The effect of grid generated turbulence on the fluidelastic instability response in parallel triangular tube array

This paper deals with the analysis of the grid generated turbulence on flow induced vibration response of the parallel triangular tube bundle with pitch ratio of 1.54. Turbulence characteristics of the upstream flow can highly influence the stability behavior of a single flexible as well as multiple flexible tubes in early rows of tube bundle. Experimentation have been performed in the wind tunnel with the free stream velocity ranging from 0 to 8.9 m/s. Distinct amplitude behavior for the monitored tube has been observed for each row in both streamwise and the transverse direction. With the increase in the upstream turbulence, the stability threshold of the monitored tube tends to be delayed for single flexible case, however for the multiple flexible case the stability threshold tends to move towards early values. This indicates the importance of stiffness mechanism in generating instability in the monitored tube, which is enhanced by upstream turbulence. Surprisingly, upstream turbulence is strongly affecting frequency response of the tube bundle. Spectral analysis depicts that translational mode dominates rocking mode for single flexible tube bundle for all cases even for highest turbulence intensities. However, for multiple flexible tube case the rocking mode tends to dominant for the fourth row subjected to highest turbulence intensity of 16.5%. This may be due to the uneven distribution of forces on the tube length due to upstream turbulence and turbulence generated by first three rows which intern to excites the rocking mode. It is revealed that multiple flexible tube bundle dominates rocking mode frequency amplitude because of fluid forces generated by tube to tube coupling. The rocking mode become significant which reflects the importance of stiffness mechanism for generating instability in the monitored tube. Stability diagram delineates that for higher upstream turbulence intensities, the data points lies on the upper side of stability boundary in unstable region for multiple flexible tube case and under the stability boundary for single flexible tube case. The results depicted that the previous theoretical stability models either underestimate or overestimates the stability behavior of tube bundle and hence requires modification in the model to predict the stability boundaries for higher upstream turbulence intensities.