Transient 1-way FSI with transient structural and external data modules

First off, thank you for including all the pics. They really help paint the picture of your setup, and you wouldn't believe how rare it is for someone to be this detailed.

I have a little experience in FSI and the External Data module myself, and with that being said I'm a little confused by your setup. You exported your pressures and nodal points correctly, but you did not take into consideration how the nodal points change with respect to time. Again, I don't know the specifics of your setup so I might be out of line with this, but let me try to walk through it with you.

Let's say this box is your fluid domain.

So, at time step 1, the pressure acting on the right hand-side might be greater than the initial pressure. And then at time step 2, it might be less. As you say, it oscillates. Keep this in mind for later.

Time step 1. Time Step 2.

Since this is purely a fluids-based simulation, I'm assuming that the walls do not move. This is an important distinction, because if the walls do move, then the resulting pressure force would change to adjust to the altered geometry. As they do not, your nodal points remain fixed in space, with the pressure varying as a function of time. I'm also assuming that the harmonic pressure attenuates. If it remains constant, I'd need to think about this a little more.

Now, you feed this information to Mechanical, and attempt to perform a 1-way FSI. Allow me to say that, while I certainly don't know everything about FSI, I'm not quite sure you can do this. I mean, you can physically do this, but I don't think you will get a right solution. Typically, 1-way FSI is performed by transferring displacement, calculated by Mechanical, to CFX with the resulting force on the walls being neglected (I am ignoring the vast majority of 1-way FSI to focus on this one case). If you attempt to go the other way with the simulation, I think you will run into serious problems. Just as CFX will calculate the resulting forces from a moving boundary, Mechanical will calculate the resulting displacement from a force.

Back to the box diagram. Within Mechanical, during the first time step, the increased force on the wall will cause the box to expand. This means that the nodal points will move from their initial positions. This answers your last question: what is going on with the mapping? Essentially, at the second time step, Mechanical is having difficulty finding the node that was previously in the center of the box, because it moved. To fix this and continue mapping, it adjusts its mapping algorithm, which is why you see the message of "The program controlled pinball grew to more than 10%...". It's trying to find the node that the External Data module says should be there. Eventually, it will find it and continue solving.

However, this introduces a flaw in your simulation. Think back to the CFX sim: at time step 2, the pressures were calculated with respect to a specific fluid domain. Now, that domain is changed in Mechanical. So, the forces experienced at time step 2 are no longer representative of your CFX simulation, meaning that the resulting displacements in Mechanical are not correct. Explained out a bit: CFX said that there is a lower pressure response on the walls than at initialization (in our example) in the second time step. In Mechanical, however, this lower pressure might not be enough to cause the shape to change much, so it will stay bulged out. What does this mean for your acceleration? It basically decreases/goes to 0. Not to mention the fact that, were this altered geometry imported into CFX you would experience drastically different results from what you previously calculated. In short, you might be doing everything correctly, but just set up your problem in such a way as to not give you the desired output.

This simulation might be better suited for a two-way FSI. Have you considered that?