Hi, In a stirred vessels it can occur that a vortex is present which is unwanted because it decreases the mixing. The dimensionless Froude number is a value that can be used to see if the vortex formation is negligible or not. It is defined as Fr=N^2D/g an you can look at it as a ratio of inertia forces to gravitation force. At low velocities you usually don't have a vortex, so this means that the froude number is small? But then I wonder, if Froude is small, then the denominator of Froude must be large and thus the gravitation force delivers a large contribution with reference to the inertia forces. I would say a vortex would certainly be present then?