How can I sum two plots that don´t have the same number of elements?

I have the following case which represents the pressure distribution over a profile:

I want to separate the intrados and extrados sections and calculate the sum of it. I´ve divided my data but the 2 parts don´t have the same number of elements. For example, I have 170 data points for the entire plot and 94 of them are data from intrados and the rest is extrados.

I can divide it and represent it:

but I have troubles with performing the sum. I need to create extra points somehow so the 2 data sets have the same number, how can I do it?

Answer 1

[Sam Chak]

I think one of the methods is to perform data interpolation using interp1 so that both curves are interpolated at the same point coordinates.

https://www.mathworks.com/help/matlab/ref/interp1.html

[Andriy Voshchenko]

Was able to solve it with the interp1 function, thanks a lot!

For now I load the case file and manually detect where the data is separated by looking at the X coordenate, it does something like this, so i can clearly see the max/min when I plot the X coordenate:

Any suggestion on how I can detect the point automatically??

Note: The X coordenate for extrados and intrados is slightly different due to the AoA in space and profile thickness but those variations are very small, I interpolate intrados and extrados on separate and then just perform the sum.

%% Detection of the separation point

% plot(pressures(:,1),pressures(:,2))
dimensions=size(pressures);
X=linspace(1,dimensions(1),dimensions(1));

%  plot(X,pressures(:,1)) %%  PLOT AND DETECT THE SEPARATION POINT (GLOBAL
%  MAX OR MIN) (manually for now)

% plot(pressures(1:93,1),pressures(1:93,2)) 
% plot(pressures(93:end,1),pressures(93:end,2))


%% Interpolations
chord=linspace(0,2.384,200); % Create chord for plot

intrados=[pressures(1:93,1),pressures(1:93,2)];
intrados_chord=linspace(intrados(1,1),intrados(end,1),200); % is the coordenate in space, not a chord from 0 to c
intradosCorrected=interp1(intrados(:,1),intrados(:,2),intrados_chord);

% plot(chord,intradosCorrected)

extrados=[pressures(94:end,1),pressures(94:end,2)];
extrados_chord=linspace(extrados(1,1),extrados(end,1),200);
extradosCorrected=interp1(extrados(:,1),extrados(:,2),extrados_chord);

% plot(chord,extradosCorrected)

[Sam Chak]

Hi @Andriy Voshchenko

You are welcome! I'm glad that it works out nicely. Regarding the next issue, perhaps you can try using the max() function to find the maximum elements of a data array.

A few examples are given here in this link:

https://www.mathworks.com/help/matlab/ref/max.html

[Andriy Voshchenko]

I would need to tind the position of the max in that case, but yeah i already figured I can use something like

maximum = max(max(A));
[x,y]=find(A==maximum)

Thank you for your help!

[Sam Chak]

You can actually use the max function directly.

[M,I] = max(A)

also returns the index into the operating dimension that corresponds to the maximum value of A.

[Andriy Voshchenko]

Oh, that´s actually more useful. Will probably need to solve more cases so the automatic detection and separation on intrados/extrados will accelerate things a lot. Will try to implement it now. Thanks a lot!

[Sam Chak]

You are welcome, @Andriy Voshchenko.