Time Step Size
The Granular Flow interface suggests a time step for the model set up and stores it in a variable, <scope>.ts (for example, gran.ts). This variable is automatically used in the solver for the time step used. It should be noted that it is just a suggestion, and users are encouraged to verify the time step size and change the value or expression as needed. The expressions for the suggested time step depends on the contact force model used. When the Linear elastic model is selected in the Contact force model list in the physics interface node’s Force section, the suggested time step is calculated as:
(3-31)
where,
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mg is the mass of the grain
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kn is the normal spring coefficients
If model other than Linear elastic is selected, then the suggested time step is calculated based on the Rayleigh time step criteria (Ref. 7):
(3-32)
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rg is the radius of the smallest grain
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ρg is the value of the least dense grain
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Gg is the maximum value of shear modulus
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νg is the maximum value of Poisson’s ratio
Equation 3-32 is calculated for each grain species, and the smallest value is stored in the variable <scope>.ts.
The coefficient 0.2 in both Equation 3-31 and Equation 3-32 is used to make the time step less restrictive so that the simulation, in most cases, does not become unstable due to large time step issue.
It is to be noted that the variable <scope>.ts is a global variable as it needs to be accessible to the solver, so it can only be computed using the properties of grains that are global variables as well. The properties of walls cannot be used directly as the variables are defined on the boundaries. Users are suggested to manually calculate the suggested time step based on the above equations if wall material properties are to be used.