Question

Find the Reynolds number if a fluid of viscosity 0.4 Ns/m2 and relative density of 900 kg/m3

through a 20 mm pipe with a velocity of 2.5 m/s. Is the flow Laminar or Turbulent?

Note: If Reynolds < 2000 the flow is laminar

If Reynolds > 4000 the flow is turbulent

If 2000 < Re < 4000 it is transitional flow

Re =

Answer 1

The Reynolds number is given by the following expression:

$$\mathrm{Re} = \frac{\rho u D}{\mu}$$

where $\rho = 900 kg/m^3$ is the relative density of the fluid, $u = 2.5 m/s$ is the velocity of the fluid, $D = 0.02 m$ is the diameter of the pipe and $\mu = 0.4 N\cdot s/m^2$.

Substituting values, obtain:

$$\mathrm{Re} = \frac{900\cdot 2.5 \cdot 0. 02}{0.4} = 112.5$$

As far as 112.5<2000, the flow is laminar.

Answer. Re = 112.5, laminar flow.