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how to determine the value to start with ? the Re and relative roughness is not given ?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 10^{5}.
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.how to determine the value to start with ? the Re and relative roughness is not given ?
when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?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.
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.when Re = 100000 , we have to choose the friction factor based on the relative roughness , right ? how to know the relative roughness?
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 ?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.
Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.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 ?
But, we do not know D, how to assume the value of D?Once you know D, you will have e/D for the next iteration. So just assume smooth for the first iteration.
1. Guess value of DBut, we do not know D, how to assume the value of D?
so , we need to assume the value of D first ?1. Guess value of D
2. Calculate Re
3. Calculated e/D
4. Determine f
5. Determine h_{f} and compare with required value
6. Determine difference
7. Go back to step 1
8. Make a graph of either h_{f} vs D or (h_{f }difference) vs D.
That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.so , we need to assume the value of D first ?
is there any tips to assume the value of D ?That's what I would do. Choose values for D that give you the exact values of e/D on your Moody chart.