How do i plot this solutions?

Hello,

I want to plot the theta2 values on the x and presures (P2_ALL) on a plot. It seems my pressures are in symbolic form so Im using fplot to plot. When I do this it gives me an error. Does anyone know how to fix this?

clc; clearvars 
format short g 
M1 = 3 ; 
P1 = 1 ; 
theta2 = 1:1:30 ; 
gamma = 1.4 ; 
phi = 4.8 ; 
syms beta2 
tantheta2 = tand(theta2 ); 
x = 2*cotd(beta2)*((M1^2*((sind(beta2)).^2)-1 )); 
y = M1^2*(gamma+cosd(2*beta2))+2 ; 
eqn2 = x/y ; 
b0=1 ; 
for ii=1:numel(theta2 ) 
T = tantheta2(ii ); 
clearvars result2 
syms beta2 
result2 = vpasolve(T== eqn2, beta2,b0 ); 
beta2 = vpa(result2 ); 
Mn1 = M1*sind(beta2); % Eqn 4.7 from Anderson's 
p2p1 = 1+((2*gamma)/(gamma+1))*(Mn1^2-1); % Eqn 4.9 from Anderson's 
Mn2sq= (Mn1^2+(2/(gamma-1)))/((2*gamma/(gamma-1))*Mn1^2-1);% Eqn 4.10 from Anderson's 
Mn2 = sqrt(Mn2sq ); 
M2 = Mn2/sind(beta2-theta2(ii )); 
P2 = p2p1*P1 ; 
P2_ALL{ii}=vpa(P2) ; 
end
fplot(theta2,P2_ALL)

Error using char
Cell elements must be character arrays.

Error in fplot (line 144)
strArgs = char(args{k});

Answer 1

[Abhaya]

Hello Benneth,

The issue you are experiencing arises because the second argument of the MATLAB 'fplot' function is a cell array. Typically, the MATLAB 'fplot' function is designed to plot expressions or functions. In this case, the variable 'P2_ALL' contains a cell array of cells. To address this, here are two potential solutions to plot variable 'P2_ALL' against variable 'theta2'.

1. Convert the Cell Array to a Vector: You can add the following lines of code to your existing script in order to create a vector from the variable 'P2_ALL'.

vecP2 = [P2_ALL{1,:}]; 
plot(theta2, vecP2); 

2. Change 'P2_ALL' from a Cell Array to a Vector: Instead of making variable ‘P2_ALL’ a cell array, you can make it a vector. Here is the updated code:

clc; clearvars  
format short g  
M1 = 3 ;  
P1 = 1 ;  
theta2 = 1:1:30 ;  
gamma = 1.4 ;  
phi = 4.8 ;  
syms beta2  
tantheta2 = tand(theta2 );  
x = 2*cotd(beta2)*((M1^2*((sind(beta2)).^2)-1 ));  
y = M1^2*(gamma+cosd(2*beta2))+2 ;  
eqn2 = x/y ;  
b0=1 ;  
for ii=1:numel(theta2 )  
    T = tantheta2(ii );  
    clearvars result2  
    syms beta2  
    result2 = vpasolve(T== eqn2, beta2,b0 );  
    beta2 = vpa(result2 );  
    Mn1 = M1*sind(beta2); % Eqn 4.7 from Anderson's  
    p2p1 = 1+((2*gamma)/(gamma+1))*(Mn1^2-1); % Eqn 4.9 from Anderson's  
    Mn2sq= (Mn1^2+(2/(gamma-1)))/((2*gamma/(gamma-1))*Mn1^2-1);% Eqn 4.10 from Anderson's  
    Mn2 = sqrt(Mn2sq );  
    M2 = Mn2/sind(beta2-theta2(ii ));  
    P2 = p2p1*P1 ; 
    
    %P2_ALL vector of sym 
    P2_ALL(ii)=vpa(P2) ;  
end 

plot(theta2,P2_ALL); 

With following either of the approach, you can plot the variable ‘P2_ALL’ against ‘theta2’. The result can be shown as the figure attached below.

For more information on MATLAB ‘fplot’ function, please refer to the documentation given below.

https://www.mathworks.com/help/releases/R2021b/matlab/ref/fplot.html

Hope this resolves the query.