i have seen this video. https://www.youtube.com/watch?v=pnbJEg9r1o8&t=96s It is not completly clear how a difference of velocity can cause a vortex, can you explain me?
There are a couple of effects causing that. As she explained, the water flows off & around the edge of the plate, to the back, causing the ring of water around the plate to spin. Also, if there is a moving fluid against a stationary (or slower moving) fluid, the bit in between gets rolled between the two, like rolling a stick between your hands.