How to solve this equation with matlab
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hadi mohammadian
on 30 Dec 2020
Commented: Walter Roberson
on 30 Dec 2020
clear all;
clc;
data8={'1','10','20','23','12','11'};
kk{1}='Internal pipe roughness (mm)';
kk{2}='Hydraulic diameter of pipe (m)';
kk{3}='Reynolds number ';
kk{4}='Pipe line length (m)';
kk{5}='Average pipe line velosity (m/s)';
kk{6}='Acceleration due to gravity (m/s^2)';
kkk=inputdlg(kk,'Head loss',1,data8);
e11=str2num(kkk{1});
d12=str2num(kkk{2});
re1=str2num(kkk{3});
l2=str2num(kkk{4});
v11=str2num(kkk{5});
g12=str2num(kkk{6});
eqn=(1/sqrt(f9)==(-2*log((e11/3.7*d12)+(2.51/re1*sqrt(f9)))));
solve('eqn','sqrt(f9)');
hf=(f12*l2*(v11^2))/(2*g12*d12);
fprintf('Friction factor in pipe = %f \n Head loss = %f (m) \n-----------------------------------------------\n',f1,hf);
Undefined function or variable 'f9'.
Error in Untitled (line 150)
eqn=(1/sqrt(f9)==(-2*log((e11/3.7*d12)+(2.51/re1*sqrt(f9)))));
2 Comments
Walter Roberson
on 30 Dec 2020
e11=str2num(kkk{1});
d12=str2num(kkk{1});
re1=str2num(kkk{1});
l2=str2num(kkk{1});
v11=str2num(kkk{1});
g12=str2num(kkk{1});
are you sure that you want to use kkk{1} as the source for all of those str2num() ? That assigns the same value to each of the variables.
Accepted Answer
Walter Roberson
on 30 Dec 2020
syms f9 f9sqrt
eqn=(1/sqrt(f9)==(-2*log((e11/3.7*d12)+(2.51/re1*sqrt(f9)))));
F9sqrt = solve(subs(eqn, f9, f9sqrt^2), f9sqrt)
5 Comments
More Answers (1)
Alan Stevens
on 30 Dec 2020
Strange way of entering data! However, one way to find the friction factor is a simple fixed point iteration method, such as
e11 = 1;
d12 = 10;
re1 = 20;
l2 = 23;
v11 = 12;
g12 = 11;
f9 = 1; % initial guess
tol = 10^-6;
err = 1;
% fixed point iteration
while err>tol
f9old = f9;
f9 = (-2*log10(e11/(3.7*d12)+2.51/(re1*sqrt(f9))))^-2;
err = abs(f9-f9old);
end
disp(f9)
Note that I've used log10 rather than log (the latter is log to the base e).
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