An experimental study is performed to determine flow instability in the diverging-flow region with flip-flop phenomenon inside diamond-shaped cylinder bundles. Both two-dimensional particle imaging velocimetry (PIV) and two-dimensional laser Doppler velocimery (LDV) are employed to visualize and monitor flow phenomena. The study leads to the following disclosures in the diverging region with flip-flop phenomenon: (i) Velocity distribution can be expressed three-dimensionally and longitudinal vortices (migrating helically) appear in the secondary flow resulting from separation of the main stream. (ii) Power spectrum distribution with identical dominant frequency prevails in every row, indicating flow oscillations propagate over the entire passage. At the third row, the magnitude of power spectra reaches maximum near the “energy supply point” but subsequently diminishes to the minimum value at the apex of the succeeding cylinder where micro vortices alternately shedding on each of the downstream surface. (iii) The flow exhibits the characteristics of a jet tone.

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