Silence to Violence…!
I was jogging in the evening at Bishan Ang Mo Kio Park in Singapore, along with my son who has just joined the National Service. On the way, we paused to have a glance at a small streak of stream that was running along the park. (He is taking some modules on the basic fluid dynamics in the service.)
One end of the stream the water flow was calm and silent, while at the other end it was turbulent and violent.
I prepared to begin my Fluid Dynamics class!
The calm smooth flow of the water is known as laminar flow, while the “rough” flow is turbulent flow.
Why does it happen? Let us begin with the Reynolds Number.
The characteristics of any fluid flow are determined by the dimensional quantity called Reynolds Number. By definition, the Reynolds number is the ratio of inertial forces to viscous forces within a fluid that is subjected to relative internal movement due to different fluid velocities.
Let us just focus on the velocity of the flow. If the velocity of water flow increases, while the other dimensions (especially D) remain the same, the Reynolds number increases only due to the “increased velocity”.
If you pour water slowly and softly, it spreads in all directions like a flow of music. If you force it by any means, the flow turns violent like a cacophony.
The velocity of the flow increases its inertial force over the viscous force. Actually, the viscous force is the force that opposes relative motion between the layers. Simply put, viscous force opposes the fluid motion while the inertial force tries to break it.
In fact, all fluids, including water, have viscous properties but in varied quantities. If water flows slowly, its viscous force dominates, and when it speeds up, the inertial force overcomes its viscous force.
Denoting it in terms of the Reynolds Number, let us consider the flow of water through a pipe. If Re is below 2300, the flow would be smooth and laminar. In case, due to some reason, the velocity of water increases and Re surpasses 4000, then it becomes turbulent. The Reynolds Number in between is called the transition of flow. So it is the Reynolds number that determines the flow of fluids!
Let us come back to the stream!
When the water flows through the narrow gaps between the stones, the velocity increases. Yes, Bernoulli’s principle plays a crucial role here. Thus the velocity of water increases.
Look at the video in which the small leaf, when passing through the narrow gap, cruises fast until it reaches the wider zone.
This velocity is sufficient to increase the Reynolds number beyond the critical value — the value that changes from laminar to turbulent.
Thus, at one end of the stream the water flow is calm and silent, while at the other end it is turbulent and violent.
