A glass tube of uniform cross-section is connected to a tap with a rubber tube. The tap is opened slowly. Initially the flow of water in the tube is streamline. The speed of flow of water to convert it into a turbulent flow is [radius of tube = $1\text{ cm}$, $\eta = 1 \times 10^{-3}\text{ Ns/m}^2$, $R_n = 2500$ and density of water = $10^3\text{ kg/m}^3$]