## Fluids

#### dynamic : deforming or rigid body (six dof on; passive on)

• Emperor
Subscriber
Hello to all,nI'm currently doing very simple tests on FSI with the fluent solver and I'm learning about dynamic meshing. Someone could please explain to me that it is the difference between deforming and rigid body (six dof on; passive on), to simulate the evolution of a semicircle (2d) in a fluid it would be better to use which one for dynamic meshing. nI have looked a bit in the documentation but I must admit that this point is not too clear to me, nnThank you
• Rob
Ansys Employee
As I understand it, deforming means it changes shape; rigid body moves but retains it's shape. We'd use the latter to model things falling off aeroplanes and the former to look at a flexing pipe. n
• Emperor
Subscriber
Thank you , nI try to simulate this flow, but I notice that the mesh gets distorted but there is no remeshing, and the crash computation, so I decrease the time step to have a better behavior, convergence can occur but the ball displacement is very small, but if I increase the number of iterations. nI am more adapted to the mechanical solver where for a transient analysis we choose the time step of the analysis and the end time of the analysis, but I notice that in fluent we only choose the time step from one iteration to the other, could you please enlighten me on these pointsnThank youn
• Rob
Ansys Employee
You'll need to add remeshing options into the model: have a look under Help as there are tutorials and/or videos covering moving mesh. nRe the start & stop times; in CFD we initialise the model at t=0 and then run some time steps with either a fixed size or using a program controlled size. As we can stop & restart the model we've never really needed to define a stop time. Having said that, in the adaptive time stepping there is a stop time. Note in CFD each data file is unique to that mesh and point in time. For a 10s simulation we'd then save several data sets to post-process. In Mech all the data is contained in one file. n