The Reynolds Number

04/06/18

“How can we predict if a fluid flow will be laminar or turbulent?”

 

Fluids have a most remarkable form of movement with their flow. Some are laminar as a calm lake while others thrush around with the turbulence of a roaring river. But how can we predict if a fluid flow will be either turbulent or laminar? Well, let’s think about it using our engineering mindset. We know that two types of forces act on a moving fluid, inertial and viscous forces. The former are forces that tend to move an object, such as a pressure difference or momentum, while the latter are ones that tend to keep a fluid’s movement neutral, such as friction or momentum loss. It would be logical that if the former were stronger, then the fluid would be freer to move and therefore create turbulence while the latter would keep everything mellow and laminar. So what if we were to take the ratio of these forces and classify fluids based on it? Well, this is known as the Reynolds Number and is used to predict the flow type of a fluid. For simple fluids, the Reynolds Number can be expressed symbolically as Re = rho *v*L/mu, where Re is the Reynolds Number rho is the density of the fluid v is the velocity L is the characteristic linear dimension of the fluid and mu is the dynamic viscosity of the fluid.