# Difference between turbulant and unstable flow?

In (1) I have read that their are two 'critical' Reynold's numbers. Above one we get unstable flow and above the other we get turbulent flow.

Since I don't think a proper formal definition of turbulent flow exists (correct me if I am wrong), I was wondering what the difference between a turbulent flow and an unstable flow is?

References

(1) Spurk, J.H. and Aksel, N. 2008. Fluid Mechanics. 2nd ed. Berlin: Springer (Page 206, link to Google books)

• Possible duplicates : Links between laminar-turbulent and steady-unsteady flows and Does steady flow imply laminar? Aug 19, 2016 at 17:27
• Steady flow does not change with time. Unsteady flow is time-varying laminar flow : the flow pattern varies predictably, possibly periodically. Turbulent flow varies unpredictably - it is chaotic. Aug 19, 2016 at 17:31
• @sammygerbil sorry my question was meant to ask the difference between turbulent flow and unstable (not unsteady) flow. I have edited it now. Aug 19, 2016 at 17:55
• Are you sure that the textbook does not distinguish between these 2 types of flow? Please can you post an extract? As far as I know, unstable is synonymous with turbulent. Aug 19, 2016 at 17:59
• @sammygerbil From the source (1) above: "However, what happens in pipe flow makes it clear that the Reynolds' number at which the flow becomes turbulent is generally different from the Reynolds' number at which the flow becomes unstable for the first time." Aug 19, 2016 at 18:05

The above description is typical of chaotic systems, in which a transition region exists between predictable (eg periodic) and unpredictable (chaotic) behaviour. This is illustrated in the following typical bifurcation map. The region up to $r=3.0$ represents stable predictable flows. At $r=3.0$ flow can switch unstably between 2 different patterns. Another instability arises at $r=3.45$. Then again at $r=3.55$ and so on. During the transition region from $r=3.0$ to $r=3.65$ the pattern of flow gets more and more complex in discrete steps which become more and more frequent as the relevant parameter (here Reynolds number) is increased.