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

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Andriy Voshchenko 2022-5-26

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评论： Sam Chak 2022-5-27

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?

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Answer 1

Sam Chak 2022-5-26

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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

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Andriy Voshchenko 2022-5-26

在 MATLAB Online 中打开

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)

Andriy Voshchenko 2022-5-27

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!