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How to slow down fluid speed


Dennis Sedov

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I have dont a fluid simulation, that I am pretty happy with, the only thing I want is it to be 3 times slower, but following the same paths. Resizing, changing forces does not work, as the fluid blend together and looses all the detail created by noise fields.

I am really ran into a wall, and don't know where to go from here rather then retiming rendered images.

What I am trying to simulate is a wispy smoke that goes from a sewer on a street at night. So I have two problems - there is not enough "whispiness" and the simmulation is too fast for what is should be

Any help would be much appreciated. Hip file is attached

fluid_smoke6.hip

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the only thing I want is it to be 3 times slower

Wow, and here we normally have people complaining that they want faster fluids :blink:

Yes, you can try the time scale in your simulation to make things behave slower. The reason you wouldn't have had any frames in .5 because you probably aren't substepping while simulating (change the substep parameter on the dopnet to do this).

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Hi there,

well at there moment I am baking out the smoke sim , Its gona take a while , My idea is getting this into the chops.

can't do nothing until the sim is complete . will give it a shot in the morning , it is 12 pm here in madrid.

A while back there was a series of videos tutorials on the chops and you could do retiming , atleast with dop rbd's

have you looked into this ,

cheers , I will try and do something in the morning ,

mangi

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Hi there.

well i did bake the sim ran out of disk space.

It looks like the chops , it's not a good idea as the dynamic node import is for the transformation. it was a while back

that I looked at this.

However, I did a simple sim and tried the Scale Timer 0.5 and Time Step 0.02 in the dop and was able to to some retiming as what allegro had mentioned.

cheers.

I will do some testing to see what the issue may be

cheers

mangi

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Edited by mangi
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See - I have exactly the same behavior on a simple scene like this. Download the hip file I have attached with my first message and see the difference in how the forces are applied. I have several fans in the scene. So in at full speed I get the result I want. However, when I put the time scale at 0.5 the sim behaves as there are no fans or forces in the scene - the smoke just goes up. Before you play the sim down the size of the container resolution ( I have like 120 there) to 20 so you'd see the feedback and be able to make a difference between a full speed simmulation and the time scaled one.

Hi there.

well i did bake the sim ran out of disk space.

It looks like the chops , it's not a good idea as the dynamic node import is for the transformation. it was a while back

that I looked at this.

However, I did a simple sim and tried the Scale Timer 0.5 and Time Step 0.02 in the dop and was able to to some retiming as what allegro had mentioned.

cheers.

I will do some testing to see what the issue may be

cheers

mangi

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See - I have exactly the same behavior on a simple scene like this. Download the hip file I have attached with my first message and see the difference in how the forces are applied. I have several fans in the scene. So in at full speed I get the result I want. However, when I put the time scale at 0.5 the sim behaves as there are no fans or forces in the scene - the smoke just goes up. Before you play the sim down the size of the container resolution ( I have like 120 there) to 20 so you'd see the feedback and be able to make a difference between a full speed simmulation and the time scaled one.

I think part of the problem is that you have $F as the seed in your noise. As soon as you add substeps, or scale the time you're going to have different results in your simulation because of this noise so offhand, I'm not entirely sure how you would get it to have an identical look, yet behave slower.

I've reworked your file slightly however, and you can see that by multiplying your noise by the timestep, you can at least get identical results when either doubling the substepping or halving the timestepping (identical to each other, not identical to your original simulation)

fluid_smoke7.hip

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Thank alot! Now I see what I did wrong. Also replaceing $F with $FS would yeld similar results!

Another question offopic. Is there a way to control amount of fluids generated? In maya you can set amount of density, heat and fuel generated per second. Is it possible to set in Houdini. I only found how to set up temperature, which just makes smoke go up really fast, but I'd rather set density.

I think part of the problem is that you have $F as the seed in your noise. As soon as you add substeps, or scale the time you're going to have different results in your simulation because of this noise so offhand, I'm not entirely sure how you would get it to have an identical look, yet behave slower.

I've reworked your file slightly however, and you can see that by multiplying your noise by the timestep, you can at least get identical results when either doubling the substepping or halving the timestepping (identical to each other, not identical to your original simulation)

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Thank alot! Now I see what I did wrong. Also replaceing $F with $FS would yeld similar results!

Another question offopic. Is there a way to control amount of fluids generated? In maya you can set amount of density, heat and fuel generated per second. Is it possible to set in Houdini. I only found how to set up temperature, which just makes smoke go up really fast, but I'd rather set density.

If you convert your torus to a fog volume using an iso offset sop before sending it into dops as a source, I believe you can control the density that gets emitted with a volume mix or volume ramp sop.

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If you convert your torus to a fog volume using an iso offset sop before sending it into dops as a source, I believe you can control the density that gets emitted with a volume mix or volume ramp sop.

Nope - have not found that unfortunatelly. And there is nothing in the docs eather. Don't really want to do this with heat, as it becomes less controlable =(

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