How do I choose the first f_try without given criteria for friction factor?

[foo9008]

Homework Statement

is there any criteria when it comes to choosing the first f_try ?

Homework Equations

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 ?

 

[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.

 

[foo9008]

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.

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

 

[foo9008]

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.

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

 

[foo9008]

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.

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

 

[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.