In near$-$wall flows (i.e., boundary layer, as well as tube and channel flows), turbulence generates in the region of the greatest near$-$wall velocity gradients throughout the flow extent. The turbulent flow is much more capable for transfering momentum, heat and suspended particles, and for propagation of chemical reactions than is laminar flow.
Hence, in a turbulent flow, the velocity of the liquid molecules in contact with the walls of the tube is may have any value.


$(A)$ $\beta=0$ when $a= g / \sqrt{2}$
$(B)$ $\beta>0$ when $a= g / \sqrt{2}$
$(C)$ $\beta=\frac{\sqrt{2}-1}{\sqrt{2}}$ when $a= g / 2$
$(D)$ $\beta=\frac{1}{\sqrt{2}}$ when $a= g / 2$