# Damping force and Morison equation

Hello everyone,
In OpenFast documentation, there is damping force.
I know that radiation force contains two contributions: from added mass and the other from radiation damping. I know also there is structural damping.
So where the damping force in the doc did come from ???

Also, until i don’t know why in articles that contains analytical MDOF models, we omit the memory effect ???

Finally, in linear wave theory, Morison equation is used when the body is small compared to wavelength. The Morison formula contains added mass. So in the equation of motion we have two times added mass matrix ???

Thank you for your help !!!

I’m not really understanding your questions; please clarify.

Regarding your damping question, can you be more specific which section you are referring to?

Best regards,

Dear @Jason.Jonkman ,

I’m sorry. My questions are not very clear. I have several questions to ask:

1- I was reading in Openfast documentation the theory behind the code. (i didn’t bookmark the page and i can’t find it ). What i read, was the cummins equation which represents the motion of a floating body.
I notice that the equation contains damping matrix which is different from damping coming from radiation. My question is where did this damping come from ???

2- n the litterature, where there is MDOF models based on few DoFs e.g 13 DoF, the authors neglect the memory effect. Why ?

Finally, in linear wave theory, Morison equation is used when the body is small compared to wave length. This formula contains added mass. My question is when we assemble the equation of motion we will {[M] + 2*[A]}*q_double_dot where [M] is the mass matrix and [A] is the added mass matrix at infinite frequency. I multiply [A] by 2 because in the general equation we have added mass and the other comes from Morison equation right ?

Thanks for your help !!!