Hi @sjoshi, as @BenLewis stayed, there’s a problem with forces. Total X force (Drag) should become steady after a few seconds. Fixed Z position of the hull shouldn’t be a problem for your velocity and Rn. If something as a Karman Vortex is raising behind the hull, it is not visible in your graphics.
By the way, @Maciek, I personally prefer complete screen captures because in that way part of the interface and settings are visible. I have a lot to learn before using the interface properly
At this stage it is premature to compare with existing and real data, but reaching that point there are many well known studies out there. Perhaps one of the most famous is the Delft Series (DSYHS), Delft University recently published all the data, including hull shapes used on tank testing for over 30 years of studies.
This document may be of interest: An extensive cfd analysis (using ANSYS CFX) of one of the Delft hulls.
The force plots indeed look quite suspicious - thank you for bringing it to notice.
I will dive in and look where the issue could be and update here.
@Maciek The visualization tactics of scaling the the important range of data values indeed do help convey a clearer message - I shall keep that in mind.
Finally I had time to catch up on this thread! Impressive work @sjoshi! Looking forward to the next iteration of the simulation results - it looks like you’re getting there. My 2 cents on the simulation:
Agreed with @Maciek, that the computational domain needs to be deeper to avoid interaction of the flow field with it (I like the expression “ridiculously big domains” ). However I doubt that this is the main reason behind the incorrect results.
@BenLewis: I think @sjoshi is using a local time-stepping method so numerically it’s possible that transient patterns appear towards the end, looking never really like a “steady state”. However I agree that the question is if what we see is physically correct - the drag and lift plots do look questionable. @gholami mentioned that a good way of assessing the quality of such simulation results is running the same simulation with different time-step length to make sure the results are independent of the time discretization. Probably too early right now but once the results look more trust-worthy
What about the mesh around the hull itself? On the image it looks quite coarse. Do the result fields close to the hull look mesh-independent? Could be worth looking into this and adding some refinement layers there.
@quequen: Thanks for linking to the Delft Series. Great data! This is definitely something we will look closer at!
As mentioned in the beginning of this thread I have little background in boat hull design. Therefore I am also very interested in learning more about the general approach to these design projects in terms of simulation. @quequen: You asked if the boat can move freely in this simulation. This method is in the product backlog and will be integrated moving forward. But how important is this kind of analysis within the design phase? Comparing it to other design processes and applying the old “Crouch, Walk, Run” principle, I would assume that this is rather something I do at the very end of a project as it’s quite computationally intensive and rely on faster steady-state, fixed-boat analysis for the main part. Would you agree or are the assumptions behind this analysis type too vast?
@dheiny, yes I agree: step by step. Regarding oscilating forces, the document I attached previously throw light in many issues about this kind of simulations. Page 20 and figures 16-17 show how forces oscilate for a while before converging and, as expected, this issue reduces when Fn increases (time-step is 0.02s). At page 28, fig. 23 shows that a fixed geometry throw good outcomes up to Fn=0.4 but higher Fn increases dynamic forces too much, therefore agreement with experiments is lost.
The problem with oscillating forces might have something to do with the geometry being bad. Here are a few snapshots showing the old geometry with selection of external and internal faces:
As one can see, there exist surface intersections between external and internal surfaces. I chose a simpler geometry that I closed from the top. Now, the geometry is free from surface intersections:
Please note that the actual values of these forces might not be accurate, since on the platform only 1 reference density is used for computing forces on the selected surfaces.
Very neat @sjoshi! @quequen and others: I think it’s worth putting together a webinar/tutorial for such kind of simulations to enable new users faster. I think especially the phase fraction concept / initialization of water/air makes this approach a bit tricky in the beginning. What is your preferred way of learning such new methods/softwares? Would be great to get some opinions on this - check out this post: How to best learn simulation?
@sjoshi, it looks great! Some details may still remain but I think you have a strog start point for this kind of simulations. @dheiny I agree, this simulation deserves some webinar/tutorials (perhaps 2 or 3, 15 minutes each?), including voice comments on how to perform the settings, particularly the phases issue. My choice is webinars or tutorials, and a good printed manual (that’s why I asked for a .pdf manual, instead of online manuals). Please remember that my geomety is still there if you want to play with it. I would now try some leeway angle (angle of attack), may be 3 or 4 degrees, bow to port, to catch appendages behavior and a more realistic X force.
regarding how to learn simulation best, we’re in the middle of collecting more feedback and a little survey to steer the type of learning material we want to provide in the near future. I added your opinion there as well!
I wanted now to follow @sjoshi’s work on the boat flow and set up one myself. Did you already upload your model to a SimScale project? If not could, you do it and share the project with me? We could see how far we come before asking @sjoshi for help ;).
@dheiny, I shared a geometry to support@simscale.com , can you pick it up from there?
If you send me your e-mail by private message I’ll share with you some other geometries too.
On my boat there is no leeway, wich is bad. A sailboat beating against the wind will heel (like mine), but will also sail at a small angle. There must be a small angle of attack for the appendages (keel and rudder) to produce some lift (“x” force). So the boat should be rotated around a vertical axis, on the xy plane, about 3 or 4 degrees counter-clockwise seeing from the top. Note that in my geometry “x” goes forward, “z” goes upwards and “y” goes sidewards.
One question regarding the CAD model: It’s in STEP format but the face topology rather reminds me of a faceted file format (e.g. STL or OBJ). Each triangle of the tessellated surface is considered a single face. I would expect that the STEP file defines the big faces (e.g. the deck or the hull) as one single NURBS surface. Would you mind sharing in what CAD system you modeled these? Because to me the STEP export behaves unexpectedly. It might be even better in that case to export it as STL if your CAD system does support such an export. If not, I am confident that we can also work with these files.
Looking forward to seeing some waves around that hull ;)!
@quequen, is there a chance that you can get a STEP file out of your CAD system that does not tessellate the surfaces but exports them as NURBS? If not, I’ll use a tool on my end to combine them again. But we might loose surface quality…
No way for me actually. I’m sending to you some .igs and .iges files by private message (Simscale Forum do not accept this kind of files so I can’t post them here).
Mhm, interesting that it exports STEP and IGES files like that. What CAD software are you using? I received the IGES files but they show the same effect. But we’re having a seat of a direct modeler here with which it should be possible to “stitch” those faceted faces together. In principle it would also work directly with the models you uploaded, but the mesher would need to deal with a lot of separate faces which might get tricky. Anyways, I’ll give those models now a spin early next week.
On a related note: I just saw my colleague @gholami release-testing the first “moving boat” capabilities. They will be included in one of the next releases!
@quequen, the last models you provided were not that fine granular tessellated and worked much better. It still puzzles me why the exporter of your CAD system works that way. SimScale is used together with a wide variety of CAD systems but that behavior was new to me. At some point I would be interested in digging deeper into this to provide a good interoperability with it.
Anyways - the first laminar sim is now running, heeled about 10 degrees and rotated 4 degrees around the vertical axis. However in terms of water level I was not sure how to adjust it properly. What workflow/approach would you recommend here?
Thanks to @sjoshi, the setup was quite straightforward - I copied it and changed a few numerical settings:
the refined mesh region around the hull influences the interface quite a bit
solution not yet converged (another simulation already kicked off)
I’ll do more post-processing tomorrow (some quantitative results regarding drag and lift) - anything else that catches your eye in these initial results?
Animation of the air-water interface color-coded by velocity approaching steady-state:
David, try with the last geometry I shared. The one you are using here has no leeway, so there will be no significative forces on the “y” direction. (Anyway your present analisys is interesting to find the “y” force caused by asymetry of the submerged shape, because of heel angle).
The last one I shared also has the waterplane in the right position (boat sinked with a crew of 4 people).
For this condition (beating upwind, 3 degrees of leeway, crew of 4 people), a typical velocity could be Fn=0.36 wich gives 3m/s for my LOA.
By the way, your work is getting cool!!
Unfortunately I right now don’t have time to dive into post-processing, but a first glance, the surface mesh needs to be finer as the surface pressure plots do show quite a mesh dependency:
With the latest platform update a new multiphase tutorial was added to the documentation:
It’s not exactly about ship hull flow, but the initialization of the phase interface and other multiphase-specific simulation steps are demonstrated. Another piece of documentation on how to calculate the forces created by a free surface flow on a solid body is right now being written. Should be published soon.
More post-processing of the simulation of @quequen’s boat design comes asap…
). However I doubt that this is the main reason behind the incorrect results.
