Flow Regimes

[Fog1]

Could someone explain the difference between Laminar Flow and Streamline Flow and in so doing enlighten my ignorance?

My confusion has arisen from the observations I have made regarding fluid flow through narrow passageways in fluid filled enclosed channels.
Here the Reynolds number is high, suggesting turbulent flow but the fluid appears not to mix and appears to flow without turbulence.

I described this to a college who simply said the flow regime was streamline.

So is streamline flow turbulent or laminar or can it be both?
What thermodynamics am I missing?

 

[saxon]

Fog1, When it comes to the Reynolds no., how high is high?There is a flow regime know as "transitional" where the flow can be either, turbulent or laminar. Your system may be operating in this region.

saxon

 

[Fog1]

Hello Saxon,

The Reynolds number as the fluid enters the system is approx 91000, well over the tanditional 10000 mark.

As the flow is then divided the overall Fd still suggests turbulance as it squirts down each passageway. The Fd of each pasageway is still sufficiently high to suggest turbulance.

The flow, as far as I can tell is not in some transitional phase and appears to the camera as lamina!!!

Streamline = Laminar is that correct in terms of the description?

 

[iainuts]

I've not heard of "streamline" flow as anything more than laminar flow. Here's a reference:

http://www.tpub.com/content/engine/14105/css/14105_26.htm

I'd suggest verifying the constants used to calculate Reynolds #, and if you're sure it's turbulent, then I'd suggest that turbulent does not necessarily indicate that mixing is going to occur. In fact, I'd be surprised if significant mixing occured in short runs of turbulent flow. Getting two media to mix well is not necessarily as simple as creating turbulent flow in a straight pipe. Perhaps after going through some elbows and other restrictions, a significant amount of mixing will occur. I'm not an expert on this.

Have you searched the web on turbulent mixing?

 

[Fog1]

Saxon,

The Reynolds number is well above 10000 at the entrance to the system.

The Fd value is also high at the entrance to the system.

Turbulent in but through the channels does it become laminar? Fd reduces.

What is the difference between laminar and streamline flow??? Is there one??? Do both descriptions describe the same thing ??

 

[Fog1]

Sorry Saxon I couldn't see your first reply our indeed that of my own but now I can.

I've searched the web without success, there seems no in depth revelations that point me in the right directions.

I am sure that in the narrow passageways, which are long compared to the entrance section, the flow does go laminar.

What has caused my confusion is a college saying that it can’t be laminar, suggesting that its streamline flow, then leaving for a week without further expansion or debate.

OK fine streamline it is, maybe, but what is meant by streamline? Is it turbulent non-mixing or is it laminar?

 

[saxon]

Fog1, This is a direct quote from the Crane Technical Paper 410, "Flow of Fluids":

"For engineering purposes, flow in pipes is usually considered to be laminar if the Reynolds number is less than 2000, and turbulent if the Reynolds number is greater than 4000. Between these two values lies the "critical zone" where the flow ... being laminar, turbulent, or in the process of change (ie, transitional), depending upon many possible varying conditions is unpredictable. Careful experimentation has shown that the laminar zone may be made to terminate at Reynolds numbers as low as 1200 or extended as high 40,000, but these conditions are not expected to be realized in ordinary service."

If you're seeing a laminar flow at high Reynolds numbers, in all likelyhood you've got a special condition, outside of ordinary service.

As it normally goes in science, the exception proves the rule, or for every rule there is an exception.

Hope this helps.
saxon