How do we know if a flow is turbulent, transitional or laminar? In the late 1800’s, Osbourne Reynolds discovered that the type of a fluid flow is related to the fluid’s density, mean velocity, diameter and viscosity. This dimensionless (no units) number helps predict changes in flow type. In simple terms, the Reynolds Number can be written as:
density x mean velocity x diameter / viscosity
It is generally accepted that flow is laminar if the Reynolds Number is less than 2000. Transitional flows have a Reynolds Number between 2000 and 4000. Flows are considered turbulent when the Reynolds Number is greater than 4000. Using the Reynolds equation, we can see that reducing the density, mean velocity and/or diameter of a turbulent fluid flow (unchanging viscosity) will make it “more” laminar. This could also be accomplished by increasing the fluid viscosity (keeping density, mean velocity and diameter the same). The inverse is true to make a flow more turbulent.