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When the Brownian Force is included in a model, the Update scaled absolute tolerance check box is cleared in the Time-Dependent solver settings. This prevents the solver from taking extremely small time steps to try to compensate for the random nature of the force.
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From the Home toolbar click a Parameters () node. Or in the Model Builder, right-click Global Definitions () and add a Parameters node.
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Set the Parameter values to be an array whose length is equal to the number of times the model should be solved. For example, to solve the model 5 times, set the Parameter values to 1 2 3 4 5.
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When 2 (the default) is selected from the Wall accuracy order list in the physics interface Advanced Settings section, the Brownian force may cause particles to undergo extremely large accelerations after they are released or when they interact with walls. This is due to the second-order Runge–Kutta extrapolation using the random force to predict the particle position at the end of the current time step. To prevent this nonphysical behavior, consider selecting 1 from the Wall accuracy order list when modeling Brownian motion.
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Brownian Motion: Application Library path Particle_Tracing_Module/Verification_Examples/brownian_motion
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