Hybrid approach

Good afternoon,
I have questions concerning the correct way to use an hybrid approach using parts of Hydrodyn calculations to avoid missing or doubling counting effects. I have already asked questions on the topic but I would like to get stronger convictions. The case I want to treat is drag and radiation for a body under forced oscillations. I am joining a document because I needed to rely on the equations. My main concern is too be sure that the output radiation force is only including the convolution term and no added mass.
If you could please clarify this point?
Many thanks in advance for any help.
Best regards.
Florence Haudin.
questions_forum.pdf (175 KB)

Dear Florence,

The radiation load outputs from the HydroDyn module of FAST / OpenFAST (RdtnF* and RdtnM*) include the effects both from radiation damping (the convolution term) and impulsive added mass. While the two contributions are computed separately, they are summed before being output. You could, of course, modify the source code if you want to output the terms separately.

And yes, from your equations, the a_p vector refers to the platform acceleration (including both translational and rotational accelerations).

Best regards,

Hi Jason,
Thanks for your answer. Things are ok for me now.

I am wondering, still in the forced oscillations case, still in an hybrid approach in the WAMITInputsMod equal to 2 , how can I add axial drag on heave plates. As a summary :

  • I am taking radiation (added mass and convolution term added as explained in your previous post) and hydrostatic forces from hydrodyn potentiel flow calculations
  • I am calculating the transverse drag from a separate Morison calculation implemented on all members
  • I want to add axial drag at the heave plates

Is it possible to get from Hydrodyn only the 3 heave plates contributions from one of the output of Hydrodyn ? Playing on that section with 9.60 in that example being the tuned AxCd for the heave plate?
---------------------- AXIAL COEFFICIENTS --------------------------------------
2 NAxCoef - Number of axial coefficients (-)
AxCoefID AxCd AxCa AxCp
(-) (-) (-) (-)
1 0.00 0.00 1.00
2 9.60 0.00 1.00

Then these coefficient are distributed in this joints’ section :

---------------------- MEMBER JOINTS -------------------------------------------
44 NJoints - Number of joints (-) [must be exactly 0 or at least 2]
JointID Jointxi Jointyi Jointzi JointAxID JointOvrlp [JointOvrlp= 0: do nothing at joint, 1: eliminate overlaps by calculating super member]
(-) (m) (m) (m) (-) (switch)
1 0.00000 0.00000 -20.00000 1 0
2 0.00000 0.00000 10.00000 1 0
3 14.43376 25.00000 -14.00000 1 0
4 14.43376 25.00000 12.00000 1 0
5 -28.86751 0.00000 -14.00000 1 0
6 -28.86751 0.00000 12.00000 1 0
7 14.43376 -25.00000 -14.00000 1 0
8 14.43376 -25.00000 12.00000 1 0
9 14.43375 25.00000 -20.00000 2 0
10 -28.86750 0.00000 -20.00000 2 0
11 14.43375 -25.00000 -20.00000 2 0 …

Is it only possible to calculate axial drag at heave plates with WAMITInputsMod equal to 2, because I think that it is not possible to get any Morison terms calculations in that case? That the reason why we implemented an independant transverse drag calculation.

In hydrodyn manual it is precised " Axial viscous-drag loads will be calculated for all specified member joints. Axial added-mass, fluid-inertia, and static-pressure loads will only be calculated for member joints of members not modeled with potential flow (PropPot = FALSE). Axial loads are only calculated at user-specified joints." So if relevant to my case, I guess I need to precise this location, in the list of output I want to have ? Like this output :*Lumped Loads at Joints
JαFDxi, JαFDyi, JαFDzi(N), (N), (N)Viscous-drag force at Jα

Many thanks in advance for any help or suggestion.
Best regards.

Hi Florence,

The HydroDyn module itself is able to model a hybrid combination of potential flow and viscous drag on the heave plates, as your indicated. However, the standalone HydroDyn driver is currently limited and it is not possible through the driver to prescribe forced motion of the strip-theory (Morison) members through the driver. You’d have to modify the standalone HydroDyn driver source code to prescribe the motion of the joints that you want to calculate the viscous drag loads on. Alternatively, you can run HydroDyn coupled to FAST / OpenFAST to model the hybrid approach.

Best regards,

Thanks Jason,
So if I move to Openfast for this case, the calculation is possible with axial and transverse drag for the hybrid approach with a forced motion prescribed from a file.
Sorry in fact I had not understood before that moving to openfast instead of using hydrodyn in the standalone mode would solve the problem.
Best regards.

Dear Florence,

OpenFAST–without modification of the source code–does not have the ability to model prescribed motion either; I was simply implying that OpenFAST simulations using HydroDyn can include hybrid combinations of potential flow and viscous drag.

Some amount of source-code modification is required to model forced motion with a hybrid approach in HydroDyn.

Best regards,