Correct Answer - Option 2 : 75 m/s
Concept:
Reynolds number signifies the ratio of inertia force to viscous force in velocity boundary layer. i.e. Re = \(\frac{F_i}{F_v}\)
Reynolds number also represents where the boundary layer changes from laminar to turbulent.
It is given in terms of viscosity:
\(Re=\frac{{\rho V L}}{\mu}=\frac{{ V L}}{ν}\)
where, ρ = density of fluid, V = velocity of fluid, L = characteristics length, μ = dynamic viscosity of fluid and ν = kinematic viscosity.
Characteristic length = \(\frac{4A}{P}=D\)
Calculation:
Given:
d = 40 mm = 0.04 m, Re = 200000 = 2 × 105, v = 1.5 × 10-5 m2/s.
\(Re=\frac{{\rho V D}}{\mu}=\frac{{ V D}}{ν}\)
\(2\times 10^5=\frac{{ V \times 0.04}}{1.5\times 10^{-5}}=75\;m/s\)