Critical velocity in a pipe flow

Velocity of liquid hydrocarbon fuels in a pipeline can not be measured by magnetic flowmeters, because their __________ is very low/small.

A. Thermal conductivity

B. Electrical conductivity

C. Specific gravity

D. Electrical resistivity

For turbulent fluid flow in pipe, the expression for Prandtl one seventh power law is (where, r = pipe radius, x = distance).

A. $$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {\frac{{\text{x}}}{{\text{r}}}} \right)^{\frac{1}{7}}}$$

B. $$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {\frac{{\text{r}}}{{\text{x}}}} \right)^{\frac{1}{7}}}$$

C. $$\frac{{\text{V}}}{{{{\text{V}}_{\max }}}} = {\left( {{\text{x}} \times {\text{r}}} \right)^{\frac{1}{7}}}$$

D. None of these

Capillary rise of mercury in a small diameter tube is proportional to (where, d = diameter of the tube, σ = surface tension of mercury)

A. d

B. $$\frac{1}{{\text{d}}}$$

C. $$\sigma $$

D. $$\frac{l}{\sigma }$$

What is the force required (in Newtons) to hold a spherical balloon stationary in water at a depth of H from the air-water iterface? The balloon is of radius 0.1 m and is filled with air.

A. $$\frac{{4\pi {\text{g}}}}{3}$$

B. $$\frac{{0.01\pi {\text{gH}}}}{4}$$

C. $$\frac{{0.01\pi {\text{gH}}}}{8}$$

D. $$\frac{{0.04\pi {\text{gH}}}}{3}$$