4.3. Convection
This demo shows another common thermodynamic effect known as convection, which is the process of heat transfer through the bulk movement of a gas or liquid. In the simulator, the right wall is heated, causing the particles to speed up and allowing them to fight against gravity towards the top, where they can spill over to the left, cool down on the left wall, and fall the the bottom-left corner. Particles here are low-energy, but so many of them bunch up that they eventually push into the less-dense right side again, and the circuit continues on.
Interestingly, by playing with the controls in the top-left, you can see that the convection current depends not only on the temperature differential between the 2 sides (enable/disabled using the 3rd button), but also on the strength and direction of gravity. Without gravity the hotter particles have no reason to prefer moving across the top or bottom path. With gravity, only the hotter particles can reach the top and therefore spill over to the empty colder side. And as expected, reversing gravity reverses the convection current as well into a clockwise path.
Using the mouse trajectory indicator, you can see how the walls affect the path of colliding particles. In actual physical reality, the wall itself would be a collection of particles as well, but this would have made simulating walls much more complex, andwe didn't want to lose the elegance of the perfect bouncing/reflections off the walls. So instead, we approximate the effect of thermal conduction between walls and particles, and the result seems to be a great approximation so far (as seen by the emergent convection behavior in this demo). Basically, the temperature of the wall indicates how much energy it has to share with colliding particles. If a colliding particle has less or more energy than the wall's target, the particle bounces off with increased or decreased energy, respectively, which is applied by changing the velocity along the normal of the wall). You may also notice that the trajectory indicator seems to jitter when reflecting off of the hot wall (and to a lesser extent off the cold wall). This is because the wall's target energy for each interaction varies by +/- 20% in order to mimic the actual random fluctuations that would occur in an actual physical system.