Friction factor inside a pipe with low very low Reynolds number locally

user9 · August 22, 2022, 07:36

I am solving a single phase Turbulent flow problem inside a pipe. Inside the domain there is some obstruction ( solid bodies) where the velocity ( locally) is very very low and therefore Reynolds number comes out to be very low[order of 10^(-6)]. I have written a custom fields function inside Ansys Fluent which calculates friction factor. The correlation for friction factor has been taken from literature where friction factor is inversely proportional to Reynolds number.

When my plot contours of friction factor, it's maximum value comes out in the range of 1500~2000 at places where Reynolds number is very low.

How can I solve this? Is it possible to have friction factor greater than 1?

Also if I consider Moody's chart , the lowest Reynolds number mentioned in the chart is 60.

Please help

FMDenaro · August 22, 2022, 12:13

Quote:

Originally Posted by user9

I am solving a single phase Turbulent flow problem inside a pipe. Inside the domain there is some obstruction ( solid bodies) where the velocity ( locally) is very very low and therefore Reynolds number comes out to be very low[order of 10^(-6)]. I have written a custom fields function inside Ansys Fluent which calculates friction factor. The correlation for friction factor has been taken from literature where friction factor is inversely proportional to Reynolds number.

When my plot contours of friction factor, it's maximum value comes out in the range of 1500~2000 at places where Reynolds number is very low.

How can I solve this? Is it possible to have friction factor greater than 1?

Also if I consider Moody's chart , the lowest Reynolds number mentioned in the chart is 60.

Please help

What kind of turbulence would you supppose at such a low Re number…

LuckyTran · August 22, 2022, 13:54

I have to say there is a lot here that is inconsistent with the general approach of pipe flows.

I'm not sure what you mean by local Reynolds number. For pipes we use the bulk averaged Reynolds numbers.

Obstructions cause local acceleration, not deceleration.

There is no meaning to a friction factor value anywhere close to or greater than 1, that would mean the flow loses more than its entire bulk share of dynamic head and pressure becomes negative. Friction factor is also a single scalar, there is no local contour of friction factor.

A lot of clarifications are needed. Can we please just start over from the very beginning?

August 22, 2022, 07:36	Friction factor inside a pipe with low very low Reynolds number locally	#1
user9 New Member anuj Join Date: Feb 2020 Posts: 17 Rep Power: 6	I am solving a single phase Turbulent flow problem inside a pipe. Inside the domain there is some obstruction ( solid bodies) where the velocity ( locally) is very very low and therefore Reynolds number comes out to be very low[order of 10^(-6)]. I have written a custom fields function inside Ansys Fluent which calculates friction factor. The correlation for friction factor has been taken from literature where friction factor is inversely proportional to Reynolds number. When my plot contours of friction factor, it's maximum value comes out in the range of 1500~2000 at places where Reynolds number is very low. How can I solve this? Is it possible to have friction factor greater than 1? Also if I consider Moody's chart , the lowest Reynolds number mentioned in the chart is 60. Please help

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August 22, 2022, 13:54		#3
LuckyTran Senior Member Lucky Join Date: Apr 2011 Location: Orlando, FL USA Posts: 5,762 Rep Power: 66	I have to say there is a lot here that is inconsistent with the general approach of pipe flows. I'm not sure what you mean by local Reynolds number. For pipes we use the bulk averaged Reynolds numbers. Obstructions cause local acceleration, not deceleration. There is no meaning to a friction factor value anywhere close to or greater than 1, that would mean the flow loses more than its entire bulk share of dynamic head and pressure becomes negative. Friction factor is also a single scalar, there is no local contour of friction factor. A lot of clarifications are needed. Can we please just start over from the very beginning?