# How can I get a volume to expand uniformily in all directions, like a nova?

I would like to simulate a star exploding into a nova, ie

I have a quick smoke where the density is keyframed, emitting smoke from a sphere for few frames only. I need a force that pushes this smoke in every directions, uniformly. Vorticity, noise, texture, collisions, high definition smoke, etc will do the job of modelizing the shape as it goes and offer nice details. I do not want to use a mesh.

But it doesn't seem to exist any force field that acts like a sphere, attracting or repulsing everything from a single point and uniformly. This is very easy programmatically so I surely missed something and I hope someone will point me to the correct feature ! Otherwise I'll have to learn a bit of python I guess.

Wind only pushes smoke in a single direction. I can make it a cone-wind, with an angle of 90° and both 2 directions, I can only get :

(Direction says : both Z, by the way gravity is off and the domain density and temperature parameters are 0).

I can put it to sphere of course, this is another frame but same :

No idea why it raises upwards, same params as before, but anyway this is not even a sphere.

At last what approaches the most my needs is the "force" force field, in "sphere" mode, but it is absolutely not uniform :

I don't know why it acts more on the horizontal plane. I tried to modify the domain resolution since the voxels grid contributes to non-uniformity anyway but even big resolutions end this way.

I have to mention I also tried to expand an invisible sphere used as a collider, however I cannot use this then to get the vorticity, smoke effects precisely, that I am looking for.

Any idea ?

The smoke by default rises upwards from the emitter. To disable this, you need to set the Density and Temp. Diff. both to 0. Also disable the influence of gravity by setting Gravity under Smoke Field Weights to 0.

Here, I've also set Dissolve to 10 frames, to make the smoke disappear. I'm using a Force type force field, with Power set to 2*. This makes the effect falloff physically correct, by making the force proportional to inverse square of the distance from the source.

And this is the result I got.

As you can see, it doesn't spread exactly equally in all directions. However, the force, and most importantly the Vorticity of the smoke domain, are quite strong. The smoke will be calmer if you reduce the vorticity, and spread more slowly if you reduce the strength of the force.

*Have a look at the Blender Manual for an explanation of the Power and other settings.

• Thanks for your quick answer ! I've been quite lazy in my question I have to admit it, but the best I could obtain is this precisely, I should have done some screenshots. I missed the word for 'exploding star', I would like a nova look. Editting Nov 4 '16 at 21:33
• Edit done, here is all I could try. I upvote your answer anyway, thanks for your time. Nov 4 '16 at 22:05
• @Atrahasis I see. I've never tried having smoke/fire spread the way you want, so I'm not sure how to do it. However, my first thought would be using particles as a source for a point density texture rather than smoke. I'll look into it over the weekend, but I can't make any promises, since this is new territory for me as well.
– user27640
Nov 5 '16 at 0:37
• Indeed, I'm trying particles, at least I easily have a spherical beginning when using a lot of them :) I still have to work on shockwave deformation now, but yet with the other answer you got me on some path. Nov 5 '16 at 15:13

I would suggest to add particle system to the sphere, and make it emit smoke. Thanks to that you will have great level of control over the explosion. This tutorial gives quite good understanding of the process.

• Had not seen this one yet, seems cool ! But, I missed the correct word for 'exploding star' in my OP, I mean something like a nova or a supernova, that is why I'm looking to use wind on smoke. Nov 4 '16 at 21:32
• I editted my question, I upvote for your time anyway, thanks. Nov 4 '16 at 22:08