# How Is the Exchanged Data of Subcycles Generated? | Non-Matching Timesteps & Fixed Time Window **URL:** \ **Category:** Is preCICE for me?\ **Tags:** time-stepping\ **Created:** [August 26, 2021, 12:48pm UTC](https://precice.discourse.group/t/how-is-the-exchanged-data-of-subcycles-generated-non-matching-timesteps-fixed-time-window/695 "2021-08-26T12:48:05Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![JulianSchl](https://yyz2.discourse-cdn.com/free1/user_avatar/precice.discourse.group/julianschl/32/692_2.png) [@JulianSchl](https://precice.discourse.group/u/JulianSchl)\ **Post date:** [August 26, 2021, 12:48pm UTC](https://precice.discourse.group/t/how-is-the-exchanged-data-of-subcycles-generated-non-matching-timesteps-fixed-time-window/695/1 "2021-08-26T12:48:05Z")
Hi everybody, I’ve got a question about the coupling with non-matching timesteps with fixed time windows. **About my current case:** I’m creating a FSI coupling with the OpenFOAM adapter. Because the Adapter doesn’t match my needs of coupling additional solid body motion, I’m sending this motion data manually via a seperate socket connection (other displacement data is mapped via preCICE). This works fine for identical timesteps. On the [documentation page](https://precice.org/couple-your-code-timestep-sizes.html#fixed-time-window) it says “Once both participants reach the end of the time window, coupling data is exchanged.”. Because I have to send some data manually as stated above, I need to know how this exchange of coupling data is performed in detail. **Question:** Is the data collected over a time window just summed up together and finally exchanged with the other coupling partner? Or are there some algorithms acting in the background to merge the timestep data and if yes, can I neglect them? ---
**Author:** ![uekerman](https://yyz2.discourse-cdn.com/free1/user_avatar/precice.discourse.group/uekerman/32/2199_2.png) [@uekerman](https://precice.discourse.group/u/uekerman)\ **Post date:** [August 27, 2021, 10:06am UTC](https://precice.discourse.group/t/how-is-the-exchanged-data-of-subcycles-generated-non-matching-timesteps-fixed-time-window/695/2 "2021-08-27T10:06:02Z")
Answer is short: preCICE simply uses the last data in the time window and sends this one. All intermediate data is neglected. Important to understand in this context: “displacement deltas” always means the delta in reference to the last time window (not the last timestep). Further information: > **[Step 5 – Non-matching timestep sizes | preCICE - The Coupling Library](https://precice.org/couple-your-code-timestep-sizes.html)** > > In this step, you learn how preCICE handles non-matching timestep sizes and a few more things about simulation time. [![](https://global.discourse-cdn.com/free1/uploads/precice/original/2X/c/c4d620fd94e95a0f0167a7a96603a77cb7166e64.jpeg "High-order and multi-rate time stepping with preCICE (Benjamin Rüth, preCICE Workshop 2020)") ](https://www.youtube.com/watch?v=7NhBmcx_MmI) For future readers: we are currently implementing waveform iteration in preCICE which will makes such scenarios much much better. ---
**Author:** ![JulianSchl](https://yyz2.discourse-cdn.com/free1/user_avatar/precice.discourse.group/julianschl/32/692_2.png) [@JulianSchl](https://precice.discourse.group/u/JulianSchl)\ **Post date:** [August 28, 2021, 12:49pm UTC](https://precice.discourse.group/t/how-is-the-exchanged-data-of-subcycles-generated-non-matching-timesteps-fixed-time-window/695/3 "2021-08-28T12:49:53Z")
Thank you for the reply 🙂 ---
**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/precice/original/1X/67d2a5fa6f80e597e2267ff17fa8c1673cd11caa.svg) [@system](https://precice.discourse.group/u/system)\ **Post date:** [August 31, 2021, 12:50pm UTC](https://precice.discourse.group/t/how-is-the-exchanged-data-of-subcycles-generated-non-matching-timesteps-fixed-time-window/695/4 "2021-08-31T12:50:45Z")
Closed on August 31, 2021, 12:50pm UTC This topic was automatically closed 3 days after the last reply. New replies are no longer allowed.