Oil is flowing through a rectangular duct of dimensions 20 cm × 3
| Oil is flowing through a rectangular duct of dimensions 20 cm × 30 cm and the viscosity of the oil is 1500 mm2/s. The velocity at which flow becomes turbulent flow is
A. 25 m/s
B. 37.5 m/s
C. 12.5 m/s
D. 15 m/s
Please scroll down to see the correct answer and solution guide.
Right Answer is: C
SOLUTION
Concept:
For flow in pipes, the Reynolds number at which flow becomes turbulent is 4000.
For rectangular ducts, the characteristic length is given by
\(D = \frac {2ab}{a+b}\)
Calculation:
Given a = 20 cm, b = 30 cm, ν = 1500 mm2/s = 15 × 10-4 m2/s;
Now the Reynolds number will be VD/ν = 2000
\(D = \frac {2\times20\times30}{20+30} = 24 \; cm\)
\(⇒ Re = \frac {V \times0.24}{15 \times {10^{-4}}}=2000\)
⇒ V = 12.5 m/s;