Help with contour plot

Hello veryone,

i am trying do a contour plot of the stability of a mechanical system: so given a meshgrid X-Y and for a varying parameter k1, i calculate Z based on two conditions (B and D) and plot for Z=0. As k1 can be one of five different values i would like to have five different curves on the same plot.

but i only get two curves (which are also not right) and in the command windows this warning appears:

"Warning: Contour not rendered for constant ZData"

could anybody help me find the problem? thank you very much

here ist the script i am using

clear; clc; clf;
M = 700; J = 400; a1 = 1.85; a2 = 1.85; L = 3.7;
n = 101;
m = 50000;
k_a12 = linspace(-m, m, n);
k_a21 = linspace(-m, m, n);
[X, Y] = meshgrid(k_a12, k_a21);
v = [0 0];
clr = ['r','g','b','c','k'];
b = 100000;
a = 5;
k1 = linspace(-b, b, a);
k2 = 0;
LegendsStrings = cell(numel(k1),1);
Z = zeros(n, n);

for q = 1:a
    for i = 1:n
        for j = 1:n
            B = k1(q)*(J + M*a1^2) + k2*(J + M*a2^2) + (X + Y)*(J - M*a1*a2);
            D = k1(q)*k2 - X.*Y;
            if B(i, j) >= 0 && D(i, j) >= 0
                Z(i, j) = D(i, j);
            else 
                Z(i, j) = -1;
            end
            
        end
    end    
    LegendsStrings{q} = ['k_1 = ', num2str(k1(q))];
    contour(X, Y, Z, v, 'LineWidth', 1.2, 'LineColor', clr(q), 'DisplayName', LegendsStrings{q});
    hold on
end

legend()
xlabel('k_a_1_2')
ylabel('k_a_2_1')
grid on

Answer 1

[Aquatris]

You actually do have 5 lines but they are on top of each other thats why you do not see them. So your Z matrix for k1(1) and k1(2) are the same. Also Z matrix for k1(3) to k1(5) are the same.

So here is you code for 4 points instead of 101 to visaulize easily. You can also click and delete the lines in your plot to see the underlying lines:

clear; clc; clf;
M = 700; J = 400; a1 = 1.85; a2 = 1.85; L = 3.7;
n = 4;
m = 50000;
k_a12 = linspace(-m, m, n);
k_a21 = linspace(-m, m, n);
[X, Y] = meshgrid(k_a12, k_a21);
v = [0 0];
clr = ['r','g','b','c','k'];
b = 100000;
a = 5;
k1 = linspace(-b, b, a);
k2 = 0;
LegendsStrings = cell(numel(k1),1);
Z = zeros(n, n);
for q = 1:a
    for i = 1:n
        for j = 1:n
            B = k1(q)*(J + M*a1^2) + k2*(J + M*a2^2) + (X + Y)*(J - M*a1*a2);
            D = k1(q)*k2 - X.*Y;
            if B(i, j) >= 0 && D(i, j) >= 0
                Z(i, j) = D(i, j);
            else 
                Z(i, j) = -1;
            end
           
        end
    end   
    fprintf('Z for k1 = %.2f is\n',k1(q))
    disp(Z)
    LegendsStrings{q} = ['k_1 = ', num2str(k1(q))];
    contour(X, Y, Z, v, 'LineWidth', 1.2, 'LineColor', clr(q), 'DisplayName', LegendsStrings{q});
    hold on
end

Z for k1 = -100000.00 is

    -1    -1    -1    -1
    -1    -1    -1    -1
    -1    -1    -1    -1
    -1    -1    -1    -1

Warning: Contour not rendered for constant ZData

Z for k1 = -50000.00 is

    -1    -1    -1    -1
    -1    -1    -1    -1
    -1    -1    -1    -1
    -1    -1    -1    -1

Warning: Contour not rendered for constant ZData

Z for k1 = 0.00 is

   1.0e+09 *

   -0.0000   -0.0000    0.8333    2.5000
   -0.0000   -0.0000    0.2778   -0.0000
    0.8333    0.2778   -0.0000   -0.0000
    2.5000   -0.0000   -0.0000   -0.0000

Z for k1 = 50000.00 is

   1.0e+09 *

   -0.0000   -0.0000    0.8333    2.5000
   -0.0000   -0.0000    0.2778    0.8333
    0.8333    0.2778   -0.0000   -0.0000
    2.5000    0.8333   -0.0000   -0.0000

Z for k1 = 100000.00 is

   1.0e+09 *

   -0.0000   -0.0000    0.8333    2.5000
   -0.0000   -0.0000    0.2778    0.8333
    0.8333    0.2778   -0.0000   -0.0000
    2.5000    0.8333   -0.0000   -0.0000

legend()
xlabel('k_a_1_2')
ylabel('k_a_2_1')
grid on

[LUCA D'AMBROSIO]

thank you, i notice it now