Intergrating using For Loop! HELP!

So, I have this assignment when I am supposed to find the distribution of certain values along a surface. The data was provided to calculate all the values needed.

this is a picture of the assignment

and this is my code..

clc;
clear;

%Reading data 
fid=fopen('airfoil.dat','r');

m=fscanf(fid,'%d',1);

for i=1:m+1
    x(i)=fscanf(fid,'%f',1);
    y(i)=fscanf(fid,'%f',1);
    cf(i)=fscanf(fid,'%f',1);
    cp(i)=fscanf(fid,'%f',1);
end


%Loop for panel calculation
%lengths (ds) orintation angle (theta)

for i=1:m 
    ds(i)=sqrt((x(i+1)-x(i))^2+(y(i+1)-y(i))^2);
    theta(i)= atan2((y(i+1)-y(i)),(x(i+1)-x(i)));
end


%CFx and CFy calculations 

for i=1:m;
    CFy_p=(-1)*cp(i)*cos(theta(i))*ds(i);; 
    CFx_p=(-1)*cp(i)*sin(theta(i))*ds(i);;
    CFy_s=(-1)*cf*sin(theta(i))*ds(i);;
    CFx_s=(-1)*cf*cos(theta(i))*ds(i);;
end

%CL and CD calculations
alpha=5
CL=((CFy_p+CFy_s)*cos(alpha))-((CFx_p+CFx_s)*sin(alpha));
CD_pressure=CFy_p*sin(alpha)+CFx_p*cos(alpha);
CD_friction=CFy_s*sin(alpha)+CFx_s*cos(alpha);
CD_total= CD_pressure+CD_friction;


%plots
plot(x,y); axis equal;
title('airfoil');
xlabel('x');
ylabel('y');

plot(x,cp);
set(gca,'ydir','revers');
title('cp distribution');
xlabel('x');
ylabel('cp');

plot(x,cf);
title('cf distribution');
xlabel('x');
ylabel('cf');

I know that the first part and last part of the code is correct but I'm not sure about the middle part.. Final values were given to us and when I compared it's not the same.

this should be thie final results

alpha=5 degrees

Cl=0.636

Cd=0.01011

please help!

data sheet attatched

[Jan]

Look at this:

for i=1:m;
    CFy_p=(-1)*cp(i)*cos(theta(i))*ds(i);; 
    CFx_p=(-1)*cp(i)*sin(theta(i))*ds(i);;
    CFy_s=(-1)*cf*sin(theta(i))*ds(i);;
    CFx_s=(-1)*cf*cos(theta(i))*ds(i);;
end

The variables are overwritten in each iteration. This cannot be useful for solving an integral. You need a sum like a = a + b.

[TF]

Still, I don't really know how to solve this problem.. Is there a way to stop the overwrite of the iteration and still use the for loop?

Answer 1

[Geoff Hayes]

TF - your alpha is in degrees but you are using the cos and sin functions whose inputs are in radians. I recommend converting alpha intio radians and trying again

alpha = 5 * pi / 180.0;

[TF]

That doesn't seem to be the problem!! CL and CD shoud be a single value but instead this is what I get when I run the code

[Geoff Hayes]

what is this code

for i=1:m;
    CFy_p=(-1)*cp(i)*cos(theta(i))*ds(i);; 
    CFx_p=(-1)*cp(i)*sin(theta(i))*ds(i);;
    CFy_s=(-1)*cf*sin(theta(i))*ds(i);;
    CFx_s=(-1)*cf*cos(theta(i))*ds(i);;
end

What is the intent behind it? From your output, it now seems that you have taken Jan's suggestion and created arrays but aren't you supposed to integrate as per your attachment?

[TF]

The CFy_p is the force coefficient due to pressure in the y direction, CFy_s is the force coefficient due to shear in the y direction... you probably got that. These should be single values. I'm new to matlab and I thought this is the way to do it

[Geoff Hayes]

hmmm...perhaps

CFy_p = 0;
CFx_p = 0;
CFy_s = 0;
CFx_s = 0;
for i=1:m;
    CFy_p = CFy_p + cp(i)*cos(theta(i))*ds(i);
    CFx_p = CFx_p + cp(i)*sin(theta(i))*ds(i);
    CFy_s = CFy_s + cf(i)*sin(theta(i))*ds(i);
    CFx_s = CFx_s + cf(i)*cos(theta(i))*ds(i);
end
CFy_p = -CFy_p;
CFx_p = -CFx_p;
CFy_s = -CFy_s;

Or something very similar to the above. Note how we keep a running total for each of the coefficients. Note also how we use cf(i) instead of just cf. Finally, we negate the first three (as per the attachment - the fourth remains as is).

[TF]

I was just thinking of defining the variables before the loop and adding them then I saw your answer!!!

Thank you so much!!! it is working wonderfully now!