experiments on Flow Noise

Waterfall and lava animations.

Principles: (see the paper)

- Rotation of the random vectors on the grid nodes (cf Perlin noise Sig'85).
- Advection of the small scales by the large scales.

• Some basic rotating noise animations (without advection):
  
  
  
• The role of rotation:
  - 1: Clouds with classical turbulent noise (i.e. t = the third dimension).
  - 2: Clouds with rotating turbulent noise (no advection).
  The second case only shows a `rolling' behavior.
  
  
  
• The role of advection:
  - 1: A noise without advection, whose the largest scale only is animated.
  - 2: The same noise with advection.
  The first example dramatically shows the drawback of adding scales independently.
  
  
  - 1: Clouds with classical turbulent noise (i.e. t = the third dimension).
  - 2: Clouds with this classical noise advected
  
  
  
• Tuning the rotation rate:
  Getting the high frequencies rotating faster or slower than low frequencies gives very different looking fluids, all of them being interesting:
  - 1: progression of 1,2,4 (i.e. increase) of the rotation rate along frequencies looks like turbulence,
  - 2: progression of 1,1,1 (i.e. constant) looks like visco-inertial flow,
  - 3: progression on 1,1/2,1/4 (i.e. decrease) looks like passive foam floatting on waves.
  
  
  
• Tuning the amount of advection:
  - 1: Clouds with rotating noise and no advection.
  It corresponds to very active phenomenons like turbulence, billowing smoke, for which high frequencies are created `on the place', thus showing no memory (this should be used togather with increase of rotation rate along frequencies).
  - 2,3: Clouds with rotating noise and advection, moderate or high (i.e. too much).
  It corresponds to non-turbulent phenomenons like viscous flows, or waves gently shaking foam (this should be used togather with decrease of rotation rate along frequencies).
  
  

• Chimney smoke:
  
  
  
• Conservative flows:
  - 1: Compressible flow (i.e. regular noise or flow noise).
  - 2: Non-compressible flow (obtained by correlating the x and y noises).
  
  
  
• Vortices:
  - forming vortices:
  the rotation rate depends on the distance to the center (in 3,4 the smaller scales only are affected).
  
  - steady vortices:
  nstead, the initial rotation angle depends on the distance to the center.
  
  
  
• Viscous fluids:
  In this experiment, the flow noise in added (in a small proportion) at each time steps to the advection of the previous result (stored in a grid). That way, the fluid shows a longer structure memory.
  NB: by changing this update proportion depending on the frequency, we can tune long memory for large scales and short memory for small scales (see 2).
  
  
  
  
• Advanced experiments on a `reverse advection model':
  In this experimental model, we reverse the scale dependency: here, the large scales are advected by the small scales (as if small disturbances were sucking large ones).
  At this stage, it gives nice still images, but poor animations.
  
  

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