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I was watching a smoke simulation tutorial (VFX), the trainer mentioned that gravity needs to be turned "on" to have a smoke simulation rise. He mentioned turning gravity "off" the density of the smoke moves around but never RISES due to no gravity.
Is this just a software "cheat" or does this happen in reality.
Just curious.
With gravity, there is a density gradient in the vertical direction. The strength of the gradient depends on the mass of the species. Since the atmosphere has a mixture of different species, you get a separation between different components in the vertical direction, that you would not get if you just mixed all the same gases together in free space (or in a chamber where you could ignore gravity).
A simple model of this is the isothermal atmosphere. The pressure of a gas varies with height $z$ as \begin{equation} p(z) = p_0 e^{-z/z_0} \end{equation} where $p_0$ is the "sea level" pressure and the scale height $z_0$ is \begin{equation} z_0 = \frac{R T}{\mu g} \end{equation} where $R$ is the gas constant, $T$ is the temperature, $g$ is the acceleration due to gravity, and $\mu$ is the molecular weight of the gas. If the molecular weight of a gas is smaller than that of air, it will tend to be more dense than air at higher altitudes.
This equation also implies that if the temperature of a gas $T$ is larger than the ambient air temperature, it will be denser than air at larger heights -- hence "hot air rises." I suspect this latter point explains the behavior of the smoke in the simulation.
In reality you can not turn of or on the gravity on earth, you could do the experiment in the space station or in free fall and see that the smoke behaves like mentioned without gravity.
“Down” in any location is generally defined as the direction of the local gravitational force (unless you are in a rotating system). “Up” is the opposite direction. So, by definition, gravity cannot act up.
I guess this is due to the fact that when gravity is turned on, the heavier gases settle down and so the lighter gases(smoke) can move up. On the other hand, if there is no gravity all gases are just mixed up together and the smoke doesn't rise.
