# Viscous damping on the floating spar buoy

Hello,
I want to better understand viscous damping in FAST (damping term proportional to the square of the velocity of the body with respect to the water). Is it implemented in Hydrodyn? if yes, How can I set or disable these forces?

Massimo Sirigu

Dear Massimo,

Yes, viscous drag is implemented in HydroDyn. Viscous drag is available locally across the structure in the strip-theory (Morison) formulation both distributed along members (by defining the member geometry and transverse drag coefficients) and concentrated at member ends (by defining the joint geometry and axial drag coefficients). Viscous drag is also available globally through the AddBQuad matrix. See the draft HydroDyn User’s Guide and Theory Manual for more information: nrel.gov/wind/nwtc/assets/d … Manual.pdf.

Best regards,

dear Jason,

1. I’m afraid I did not understand the term Cp.
quoting from Hydrodyn manual:
“The inertial force component of Morison’s equation is associated with the fluid acceleration, and includes two terms. The first is the Froude-Kriloff term associated with the unsteady pressure field generated by the undisturbed waves. This term can be controlled using the CP coefficient, with a value of 1.0 being the default”.
Does it mean that if I don’t generate waves (wavemod=0, still water) Cp is not used?

2. another question: if I want to neglect the damping term I only need to put to zero the Cd term?

3. if I want to discretize better the nodes along with the spar, how can I do it? for example, I want to add a node in -60m. How can I add this node in hydrodyn?

---------------------- MEMBER JOINTS -------------------------------------------
4 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 -120.00000 1 0
2 0.00000 0.00000 -12.00000 1 0
3 0.00000 0.00000 -4.00000 1 0
4 0.00000 0.00000 10.00000 1 0
---------------------- MEMBER CROSS-SECTION PROPERTIES -------------------------
2 NPropSets - Number of member property sets (-)
PropSetID PropD PropThck
(-) (m) (m)
1 9.40000 0.00010
2 6.50000 0.00010

Dear Massimo,

For (1) and (2), my answer is yes. Cp is not used in the absence of waves and Cd will effect the damping.

For (3), the joints define the endpoints of members. You specify the discretization along the member via input MDivSize in the members table lower down in the HydroDyn input file. You could also add additional joints and members, but this would require many more edits to the input file.

Best regards,