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Importing ANSYS AQWA diffraction results for multiple bodies (N = 3) into QBlade WAMIT format files
Hello everyone,
I am currently working on a hydrodynamic coupling workflow between ANSYS AQWA and QBlade on the floating offshore wind turbine supported by the Triceratops platform (contains 3 BLS) and seek guidance on the correct WAMIT-format representation for multi-body diffraction-radiation data.
Problem Description:
I performed a diffraction-radiation analysis in ANSYS AQWA for a FOWT supported by a triceratops platform consisting of three bodies (N = 3). From AQWA, I obtained the following hydrodynamic results:
- Radiation forces
- Added mass matrices
- Radiation damping matrices
- Sum-frequency QTF’s
- Difference-frequency QTF’s…these results are available for particular BLS in tabulated Excel form.
QBlade, however, requires hydrodynamic input strictly in WAMIT-compatible file formats. While QBlade example files and documentation primarily demonstrate single-body (N = 1) cases, my platform involves three hydrodynamically coupled bodies.
The objective is to correctly convert ANSYS AQWA diffraction–radiation results for a three-body floating system into WAMIT-format files that are fully compatible with QBlade’s hydrodynamic solver.
Kind regards,
Divyadeepsinh
Hi Divyadeepsinh,
you can find the documentation of the NBODY>1 feature here:
https://docs.qblade.org/src/user/turbine/substructure.html#nobody-1-feature
That said, the documentation is not very detailed, but all relevant keywords are explained. Regarding the required WAMIT formats you can checkout WAMIT’s User Manual: https://www.wamit.com/manual.htm
Generally, each body requires 6 DOF in all WAMIT files, so if you want to model 3 bodies you require 18 DOFs in each WAMIT file. The off diagonal entries define the interaction between the different bodies.
There is also a QBlade example existing. In the OC7 Phase I project, where we participated, a QBlade model was build using the NBODY>1 feature. You can find and download the OC7 Phase Ia Setup project file here: https://qblade.org/downloads/#Test_Data
When you open this project file it takes quite a bit of time until the hydrodynamic databases are build (QBlade is not hanging, just wait). You can then inspect LC6.X-LC8.X. These models contain the full WAMIT data for NBODY=3. You can also export the models and inspect the WAMIT files directly.
BR,
David
Thank you so much for your support.
There are further two question regarding WAMIT format.
Question 1.
For a hydrodynamic model involving multiple rigid bodies (N > 1), how should the potential flow input be provided to QBlade?
“Specifically, should a separate POT file be supplied for each individual body, or is it acceptable (and expected) to provide a single POT file that contains the hydrodynamic coefficients and interaction terms for all bodies in the multi-body system?”
Question 2.
In the case where a single POT file is used to represent a multi-body system (e.g., N = 3 bodies), how does QBlade internally distinguish and map the hydrodynamic data to the corresponding bodies and degrees of freedom?
“Is there a predefined data structure, particular code, indexing convention, or body-identification scheme within the POT file format that allows the software to correctly associate added mass, damping, excitation forces, and coupling terms with the respective bodies?”
Kind Regards,
Divyadeepsinh
Hi Divyadeepsinh,
If the purpose is to have the bodies interacting hyndrodynamically only a single set of files needs to be specified, now containing 6 DOFs per body, so in the case of 2 bodies 12 DOF’s are included in the pot file. As mentioned earlier, I recommend having a look at the example provided on the website. You can export the potential-flow files from that example and inspect them directly, which usually makes the setup much clearer.
The relevant keywords for QBlade are explained in the documentation here:
https://docs.qblade.org/src/user/turbine/substructure.html#nobody-1-feature
In short:
POT_NBODY_<X>: specifies the number of hydrodynamic bodies. Each body is associated with 6 DOF’s. By default, NBODY is equal to 1. To use NBODY>1, the potential flow files must contain the data for the additional DOF. If NBODY=3, then the data for 18 DOF’s is required.
POT_NBODY_NODES_<X>: specifies the list of joint id’s at which the hydrodynamic loads are applied. One joint per NBODY needs to be defined here. The first joint defined in the list maps to DOF’s 1-6, the 2nd joint to DOF’s 7-12 and so on…
For more details and context, I would strongly recommend going through the documentation section linked above.
BR,
David
Hello David,
As per our previous conversation ,currently i am working on a multi-body column platform (N=3) and am encountering two specific issues when trying to integrate hydrodynamic data into my simulation. I’ve outlined the scenarios below:
1. Hydrodynamic Stiffness Matrix (.hst) Error
Whenever I include the (.hst) file in my script to account for hydrostatic and gravitational stiffness, the software returns an error (please see Photo 1 for the specific error message).
2. Simulation Divergence (Without .hst)
To troubleshoot, I attempted to run the simulation using only the radiation (.rad) and excitation (.ext) files (excluding the .hst file). However, the simulation fails at the very first time step with a “Divergence Error” (shown in Photo 2).
I suspect there may be a formatting issue in how I’ve defined the multibody parameters for three bodies. I have attached:
- A screenshot of my script showing the recent changes.
- The Radiation (.rad) file I am using
Could you please verify if my multibody format is correct for N=3? Specifically, I am looking for guidance on whether the .hst matrix structure or the body definitions in the script might be triggering the divergence.
Kind Regards,
Divyadeepsinh
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Hi Divyadeepsinh,
It’s hard to tell exactly what’s going wrong with the limited information.
First, I see you’re using CE 2.0.9, and I’d strongly suggest updating to the latest release (CE 2.0.9.6).
The assertion error suggests the matrices aren’t initialized to the correct size, but your keywords look correct and are in line with the OC7 example I mentioned previously.
Does this error also occur with the latest release of QBlade, or only when importing the .hst file?
Regarding the divergence of the structural model: this can have many causes—extreme inertia ratios, large forces, problematic constraints, insufficient temporal discretization, etc., just to name a few.
It’s not necessarily caused by the potential flow input files, but it could be if the resulting forces are unrealistically large.
Try simulating without any potential flow files, and try reducing the timestep, etc.
To debug your model you can also use the “debug colsole”, see:
https://docs.qblade.org/src/user/guigraph/guigraph.html#debug-console-dialog
If you’re still getting the assertion error, I’d be interested in a .qpr file that can reproduce this behavior (if you can share it) so we can fix this issue on our side. In general, .hst files with 6xPOT_NBODY DoF’s can be loaded, I just tested this.
Best regards,
David
