Dimensionless numbers in fluid mechanics

A dimensionless number is a way to measure how different things affect each other. It's like a way to compare two different objects, even if they are completely different from each other. In fluid mechanics, a dimensionless number is used to measure the relationship between different forces acting on a fluid. For example, if you're looking at a river, the dimensionless number might measure the amount of force exerted by the current on the water and how it changes the direction or speed of the flowing water.