How can I interpolate from a table using an input value?

%Number of Blades
B = 3;        
%Relative Velocity
U = 10; %m/s
rho = 1.2754; % kg/m^3
a = (1/3); 
ar = (1-3*a)/(4*a-1);
om = 11/60; %rps
%Wind Velocity at the rotor
Vw = U*(1-a);

%% Read csv files
filename = 'DBAeroprop.csv'
filename2 = 'AirfoilData.csv'
A = readtable(filename);
A.Properties.VariableNames = {'Node' 'RNodes' 'AeroTwst' 'DRNodes' 'Chord'};
B = readtable(filename2);
B.Properties.VariableNames = {'Angle' 'Cl' 'Cd'}

%% Finding Torque
dr = 1;
r = [0:dr:100];
Q=0; %initialization
for i=1:length(r)
%     velocity
    v = om*2*pi*r(i);
%     Circuferential velocity
    Vr = v*r(i)*(1+ar);
%     chord
    c = interp1(A.RNodes, A.Chord, r(i));
%     phi angle
    phi = atan((U*(1-a)) / (v*r(i)*(1+ar)));
%     Twist angle 
    theta = interp1(A.RNodes, A.AeroTwst, r(i));
%     Angle of attack
    alpha = phi - theta;
%     Cl and Cd (start assuming Cl and Cd fixed then you have to find the way to define Cl and Cd as function of alpha using the tables)
    Cl = find_cl(alpha);
    Cd = find_cd(alpha);
%     Relative velocity
    Ur = sqrt(Vr^2 + Vw^2);
%     Torque
    dQ = B*(1/2)*rho*Ur^2*(Cl*sin(phi)-Cd*cos(phi))*c(i)*r(i)*dr;
    Q = Q+dQ;
end

%% Finding Cd and Cl based on the angle alpha
function Cl = find_cl(x)
    Cl = interp1(B.Angle, B.Cl, x)
end 
function Cd = find_cd(x)
    Cd = interp1(B.Angle, B.Cd, x)
end 

I believe I am either misusing the interpolation function or have something confused when utilizing it in a for loop. My error reads:

Unable to resolve the name B.Angle.

Error in BET_review>find_cl (line 56)

Cl = interp1(B.Angle, B.Cl, x)

Error in BET_review (line 45)

Cl = find_cl(alpha);

But I don't see any utilization of the interpolation function working. Any help appreciated. Thanks!

Answer 1

[Walter Roberson]

You have a script, followed in the same file by some functions.

You assign some variables in the script, and you want to use them in the functions.

Variables created in a script are not "shared variables" for the functions defined in the script.

Variables are only shared when you have nested functions.

If you were to put a function heading on the top of the file, and you were to put another end at the end of the file, so that the two other functions would be nested inside the first function, then that should work.

Example:

function testtab
XYZ = sortrows(randi(99, 10, 3), 1);
B = array2table(XYZ, 'VariableNames', {'Angle' 'Cl' 'Cd'});
x = 3.8;
pqr = find_cl(x);
disp(pqr)

function Cl = find_cl(x)
   Cl = interp1(B.Angle, B.Cl, x, 'linear', 'extrap');
end
end

[Rhianna McFarlen]

Do I need to utilize the XYZ variable that you did for my own code? Or was that just for an example?

[Walter Roberson]

That was example code showing the form -- "function" header first, then code that invokes the functions, then the functions (each with its own "end" statement), then a final "end" statement.