Explain Reynolds Number.

(N/A) Osborne Reynolds observed that turbulent flow is less likely for viscous fluid flowing at low rates. He defined a dimensionless number,whose value gives an idea of whether the flow would be turbulent. This number is called the Reynolds number,$R_{e}$.
$R_{e} = \frac{\rho v d}{\eta}$
Where $\rho$ is the density of the fluid,$v$ is the velocity of the fluid,$\eta$ is the viscosity of the fluid,and $d$ is the characteristic dimension (diameter) of the pipe.
The nature of the flow can be determined from the value of the Reynolds number:
$1$. If $R_{e} < 1000$,the flow is streamline or laminar.
$2$. If $R_{e} > 2000$,the flow is turbulent.
$3$. If the value of $R_{e}$ is between $1000$ and $2000$,the flow is unsteady (transition state).