What are the limitations of the concept of turbulence in fluid dynamics?

The Reynolds number of a flow is the ratio of

Two capillary tubes of the same length but different radii $r_1$ and $r_2$ are fitted in parallel to the bottom of a vessel. The pressure head is $P$. What should be the radius of a single tube that can replace the two tubes so that the rate of flow is the same as before?

In the case of a fluid flowing on a horizontal surface, the flow of the fluid becomes unsteady when the Reynolds number $(R_n)$ is:

What is turbulent flow? Give its illustrations.

Water from a pipe is coming at a rate of $100\, L/min$. If the radius of the pipe is $5\, cm$, the Reynolds number for the flow is of the order of: (density of water $= 1000\, kg/m^3$, coefficient of viscosity of water $= 1\, mPa\, s$)