How to interpolate these values?

Hi everyone. I have a table like this:

Well, I have to calculate for example the value for (ALFA = 14,2-CD=150) that there isn't in this table. I tried to calculate the equation in Excel with polynomial regression and also with INTERP in Matlab but it isn't accurate. I don't know where I am wrong. Can you help me? Thanks a lot.

[Matt J]

By the way, commas for decimals, e.g., ALFA = 14,2 are very prone to confusion in MATLAB forums since commas are used in MATLAB for other things.

[Jan]

Please show us, how you use INTERP. Otherwise we cannot guess what's going wrong.

[Giacomo]

I know. I'm sorry..

Answer 1

[Matt J]

You should use INTERP2 or griddedInterpolant.

[Giacomo]

Ok, let's try.

[Matt J]

Glad it worked. You may not be aware of the practice of Accepting answers. Please accept this one if it was what you needed.

Answer 2

[Giacomo]

Ok, I wrote:

 load cl.txt
 alfa=cl(2:61,1);
 Reynolds=cl(1,2:8);
 values=cl(2:61,2:8);

And if I ask

 CL(150,14.2)= interp2(Reynolds,alfa,values,150,14.2)

it returns the right value. The new problem is that I have two matrices 17x11, Alpha and Re. Now, I have to calculate CL matrix (17x11) where (i,j) elements are calculated combined the (i,j) values of Alpha and Re through INTERP2 command.

Suggestions?

Answer 3

[Giacomo]

I tried with:

 for i=1:length(r)
    for j=1:n+1
 CL(i,j) = interp2(Reynolds,alfa,values,Re(i,j),alpha(i,j));
    end
 end

and seems work.

Thank you, Matt.

[Matt J]

You could also have done it with a single call to INTERP2 (which is faster) by doing

 idx=sub2ind(size(Re),1:length(r),1:n+1);
 CL(idx) = interp2(Reynolds,alfa,values,Re(idx),alpha(idx));