Flow Induced Vibration Part 2: Using CFD to find the forcing frequencies

Combining CFD with FEA can provide a powerful method to investigate flow induced vibration. In Part 1 of this post, the use of FEA to find the natural frequency of the spray lance was discussed. In this post, the use of CFD to find the frequencies found in the velocity fluctuations as the fluid flows across the spray lance.

The spray lance shown in the previous post was designed to be inserted into a vertical column with a turbulent airflow. An important question that had to be answered was whether the flow across the spray lance would induce vibrations within the spray lance that would lead to vibration or even to failure. Since the system was in the design phase, it was necessary to use computer simulations to investigate the expected performance of the system. A transient CFD analysis was performed to estimate the velocities that would be found within the system during normal operation.

The plot below shows the fluctuation in the average velocity along the length the spray lance as determined from the transient CFD analysis. This plot shows that significant fluctuations in the velocity can be expected and, as a result, fluctuations in the forces acting on the spray lance. 

A Fourier analysis of this velocity signal yielded a frequency spectrum for this velocity profile as shown in the figure below. This plot shows that the primary frequencies of the velocity are found below 10 Hz, with the two main frequencies at approximately 2-4 Hz.

This second look shows that there are no strong forcing frequencies found at the first or second natural frequencies of the spray lance. Like the results from the velocity analysis, these results also suggest that the configuration is unlikely to experience vibration due to the flow across the spray lance.

The method outlined in these two posts is a simple approach to looking at flow induced vibration. A complete analysis is far more complicated and takes more factors into account. This method however can be used as a screening tool to find potential problems. For example, if the velocity were at or near the critical velocity or if the frequencies in the velocity were found to be near the natural frequency of the spray lance, it would indicate that there was a strong possibility that excessive vibration would exist when the system was started. In this were indeed the case, more investigation or a redesign of the spray lance would be required.