"Gravity" is surely the most often used daemon in RealFlow and necessary for almost any scene. It is a global daemon, acting with equal strength at any point of the 3D domain. Since this daemon is commonly used, it is important to know a few things about it:
Gravity is mass-independent. This means that higher masses will not lead to stronger acceleration or higher velocities. If you want to achieve stronger acceleration, it is necessary to raise the daemon’s strength value.
Gravity strength depends on the particular location. Here on Earth, gravitational acceleration has an average value of 9.8 m/s2, on Mars it is just around 3.7 m/s2, for example.
Mass and weight are not the same. Weight is a force and depends on the particular gravitational acceleration of a location, but mass is determined by the amount of atoms. On Earth, weight and mass have the same absolute value and that is the reason why they are often used equally.
Each body produces a certain amount of gravitational acceleration, but it can be neglected for very small bodies.
If you want to experiment with different strength values please be careful. Since gravity is an accelerating force, it strongly affects the behaviour of fluids. It is possible to create “denser” fluids with higher values or to avoid exaggerated splashes, but changes cause the fluid to behave completely differently.
Affect
Particles can be affected in two ways: either by “Force” or “Velocity”. The first option applies an external force, resulting in an acceleration, while the second one only modifies the velocities of the particles without introducing an additional acceleration. Forces take a little time to display their full influence. This means that they accelerate the particles over a certain time span depending on their strength. High forces exert stronger accelerations and the particles or bodies become faster and faster as long as the force acts on them. The result is a curved stream of particles.
“Velocity” directly affects the particles from the very beginning without any delay or deceleration. The result is an apparently stronger influence, because the deflection of the particles starts with the very first moment. The result in this case is a linear particle stream. “Velocity” is not available for rigid bodies.
Strength
This is the dimension of the gravitational acceleration and it depends on your current location. Please read the annotations above to get a better understanding of RealFlow’s "Gravity" daemon. “Strength” does not accept negative values. If you want to change direction of the force, you have to rotate the daemon. The "Strength" parameter is also known as “g” and is different for any location, because it strongly depends on a body’s mass. Even here on Earth, gravitational acceleration differs from place to place. Hence the given values are only averages.
Celestial body | g [ m/s^2 ] | Celeatial body | g [ m/s^2 ] |
Sun | 274.0 | Jupiter | 24.9 |
Mercure | 3.70 | Saturn | 11.1 |
Venus | 8.87 | Uranus | 9.0 |
Earth | 9.81 | Neptune | 11.4 |
Moon | 1.62 | Pluto | 0.17 |
Mars | 3.37 | Ceres | 0.27 |
Bounded
If this option was available in real life, it would be fantastic, but unfortunately it is not and so the only way to restrict the scope of gravity is inside RealFlow. You can choose from four options: “No”, “Box”, “Plane” and “Push”. “No”, of course, turns on the global force. “Box” restricts gravity to the inside of a box that can be scaled like any other object. “Plane” restricts the effect to every particle at one side of the plane that becomes visible in the viewport. “Push” applies the force to an object acting, in a way similar to an engine. This option is not meant to be used with particles.
The daemon's bounding box can be distorted with the “Shear” parameter under “Node”, but this only affects the visual representation in the viewport. The daemon's scope is not affected.
Underwater
This parameter is only available with the “Push” method from the “Bounded” parameter and exclusively acts on objects in combination with a RealWave surface. It applies the force to the connected object when the point of action is below the surface – for example, you want to simulate a ship pushed by its propeller, but you do not want it to move when the propeller is above the surface.