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[FAQ]

How do I determine the numerical damping (GAMMA) value in Transient Analysis? For transient analyses using the full method, the HHT-alpha algorithm is the recommended algorithm due to its accuracy. Mechanical APDL, by default, uses the Newmark method, but this can be changed with the TRNOPT command. Workbench Mechanical uses the HTT-alpha method. It is worth noting that the default value of gamma (specified in the TINTP command) in Mechanical APDL is 0.005. On the other hand, in Workbench Mechanical, the default value of gamma (specified in the Damping Controls section of the “Analysis Settings” branch) is 0.1. The gamma parameter, also known as the “amplitude decay factor,” provides numerical damping (dissipation) to provide numerical stability to the time integration scheme. It does not represent any physical form of damping or energy dissipation. Because of this, some trial-and-error may be required to determine the optimal value of gamma for a given problem. A value of gamma=0.05 is recommended by the original authors of the HHT-alpha algorithm to remove any spurious participation of higher modes without influencing the lower modes. The default value in Mechanical APDL is a lower numerical damping value while that in Workbench Mechanical is higher. The values of gamma should usually be equal to or greater than 0.005 but not more than 0.33 (setting gamma=0.33 will introduce a large amount of numerical damping and is therefore not recommended in general). It is suggested to use the default values but change gamma only when needed. For example, with the default value in Mechanical APDL, the gamma parameter may be increased if undesirable high frequency response is observed in the output.