolver: ATTENTION! setting the RHS to zero!
  PartitionedFSIBaseSolver: FSI non-linear iteration = 1 |res|/sqrt(nDOFS) = 0.0
  PartitionedFSIBaseSolver: FSI non-linear iteration convergence achieved in 1 iterations. - I ran a test case to compare and the residual is non zero so I assume something goes wrong in my simulations.

• I use the FSI module to get access to the "ALE mesh displacement BC" but then I am forced to specifiate a solid with mechanical properties, and mesh it, while I am only interested on the fluid properties and the forces but not the cylinder dynamic. So my question is : is there a way to run the simulation without resolving the dynamics of the cylinder ? Right now I am using the model "Solid small displacements" with the constitutive law "Linear Elastic Plane Strain" and random values of density, young modulus and Poisson ratio.

Any insights or guidance would be greatly appreciated! 🙌

Thanks in advance for the help!

[rubenzorrilla]

Hello,

I am working on a 2D FSI problem involving two cylinders immersed in a viscous stagnant fluid. One cylinder is fixed while the other one is oscillating around its initial position. My goal is to estimate the hydrodynamic forces (pressure and viscous force) applied onto the two cylinders.

I managed to run simulations but I have some concerns :

• How to access to the forces ? I tried converting the bin files to vtk files to calculate the forces with Paraview but it is quite challenging. In fact, I would like to get at the end of the simulations a file that contains all the informations needed (pressure, velocity, normal vectors) at the nodes of the cylinders' boundaries.

There are some utilities to calculate the drag (see the ComputeDragProcesses in the FluidDynamicsApplication). However, these are admittedly a bit basic so depending on your needs would possibly be insufficient. If you really need the information above, you'll need to query and write it by yourself. The way to do it will depend on whether you need this information at the nodes or at the integration points.

• At each time iteration, i have this Warning : - ResidualBasedBlockBuilderAndSolver: ATTENTION! setting the RHS to zero!
  PartitionedFSIBaseSolver: FSI non-linear iteration = 1 |res|/sqrt(nDOFS) = 0.0
  PartitionedFSIBaseSolver: FSI non-linear iteration convergence achieved in 1 iterations. - I ran a test case to compare and the residual is non zero so I assume something goes wrong in my simulations.

This normally appears at the very first iteration of the first step as everything is zero. If you're getting this at each time iteration I'm afraid you're right, there's something wrong in your simulation. It seems to me that you're effectively not coupling the two solver.s

• I use the FSI module to get access to the "ALE mesh displacement BC" but then I am forced to specifiate a solid with mechanical properties, and mesh it, while I am only interested on the fluid properties and the forces but not the cylinder dynamic. So my question is : is there a way to run the simulation without resolving the dynamics of the cylinder ? Right now I am using the model "Solid small displacements" with the constitutive law "Linear Elastic Plane Strain" and random values of density, young modulus and Poisson ratio.

I'm not 100% sure of what you actually target. If you need a CFD in which the motion of the boundaries is known, I'd use the CFD ALE solver (NavierStokesAleFluidSolver) and impose the moving cylinder MESH_DISPLACEMENT directly in the main script. If the movement of the boundaries is not known, i.e., you need to couple the CFD solver with a rigid body motion one, there's a recent implementation in the CoSimulationApplication which enables such capabilities.

Any insights or guidance would be greatly appreciated! 🙌

Thanks in advance for the help!

[rubenzorrilla]

The main script (i.e. MainKratos.py) is missing.

[Florian660]

MainKratos.py

[rubenzorrilla]

I just realized that the geometry (i.e. mdpa file) is also missing.

[Florian660]

I can't directly add .mdpa file here but you can find it on the link below.

https://filesender.renater.fr/?s=download&token=faee4d13-1827-41d0-8af2-36c7c09cae8f

[rubenzorrilla]

Here it goes. On top of setting the ALE stuff, I fixed the pressure at the bottom left corner node to improve convergence. Aside of creating the corresponding model part in the mdpa, the unique changes are in the ProjectParameters.json. Feel free to ask any doubt when comparing to your original version.

florian_ALE.zip