Question

In which one of the following cases will the liquid flow in a pipe be most streamlined

• A. Liquid of high viscosity and high density flowing through a pipe of small radius
• B. Liquid of high viscosity and low density flowing through a pipe of small radius
• C. Liquid of low viscosity and low density flowing through a pipe of large radius
• D. Liquid of low viscosity and high density flowing through a pipe of large radius

Answer 1

The correct option is B Liquid of high viscosity and low density flowing through a pipe of small radius

For a streamline or laminar flow, Reynold's number, $N_R=\frac{\rho vD}{\eta }$ should be less.
So, for lesser value of $N_R$, diameter (or radius) and density should be small and viscosity should be high.