MATLAB: Most recent value incremented variable from ‘while’ loop

hydrostatic pressurewhile loop

This code is meant to determine how low a cone will sink in water before the buoyancy force is equal to the weight of the cone. The buoyancy force is determined by summing the force on each area element and increasing the depth (increases the pressure) until the buoyancy force is equal to the weight of the cone. My code seems to be running ok as my while loop terminates when my "sumFz" is lower but similar in size to "'wCo", but when I go to retrieve the last 'z' (the depth) it just says it's zero. Can someone figure out why, please?

rhoFluid = 1000;
beta = 30*pi/180;
dtheta = pi/50;
mCo = 15;
wCo = g*mCo %Weight of cone
N = 100;
h = 0.5;
dz = h/N;
sumFz = wCo+1;
z = 0;
while sumFz <= wCo
    pressure = rhoFluid*g*z;
    for j = 1:N
        r = z*tan(beta);
        Fz(j) = pressure*1/2*r^2*dtheta*dz*cos(beta);
    end
    sumFz = sum(Fz,'all')   %Total buoancy force at current depth
    z = z+dz;
end
sum(Fz,'all')
z

Best Answer

sumFz = wCo+1;

You initialize sumFz to be greater than wCo

while sumFz <= wCo

but your while only happens when sumFz is less than (or equal to) wCo . Which it never is, because you initialized it to be greater than.

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Because forceErr is never <= 0.1.

Your while loop is actually unnecessary here. It doesn't add to the solution. It will just recompute the exact same results as before. Everything is actually happening inside your for loops.

What you might want to do is replace the outer for loop with a while loop. Before the while loop, define an initial value for z. Then, inside the while loop, increment that value of z by some amount each time. This will allow your value for pressure to continue changing until a result is found that meets your stopping criteria.

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maxIterations = 50000; % Some big number that you know should never be reached.
loopCounter = 1;
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Now, after you've made those modifications, what do you see? It should kick out after it hits the max iteration number. Does it? Why? You should check the condition for when the two loops you have should exit. Are they ever met?

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