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What is the definition of Reynold's Number, and how is it used in the study of aerodynamics?
The Reynolds number is defined as the product of density times velocity times length divided by the viscosity coefficient. This is proportional to the ratio of inertial forces and viscous forces (forces resistant to change and heavy and gluey forces) in a fluid flow.
The Reynolds number is used to study fluids as they flow. The Reynolds number determines whether a fluid flow is laminar or turbulent. Flowing fluids normally follow along streamlines. If a flow is laminar, fluids will move along smooth streamlines. If the flow is turbulent, these streamlines breakup and the fluid will move in an irregular manner. You can see of laminar and turbulent flow in our explanation of drag. Turbulent flow creates more friction drag on an airplane. However, it also keeps the flow attached over its surface.
Since the Reynolds number is a measure of how a fluid behaves, you can also substitute dynamically similar fluids, which are fluids with the same Reynolds number, when testing models to see how they would behave in a specific environment. If the air in your wind tunnel is not dense enough for what you need to test (i.e. Reynolds number too low), you can make it less sticky by cooling the air to increase the Reynolds number to that of the dense air you need.
