Controlling Fluid Simulations with Custom Fields in Houdini Master ...

Figure 18: The node tree for the dierent steps for the advect by curve simulationas described in (5.3.2).1. Dene a few custom sop vector elds to hold to custom dened volumesfrom sops, such as the density source (the little sphere), the custom densityof the curve, the tangent velocity and the gradient velocity (which will becalculated at a later step).2. Calculate noise by dening new custom elds with the Gas Match Fieldmicrosolver, a scalar noise_eld and a vector noise_vel eld and denethe noise in the noise_eld inside of a Gas Field Vop using an Anti-Aliased Flow Noise vop. By using a Gas Analysis microsolver we cancalculate the gradient of the noise_eld and store it in the noise_vel.3. Calculate the gradient of the blurred density of the curve. Create a newtemporary eld to store the blurred_custom_density using a Gas MatchField, matching the custom density eld of the curve. Copy the custom_densityinto the blur_custom_density using a Gas Calculate. Blurthe blur_custom_density eld using a Gas Blur. Calculate the gradientof the blur_custom_density and store the result in the gradient_velocity.Finally normalize the gradient_vel using a Gas Analysis. The resultantforce will appear to be always pointing towards the helix shape.4. Add the forces to density. By using Gas Calculate nodes, the variouscustom velocity elds can be premultiplied by higher values for a morepowerful eect and added to the velocity eld.The simulation can get unstable when too much force is added, so it can beuseful to add a low velocity damp on the pyrosolver, or even turn on speedlimits.A few stages of the simulation can be seen in Figure (19).28