# Homework Help: Choosing friction factor

1. Mar 7, 2016

### foo9008

1. The problem statement, all variables and given/known data
is there any criteria when it comes to choosing the first f_try ?

2. Relevant equations

3. The attempt at a solution
in my opinion , there must be at least either relative roughness(ε) given or Reynold number given , so that we can find the first f_try ....But , it is not given in this question , is the question wrong ?

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2. Mar 7, 2016

### BvU

I don't think they worry about roughness - it's considered to come as a given with the choice of pipe . I suppose it can be ignored to pick the first f. The point is that the friction factor depends on Re and Re depends on the friction factor. So the solution has to be iterative.

By the way, which of the wavy vague pictures are you referring to ? I could only find something in 4.9 but I hate reading these jpgs.

3. Mar 8, 2016

### foo9008

y , i uploaded the wrong pictures, here's the second picture which is continuous from the first one.
pls ignore the dsc0501

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4. Mar 8, 2016

### foo9008

when it comes to choosing friction factor , it depends on the Reynold number and the relative roughness, am i right ?

5. Mar 10, 2016

### foo9008

but the Reynold number is also UNKNOWN from the start ? the D is unknown , so how to choose friction factor in this case?

6. Mar 11, 2016

### BvU

It's a starting value: a guestimate. The idea is that another initial value should lead to the same final result after a (possibly different) number of iterations.