Using fzero function to solve a nonlinear equation with one input
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Hi,
I am trying to solve this non-linear equation for alpha using a range of values of X (0-100) [so that I can plot alpha vs X].
function y=equations(alpha)
[D, A, R] = geom();
[n, m] = setglbl();
[X] = setX();
Ag = (R.^2)*(pi-alpha+0.5*sin(2*alpha)) ;
Sgw = 2*R*(pi-alpha) ;
Si = 2*R*sin(alpha) ;
Dg = 4*Ag/(Sgw+Si) ;
Al = (R.^2)*(alpha-0.5*sin(2*alpha)) ;
Slw = 2*R*alpha ;
Dl = 4*Al/Slw ;
ag = Ag/A ;
y=((X.^2).*(1-ag).^(n-2)*(D/Dl).^(n+1))-(ag.^(m-2)*(D/Dg).^(m+1)*(1+(Dg*Si/Al)));
end
WHERE
function [X] = setX
global X ;
X=linspace(0,100,101) ;
end
AND
function [Cg, Cl, n, m] = setglbl
global Cg Cl n m
n=1;
m=1;
end
AND
function [D, A, R] = geom()
D = 0.3; %Pipe Diameter (m)
R = D/2; %Pipe Radius (m)
A = pi*(R^2); %Cross-sectional Area (m^2)
end
I KEEP GETTING ERRORS:
Error using fzero (line 301)
FZERO cannot continue because user-supplied function_handle ==> equations failed with the error below.
*Matrix dimensions must agree.*
>> equations
*Not enough input arguments.*
Error in equations (line 7)
Ag = (R.^2)*(pi-alpha+0.5*sin(2*alpha)) ; % [m^2] Cross-sectional area of gas phase in pipe
Does anyone have some advice on what the problem could be?
Should the X interval be defined differently? Or should X be used as a second input to the function?
Any help appreciated!
0 Comments
Accepted Answer
Walter Roberson
on 14 Feb 2017
You need to change
function [n, m] = setglbl
global n m
n=1;
m=1;
end
However, you have
X=linspace(0,100,101) ;
so X is a vector of length 101, and you have
y=((X.^2).*(1-ag).^(n-2)*(D/Dl).^(n+1))-(ag.^(m-2)*(D/Dg).^(m+1)*(1+(Dg*Si/Al)));
so y is going to be a vector the same length as X. You cannot fzero an entire vector.
Your formula has some numeric problems that need to be worked around. Revised source is:
function alphaVsX
[D, A, R] = geom();
[n, m] = setglbl();
X = setX();
alpha = arrayfun( @(x) fzero( @(alpha) equations(alpha, x, D, A, R, m, n), [1E-4, pi-1E-4]), X );
plot(X, alpha);
xlabel('X')
ylabel('alpha')
end
function [D, A, R] = geom()
D = 0.3; %Pipe Diameter (m)
R = D/2; %Pipe Radius (m)
A = pi*(R^2); %Cross-sectional Area (m^2)
end
function [n, m] = setglbl
n=1;
m=1;
end
function [X] = setX
X=linspace(0,100,101) ;
end
function y = equations(alpha, X, D, A, R, m, n)
Ag = (R.^2)*(pi-alpha+0.5*sin(2*alpha)) ;
Sgw = 2*R*(pi-alpha) ;
Si = 2*R*sin(alpha) ;
Dg = 4*Ag/(Sgw+Si) ;
Al = (R.^2)*(alpha-0.5*sin(2*alpha)) ;
Slw = 2*R*alpha ;
Dl = 4*Al/Slw ;
ag = Ag/A ;
if X == 0
y = 0;
else
y=((X.^2).*(1-ag).^(n-2)*(D/Dl).^(n+1)) - (ag.^(m-2)*(D/Dg).^(m+1)*(1+(Dg*Si/Al)));
end
end
7 Comments
Walter Roberson
on 17 Mar 2017
You have
alpha = arrayfun( @(x) fzero( @(alpha) equationsch(alpha, X, A, m, n, Z, al), [1E-4, pi-1E-4]), X, al );
which has three arguments for arrayfun: the function handle, X, and al . arrayfun() takes a function handle and passes each of the remaining arguments to it, so because you are providing two arguments after the function handle, the function handle has to take two arguments. But it does not: the function handle takes only one argument, lower-case x. If each X entry is to be paired with each al entry, then you need to have your function handle
@(x) fzero( @(alpha) equationsch(alpha, X, A, m, n, Z, al), [1E-4, pi-1E-4])
take two arguments instead.
After that, notice that your function handle has a formal parameter lower-case x as an argument, but it does nothing with that argument. Instead, it passes upper-case X to the function, which is the entire vector of X values.
Perhaps you want
alpha = arrayfun( @(one_X, one_al) fzero( @(alpha) equationsch(alpha, one_X, A, m, n, Z, one_al), [1E-4, pi-1E-4]), X, al );
More Answers (2)
Torsten
on 17 Mar 2017
Does this work ?
function [alpha] = getalphach
[A] = geom();
[n, m, Z] = setglblch();
X = setX();
al = setal();
for i=1:numel(X)
Xloc = X(i);
for j = 1:numel(al)
alloc = al(j);
alpha(i,j) = fzero(@(x)equationsch(x,Xloc,A,m,n,Z,alloc),[1e-4, pi-1e-4]);
end
end
Best wishes
Torsten.
3 Comments
Torsten
on 17 Mar 2017
Edited: Torsten
on 17 Mar 2017
Try to call fzero with a single starting value, e.g.
alpha(i,j) = fzero(@(x)equationsch(x,Xloc,A,m,n,Z,alloc),pi-1e-4);
If fzero cannot find a solution, you should first plot your function to see whether a zero really exists.
By the way: Your call to fzero must be with a small x:
Not:
alpha(i,j) = fzero(@(x)equationsch(X,Xloc,m,n,Z,alloc),[1e-4, pi-1e-4]);
But:
alpha(i,j) = fzero(@(x)equationsch(x,Xloc,m,n,Z,alloc),[1e-4, pi-1e-4]);
Best wishes
Torsten.
Torsten
on 22 Mar 2017
Sure.
function [alpha] = getalphach
[A] = geom();
[n, m, Z] = setglblch();
[X,al] = setmartvar();
for i=1:numel(X)
Xloc = X(i);
alloc = al(i);
alpha(i) = fzero(@(x)equationsch(x,Xloc,A,m,n,Z,alloc),[1e-4, pi-1e-4]);
end
Best wishes
Torsten.
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