Fluid Simulations

One of the most significant and commonly requested areas of real world simulation is fluid simulation. From pouring shots to ocean vistas, directors and artists have come to rely on computer simulated water and similar fluids.  Fluid dynamics is a complex area and fluid simulations are notoriously computationally expensive, yet when they work they can provide magnificent production value and breathtaking visual effects.

Newer simulations are concerned with volumes, both in terms of say a pouring shot, or a flood of water rushing around rigid bodies but also how say where the floor under the water is located in height affects the surface – ie. the way land fall causes breaking waves on a beach.

 

Some of the fundamental issues that are at the basis of these second main class of fluid sims are:

 

  • conservation of mass. Literally the water doesn’t disappear during the sims (or the maths that it is built on)
  • conservation of momentum or energy
  • conservation of volume – incompressible – unlike say real water this may seem odd (doesn’t sound travel under water?) but for visual effects this assumption is close enough to reality and makes the maths vastly simpler
  • connective acceleration – space controls acceleration, for example as water spreads out or funnels in
  • there are two key forces that act on fluid – fluids are assumed to be affected by gravity and itself (pressure) In a paragraph or two we can even show that mathematically (see below)
  • we ignore viscosity for most but not all fluid sims, viscosity plays a small role and we ignore it
  • boundary conditions – this last point is key – the edge of the fluid is very important, be that a surface or a boundary. Bridson in his book, Fluid simulation for CG (2008), stated that “most of the, ahem, “fun” in numerically simulating fluids is in getting the boundary conditions correct.” There are three things – solid walls, free or freely moving surfaces, and the hardest, other liquids (this last one is rarely seen in films).

 

To understand volume simulations one needs to think of forces acting on ‘fluids” as being simply: high pressure to low pressure in a 3D space. In the water or fluid sim one has a gradient from low to high pressure and this “vector field” will define the movement based on some reasonable time interval.

(text from fxguides.com)

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