What is the best tool for simulating Vacuum and Fluids together? I require a software to simulate Fluid simulation with the capability of supporting vacuum simulation. My requirements are that all numbers must reflect their real counterparts almost exactly. For example I need to mix Fluid, Air and Vacuum.
I have tried RealFlow but it doesn't support Vacuum. 
Any body knows any software for this?
 A: USim can do vacuum + gas http://www.txcorp.com/home/usim/usim-overview.  Not technically true vacuum, but 9 orders of magnitude density jumps.  It doesn't support an incompressible fluid (liquid) though.
A: What you're looking for doesn't entirely make sense.
Vacuum ins't anything, per se.  Vacuum is the absence of things, e.g. particles, pressure, etc.  A perfect vacuum doesn't exist, thus generally one is interested in varying degrees of low-density fluids.  Thus one would most likely rephrase that you're looking for a simulation of two fluids, with strongly varying density. 
The standard method of simulating fluids is with either a grid based hydrodynamics code (a so-called 'Eulerian' code), or a smoothed particle hydrodynamics (SPH, a `Lagrangian' code).  Grid codes have trouble simulating large differences in density (especially over small spatial scales), which suggests that you should try using an SPH code.  In astrophysics, which is the field I am most familiar with, the standard SPH code is called Gadget (note that it can be used for simulations of small scale objects, instead of galaxies and larger---which it is generally advertised for).
If that is insufficient, you should google it, or be more specific.
