Learning Forum

[andres.baeza]

Hi everyone, I'm running a FSI simulation (Fluent + Transient Structural + Ansys Coupling) that's taking me quite a while to solve (even days). Sometimes, when I run tests in Transient Structural, after the simulations (which take a couple of hours to complete), I get a recommendation to use an HPC license. Is it possible for you to explain in detail what this is about and if I can use it with Ansys Coupling?

[peteroznewman]

A two-way FSI coupled solution has a Transient Structural analysis and a Transient Fluent analysis linked by a System Coupling block.

Transient Structural uses the MAPDL solver and in a nonlinear model, performs several iterations at each time step to converge before sending an update to Fluent.

The Fluent solver performs several iterations at each time step to converge before sending an update to Structural.

The time taken to compute one iteration depends on the model size, the amount of RAM, the clock speed of the CPU, the number of cores, the speed of the storage media (HDD vs SSD).

With no HPC licenses, the solvers can use a maximum of 4 cores.  You can set the solvers to use only 2 cores and observe how long each iteration takes compared with 4 cores.

If you have HPC licenses, you can set the solvers to use 8 cores (if you have them).  This does not reduce the time per iteration in half (50%).  It reduces the time by a smaller percentage.

The time reduction of using 8 cores instead of 4 cores may be larger in Fluent than in Structural due to the nature of the calculations being performed.

An HPC Pack license allows you to solve using a compute GPU (if you have one). This is separate from the GPU driving the display.  The Fluent solver will get more benefit from a compute GPU than the MAPDL structural solver in terms of reducing iteration time.

If you have a high core count computer with sufficient RAM such that the structural model is solving entirely in RAM (incore, not out-of-core), you can request an HPC trial license from your ANSYS distributor and perform some testing on the total compute time of your model with higher number of cores.  If the structural model is solving out-of-core, you should first install additional RAM and make sure you have a fast SSD to use while solving.