How to solve this equation with matlab

Question

hadi mohammadian 2020-12-30

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/705348-how-to-solve-this-equation-with-matlab

评论： Walter Roberson 2020-12-30

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 个评论
显示无隐藏无

Walter Roberson 2020-12-30

在 MATLAB Online 中打开

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.

hadi mohammadian 2020-12-30

编辑：hadi mohammadian 2020-12-30

yes . you're right . I changed them . thanks alot .

in matlab they're correct but they were wrong in here .

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Walter Roberson 2020-12-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/705348-how-to-solve-this-equation-with-matlab#answer_587398

在 MATLAB Online 中打开

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 个评论
显示 3更早的评论隐藏 3更早的评论

hadi mohammadian 2020-12-30

thanks man . my issue solved .

Walter Roberson 2020-12-30

What was the solution ?

请先登录，再进行评论。

Answer 2

Alan Stevens 2020-12-30

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/705348-how-to-solve-this-equation-with-matlab#answer_587318

在 MATLAB Online 中打开

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)