# How to apply a certain harmonic force to the foundation

Dear Dr.Jason
I want to apply a certain harmonic force P=asinwt to the floating platform foundation.But I don’t know how to operate in the software FAST V8 .I would be very happy if you could give me some guidance.
Best regards

Dear Yuanzhao.Han,

The use of regular (periodic) waves (WaveMod = 1) in HydroDyn will result in sinusoidal forcing of the floating foundation at the frequency of the regular wave.

Best regards,

Dear Dr Jason
I carefully read the method you said. But I still have some doubts. Because your method is to simulate the regular wave to realize the harmonic force applied on the platform. But the harmonic force P=asinwt that I mentioned above is not generated from the wave, but from the vortex motion. So I would like to calculate the response of the platform in the condition that the floating wind power generator system is under the effect of the vortex force and wind load. From my point of view, the use of regular (periodic) waves (WaveMod = 1) in HydroDyn can not achieve what I want .
Best wishes

Dear Yuanzhao,

You first post you mention that you want to apply a harmonic load on the floating platform. Now, I’m not sure I understand enough about what load you want applied where to comment.

Best regards,

Dear Dr Jason
Sorry, I didn’t make it clear. I repeat my question. I would like to apply the wave forces and an additional harmonic force (P = asinwt) on the floating foundation . In this formula ，a and w have been already calculated by external program. What I want to know is how to add the harmonic force P in the FAST program. And if the original function of FAST 8 can not achieve it， whether I modify the FAST source code is a viable solution or not?
Best regards

Dear Yuanzhao,

Ah, now I understand. You’ll need to make a small modification to the source code. I suggest you look in the HydroDyn source code to see how the constant load AddF0 is added to the platform load. You can add your new harmonic force at the same place in the source code and recompile FAST.

Best regards,

Dear Dr Jason,