AN EXPERIMENTAL INVESTIGATION OF VORTEX INDUCED ACOUSTIC RESONANCES IN A RECTANGULAR DUCT

Ali  H. HAMMOUD

This thesis reports the results of an experimental investigation of vortex induced acoustic resonances in a rectangular duct containing two plates located in tandem. The effects of varying the plate spacing and the leading edge shape were investigated. Tests were conducted in both an open  channel. and in a rectangular wind tunnel.

Measurements were made of acoustic amplitude, frequency and phase relationship between the acoustic field and vortex shedding.

Short  tests were also conducted with an absorber  unit mounted in the wind tunnel  wall .

Results of tandem plate configuration showed in most cases the presence of two discrete resonances during which the phase of shedding of the first resonance is similar to that of the upstream plate alone while during the second resonance, there was a much smaller variation in phase. In between the two resonances, there was a sudden jump in phase and amplitude as the flow velocity was further increased due to the re-excitation of the resonance.

The effect of varying the plate spacing has a marked effect on both the velocity range at which the resonances were excited and the natural  Strouhal  number.

At some plate spacing, these two resonances were excited alternately while at other spacing, there was a wide break between them.

The application of tandem plates as a general means to suppress resonance was unsuccessful.

It was also found that resonance can be totally suppressed by using an absorber unit .