The best way to do this is time consuming but worth the effort . In the project panel select all the 11FPS clips and duplicate them. Move the duplicates to a bin labelled Sound. Select all the original clips and right click. Select the Modify> Interpret Footage options and then Assume frame rate to be 24fps (23.976). Drop one of these Clips onto a 24 p timeline and it will have a duration of 11/24 ie roughly half the original length. Select the clip on this timeline and right click. Select the Speed/Duration option and then enter the Speed to be 100 *11/24 ie 45.83%. Select Optical flow and OK. If you need the original sound, take the audio from the duplicate clip in the Sound bin and drop it on the timeline to sync to the time changed video clip. I personally would export and re-import the clip into the project as a regular 24 fps 4K clip as the alternative of dealing with the optical flow effect on a 4K clip needs a large amount of processing power.
You can process more than one clip at a time in this way but be aware that applying the Optical flow over a cut produces a weird effect over the two frames that comprise the cut.
Depending on the nature of the images, the Optical flow usually gives excellent results, however, certain movement, for example rotating helicopter blades, can create undesirable motion artifacts. If some of your material produces odd effects, use one of the other options in the Speed/Duration menu other than Optical flow.
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