Suppose you wanted to study current patterns in a tidal basin or the effect of gale winds on a tall building. One approach would be to construct scale models and run water through one and blow air on the other. Well, it's not really quite that simple. In trying to relate flow patterns around a model with what would be observed in the real thing, there are fully eight parameters that must be taken into consideration. In most applications, the most important of these are the relative size of the model, and the viscosity, speed and density of whatever is flowing past it.
Some surprises can pop up.
These are very simplistic examples of very complex problems. Nature, however, has no difficulty in providing us with examples illustrating the mechanics of fluids in motion, even if we don't always recognize them. For example, the equations of motion for aurora-producing plasma in the earth's magnetic field are essentially the same as those governing liquid particle motion in ocean turbidity currents or cream in a cup of coffee.