The Validation of Fluid-Elastic Instabilities in a Flexible Rod Bundle using STAR-CCM+ coupled to Abaqus

Fluid-elastic instabilities of a flexible rod bundle in turbulent water cross flow have been well studied in laboratory scale experiments. As a validation, unsteady RANS simulation in STAR-CCM+ are coupled to Abaqus via co-simulation and the results compared to experiments. Because of the strong rod-water coupling, this validation represents a complex test for implicit coupling technique realized in STAR-CCM+.

STAR-CCM+ and Abaqus models were constructed based on the description and dimensions provided in the reference documentation.* The rod fastening conditions and properties were tuned to correspond to the measured natural bending frequency in air. Coupled simulation of free vibration under quiescent water conditions have been conducted and compared to the measured vibration frequency and fluid damping under these conditions. Fluid standalone solutions (rigid rods) are conducted to evaluate vortex-shedding frequencies and the Root Mean Square (RMS) pressure for each inlet cross flow velocity from the test matrix. Finally, fully coupled solutions were launched to obtain Limit Cycle Oscillations (LCO) of the rods for each inlet velocity. RMS rod displacement values in stream-wise and transverse directions in the data points were evaluated and compared to experimental values.

The following points can be outlined on the base of the performed study:
• Fluid-elastic instability phenomena of the tube bundle are apparent in the simulations with the main phases of the tube bundle behavior described in the reference documentation detected in the provided solutions.
• Although the critical velocity is over predicted, 0.24 vs. 0.17 m/s, the prediction is considered quite satisfactory at this stage of the model development.
• The trend of LCO amplitude growth above the critical velocity satisfactorily corresponds to the experimental data in the range of inlet velocities studied.
• LCO frequencies satisfactorily correspond to the experimental spectra.

* D. S. Weaver, A. Abd-Rabbo: A Flow Visualization Study of a Square Array of Tubes in Water Crossflow. Journal of Fluids Engineering. September 1985. Vol. 107, p. 354-363. publi_Weaver_Abd-Rabbo.PDF.

Alan Mueller
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