The Reynolds number (Re) is a dimensionless quantity used in fluid mechanics to predict the flow regime around an object or within a fluid flow. It’s named after Osborne Reynolds, a British engineer who popularized its use. The Reynolds number is defined as the ratio of inertial forces to viscous forces and is given by the formula:

Where,

• $\rho$ is the density of the fluid.
• $V$ is the velocity of the fluid.
• $L$ is a characteristic length or characteristic dimension of the object or flow.
• $\mu$ is the dynamic viscosity of the fluid.