Laminar Flow Demonstration

From Steve Spangler's website:

"What's happening here is very low Reynold's number laminar flow, which means that there are many parallel layers of "sticky" (or viscous) fluid. As the handle is turned, the dyes remain within their original layers and do not mix with each other, even as they are spread out over their individual layers. As the crank is turned backwards, the process is inverted, and since there is no fluid turbulence in laminar flow, the process can be inverted almost perfectly."

The Reynold's number is the relationship between viscosity and velocity. As we increase velocity, the Reynold’s number increases and we get a more turbulent flow.  Additionally, at the same velocity, the more viscous a fluid is the smaller the Reynold’s number becomes, and if low enough a slow, steady, and laminar flow is created.

Below is a video example of laminar flow of water in a stream.  At a very low velocity water will flow in a stream as smooth sheets running parallel to the stream bed.  If you could look at the speed of the flow in profile it would have an appearance much like that of a deck of cards with the top card jutting forward over those below. This occurs because the tug of the channel bottom slows the water near the bed while the water layers nearer the surface flow somewhat faster.  The intensity of the flow is also so slow that the water has a very low sediment load as only the finest particles can be detached from the bed of the stream, further reducing friction.

 

If the velocity of the flow were to increase, the friction between the flowing water and the bed would rise.  Sediment load would also increase as the water would be able to pick-up or move increasingly larger grains of sediment from the bed.  As such, the Reynold's number would also increase, and eventually the flow would become turbulent, the drag creating eddies that would disrupt the water layers above the bed.

However, because water is not very viscous, in nature, conditions for laminar flow are not as common as those that cause turbulence.  As such water is is mostly found in states of turbulence.