MATLAB: How can ı solve these equations

warning:not enough input arguments.

function dc=pbr(w,c)
dc=zeros(2,1);
Cao=0.0454;
Fao=13.2;
R=0.082;
T=443;
k=1.74;
teta=2.0303;
eps=-0.66;
alfa=0.027;
dc(1,:)=k*Cao*R*T*c(2)*(teta-2*c(1))/((1+eps*c(1))*Fao);
dc(2,:)=-alfa*(1+eps*c(1))/(2*c(2));
[w,c]=ode45(@pbr,[0 1],[0 70]);
plot(w,c(:,1))

Best Answer

This works:

dc=zeros(2,1);
Cao=0.0454;
Fao=13.2;
R=0.082;
T=443;
k=1.74;
teta=2.0303;
eps=-0.66;
alfa=0.027;
pbr = @(w,c) [k*Cao*R*T*c(2).*(teta-2*c(1))./((1+eps*c(1))*Fao);  -alfa*(1+eps*c(1))./(2*c(2))];
[w,c]=ode45(pbr,[0 1],[0 70]);
figure(1)
plot(w,c(:,1))
grid

It is much easier to use an Anonymous Function for simple ordinary differential equations such as yours. Please see the section on Anonymous Functions (link) in Function Basics (link).

Some of the problems you are experiencing are because you are not using element-wise operations. See the documentation on Array vs. Matrix Operations (link) for a discussion and use of the dot ‘.’ operator for element-wise multiplication (.*) and division (./).

Related Solutions

MATLAB: Ode 45 and for loop

k = 0;
T_sol = cell(1,100);
X_sol = cell(1,100);
for ii = 0.1:0.1:10
    k = k+1;
    [T,X] = ode45(@(t,x)fx(t,x,ii), [0 4], [0 0]);
    T_sol{k} = T;
    X_sol{k} = X;
end
% plot first and last solution
plot(T_sol{1}, X_sol{1})
hold on
plot(T_sol{100}, X_sol{100})
hold off
function dx = fx(~,x,ii) % Can I give i as an input here.
     rho = 1.225;
     C_w = 0.3; 
     A = 0.005; 
     m = 0.314; 
     C_r = 0.035; 
     R_T = 0.7;
     n_0 = 378.33;
     T_stall = 0.1868;
     r = 0.030;
     
     dx = zeros (2,1); 
     dx(1) = x(2);
     dx(2) = (-T_stall * R_T * ii.^2 * ((1000 * x(2)/ pi ) - n_0/ii)/(n_0 * r) - m * 9.81 * C_r - 0.5 * rho * A * C_w * x(2)^2)/m;
     % the diff. eq. with i
end

MATLAB: Extracting particular x and y data points from a figure

Thinking in terms of graphs is useful but only takes you so far. The point is how to think about the data and/or functions underlying the graph.

You are looking for "interpolation" strategies.

Since both x and y look well behaved, you can achieve what you want simply by

x = [0 10 20 30 40 50 60 70 80]
y = [0 3 6 9 12 15 18 21 24]
yTarget = 8
xTarget = interp1(y,x,yTarget,"linear")
% check graphically
hold on;
plot(x,y)
plot(xTarget,yTarget,'*')

In fact, upon trying above and actually looking at your data, you have labored to define a very simple line, which by inspection has the form

So for your specific problem just do algebra

But anyway the procedure in the code above will work well as long as y is monotonic.

Best Answer

Related Solutions

MATLAB: Ode 45 and for loop

MATLAB: Extracting particular x and y data points from a figure

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