Geometric nonlinearity in Abaqus

Hi all, 

We have a 38 m long beam of OD 30 mm. We want to know if geometric nonlinearities are playing a role when the beam is subjected to a uniform current Of U  = 0.5 m/s. The predicted response frequencies from using analytical eigenvalues in conjuction with a CFD code do not match those reported from test data. We would like to check if geometric nonlinearities are the reason for the discrepancy by comparing the eigenvalues. We have an abaqus model of the beam but am not sure how to confirm the presence of geometric nonlinearities. Any suggestions would be greatly appreciated.

cherers, 

RDG