# Drag on a rotating cylinder

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I really tried on this one, for the last two days, and I just couldn't get it to work. I don't know how far off I am. Basically I have a bnch of rigid body sticks that are floating around in my simulation. I define these sticks as the two end points, and any forces that I calculate on it must be applied via these two points. Now, imagine one of these sticks is spinning. The velocities of the two points are given by the arrows in the picture. I found a function for getting the Cd of a cylinder given fluid viscosity, velocity of flow around cylinder, diameter of sphere, and density of fluid. I also have a function for finding the Drag force per unit length. It is 1/2 * density * flow velocity ^ 2 * Cd * diameter. Here is the algorithm I invented. I tried it out and it doesn't seem to be working. I need to know if it's a logic error or just a problem with the implementation. written from the framepoint of the current point being updated. "Other" is the other point on the stick.
v1 = my velocity m1 = my mass / me p1 = my position v2 = other's velocity m2 = other's mass / other p2 = other's position 1. Find unit vector for stick = u = (p2 - p1) / length(p2-p1) 2. Vc = velocity of center of mass = (m1v1+m2v2)/(m1+m2) 3. a = v1 cross u b = vc cross u a and b are the cross velocities, that is, the velocity perpindicular to the movement of the tie, which is the direction that causes drag. c = (a + b) / 2 4. Find force of drag per unit length using c as the velocity <--- this assumption that drag and velocity are linearly related is incorrect for turbulent flows, but probably accurate enough. 5. The torque applied to "me" = Drag Force per length / 2, since it's the area of the triangle formed by length and dragforces at Mc and m1, divided by the length since we 're applying them all to m1
I can't figure out exactly what isn't working, so I have no way of "fixing" it. Is there a better way? I use the center of mass instead of the otehr point because anything past the center of mass is going to apply torque in the opposite direction. I don't want to use kinematics. I'm trying to keep it all in forces. Perhaps there's a way to do this using rotational units instead of trying to translate everything into translational forces?

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Arrg! I need an answer :P

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