fluid hovers away from some - but not all - object surfaces

[fip]

I've created a hybrido sim in RF2012 using an SD scene imported from 3dsMAX 2012. because of scale issues between the two programs I had to resize the MAX scene before import and also adjust the scale of the scene in RF. I'm creating a simple cube emitter that fills a closet and pours into a larger space once the door opens.

when simulating, the fluid reacts oddly to various surfaces - hovering about 1 foot above the floor while reacting with a much tighter tolerace when interacting with other objects. it seems like sometimes objects that overlap repel the fluid more strongly than if they are side by side. I thought it might be an issue with the SD file so I've reduced any instances of object overlap and replaced many of the MAX objects with primitives from RF but see variations of the same problem.

has anyone seen something like this - maybe offer a direction I can begin to look into? I searched around for mention of this issue on the forum but didn't find anything - not sure what search terms to use. thanks in advance for any help with this.

-frank

[fip]

here is a screen cap. the upper port is the top view. you can see how the fluid stays farther away from the lefthand wall than from the one lowest in the view. likewise, the fluid coming down the steps in the side view spills close to the steps but then comes to a complete stop about a foot from the level that is the floor.



-f

[tsn]

Hi fip,

It's very likely that this is related to your grid fluid domain's resolution parameter. The domain is subdivided into regular cells and when you activate its display properties you'll see the your object intersects the cells at different positions (hard to explain what I actually mean...). Just turn on

Grid_Fluid_Domain > Node Params > Display > Domain > Back cells

and switch into side view. Now you can see that there are different offsets between the borders of your objects and the cells: some borders intersect a cell directly in its middle others more to the left or right. To reduce this effect you have to drastically increase the "Resolution" parameter. This issue is a fundamental thing with all solvers based on regular grids, not only Hybrido. With particle fluids you won't see this effect, because it works completely different and both methods can't be compared.

[fip]

thanks for the simple and consise answer TSN. it makes sense. we're talking about the res that is shown by those corner boxes in the grid box. my domain is scaled pretty big and those coner boxes seem small until I pull a corner over an area of geometry detail. I imagine nudging the domain slightly might tweak how strong this effect is seen on a particular section of the scene. one concern I have is that the current res is 500000 and it looks like I would need to bump it tenfold to get a tight sim. is 5 mil too much to ask of a modest workstation or is the hybrido feature able to juggle that high a number easily?

[fip]

the resolution change helped the quality but there is a problem still. here's a screen cap of two views of a followup sim. at the bottom of the fluid, the two stair-steps on either side of each viewport is fluid responding to non-existent geometry. not sure what is going on.



thanks again if anyone can point me in the right direction.

-f

[tsn]

Hi again fip,

500,000 cells is really not very much for Hybrido and you can easily use 5, 10 or 20 million voxels. Don't be shy

It's not only that the grid will fit much better to the surrounding geometry with high resolutions, it also affects meshing. A mesh from 500 K cells will look coarse and blobby. Only with much higher resolution you'll be able to create a good and realistic mesh. Give it a try with 10 or 15 million voxels. But, as I already said before, this effect cannot be totally suppressed, because it depends on where the polygons intersect the grid cells of the domain.

-Thomas

[fip]

thanks again for the reply thomas. I did bump up the res for this to 3 mil and saw an importment in detail. but the phantom geometry still persists. here's the same JPG with red lines drawn to show where the fluid is reacting to non-elements. they are 2 and 3 grid units high respectively. I didn't hide any elements but maybe something got created during export/import. I am going to dig around in the list of objects and see if I can find anything unexpected.



-f

[tsn]

Hi again fip,

It's much better now, but I can see what you mean and this seems to be an issue with Hybrido's voxelization process. I also observed in some scenes with complex geometry that Hybrido virtually extends some objects and the result was exactly the same as in your screenshot: the voxelization acts more like a hull around the objects and doesn't represent the geometry's outlines. I'm afraid that the only solution is to change geometry and/or the position of the objects. Unfortunately I don't know a workaround for this problem. I'm very sorry. Maybe the Hybrido programmers can help you out here.

Cheers,
Thomas

[fip]

thanks again for the feedback thomas. yes. I've been tweaking position of objects and am running a sim. earlier I noticed that the issue might be created when two objects overlap in space. so I've reduced instances of this and am keeping fingers crossed.

-f

[LuisMiguel]

Hi flip,

Could you tell me if when you exported the objects from 3dsmax 2012, you did a resetxform for them before exporting?. If the objects has been modified inside 3dstudio max, they'll need to apply the resetxform. If not, once imported to RealFlow, you might see the objects in the right place, but their axis transformation won't match the same you had in 3dsmax, so realflow can think that the objects are really in that ghost place. In order to avoid that, is necessary to apply the resetxform in 3dsmax before exporting to sd.

luisM.