# Fluid solver with subcycles

I want to use subcycles in my fluid solver FASTEST. like this

where W is the fluid velocity, and u structural displacement.

I do not sure if this is the idea for preCICE time-windows. If it is, Do I need to save my last velocity and position before the subcycle ( Wn, Un) to calculate (Wn+1)?

Hi Jessica,

as far as I understand your drawing you want to couple explicitly.

Your Fluid solver does n_{S/F} steps of timestep size \delta t_F = \Delta t / n_{S/F} to compute W_{n+1} at the end of the window of size \Delta t. Every single one of these timesteps uses the same value for u_n (provided through precice.readBlockScalarData). Then your Structure solver uses p_{n+1} (again, provided through precice.readBlockScalarData) and U_n to do one big timestep of size \delta t_S = \Delta t to compute U_{n+1}. This gives us u_{n+1} and the computation of the first window is now complete; no iteration happens.

preCICE supports this workflow through subcycling. You do not have to save U_n or W_n (or worry about checkpointing), if you are using explicit coupling.

Note that accuracy might suffer if you do not use any iterations (=implicit coupling) or interpolation in time (which is a work-in-progress feature). If you need something more accurate than explicit coupling + subcycling (= approach described above), I would be glad to provide further help.

Best regards,
Benjamin