Help with for loops

Trying to run this loop for 3 different varibles for H and T, i have it working to find the different forces of a wave for one scienario but cant figure out the loop to run the other two scienarios (H,T change). Ive attached the code below.

%%
g=9.807;

d=30;
H=[4 2 7];
T=[11 7 20];
D=1;
rho=1025;

l=length(H);

for i=[1:l]
% dispersion relation to find wave length
[ka,L] = dispersion_relation(T(i),d);

k=((2*pi)./L);
w=((2.*pi)./T(i));

H_D=H(i)./D;
PiD=(pi.*D)./L; 


% Calculating Orobital Velocity
Vmax=((g.*H(i))./(2.*w)).*k;

%KC
KC=(Vmax.*T(i))./D;

%kinematic velocity
v=1.004.*10^-6;

%reynolds number
Re=(Vmax.*D)./v;

%due to wave forces on slender cylinder falling in V must calculate
%Intertia and drag force.

%assume Cm to be 1.755
%assume Cd to be 0.8

Cd=0.8; %Must change each example
Cm=1.755; %Must change this each example

t=linspace(0,22);
a=(H(i)./2);
f=(1/T(i));

y=a.*sin(2.*pi.*f.*t);
u=gradient(y);
u_dot=gradient(u);

FT(i)=-Cm.*((pi.*rho.*D^2.*w^2.*H(i))./(8.*k)).*sin(w.*t)+Cd.*(rho.*D.*w^2.*H(i)^2)./(16.*sinh(k.*d)^2).*(((sinh(2.*k.*d))./(2.*k))+d).*cos(w.*t).*abs(cos(w.*t));
Fi(i)=-Cm.*((pi.*rho.*D^2.*w^2.*H)./(8.*k)).*sin(w.*t);
Fd(i)=Cd.*(rho.*D.*w^2.*H(i)^2)./(16.*sinh(k.*d)^2).*(((sinh(2.*k.*d))./(2.*k))+d).*cos(w.*t).*abs(cos(w.*t));


subplot(3,1,1)
plot(t,Fi)
xlabel('t')
ylabel('F_inertia')
subplot(3,1,2)
plot(t,Fd)
xlabel('t')
ylabel('F_drag')
subplot(3,1,3)
plot(t,FT)
xlabel('t')
ylabel('F_Total')

end 

Answer 1

[Walter Roberson]

l=length(H);
for i=[1:l]
% dispersion relation to find wave length
[ka,L] = dispersion_relation(T(i),d);

That always uses corresponding H and T values. If you want the vectors to be independent then use another for loop such as

for i = 1 : length(H)
    for iT = 1 : length(T)
        [ka,L] = dispersion_relation(T(iT),d);
        the other stuff
    end
end

Depending exactly what you wanted to do, you might end up assigning to variables indexed at (i,iT) instead of at (i)

t=linspace(0,22);

That is a vector.

FT(i)=-Cm.*((pi.*rho.*D^2.*w^2.*H(i))./(8.*k)).*sin(w.*t)+Cd.*(rho.*D.*w^2.*H(i)^2)./(16.*sinh(k.*d)^2).*(((sinh(2.*k.*d))./(2.*k))+d).*cos(w.*t).*abs(cos(w.*t));

The right hand side uses the entire t vector in multiple places, so the right hand side is going to be a vector. But you are assigning the vector to a scalar location. You should do one of the following:

1. assign to FT(i,:) or FT(:,i) so that each iteration you are recording one entry for every t value, into a 2D array (you might even want to go 3D if T is to be independent of H; OR
2. assign to FT{i} instead of FT(i): cell arrays can hold multiple values for each index location; OR
3. if the situation is mathematically suitable, use some kind of summarizing to transform the vector of values into a single value; for example sum() or var()

[William Harris]

Keep getting this error

Arrays have incompatible sizes for this operation.

says its happening in line:

Fi(i,:)=-Cm.*((pi.*rho.*D^2.*w^2.*H)./(8.*k)).*sin(w.*t);

[Walter Roberson]

We do not have dispersion_relation to test with.

[William Harris]

function [ka,L] = dispersion_relation(T,d)
%UNTITLED8 Summary of this function goes here
%   Detailed explanation goes here
g=9.81;
w=2*pi./T;


if length(d)==1
    d=d*ones(size(T));
end
L=T;

for i=1:length(T)

    Tl = T(i);
    dl = d(i);
wl=w(i);

f=@(k) wl.^2-g.*k.*tanh(k.*dl);



% deep water approx
k_DW=wl.^2/g;

%shallow water approx


k_SW=wl./sqrt(g.*dl);

kl=fzero(f,k_DW);
ka=abs(kl);

L(i)=2.*pi./ka;
end

[Walter Roberson]

Fi(i, :) = -Cm.*((pi.*rho.*D^2.*w^2.*H(i))./(8.*k)).*sin(w.*t);

You were missing the (i) for the H