Why am I getting different frequencies in ANSYS 8.1 than I did in ANSYS 7.1 with lumped mass on?
The frequencies in 7.1 were 43.3, 51.8, 59.6, 67.9. In 8.1 they are 51.8, 57.3, 67.8.
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This is an error with LUMPM,ON for models with couples and/or constraint equations [Defect 16666]. It was fixed in 8.0. Here is what happened - when choosing lumped mass with CP/CE present in the model, ANSYS issues a warning about off-diagonal terms being dropped when applying the CP/CE to the lumped mass matrix. This approximation is dependent on the selection of slave and master DOF for the CP/CE. Internally, symbolic assembly reorders the slave and master DOF for the CE's in this model and this changed the accuracy of the approximation. We now stop symbolic assembly from doing this for lumped mass modal analyses so we obtain the same approximation as the frontal assembly code. However, the user should only use a consistent mass matrix when CP/CE are present in the model to avoid any loss of potentially significant accuracy. Note that if this is run with either LUMPM,OFF or with ASSOPT,,FRONT it runs correctly. |
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