MATLAB: Numerical root finding procedures

elseif

trying to solve the part under else. eover and Re are known, but still leaves me with (f) on both side of the original colebrook equation. 1/sqrt(f) = -2*log10(eoverD/3.7 + 2.51/Re/sqrt(f)). Please help, a bit stuck.

Re = V*D1 / nu;
% Check for laminar flow.
if Re < 2300
F = 64 / Re;
else
    F(f)=1/sqrt(f)+2*log10(eoverD/3.7 + 2.51/Re/sqrt(f));
end

Best Answer

You should be able to use the function fzero() to solve for f in your implicit equation.

>> doc fzero

for details.

I think this will do it, but definitely check:

darbyFormula = @(x) 1/sqrt(x)+2*log10(eoverD/3.7 + 2.51/Re/sqrt(x));
f = fzero(darbyFormula,1)

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MATLAB: Function handle error with matirx

You cannot call fzero() with Re as a vector, in the way that you have done. You need to solve for each value of Re individually. (Also, your if statement will not work as you intend, as written.)

Rather than trying to do all values of (D,L) at once, I suggest you just write a loop over those values.

MATLAB: Returning values for iterations

Just set the equality:

Vo = Va; % right after your last line of code.

Like this:

Vn = 2:0.1:30;% iteration length. 

Vo = 2;% initial Vo to calc Re and f.

%



Re = (Vo.*D) / nu;
% 

if Re < 2300
f = 64 / Re;
else
   darbyFormula = @(x) 1/sqrt(x)+2*log10(eoverD/3.7 + 2.51/Re/sqrt(x));
f = fzero(darbyFormula,0.01);
end
%
dPloss_1 = (f*rho*(L/D)) * ((Vn*Vo)./2);
Va = (alpha*Vn)+((1-alpha)*Vo); % this to become the new Vo for next iteration.

Q=Va*A;
Vo = Va;

But shouldn't you use a while or for , instead of a if in your code? I don't know how it is implemented, but it should go more or less like this:

Vo = 2;% initial Vo to calc Re and f.
for Vn = 2:0.1:30;% iteration length. 
%
Re = (Vo.*D) / nu;
% 
if Re < 2300
f = 64 / Re;
else
   darbyFormula = @(x) 1/sqrt(x)+2*log10(eoverD/3.7 + 2.51/Re/sqrt(x));
f = fzero(darbyFormula,0.01);
end
%
dPloss_1 = (f*rho*(L/D)) * ((Vn*Vo)./2);
Va = (alpha*Vn)+((1-alpha)*Vo); % this to become the new Vo for next iteration.
Q=Va*A;
Vo = Va;
end % for Vn = 2:0.1:30;

Best Answer

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MATLAB: Function handle error with matirx

MATLAB: Returning values for iterations

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