# Excitation force floating body

Discussion in 'Hydrodynamics and Aerodynamics' started by Troglo, Nov 30, 2012.

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### TrogloNew Member

I am working on a heave model , I am trying to make a model in heave in the time domain. I am using wamit the identify the hydrodinamic parameters as added mass, radiation damping and excitation force. I have some problem with the result, indeed using the diffraction force and the haskind force coming from wamit I have a response of my model not reasonable, I have a amplitude of the motion 2 or 3 times the incident wave. I know that the parameters of the force are normalized by rho*g*H/2, but made a total excitation force due to the sum of Haskind+diffraction I could find any reasonable results. Can you explain how use this values?

I write the equation that I am solving through the matlab function ODE45, where bvz is the viscous factor damping, brz is the radiation damping, Awp is the restoring coefficient, m and awz are mass and add mass and Fz0 is the total excitation force
y(2) is the velocity in heave of my body and y(1) is the displacement.

dydt=[y(2);
(-(bvz*y(2)*abs(y(2))))-brz*y(2)-(((Awp))*y(1))+Fz0*cos((ome.*t))))/(m+awz)];

I would be very grateful if someone could help me to understand where is the error!

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### philSweetSenior Member

your parentheses are a mess. (check (your)) ttyping)))

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### TrogloNew Member

You are right, sorry!
I fix it a bit

dydt=[y(2);
(-(bvz*(y(2)*|(y(2)|-brz*y(2)-Awp*y(1)+Fz0*cos(ome.*t))/(m+awz)];

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### philSweetSenior Member

next item-

The quantity on the left is an acceleration - y" or d^2y/dt^2. Are you integrating it twice to get heave? If I understand the code, it will return an acceleration, but I've never used Matlab.

Is Fz0 a surface displacement, or a force? if the former, wouldn't it just get rolled into the y(1) term? ie Awp*(y(1)+Fz0*cos(ome.*t))

when all else fails, write the whole thing out with units explicitly and make sure the units add up.

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### TrogloNew Member

ok, I was not really clear, my fault. Fz0 is the amplitude of the excitation force.
My question is: Should I consider the total amplitutde of the excitation force acting on my body as the sum of the two output coming from wamit, ie. HASKIND and DIFFRACTION excitation force, or these two parameters are two way to identify the same quantity?

Thanks and sorry for the lack of clarity!

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### CWTeebsAnomalyGenerator

Are you simulating in regular or irregular seas? Can you post your MatLab code? What do your variables mean? What coordinate system are you using?

It's the "Haskind relations" not the "Haskind forces," meaning the pressure in the fluid domain at the location of the structure without the ship, plus the force required to scatter/diffract the water away from the structure. Together these constitute the excitation forces. Well, I guess you could call these the Haskind forces, I'm just used to seeing it referred to as the Haskind relations, they're just two distinct components that makeup the inputs to the system.

Your problem may be related to how your undimensionalizing the WAMIT parameters. I've not personally used WAMIT. See the following thread:

http://www.boatdesign.net/forums/de...tation-force-nondimensionalization-45057.html

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### TrogloNew Member

I am simulating in regular waves, the coordinate system is refered to the center of gravity of the body.
bvz=coeff. of viscous damping
Awp=waterplane area
Fz0=Excitation force amplitude
m=bodu mass