- Compute the exact derivative with a symbolic asistant (or by hand ...) and then add to your code in line 16.
- Approximate the derivative with a centered finite difference (or backward,forward).
Help Using Newton Rhapson Method for Colebrook equation
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Mary Fasulo
on 18 Feb 2020
Commented: Yagmur Atalayin
on 26 May 2020
For context, the homework assignment wants us to use the Newton Rhapson method to solve for the friction value using Colebrook's equation. I have my code below but I keep getting errors saying that in line 18 matrix dimensions must agree. I'm not exactly sure what the issue is and our professor told us to approach the problem this way. Any help would be greatly appreciated. Code is attached
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/272259/image.jpeg)
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Accepted Answer
Lemuel Carlos Ramos Arzola
on 18 Feb 2020
Hello Mary,
The problem is with the function:
diff(h(x))
This function operates with vectors and it only computes the difference. Also, to obtain the derivative you need to divide by an appropriate step size. You have two options here:
I used here the second option,
x0 = 0.01; %Guess for initial x
x=x0;
dx = 1e-3; % Step size for the centered finite difference
for i=0:100000
y=x;
m = (h(x+dx) - h(x-dx)) / (2*dx); % centered finite difference
x = y-(h(x)/m);
if x==y
break
end
end
Best.
5 Comments
Lemuel Carlos Ramos Arzola
on 19 Feb 2020
Edited: Lemuel Carlos Ramos Arzola
on 19 Feb 2020
You are welcome. I am a hydraulic engineer and in a few occasions I used that formula. But I prefer the explicit formula of Swamee-Jain.
More Answers (1)
Yagmur Atalayin
on 26 May 2020
Can I help me please
3 Comments
Yagmur Atalayin
on 26 May 2020
How can I write marry's question at Matlab programme ,using newton Raphson and colebrook ? Please help me! Pleaseeeeee :( if I don't solve this problem ,I can failure at my lesson
Yagmur Atalayin
on 26 May 2020
How can I solve marry's problem at matlab using newton Raphson and colebrook , help me please if I don't solve this problem I can failure in my lesson
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