Fluid Dynamics

The speed of flow is another factor that determines the nature of flow. The speed of flow is either that of a liquid or gas moving across a solid surface or, alternatively, the speed of a solid object moving through a fluid. The flow patterns in either case are exactly the same. That is why airplane designs can be tested in wind tunnels where air is made to flow over stationary test models to simulate the flight of actual planes moving through the air.

The speed of flow is related to the viscosity by virtue of the fact that a faster moving fluid behaves in a less viscous manner than a slower one. Therefore, it is useful to take viscosity and speed of flow into account at the same time. This is done through the Reynolds number named after the English scientist Observe Reynolds (1842-1912). This number characterizes the flow. It is greater for faster flows and more dense fluids and smaller for more viscous fluids. The Reynolds number also depends on the size of the solid object. The water flowing around a large fish has a higher Reynolds number than water flowing around a smaller fish of the same shape.

As long as the shape of the solid surface remains the same, different fluids with the same Reynolds number flow in exactly the same way. This very useful fact is known as the principle of similarity or similitude. Similitude allows smaller scale models of planes and cars to be tested in wind tunnels where the Reynolds number is kept the same by increasing the speed of air flow or by changing some other property of the fluid. The Ford Motor Company has taken advantage of the principle of similarity and conducted flow tests on cars under water. Water flowing at 2 MPH (3.2 km/h) was used to simulate air flowing at 30 MPH (48 km/h).