Why Are Reynolds Numbers Higher When Reducing Flow Rate?

[tony_engin]

Hi all!
In a pipe flow, when the flow rate of water is increasing, reaching a critical flow rate, the flow will switch from laminar to transition flow, right? And continue the increase of flow rate will further change it to turbulent flow. The Reynolds number for these 2 transitions are obtained. When reversing the process. i.e. decreasing the flow rate of the water starting from a turbulent flow back to the laminar flow, the 2 Reynolds numbers obtained in this case (from turbulent to transition)(from transition to laminar) would be higher than that when increasing the speed. How would you explain this? Intuitively, I would attribute this to the greater vibration existed when increasing the flow rate than decreasing the flow rate..But I'm not sure if this is true.

 

[MR. P]

this might help I'm not a member but looks promissing...Toward a theory of turbulence...
www.springerlink.com/index/LTR377P413316574.pdf

looking for information on reynolds number determination is an art that leads to 'pay per view' sites. Do you know what the secret is?


wondering myself

frank MR. P

 

[Clausius2]

Hi all!
In a pipe flow, when the flow rate of water is increasing, reaching a critical flow rate, the flow will switch from laminar to transition flow, right? And continue the increase of flow rate will further change it to turbulent flow. The Reynolds number for these 2 transitions are obtained. When reversing the process. i.e. decreasing the flow rate of the water starting from a turbulent flow back to the laminar flow, the 2 Reynolds numbers obtained in this case (from turbulent to transition)(from transition to laminar) would be higher than that when increasing the speed. How would you explain this? Intuitively, I would attribute this to the greater vibration existed when increasing the flow rate than decreasing the flow rate..But I'm not sure if this is true.

I am not an experimentalist of Fluid Mechanics at all, but I bet those Re Numbers are different. The threshold for turbulence onset could not be the same than the laminar onset. Once the onset of turbulence occurs the flow is mainly instabilized and a reduction in velocity could not be felt in same way than a incresing of velocity from laminar stage.

If some other member is not agree, he is welcome.

 

[Astronuc]

Here are some discussions on transition from laminar to turbulent flow in water.

http://www.princeton.edu/~asmits/Bicycle_web/transition.html

Interesting discussion on the Reynolds Number -
http://www.flowcontrolnetwork.com/PastIssues/novdec2002/8.asp

One either wants to be in the laminar region Re < 2000 or in the turbulent region Re > 4000.
There are practical reasons to stay out of the transition region.

See also Turbulent transition for fluids.

I believe the reason for the difference in transition points is the fact that going from laminar to transition to turbulent, one is increasing fluid velocity, so one is adding energy to the system, and that energy is also available to drive local instabilities. When going from turbulent to laminar, one is reducing the energy into the system and allowing the dissipative forces to reduce the flow, and the instabilities are not 'driven' as much, i.e. the instabilities are damped more than they would be if the flow was accelerating.

An interesting experiment would be to increase/decrease the flow velocity at a slow rate vs a fast rate to see if that has an effect on the transition points.

In energy production, one does forced convection, so the flows in the heated regions are turbulent, which is desirable from a heat transfer standpoint.