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.
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.
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.
But, we do not know D, how to assume the value of D?
1. Guess value of D
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.
is there any tips to assume the value of D ?
