This shelf tool sets up a sparse pyro simulation of a large ground explosion. The source and simulation settings used by this tool are identical to the Ground Explosion tool on the Sparse Pyro FX tab. However, rather than building a DOP network, it uses the SOP-level tools to build a simple network to work with sparse pyro.

If you have a Houdini FX License, you can dive inside the solver and edit the subnet with custom DOP forces. Otherwise, you can stay at the SOP level and use the exposed parameters on the node.

• The node chain leading up to merge_for_sim is responsible for setting up density, temperature, divergence, burn, and velocity sources. Burn gets merged with the pyro flame field to refresh the available reactants.

  pyroburstsource sets up sources for the main explosion. The input to this node contains several points with different startframe attributes. This gives rise to multiple parts of the explosion that engage at slightly different times.

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  Change the number of bursts	Modify the Force Total Count parameter on the scatter node. This value directly controls the number of separate bursts that make up the explosion.
  Animate rates of the sourced values	Turn on the Scale Over Duration checkbox on the Burst Components tab of the the Pyro Burst Source node. This will reveal the Duration Ramp that controls a global multiplier for source values over the duration of the burst.
  Set the size of the initial blast	Change the Initial Size or Outward Expansion parameters on the Pyro Burst Source node. Initial Size controls the starting scale of the explosion, whereas Outward Expansion affects its inflation over the duration of expansion.
  Change the shape of the initial blast	Modify Shape Offset on the Burst Shape tab of the Pyro Burst Source node. This parameter acts as a seed for the random shape. To more drastically modify the shape, you can adjust the values of the other parameters on the Burst Shape tab, such as Spread Angle, Roundness, and Number of Trailings.

  The pyrotrailpath chain sets up the trails flying out from the explosion. The Pyro Trail Path node directs the trails, while the Pyro Trail Source node sets up the density and temperature sources for the trails. The generated temperature volume is not sourced into the simulation, but combined with simulation results to light up the leading tip of the trails with fire.

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  Set the number of trails	Change the Number of Trails on the Pyro Trail Path node.
  Control where the trails are ejected	Modify the Spread Angle and Azimuth Angle parameters on the Pyro Trail Path node. Setting Guide Display to Distribution Guide can help you visualize where the trails can be emitted.

• pyrosolver_ground_explosion performs the simulation. Parameters on this node have the greatest influence on the motion and emergent shape of the fireball and its smoke. Parameters on the Solving tab allow you to make the following modifications.

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  Set the expansion rate of the rising fireball	Adjust the Expansion Rate and its corresponding Flame Range on the Pyro Solver. These parameters are located on the solver’s Flames tab, in the Expansion collapsible menu. If the rate is increased, you might need to also increase minimum value of the Flame Range parameter to avoid undesirably high expansion.
  Change how fast the fireball fades out	Modify the Flame Lifespan parameter. The flame field will be reduced more slowly when value of this parameter is larger, resulting in a longer-lasting fireball.
  Modify the overall rising rate	Experiment with Buoyancy Scale values. Boosting its value will increase the rising rate and vice versa.
  Control how the fireball rises	How the fireball will rise is determined by the temperature field, whose evolution depends on the Cooling Rate and the parameters found in the Temperature collapsible menu on the Flames tab. A higher Cooling Rate will result in more rapid temperature loss, making the simulation more reliant on the temperature output from the flame field to drive the upward motion.
  Specify how the smoke is emitted	Modify the Flame Range and Flame Ramp parameters in the Smoke collapsible menu on the Flames tab.
  Control how fast the smoke disappears	Adjust the Dissipation parameter. Higher values will make the smoke rapidly disappear and smaller values will force the smoke to stick around for longer.

• pyrolook sets the proper viewport visualization settings and creates a matching render material. If you dive inside this node, you modify some of the parameters on the Pyro Shader to do the following.

  To...	Do this
  Change the color of the smoke based on density	Change the menu next to Smoke Color from Constant to Use Ramp. Set the Density Range parameter to control the color of the smoke at given density values. Set the Smoke Color Ramp to change the color of the smoke based on Density Range.
  Change the color of the smoke based on how light scatters through the volume	Change the Absorption Color. The value of 0.425 (red),0.36(green), 0.3(blue) could be a good starting point for more realistic looking smoke.
  Scale smoke density using an other volume	Turn on the Use Control Volume checkbox. Set Density Control Volume on the Bindings tabs to the field that you want to use to affect the density. Usually temperature or flame are good choices for this purpose.