I recently saw an experiment in youtube (vortex collision experiment of 1992 recreated). I found it very impressive and I was wondering if someone can give me a brief explanation, since in the internet there are many detailed documents with unclear results. If someone have an idea why this is happening, is pleased to write it. (In the video is said that pressure may cause it, but it does not explain it well.) A specific question I have is related to the number of rings that are created. Is it the same number everytime despite the speed or/and the size of vortex?