The stability of laminar flow (or streamline flow) is maintained by viscous forces.

At high flow rates, irregular, unsteady flow sets in and flow no longer remains laminar and is called Turbulent flow.

Reynolds Number `(R_e)` is a dimensionless number whose value gives an approximate idea whether the flow rate would be turbulent or laminar.

`R_e = (rho V d)/eta`

`rho = ` density of fluid `eta =` viscosity of fluid `V =` velocity of fluid

d = dimension of boundary to fluid flow

Laminar flow occurs at low Reynolds numbers where viscous forces are dominant.

Turbulent flow occurs at high Reynolds number where inertial forces are dominant `(R_e > 5000)`.