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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.

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    $\begingroup$ In reality it is difficult to cheat and turn off gravity. In software you can specify acceleration, but gravity is common and available as an option. Less dense material is displaced upwards by more dense material by gravity (acceleration in general). If you hold a helium filled balloon by an attached string in a car just below the car's ceiling (with closed windows) and increase speed (positiely accelerate forward) the balloon moves forward relative to the ocupants as the air molecules are shifted rearwards during the acceleration. The vector of shift is in combination with gravity. $\endgroup$
    – Jim Clark
    Aug 12, 2021 at 16:51
  • $\begingroup$ Imagine what striking a match would look like without gravity to cause the flame to rise. $\endgroup$
    – Jim Clark
    Aug 12, 2021 at 16:56
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    $\begingroup$ FWIW, here's a question about the behaviour of flames in microgravity: physics.stackexchange.com/q/399688/123208 $\endgroup$
    – PM 2Ring
    Aug 12, 2021 at 17:04

4 Answers 4

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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.

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  • $\begingroup$ you explained it beautifully, thanks. $\endgroup$
    – icYou520
    Aug 12, 2021 at 16:52
  • $\begingroup$ @icYou520 Actually thinking about it, the temperature is probably more relevant than the molecular weight for smoke. I'm updating the answer to state this. $\endgroup$
    – Andrew
    Aug 12, 2021 at 16:56
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    $\begingroup$ Also, because smoke particles are so small they have a very high surface to volume ratio, which gives them a very low terminal velocity. $\endgroup$
    – PM 2Ring
    Aug 12, 2021 at 17:02
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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.

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“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.

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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.

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