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Old Feb 28th 2016, 02:05 AM   #1
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Reynold number and thickness of viscous layer

why as the velocity increases , the thickness of the viscous sublayer will also increase ? i know only that when velocity increases , the Reynoldf number increses . How to relate viscous sublayer thickness with the Reynold number?
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Old Mar 1st 2016, 12:17 PM   #2
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As the average flow velocity increases, the thickness of the viscous sublayer will DECREASE.
This is because the viscous sublayer is laminar flow, and as you increase the flow velocity the additional energy will increase the turbulent portion of the flow at the expense of the laminar portion.
Note that this is in accord with the usual Reynolds related effect where the distance over which a laminar flow can continue, before it becomes turbulent, decreases as the Reynolds number increases.

If you have a very low flow velocity, then all the flow can remain laminar.
As the average velocity increases, the central region of the flow will become turbulent,
but the flow velocity at the wall is zero and the flow very close the wall is low enough to remain laminar.
As the average velocity increases still further, the distance away from the wall at which the flow velocity is low enough for laminar flow (below the Reynolds threshold) becomes thinner.
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