Parallel triangle tube arrays with pitch ratios in the range of 1.2 to 4.2 have been tested at Reynolds numbers up to 90000 to investigate the vorticity shedding and acoustic resonance mechanisms. The tests included measurements of the pressure fluctuations on the tubes and on the test section wall, as well as extensive flow visualisation and hot film measurements of the unsteady flow activities.

Three different components of flow periodicities have been observed. The flow periodicity with the highest Strouhal number (S3) is caused by a shear layer instability and is the weakest component. The S2 component is associated with small scale vortex shedding at the first row. The third component has the lowest Strouhal number (S1) and is the strongest. It is generated by large scale, alternating vortex shedding at deeper rows, and it becomes dominant at all rows at high Reynolds numbers.

For tube arrays with pitch ratio less than 3.4, the onset of acoustic resonances could not be related to the natural flow periodicities mentioned above. This behaviour is in contrast with that of normal triangle arrays, but similar to the acoustic behaviour of in-line arrays. The similarity with in-line arrays seems to stem from the fact that both arrays allow the flow to proceed along free flow lanes between the tube columns, resulting in a similar acoustic resonance mechanism. Strouhal number charts for the natural vorticity shedding and the onset of acoustic resonances have been developed.

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