One of the things that initially caused problems for me with volumes and volume tools is that smoke simulations have a very specific look – they are prone to mushroom-like shapes or more pyroclastic shapes. Looking at pictures of nebulas, it appeared to me that these fluid-like phenomena were not happening at this large of a scale, and perhaps not in the vacuum of space.
So, I needed a solution that allowed me to simulate volumes through velocity fields, flow, and advect in art-directable ways, and not have that typical smoke-simulation look. One solution I ended up with is essentially a rebuild of a smoke solver, but without the project non-divergent step. With this setup, I could add a lot of the usual enhancement microsolvers (turbulence, dissipation, disturbance, wind, dynamic resize, and even OpenCL compatibility) while staying away from the smoke look. Later, I enhanced that setup by introducing my own divergence field to fake-simulate the attractive gravitational forces with the gas clouds. With this setup, I can also inject rest fields or color fields, which can be used later in the shading networks.
The overall process is to art-direct both an initial density field (and a temperature field if needed) – seed it with the base nebula shapes, then run it through the custom solver, which then creates the gravitational attractive forces, advects the volume through the shaping velocities, and creates the low-frequency details. After the solver, it goes through my up-res process, where it gets sharpened and high-frequency detail is added. I go through this process for many elements. Both the smoke and emissive volumes (nebulas are usually made up of some combination of both) run through their own setups. I usually re-use the shaping velocity fields, as that provides cohesion to the elements, but allows for unique behavior with each element.
An example of one of the custom nebula solvers, with faked internal gravitational attraction:
