Maths help / Excel

crisbduck · 28 Apr 2011 at 18:21

Hey I've spent ages working out how to get this into excel and to arrange it to find λ

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Basically trying to find the friction factor of a pipe (y)

I got 1/SQRT λ = -5.2
so λ = ~25
Cheers for any help

Pudney · 28 Apr 2011 at 18:24

What's Re?

Edit: Even k or d. Actually, what are you asking, whether you're right?

Amleto · 28 Apr 2011 at 18:28

if 1/sqrt(x) = 'less than zero' then you prob have a problem if looking at physical properties.

nydryl · 28 Apr 2011 at 18:33

Is Re the real part of some variable that you haven't told us about or is it a variable itself? What values of k and d are you using?

crisbduck · 28 Apr 2011 at 18:36

sorry thought id added the caption to the pick!
Re - 21895
d - 13.6
k - 0.0015

Judgeneo · 28 Apr 2011 at 18:38

invert
square
done?

Also is that log base 10 or base e? You have the constraint of 6.9/Re + (k/3.71d)^1.11 < 10 or e respectfully

crisbduck · 28 Apr 2011 at 18:39

Would like to know if i'm right and how to put it into an excel document, I keep getting errors with the log and power functions..

edit - pretty sure its base 10, it doesn't actually say in the lecture notes but id guess they would put ln for e?

Judgeneo · 28 Apr 2011 at 18:40

crisbduck said:
I got 1/SQRT λ = -5.2
so λ = ~25
Cheers for any help

λ = ~1/25 that would be

I only ask about the log base because my course uses log to mean base e, but its common in engineering to use it as base 10 and ln as bsae e

Dave · 28 Apr 2011 at 18:43

OP - what are you finding difficult? I assume you're working out your roughness factor k from either a given or some other predisposed method like a Moody diagram. Just plug your constants in to the right hand side, and then take your answer to the power of -1 and then square it to get your value of lambda. The equation is explicit in lambda so you don't need a numerical method to solve for it.

nydryl said:
Is Re the real part of some variable that you haven't told us about or is it a variable itself? What values of k and d are you using?

Re is the Reynolds number, a dimensionless number used in Fluid Dynamics to describe the ratio of inertial to viscous forces. (equal to density*length(reference)*velocity / dynamic viscosity)

Judgeneo said:
I only ask about the log base because my course uses log to mean base e, but its common in engineering to use it as base 10 and ln as bsae e

This is important - if I write log I generally mean base 10, and use ln for base e! My area is Fluid Dynamics, so this is probably relevant to the calculation

Mason- · 28 Apr 2011 at 18:44

=(1/(-1.8*log((6.9/21895)+((0.0015/13.6)/3.71))^1.11))^2

0.020896244...

Probably wrong but oh well.

nydryl · 28 Apr 2011 at 18:46

Dave said:
Re is the Reynolds number, a dimensionless number used in Fluid Dynamics to describe the ratio of inertial to viscous forces. (equal to density*length(reference)*velocity / dynamic viscosity)

Funny that I always read it as being the real part of a number now even though my entire honours project revolved around how fluid flowed through porous media for different Reynolds numbers.

Dave · 28 Apr 2011 at 18:49

nydryl said:
Funny that I always read it as being the real part of a number now even though my entire honours project revolved around how fluid flowed through porous media for different Reynolds numbers.

Ahaha yes I suppose - it's quite unusual to be dealing with complex numbers in the realm of Fluid Dynamics though, so Re is always taken to mean Reynolds number. I think if you want to refer to the real and imaginary components of a complex you use the stylised R and I.

Fluid flow through porous media is what I'm trying to model with CFD at the moment - trying to leave out as many fudge-factors as possible is difficult!

crisbduck · 28 Apr 2011 at 18:54

Mason- said:
=(1/(-1.8*log((6.9/21895)+((0.0015/13.6)/3.71))^1.11))^2

0.020896244...

Probably wrong but oh well.

Hey that excellent, using the table a few scrolls down this page (http://knol.google.com/k/how-to-calculate-pressure-drop-and-friction-losses-in-a-pipe#) the table says about 0.02
I can't get it to work in excel though, i get a number error, Thanks though that's a great step forward

=/ I want to kill myself 13.6 should be 0.016 as its meant to be in metres...

nydryl · 28 Apr 2011 at 18:54

Dave said:
Ahaha yes I suppose - it's quite unusual to be dealing with complex numbers in the realm of Fluid Dynamics though, so Re is always taken to mean Reynolds number. I think if you want to refer to the real and imaginary components of a complex you use the stylised R and I.

Fluid flow through porous media is what I'm trying to model with CFD at the moment - trying to leave out as many fudge-factors as possible is difficult!

Aye suppose I only ever saw them come up when looking at stream functions etc.

Ha the book by Nield & Bejan about convection in porous media was my bible for a year. Would quite like to have kept going with fluid mechanics but hey-ho.