Skip Navigation

Aerosol Transport – Inertia


:: Section 2

Reynolds Number

Since an aerosol is a particle in air, it is important to understand how air moves before we can characterize the movement of an aerosol.

Have you ever seen a peaceful creek flow? Flowing so smoothly, it gives you a sense of tranquility. In contrast, the whirling wind of a hurricane has the power strong enough to destroy anything along its path. In the fluid field, we call the calm flow the laminar flow, whereas the rapid one as turbulent flow.

laminar flow
turbulent flow

Now that the concept of these two types of flow is established, we then need to quantitatively define each flow regime. Reynolds number is one index we can use. The aerosols' Reynolds number (Rep) is a dimensionless parameter to characterize fluid flow around an object. It is the ratio of inertial forces to viscous forces acting on the aerosol, as

where ρg is gas density, V is gas velocity, dp is aerosol diameter, and η is gas viscosity

We Define:
When Rep > 1000, the flow is turbulent. For laminar flow, Rep < 1. If Rep is between 1 and 1000, it's in the transition regime.

It should be pointed out that the Reynolds number can be for a flow around a particle or through a pipe, and the criteria are different. For a flow in a pipe, the commonly accepted Re value for turbulent flow is greater than 4000; for laminar flow, it is less than 2000.


1. Does a water flow or a gas flow have a larger Rep? Why?
2. For a turbulent flow around a tennis ball, will it be laminar or turbulent if the ball is a basketball? Why?