I'm currently conducting a project which involves a cylindrical pipe in a river, and I'm trying to determine whether vortex shedding will be an issue for the design. I tested a prototype a couple of days ago in a local river and was expecting vortex shedding to occur - from what I've read it should happen when Reynold's number (Re) > 40, and with a pipe diameter of 50 mm and a river speed of (approximately) 0.11 m/s I calculated Re = 4200. However, although vortex shedding was apparent at the point in the river where the velocity was 0.11 m/s, I couldn't see any shedding at faster points in the river, and I'm not sure why. My theory is that in faster areas of the river the water was more turbulent, which would have made the vortices more difficult to see but doesn't mean that they weren't there - is this correct? The only problem I have with this theory is that I didn't notice the pipe oscillating in the higher velocity sections of the river either, which again I would have expected if there was vortex shedding. I'd be grateful if someone could explain to me why there appears to be no vortex shedding at faster locations in the river, and whether I'm right about the turbulence affecting the prototyping test - thanks very much.