Could you tell me what is the significance of the Reynolds number. It is defined as

inertial force/viscous force

But what is an inertial force in fluid mechnanics

Thanks

The Reynolds Number is possibly the most important number in Aerodynamics because it incorporates all aspects of a flow condition: speed, density, object length, viscosity. In fact, many CFD packages don't let you enter speed, density, etc. separately; instead having you enter the Reynolds Number only.

As you mentioned, the Reynolds Number is essentially a ratio of the inertial (momentum) forces divided by the viscous forces. The inertial forces are those whose magnitude depend on speed and mass (density). The viscous forces depend on the material properties of the fluid.

As your Reynolds Number increases, this typically means that your flow speed is increasing and that boundary layer development is more likely to follow the laminar/transition/turbulent scheme. At low Reynolds Numbers (think of model airplane speeds), it is possible for the flow to go from laminar to separated; skipping turbulent development.

Low Reynolds Numbers are typically considered the most difficult to model in CFD.

John Dreese Maker of FREE Airfoil Development Software www.dreesecode.com