# Find diameter of pipe

## Homework Statement

when we find the diameter of a pipe , we have to found the value of D and f by iteration , how to choose the start value of friction factor when we do the iteration ?

## The Attempt at a Solution

#### Attachments

• 19.5 KB Views: 272
• 17.6 KB Views: 273

Related Introductory Physics Homework Help News on Phys.org
Chestermiller
Mentor
Have you looked at the graph of friction factor vs Reynolds number?

• foo9008
you mean moody chart ?

Chestermiller
Mentor
Yes. You can see that on the chart, the friction factor does not very by several orders of magnitude, at least not for turbulent flow. I would start out with the value at a Re of 105.

• foo9008
Yes. You can see that on the chart, the friction factor does not very by several orders of magnitude, at least not for turbulent flow. I would start out with the value at a Re of 105.
how to determine the value to start with ? the Re and relative roughness is not given ?

Chestermiller
Mentor
how to determine the value to start with ? the Re and relative roughness is not given ?
Like I said, start with the friction factor f for Re = 100000. As far as roughness is concerned, you would have to know that for the particular pipe from its specs or just assume a worst-case value.

• foo9008
Like I said, start with the friction factor f for Re = 100000. As far as roughness is concerned, you would have to know that for the particular pipe from its specs or just assume a worst-case value.
when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?

Chestermiller
Mentor
when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?
As I said, the pipe may come with that kind of information when it is purchased. Or, you could choose what you would consider a worst case value. Or, if it a homework problem, they will typically tell you what roughness to use.

• foo9008
As I said, the pipe may come with that kind of information when it is purchased. Or, you could choose what you would consider a worst case value. Or, if it a homework problem, they will typically tell you what roughness to use.
yes , this is a homework -like problem , the roughness , e is given , but we dont know the D , how to get the relative roughness(e / D) so that we can choose the friction factor ?

Chestermiller
Mentor
yes , this is a homework -like problem , the roughness , e is given , but we dont know the D , how to get the relative roughness(e / D) so that we can choose the friction factor ?
Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.

Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.
But, we do not know D, how to assume the value of D?

Chestermiller
Mentor
But, we do not know D, how to assume the value of D?
1. Guess value of D
2. Calculate Re
3. Calculated e/D
4. Determine f
5. Determine hf and compare with required value
6. Determine difference
7. Go back to step 1
8. Make a graph of either hf vs D or (hf difference) vs D.

1. Guess value of D
2. Calculate Re
3. Calculated e/D
4. Determine f
5. Determine hf and compare with required value
6. Determine difference
7. Go back to step 1
8. Make a graph of either hf vs D or (hf difference) vs D.
so , we need to assume the value of D first ?

Chestermiller
Mentor
so , we need to assume the value of D first ?
That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.

That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.
is there any tips to assume the value of D ?