Dear Sir,
I have a question about Wave Force based on Morison Equation using for Frequency Domain Analysis.
As you know that wave force based on Morison equation is 'Nonlinear Behaviour on Drag term' which is square of wave particle velocity.
This is complicated situation when it is used in Frequency Domain analysis which is based on Fourier series i.e. linear superposition of sin, cos wave.
I am curious about 'SACS wave force calculation used for transfer function based on Morison equation'.
Is SACS make some linearisation in the drag term when generating transfer function? And How?
Is it based on the procedure recommended in "Borgman (1967) Ocean Wave Simulation for Engineering Design"
I cannot found anything about this issue in SACS manual.
It is much appreciate, if you can explain some background of SACs calculation.
Regards,
Kasiphon
Kasiphon,
The nonlinear drag component in spectral wave fatigue analyses is addressed by using constant steepness waves when generating transfer functions. See Section 16.7.2.3 of ISO 19902.
Geoff
Answer Verified By: Kasiphon Kurojjanawong
Hi Wen Meng,
I mean that we use 'Spectral Analysis (Frequency Domain Analysis)' which it shall be decomposition by using Fourier series i.e. sin, cos wave.
According to Morison equation based on deterministic wave,
F (t) = Fi (t) + Fd (f)
when,
Inertia Force Fi (t) = Cm*Ai*a(t)
Drag Force Fd (t) = Cd*Ad* [u(t)]^2
Then,
F (t) = Cm*Ai*a(t) + Cd*Ad* [u(t)]^2
Then, you will see that even deterministic wave force is still "Nonlinear on the drag force" which is not valid for linear superposition according to Spectral Theory.
I understand that most of program tried to linearisation of drag term by proposing some factor rather than keep square term of wave particle velocity i.e. changing [u(t)]^2 term to be A*u(t) which A might be sqrt (8/pi)*urms recommended by Borgman (1967).
Now, I am curious how SACS performed drag linearisation for Spectral Analysis (Frequency Domain Analysis)