Here are the key points regarding drag coefficient, Reynolds number, and their relationship:
- Drag coefficient (Cd) is a dimensionless number that quantifies the drag or resistance of an object in a fluid environment. It depends on the size, shape, and surface roughness of the object.
- Reynolds number (Re) is also a dimensionless number that indicates the flow regime - laminar or turbulent. It depends on object size, fluid velocity, fluid viscosity, and fluid density.
- At low Reynolds numbers (laminar flow), drag coefficient is higher due to smooth laminar flow and lack of turbulence to trip the boundary layer.
- As Reynolds number increases (transition to turbulent flow), drag coefficient drops due to turbulent eddies that transfer momentum to the boundary layer and delay separation.
- In the turbulent flow regime, drag coefficient levels off and becomes independent of Reynolds number.
- A low drag coefficient generally indicates an aerodynamic, streamlined shape. A high drag coefficient indicates a blunt, non-streamlined shape.
- A low drag coefficient is desirable for most applications to reduce drag force (since drag force is proportional to Cd). However for braking systems a high drag is preferred.
- For streamlined shapes like airfoils, a low Cd of 0.02 - 0.1 is typical. For bluff bodies like spheres Cd can be 0.5 or more.
So in summary, a low drag coefficient is associated with streamlined shapes and is desirable in most applications. A high drag coefficient is seen in blunt shapes and may be useful in braking applications. Reynolds number affects Cd but once in the fully turbulent regime, Cd becomes independent of Re.