Computational Fluid Dynamics (CFD) workbench using OpenFOAM

[thschrader]

Hi Thomas

it`s more the problem you`ve solved with the pirat in the german forum. I tried to use that example, but it seems that the program changed a little bit.

regards
Peter

Yes, I know.
but I have deleted the FC-file
You should define inlet/outlet boundaries. Then trigger the inlet velocity
to get a constant flow around the hull. I am not very familiar with interfoam,
the calculation of the pirat hull was just for fun, numerics was garbage...

pirat.JPG (46.13 KiB) Viewed 2063 times

[wafi]

hm Thomas

the flow of cause would be 4,8m/s for my example due to have the max velocity of the boat. The problem indeed is the constant flow around the hull and I think that there must be another trick, if I compare this with the openfoam tutorials as Wigley hull etc.
I just trying a new run with inlet velocity of 4,8m/s AND outlet velocity same value but initialation zone with no velocity. Will see what happends now.
For the numeric numbers ... I know the reality of that boat so easy to have a clear picture of the results.

[thschrader]

hm Thomas

the flow of cause would be 4,8m/s for my example due to have the max velocity of the boat. The problem indeed is the constant flow around the hull and I think that there must be another trick, if I compare this with the openfoam tutorials as Wigley hull etc.
I just trying a new run with inlet velocity of 4,8m/s AND outlet velocity same value but initialation zone with no velocity. Will see what happends now.
For the numeric numbers ... I know the reality of that boat so easy to have a clear picture of the results.

OK mate,
give me the next weekend. I will try to build a model.
Can you provide the hull you use (I love step-files)
If not, I imagine something.

However:
For sure you can adapt the OF-tutorials for your needs.
But this is what I not want (for my work). Like in FEM-wb,
the calculation model can directly generated from CAD. Thats why I like FC!
My english is nice, isnt it.. sorry folks
Thomas

[wafi]

hm Thomas
will drink a Jever, I heared the rumor that this should improve the nice english or something like that Skoll

Please note attached, only the hull incl keel and rudder and 15 degree heeling. In general I aggree with your point of view to use freecad as direct input to work with openfoam. A great program, but I think due to missing gui the problem is that it take a hell of time and luck to work with the tutorials and change them more or less to the actual need.
Btw I will try your idea to use DesignSPHysic for a motion analyze, but first I need a little bit more time.

[thschrader]

@wafi:
There are some parts in the geometry the mesher does not like.
so at the moment I try to "unrefine" the model and reconstruct the hull-body.
(I need to refresh my curves-wb skills... ==> gordon-surface tool)
Thomas

hull_ravell42.JPG (39.14 KiB) Viewed 1965 times

reconstruction.JPG (39.56 KiB) Viewed 1965 times

[wafi]

hm Thomas

the parts above water are not very interesting, I created this file via the gf file (that`s an input file for hydrostatic calculation) and I left the lines above water for testing the model including heeling. The normal way would be to cut the hull above waterline in the situation of heeling.
The problem for a sailing boat is, that you are sailing with a heeling, for this boat I would assume max 20°, for more heeling the boat will weather helmed. Also of cause the boat will have a certain drift angle through the water, because a sailing boat is sailing in the air with the sails, but as well under water with hull, rudder and keel.
In the end the best would be, to find a method to test such model with different drift angles, heeling angles and rudder angles with the aim to check lift and drag as well as moment ... to do same with a sail model and than to compare the moments and the ability of the hull to find an equilibrium of moments of sails and moment of under water lines to have an impression if it`s possible to sail in such equilibrium or if one have to modify the under water lines or the sail plan ... The problem is a little bit that this way of research was done in the circulation tank of FH Kiel, in Delft they only looked for drag for different heeling situation (which is base of IMS racing rules) but without testing different rudder angles or to ask where is an equilibrium of the different moments of under- and above water systems.
To do tank testing is of cause much too expensive, special for small yachts. If this could be an alternative ... it would be great. Of cause such could only be an alternative if the way to create a model is easy. Of cause still the time for such calculations is a problem, but for this I have a single linux computer, doing such jobs and does not interfere my normal work flow.

[wafi]

ah ... forgot ...
you can see on which boat you are working on, take a look here: https://yachtwerft-dick.de/index.php?cl ... ats_ravell

Ok ... it is long time ago when I made this design and the last years I only looked into heavy lift problems, but my aim is to go back to yacht design, due more fun, with less money ... ok, something is wrong, maybe I will find out later

[thschrader]

Here is a first approach. I simply subtracted the ravell from the cube 24x10x4 m with boolean-cut. No problem.
Basic mesh cube length=250 mm, 60000 cells. At 2,5 m/s sailing-speed and a boat-length of
12,5 m I get Re=3*10^7. Turbulent flow, so we must use cfmesh with boundary-layers and a lot of refinement. OK?
BTW: nice boat from yachtwerft-dick

model.JPG (62.72 KiB) Viewed 1919 times

basic_mesh.JPG (163.77 KiB) Viewed 1919 times

[wafi]

hm Thomas
that`s the way I went also up to now. For the refinement I took 5 layers with cfmesh, to reduce a little bit the calculation time. I used a much bigger cube with 50m length, 30m width, 5m draft plus 2m airdraft to reduce side effects.The velocity with 2,5m/s is a little bit too slow. "Rumpfgeschwindigkeit" or Fn = 0,4 will be about 4 m/s. The max vel. I sailed with this boat were 4,8 m/s (9,33 kn). And ... yes, Helmut Dick did a very good job.

But I in the end, I will just follow your way to do, changings afterwards should be no problem.

[wafi]

Hi Thomas
I think I have managed now the problem to show the water surface including waves. The problem seems to be the meshing of the water surface. If you only use the initialisation zone you are defining with 1/0 the box for water, but that it. I defined now 2 zones, one atmosphere and one water and the top face of the water zone I`ve put into the mesh region to be refined as surface. Doing so, I am able to extract the isosurface in paraview and I getting a nice view of the water surface waves. (meshed with cfmesh). I have tried to do same with snappyhexmesh, but that did not work, I had to change some dicts with atmosphere and water box, it seems that snappyhexmesh would work with this named items and creating a refined mesh of water surface.
@Oliver
I think we need a tool to define a box for phases i.e. water / atmosphere including the material properties of each phase and an automatic refinement of the boundary of the phases.