# How to conduct multi-body time-domain coupled analysis

Dear everyone,
I am a user of OpenFast, and recently, I’ve discovered the capability for multi-body coupled hydrodynamic analysis within HydroDyn, specifically the example with NBODYMOD=1. However, in my computations, I’ve only obtained forces and moments without any structural motion response.Therefore, I have the following questions:
1.Is HydroDyn decoupled when solving wave forces for each degree of freedom?
2.How to achieve motion solving for multiple floating bodies?

Dear @Chuancheng.Shi,

I’m not sure I fully understand your questions, but when HydroDyn is coupled within OpenFAST and you have multiple potential-flow bodies enabled in HydroDyn (`NBody` > 1) and you are using SubDyn to model the structural flexibility of the floater, each potential-flow body can have different motion.

Best regards,

Dear Jason,

Thank you very much for your response, and I’m sorry I didn’t clarify my question well. I intend to analyze the motion of two vessels in a moored condition. Both vessels are considered rigid bodies and are separated by a certain distance. I’ve accounted for the coupled effects in the frequency domain, which implies NBODYMOD=1, NBODY=2, with a 12x12 matrix for the stiffness and damping coefficients of the coupled system. While HydroDyn enables hydrodynamic analysis, can ElastoDyn be used for coupled analysis when solving for motion? Or should I model one of the vessels using the SubDyn module?

Best regards,

Dear @Chuancheng.Shi,

ElastoDyn is only coupled to HydroDyn within OpenFAST through a single 6 degree of freedom rigid body.

SubDyn is coupled to HydroDyn within OpenFAST through many degrees of freedom (depending on how many Craig-Bampton modes are enabled in SubDyn), and while SubDyn supports multiple rigid bodies (called rigid-link members in SubDyn), they must be interconnected by other members such as beams so as to avoid rigid-body modes within SubDyn. (That is, SubDyn can only support elastic modes, not rigid-body modes.)

A similar question was discussed in the following forum topic: Wind wave hybrid systems.

Best regards,

Dear Jason,

Thank you very much for your response. I’ve gone through the forum topic, and I believe our shared issue revolves around how to set up an ElastoDyn solver for each BODY. The consistent motion demonstrated in that forum topic for B1, B2, and B3 is due to the configuration of only one ElastoDyn input file, which means the motion solving was performed for only one BODY. Therefore, I think the question lies in how to configure two ElastoDyn files for two separate BODY instances (used to define parameters like mass, moments of inertia, etc.) or rather, does OpenFAST allow the input of multiple ElastoDyn files?

Best regards,

Dear @Chuancheng.Shi,

OpenFAST is not currently set up to support multiple instances of ElastoDyn, although we intend to kick-off work soon to make this happen in support of modeling the aero-elastics of multi-rotor systems.

Currently, OpenFAST supports multiple hydrodynamic potential-flow bodies when the motion of each body is represented through the SubDyn module (but this requires elastic coupling between the bodies as opposed to independent rigid-body motions).

Best regards,

Dear Jason,

Thank you very much for your response. I don’t have any other questions now. I believe if I want to achieve motion solving for multiple floating bodies (rigid bodies), I’ll need to make some modifications to the source code, and I’m unsure if this task can be accomplished, as it might take quite a long time and be extremely challenging. Once again, I appreciate your assistance with my questions.

Best regards,

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