Current modeling in Hydrodyn

Wind & Water General
1

Hello everybody,

I want to perform some tests with current. I’m a bit confused on how should I configure my hidrodyn file. On the one hand, it seems that to be able to obtain the current induced viscous-drag forces, I need to use strip-theory (wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf, chapter 4.3.4). On the other hand, in an answer given here:

Re: OC4 semi submersible
Postby Jason.Jonkman » Thu Feb 14, 2019 2:02 pm

Dear Florence.

The PropPot flag in HydroDyn only effects the strip-theory solution and has no effect on the potential-flow solution. Setting PropPot = TRUE for a member disables hydrodynamic terms of the strip-theory solution (keeping only the viscous-drag and ballasting terms for that member). Setting PropPot = FALSE for a member enables all hydrodynamic terms of the strip-theory solution (keeping the inertia, added mass, viscous, and buoyancy terms for that member). If the potential-flow solution is enabled, its calculations are not effected by the PropPot flag. The PropPot flag is useful in hybrid potential-flow plus strip-theory solutions to ensure that hydrodynamic terms like inertia and added mass are not double-counted in both solutions.

For steady flow past a fixed cylinder, the only transverse load is the viscous drag term, so, the transverse load should be unaffected by the PropPot flag and/or the enabling/disabling of the potential-flow solution.

I hope that clarifies things.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

it seems that I can get the current-induced forces with the simple potential flow theory, without needing to enable the strip-theory for a few members… In fact, I run some simulation without using strip theory and the current definitely seems to have played a role. I wanted to understand if I can trust such results or if I need to turn on strip theory and repeat it. Here’s an image of my test:

aaaaa.JPG

(both current and waves direction are aligned with xx)

2

Dear Tomas,

Strip theory must be enabled with viscous drag for current-induced loads to be calculated by HydroDyn. Perhaps the forum post you referenced is not written clearly, but my statement in that post is consistent with the HydroDyn documentation.

You haven’t stated how your HydroDyn input file is set up, but seeing that current is affecting your solution, I would guess that you’ve enabled the strip theory solution with viscous drag enabled in your HydroDyn model.

Best regards,

3

Dear Jason,

All my PropPot flags are set as TRUE, thus I would conclude that strip theory is not enabled at all… Anyways, I’ll update my hydrodyn file so that you may take a look at it and possibly figure out what’s going on. The updated file corresponds to the one used to obtain the blue “CURRENT” line in the initial post on this discussion.

Thanks for the help,

Tomás

P.S. I was not able to upload the hydrodyn file in the attachment option (error hhtp is displayed.). Then, sorry for not uploading the file correctly formatted… Hope you can read it

------- HydroDyn v2.03.* Input File --------------------------------------------
NREL 5.0 MW offshore baseline floating platform HydroDyn input properties for the OC4 Semi-submersible.
False Echo - Echo the input file data (flag)
---------------------- ENVIRONMENTAL CONDITIONS --------------------------------
1025 WtrDens - Water density (kg/m^3)
200 WtrDpth - Water depth (meters)
0 MSL2SWL - Offset between still-water level and mean sea level (meters) [positive upward; unused when WaveMod = 6; must be zero if PotMod=1 or 2]
---------------------- WAVES ---------------------------------------------------
0 WaveMod - Incident wave kinematics model {0: none=still water, 1: regular (periodic), 1P#: regular with user-specified phase, 2: JONSWAP/Pierson-Moskowitz spectrum (irregular), 3: White noise spectrum (irregular), 4: user-defined spectrum from routine UserWaveSpctrm (irregular), 5: Externally generated wave-elevation time series, 6: Externally generated full wave-kinematics time series [option 6 is invalid for PotMod/=0]} (switch)
0 WaveStMod - Model for stretching incident wave kinematics to instantaneous free surface {0: none=no stretching, 1: vertical stretching, 2: extrapolation stretching, 3: Wheeler stretching} (switch) [unused when WaveMod=0 or when PotMod/=0]
4600 WaveTMax - Analysis time for incident wave calculations (sec) [unused when WaveMod=0; determines WaveDOmega=2Pi/WaveTMax in the IFFT]
0.2 WaveDT - Time step for incident wave calculations (sec) [unused when WaveMod=0; 0.1<=WaveDT<=1.0 recommended; determines WaveOmegaMax=Pi/WaveDT in the IFFT]
6 WaveHs - Significant wave height of incident waves (meters) [used only when WaveMod=1, 2, or 3]
10 WaveTp - Peak-spectral period of incident waves (sec) [used only when WaveMod=1 or 2]
2.87 WavePkShp - Peak-shape parameter of incident wave spectrum (-) or DEFAULT (string) [used only when WaveMod=2; use 1.0 for Pierson-Moskowitz]
0.314159 WvLowCOff - Low cut-off frequency or lower frequency limit of the wave spectrum beyond which the wave spectrum is zeroed (rad/s) [unused when WaveMod=0, 1, or 6]
1.570796 WvHiCOff - High cut-off frequency or upper frequency limit of the wave spectrum beyond which the wave spectrum is zeroed (rad/s) [unused when WaveMod=0, 1, or 6]
0 WaveDir - Incident wave propagation heading direction (degrees) [unused when WaveMod=0 or 6]
0 WaveDirMod - Directional spreading function {0: none, 1: COS2S} (-) [only used when WaveMod=2,3, or 4]
1 WaveDirSpread - Wave direction spreading coefficient ( > 0 ) (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1]
1 WaveNDir - Number of wave directions (-) [only used when WaveMod=2,3, or 4 and WaveDirMod=1; odd number only]
0 WaveDirRange - Range of wave directions (full range: WaveDir +/- 1/2*WaveDirRange) (degrees) [only used when WaveMod=2,3,or 4 and WaveDirMod=1]
123456789 WaveSeed(1) - First random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
1011121314 WaveSeed(2) - Second random seed of incident waves [-2147483648 to 2147483647] (-) [unused when WaveMod=0, 5, or 6]
FALSE WaveNDAmp - Flag for normally distributed amplitudes (flag) [only used when WaveMod=2, 3, or 4]
“” WvKinFile - Root name of externally generated wave data file(s) (quoted string) [used only when WaveMod=5 or 6]
1 NWaveElev - Number of points where the incident wave elevations can be computed (-) [maximum of 9 output locations]
0 WaveElevxi - List of xi-coordinates for points where the incident wave elevations can be output (meters) [NWaveElev points, separated by commas or white space; usused if NWaveElev = 0]
0 WaveElevyi - List of yi-coordinates for points where the incident wave elevations can be output (meters) [NWaveElev points, separated by commas or white space; usused if NWaveElev = 0]
---------------------- 2ND-ORDER WAVES ----------------------------------------- [unused with WaveMod=0 or 6]
FALSE WvDiffQTF - Full difference-frequency 2nd-order wave kinematics (flag)
FALSE WvSumQTF - Full summation-frequency 2nd-order wave kinematics (flag)
0 WvLowCOffD - Low frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
1.256637 WvHiCOffD - High frequency cutoff used in the difference-frequencies (rad/s) [Only used with a difference-frequency method]
0.618319 WvLowCOffS - Low frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
3.141593 WvHiCOffS - High frequency cutoff used in the summation-frequencies (rad/s) [Only used with a summation-frequency method]
---------------------- CURRENT ------------------------------------------------- [unused with WaveMod=6]
1 CurrMod - Current profile model {0: none=no current, 1: standard, 2: user-defined from routine UserCurrent} (switch)
0.5 CurrSSV0 - Sub-surface current velocity at still water level (m/s) [used only when CurrMod=1]
0 CurrSSDir - Sub-surface current heading direction (degrees) or DEFAULT (string) [used only when CurrMod=1]
20 CurrNSRef - Near-surface current reference depth (meters) [used only when CurrMod=1]
0 CurrNSV0 - Near-surface current velocity at still water level (m/s) [used only when CurrMod=1]
0 CurrNSDir - Near-surface current heading direction (degrees) [used only when CurrMod=1]
0 CurrDIV - Depth-independent current velocity (m/s) [used only when CurrMod=1]
0 CurrDIDir - Depth-independent current heading direction (degrees) [used only when CurrMod=1]
---------------------- FLOATING PLATFORM --------------------------------------- [unused with WaveMod=6]
1 PotMod - Potential-flow model {0: none=no potential flow, 1: frequency-to-time-domain transforms based on WAMIT output, 2: fluid-impulse theory (FIT)} (switch)
“…/5MW_Baseline/HydroData/marin_semi” PotFile - Root name of potential-flow model data; WAMIT output files containing the linear, nondimensionalized, hydrostatic restoring matrix (.hst), frequency-dependent hydrodynamic added mass matrix and damping matrix (.1), and frequency- and direction-dependent wave excitation force vector per unit wave amplitude (.3) (quoted string) [MAKE SURE THE FREQUENCIES INHERENT IN THESE WAMIT FILES SPAN THE PHYSICALLY-SIGNIFICANT RANGE OF FREQUENCIES FOR THE GIVEN PLATFORM; THEY MUST CONTAIN THE ZERO- AND INFINITE-FREQUENCY LIMITS!]
1 WAMITULEN - Characteristic body length scale used to redimensionalize WAMIT output (meters) [only used when PotMod=1]
13917 PtfmVol0 - Displaced volume of water when the platform is in its undisplaced position (m^3) [only used when PotMod=1; USE THE SAME VALUE COMPUTED BY WAMIT AS OUTPUT IN THE .OUT FILE!]
0 PtfmCOBxt - The xt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
0 PtfmCOByt - The yt offset of the center of buoyancy (COB) from the platform reference point (meters) [only used when PotMod=1]
1 RdtnMod - Radiation memory-effect model {0: no memory-effect calculation, 1: convolution, 2: state-space} (switch) [only used when PotMod=1; STATE-SPACE REQUIRES *.ss INPUT FILE]
60 RdtnTMax - Analysis time for wave radiation kernel calculations (sec) [only used when PotMod=1; determines RdtnDOmega=Pi/RdtnTMax in the cosine transform; MAKE SURE THIS IS LONG ENOUGH FOR THE RADIATION IMPULSE RESPONSE FUNCTIONS TO DECAY TO NEAR-ZERO FOR THE GIVEN PLATFORM!]
0.0125 RdtnDT - Time step for wave radiation kernel calculations (sec) [only used when PotMod=1; DT<=RdtnDT<=0.1 recommended; determines RdtnOmegaMax=Pi/RdtnDT in the cosine transform]
---------------------- 2ND-ORDER FLOATING PLATFORM FORCES ---------------------- [unused with WaveMod=0 or 6, or PotMod=0 or 2]
0 MnDrift - Mean-drift 2nd-order forces computed {0: None; [7, 8, 9, 10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero]
0 NewmanApp - Mean- and slow-drift 2nd-order forces computed with Newman’s approximation {0: None; [7, 8, 9, 10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero. Used only when WaveDirMod=0]
12 DiffQTF - Full difference-frequency 2nd-order forces computed with full QTF {0: None; [10, 11, or 12]: WAMIT file to use} [Only one of MnDrift, NewmanApp, or DiffQTF can be non-zero]
12 SumQTF - Full summation -frequency 2nd-order forces computed with full QTF {0: None; [10, 11, or 12]: WAMIT file to use}
---------------------- FLOATING PLATFORM FORCE FLAGS -------------------------- [unused with WaveMod=6]
True PtfmSgF - Platform horizontal surge translation force (flag) or DEFAULT
True PtfmSwF - Platform horizontal sway translation force (flag) or DEFAULT
True PtfmHvF - Platform vertical heave translation force (flag) or DEFAULT
True PtfmRF - Platform roll tilt rotation force (flag) or DEFAULT
True PtfmPF - Platform pitch tilt rotation force (flag) or DEFAULT
True PtfmYF - Platform yaw rotation force (flag) or DEFAULT
---------------------- PLATFORM ADDITIONAL STIFFNESS AND DAMPING --------------
0 0 0 0 0 0 AddF0 - Additional preload (N, N-m)
0 0 0 0 0 0 AddCLin - Additional linear stiffness (N/m, N/rad, N-m/m, N-m/rad)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 1451298897 0 0
0 0 0 0 1451298897 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBLin - Additional linear damping(N/(m/s), N/(rad/s), N-m/(m/s), N-m/(rad/s))
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0 AddBQuad - Additional quadratic drag(N/(m/s)^2, N/(rad/s)^2, N-m(m/s)^2, N-m/(rad/s)^2)
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
0 0 0 0 0 0
---------------------- 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
---------------------- 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
12 9.23760 22.00000 10.00000 1 0
13 -23.67130 3.00000 10.00000 1 0
14 -23.67130 -3.00000 10.00000 1 0
15 9.23760 -22.00000 10.00000 1 0
16 14.43375 -19.00000 10.00000 1 0
17 14.43375 19.00000 10.00000 1 0
18 4.04145 19.00000 -17.00000 1 0
19 -18.47520 6.00000 -17.00000 1 0
20 -18.47520 -6.00000 -17.00000 1 0
21 4.04145 -19.00000 -17.00000 1 0
22 14.43375 -13.00000 -17.00000 1 0
23 14.43375 13.00000 -17.00000 1 0
24 1.62500 2.81500 10.00000 1 0
25 11.43376 19.80385 10.00000 1 0
26 -3.25000 0.00000 10.00000 1 0
27 -22.87000 0.00000 10.00000 1 0
28 1.62500 -2.81500 10.00000 1 0
29 11.43376 -19.80385 10.00000 1 0
30 1.62500 2.81500 -17.00000 1 0
31 8.43376 14.60770 -17.00000 1 0
32 -3.25000 0.00000 -17.00000 1 0
33 -16.87000 0.00000 -17.00000 1 0
34 1.62500 -2.81500 -17.00000 1 0
35 8.43376 -14.60770 -17.00000 1 0
36 1.62500 2.81500 -16.20000 1 0
37 11.43376 19.80385 9.13000 1 0
38 -3.25000 0.00000 -16.20000 1 0
39 -22.87000 0.00000 9.13000 1 0
40 1.62500 -2.81500 -16.20000 1 0
41 11.43376 -19.80385 9.13000 1 0
42 14.43376 25.00000 -19.94000 1 0
43 -28.86751 0.00000 -19.94000 1 0
44 14.43376 -25.00000 -19.94000 1 0
---------------------- MEMBER CROSS-SECTION PROPERTIES -------------------------
4 NPropSets - Number of member property sets (-)
PropSetID PropD PropThck
(-) (m) (m)
1 6.50000 0.03000 ! Main Column
2 12.00000 0.06000 ! Upper Columns
3 24.00000 0.06000 ! Base Columns
4 1.60000 0.01750 ! Pontoons
---------------------- SIMPLE HYDRODYNAMIC COEFFICIENTS (model 1) --------------
SimplCd SimplCdMG SimplCa SimplCaMG SimplCp SimplCpMG SimplAxCa SimplAxCaMG SimplAxCp SimplAxCpMG
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
0.00 0.00 0.00 0.00 1.00 1.00 0.00 0.00 1.00 1.00
---------------------- DEPTH-BASED HYDRODYNAMIC COEFFICIENTS (model 2) ---------
0 NCoefDpth - Number of depth-dependent coefficients (-)
Dpth DpthCd DpthCdMG DpthCa DpthCaMG DpthCp DpthCpMG DpthAxCa DpthAxCaMG DpthAxCp DpthAxCpMG
(m) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-)
---------------------- MEMBER-BASED HYDRODYNAMIC COEFFICIENTS (model 3) --------
25 NCoefMembers - Number of member-based coefficients (-)
MemberID MemberCd1 MemberCd2 MemberCdMG1 MemberCdMG2 MemberCa1 MemberCa2 MemberCaMG1 MemberCaMG2 MemberCp1 MemberCp2 MemberCpMG1 MemberCpMG2 MemberAxCa1 MemberAxCa2 MemberAxCaMG1 MemberAxCaMG2 MemberAxCp1 MemberAxCp2 MemberAxCpMG1 MemberAxCpMG2
(-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) (-) ! Main Column
1 0.56 0.56 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Upper Column 1
2 0.61 0.61 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Upper Column 2
3 0.61 0.61 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Upper Column 3
4 0.61 0.61 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Base Column 1
5 0.68 0.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Base Column 2
6 0.68 0.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Base Column 3
7 0.68 0.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Base column cap 1
23 0.68 0.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Base column cap 2
24 0.68 0.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Base column cap 3
25 0.68 0.68 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Delta Pontoon, Upper 1
8 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Delta Pontoon, Upper 2
9 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Delta Pontoon, Upper 3
10 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Delta Pontoon, Lower 1
11 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Delta Pontoon, Lower 2
12 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Delta Pontoon, Lower 3
13 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Y Pontoon, Upper 1
14 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Y Pontoon, Upper 2
15 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Y Pontoon, Upper 3
16 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Y Pontoon, Lower 1
17 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Y Pontoon, Lower 2
18 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Y Pontoon, Lower 3
19 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Cross Brace 1
20 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Cross Brace 2
21 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ! Cross Brace 3
22 0.63 0.63 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
-------------------- MEMBERS -------------------------------------------------
25 NMembers - Number of members (-)
MemberID MJointID1 MJointID2 MPropSetID1 MPropSetID2 MDivSize MCoefMod PropPot [MCoefMod=1: use simple coeff table, 2: use depth-based coeff table, 3: use member-based coeff table] [ PropPot/=0 if member is modeled with potential-flow theory]
(-) (-) (-) (-) (-) (m) (switch) (flag)
1 1 2 1 1 1.0000 3 TRUE ! Main Column
2 3 4 2 2 1.0000 3 TRUE ! Upper Column 1
3 5 6 2 2 1.0000 3 TRUE ! Upper Column 2
4 7 8 2 2 1.0000 3 TRUE ! Upper Column 3
5 42 3 3 3 1.0000 3 TRUE ! Base Column 1
6 43 5 3 3 1.0000 3 TRUE ! Base Column 2
7 44 7 3 3 1.0000 3 TRUE ! Base Column 3
23 9 42 3 3 1.0000 3 TRUE ! Base column cap 1
24 10 43 3 3 1.0000 3 TRUE ! Base column cap 2
25 11 44 3 3 1.0000 3 TRUE ! Base column cap 3
8 12 13 4 4 1.0000 3 TRUE ! Delta Pontoon, Upper 1
9 14 15 4 4 1.0000 3 TRUE ! Delta Pontoon, Upper 2
10 16 17 4 4 1.0000 3 TRUE ! Delta Pontoon, Upper 3
11 18 19 4 4 1.0000 3 TRUE ! Delta Pontoon, Lower 1
12 20 21 4 4 1.0000 3 TRUE ! Delta Pontoon, Lower 2
13 22 23 4 4 1.0000 3 TRUE ! Delta Pontoon, Lower 3
14 24 25 4 4 1.0000 3 TRUE ! Y Pontoon, Upper 1
15 26 27 4 4 1.0000 3 TRUE ! Y Pontoon, Upper 2
16 28 29 4 4 1.0000 3 TRUE ! Y Pontoon, Upper 3
17 30 31 4 4 1.0000 3 TRUE ! Y Pontoon, Lower 1
18 32 33 4 4 1.0000 3 TRUE ! Y Pontoon, Lower 2
19 34 35 4 4 1.0000 3 TRUE ! Y Pontoon, Lower 3
20 36 37 4 4 1.0000 3 TRUE ! Cross Brace 1
21 38 39 4 4 1.0000 3 TRUE ! Cross Brace 2
22 40 41 4 4 1.0000 3 TRUE ! Cross Brace 3
---------------------- FILLED MEMBERS ------------------------------------------
2 NFillGroups - Number of filled member groups (-) [If FillDens = DEFAULT, then FillDens = WtrDens; FillFSLoc is related to MSL2SWL]
FillNumM FillMList FillFSLoc FillDens
(-) (-) (m) (kg/m^3)
3 2 3 4 -6.17 1025
3 5 6 7 -14.89 1025
---------------------- MARINE GROWTH -------------------------------------------
0 NMGDepths - Number of marine-growth depths specified (-)
MGDpth MGThck MGDens
(m) (m) (kg/m^3)
---------------------- MEMBER OUTPUT LIST --------------------------------------
0 NMOutputs - Number of member outputs (-) [must be < 10]
MemberID NOutLoc NodeLocs [NOutLoc < 10; node locations are normalized distance from the start of the member, and must be >=0 and <= 1] [unused if NMOutputs=0]
(-) (-) (-)
---------------------- JOINT OUTPUT LIST ---------------------------------------
0 NJOutputs - Number of joint outputs [Must be < 10]
0 JOutLst - List of JointIDs which are to be output (-)[unused if NJOutputs=0]
---------------------- OUTPUT --------------------------------------------------
True HDSum - Output a summary file [flag]
False OutAll - Output all user-specified member and joint loads (only at each member end, not interior locations) [flag]
2 OutSwtch - Output requested channels to: [1=Hydrodyn.out, 2=GlueCode.out, 3=both files]
“ES11.4e2” OutFmt - Output format for numerical results (quoted string) [not checked for validity!]
“A11” OutSFmt - Output format for header strings (quoted string) [not checked for validity!]
---------------------- OUTPUT CHANNELS -----------------------------------------
“Wave1Elev” - Wave elevation at the platform reference point (0, 0)
END of output channels and end of file. (the word “END” must appear in the first 3 columns of this line)

4

Dear Tomas,

The PropPot flag disables all terms in the strip theory formulation, except for viscous drag, which is not otherwise accounted for in the potential-flow solution. So, your model does have viscous drag, which is why it is effected by current.

Best regards,

5

Dear Jason,

So if I wanted to disable the viscous effect at all, I would necessarily need to make NCoefMembers=0?

Best Regards,

Tomás Cortes

6

Dear Tomas,

If you only want to eliminate viscous effects, you should set all viscous drag coefficients (Cd) to zero in your HydroDyn input file.

If you want to eliminate the strip-theory solution altogether (resulting in potential-flow-only model), you should eliminate all joints and members from your HydroDyn input file (NJoints = NMembers = 0). Until recently, there was a small issue in HydroDyn preventing one from eliminating the strip-theory solution, but this issue has been resolved with a recent pull request into the dev branch of OpenFAST–see: github.com/OpenFAST/openfast/pull/258.

Best regards,