I have a question about the WaveMod = 4 feature of HydroDyn.
My question is when I set WaveMod = 4, why FAST is asking me to set 3 for WaveStMod (Wheeler stretching) ?
My understanding is, when I set WaveMod = 4 then FAST does not calculate wave kinematics internally and I have to provide wave elevation and wave kinematics in these two files *_surface.txt and *_kinematics.txt.
If I have calculated wave kinematics by linear theory with including wheeler stretching ( defined in *_kinematics.txt), Why do I have to set WaveStMod at 3?
Is FAST going to apply wheeler stretching to the wave kinematics I have defined in *_kinematics.txt ?!
You understanding of how external wave kinematics data is read into HydroDyn when WaveMod = 4 is correct.
The reason why WaveStMod must be set to 3 when WaveMod = 4 is as follows. WaveStMod is used within HydroDyn for two purposes: (1) to determine how to stretch the wave kinematics to the instantaneous free surface {0: none=no stretching, 1: vertical stretching, 2: extrapolation stretching, 3: Wheeler stretching} and (2) to determine whether Morison’s equation applies up to the still water level (WaveStMod = 0) or to the instantaneous free surface (WaveStMod = 1, 2, or 3). As you rightly pointed out, purpose (1) does not apply when the wave kinematics are not created internal to HydroDyn (when WaveMod = 4). However, purpose (2) still applies. The WaveMod = 4 feature was added so that HydroDyn could read data generated externally by GH Bladed or an equivalent code. Because GH Bladed applies Wheeler Stretching, the option WaveStMod = 3 was set as a requirement when WaveMod = 4. In the future, we will modify HydroDyn so that external wave data in different formats is permitted.
In this post, you mentioned that the WaveMod = 4 feature of HydroDyn is only available for fixed-bottom offshore turbines. I wonder if this feature could be used for floating platform or not at this time. Also I want to know if it is not possible, how can FAST read wave data when we want to model the floating platform.
Thanks for the consideration.
The HydroDyn module of FAST has not yet been upgraded to allow it to read externally provided wave data for floating platforms. This is a feature on our near-term to-do list, but will likely not be available until sometime in 2014.
Dear Jason,
I use fast to simulate OC4 jacket type.
Fist, I use equivalent monopile method to replace OC4 jacket structure,
and then I download wind.nrel.gov/public/jjonkman/FAST/GHWvFile,
but the Stream Function waves data fit for 20m water level deep.
How can I use it for OC4 project (50m water level deep)?
In OC4 Phase I (jacket analysis), wave kinematics where not shared among participants. Instead, you can use the HydroDyn-generated wave kinematics based on the given sea-state specification.
I am simulating some scenearios with WavePkShape set to ‘DEFAULT’ and WaveMod=2, but what is that default value?
I need to know that Jonswap Parameter (Peakness factor) in order to get the spectrums of different parameters, does anybody know it?
The DEFAULT value of the peak shape parameter input in HydroDyn (WavePkShp ) is the value recommended in the IEC 61400-3 Annex B, derived based on the peak-spectral period and signficant wave height. It is also documented in Eq. (2-19) in my PhD thesis-turned NREL report available from here: nrel.gov/docs/fy08osti/41958.pdf.
Dear Jason,
How can I create the Stream Function (50m water level deep) waves data into fast for OC4 Phase I (jacket analysis)?
I use GH-bladed software but I only get water particle x,z direction velocity and acceleration.
I don’t know how to produce dynamic pressure.
If HydroDyn-generated wave kinematics can base on the given sea-state specification to produce Stream Function, can you tell me how to do it or maybe you can offer other method?
Thanks for your help.
The current version of HydroDyn (v2.01.01c-gjh) is not capable of generating Stream Function waves. In FAST v7, we had the option to import externally generated wave kinematics time histories into HydroDyn for use in the FAST simulation. While this feature is not yet available in HydroDyn v2.01.01c-gjh for FAST v8, it is on our near-term to-do list, but we will not likely release it until early 2015. If you cannot wait that long, I suggest modifying the source code to read the data in yourself.
The dynamic pressure-generated hydrodynamic loads are likely negligle for the OC4-jacket, so, you can probably safely assume that the dynamic pressure is zero.
Dear Jason,
Thanks to your reply.
I use TurbSim_v1.06.00 to generate NTM(Kaimal):V[size=80]hub[/size]=11.4m/s or 18m/s but the FAST software PDF result is different from LC2.4b and LC3.4b.
(reference: OC4 Phase I - Results of Coupled Simulations of an Offshore Wind Turbine with Jacket Support Structure)
Does the randseed number or other cause result in?
Does OC4 already agree to release wind condition to refer?
The wind data used in the OC4 Phase I project was supplied by DTU Wind Energy and is available on the OC4 SharePoint site, although some participants chose to generate there own wind data.
In your results, RootFyc1 and RootMzc1 look reasonable, but RootFxc1 is off. It doesn’t really look like the problem is a difference in turbulence though, because the mean value of RootFxc1 is very different (250 kN versus 350 kN). Perhaps there is another difference in your input file settings (aerodynamic or control related, perhaps?).
Dear Jason,
I use FAST_v7.01.00a-bjj_AeroDyn_v13.00.01a-bjj_BladedDLLInterface.exe to validate OC4 results and I have some questions as follows:
(1) My DEL value 428kN in LC2.4b is larger than American Bureau of Shipping (ABS) result .
How do I to revise it?
Is it correct about PDF-Tower-top fore -aft shear force with PDF-Fore-aft shear force are the same value?
(2)I get a larger value (-18750kN) to compare with ABS result in LC2.1a and my MudFz is the constant value (-15500kN) other than a curve in LC2.3a.
How do I to revise it?
(3)In LC5.6, mudline overturning moment value is smaller than ABS result but I get the same result about Mudline shear force.
I use plane progressive (regular) or stream function wave model in fast file and then I get the same smaller results than ABS result .
My x-dir velovity and acceleration in the stream function get from GH bladed software. (y,z-dir velovity , acceleration and dynamic pressure that I suppose to set zero)
I’m sorry, but I don’t really understand any of your questions. Please clarify what you are asking.
Are you using the latest version of FAST v8 (released July 2014) with HydroDyn and SubDyn? Have you modified this version of HydroDyn to read in Stream Function waves from GH Bladed?
OC4 LC2.4b involves aerodynamic loads on a rigid structure without waves. So, I would expect the same shear force at the tower-top and tower base. If you’re matching the PDF, you should be able to match the DEL.
I don’t think I can answer this question because I don’t know how you are modeling the OC4-jacket in FAST v7, which is only capable of modeling wind turbines atop a monopile or floating platform. Are you modeling the OC4-jacket as an effective monopile? In a monopile, the vertical force at the mudline won’t change as waves pass, whereas they will change if the hydrodynamics are properly modeled on a jacket structure.
Again, I don’t think I can answer this question because I don’t know how you are modeling the OC4-jacket in FAST v7.
FAST v8 is capable of modeling wind turbines atop jacket substructures, and we’ve verified the FAST v8 solution against the results of the OC4 Phase I (jacket). Perhaps its time to upgrade from FAST v7 to FAST v8.
Dear Jason,
Thanks to your reply.
(1) I use Mcrunch software to match the DEL in OC4 LC2.4b again.
I set SN slope to be “4” and My DEL value 180kN is close to American Bureau of Shipping (ABS) result (about 155kN) .
Is it correct?
(2)&(3) I am modeling the OC4-jacket as an effective monopile in FAST v7.
In LC5.6, mudline overturning moment value is smaller than ABS result but I get the same result about Mudline shear force.
This question confuses me for a long time.
I will use FAST v8 to model wind turbines atop jacket substructures in the future.
If I want to modify this version of HydroDyn to read in Stream Function waves from GH Bladed that I must compile new fast.exe.
Is it correct?
(1) I didn’t do the data processing for OC4 Phase I, so, I forwarded your question to Wojciech Popko of Fraunhofer IWES who did do it. Comparing his MCrunch input file with yours, I see the following differences:
RF_Per = 3.1536 - This comes from 20 years (in seconds) divided by an assumed 2E8 cycles
UCMult = 0.5 - Weight unclosed cycles as half a cycle
BinWidth = 5 - I don’t think this matters because you aren’t binning cycles
TypeLMF = 0 - Assume zero mean
LUlt = 1.00E+100 - The DEL asymptotically approach a fixed value for large values of LUlt
(2) You can get the same shear force but a different overturning moment if the moment arms are different. My guess is this is related to the assumptions used to make the effective monopole.
(3) It is not currently possible in FAST v8 to import external wave kinematic time histories for use within HydroDyn (e.g., Stream Function waves from GH Bladed). This is on our to-do list for next year. If you need that feature be we get to it, you’ll need to modify the source code yourself.
Dear Jason,
I am confused about OC4 Phase I jacket-type WT load case 4.5 question recently.
I can get similar results with other research team in other load case. (Besides load case 4.5)
I have attached the simulation file, could you please take a look of it?
Load case 4.5
Enabled DOF: Support structure
Wind Conditon: NO air
Wave Condition: Irregular Airy: Hs=6m, Tp=10s, PM wave spectrum
Does B59Ax is M5N1FKZe in the SubDyn?
The unit of B59Ax in PSD is dB/Hz not (kN)^2/Hz. Is it correct?