Resonance Frequency of Transverse Waves Due to Vortex Shedding of Obstacles with Different Arrangements

Water flow through obstacles such as bridge piers results vortex waves. The overlap of vortices resulted from any obstacles, contributes to transverse waves, perpendicular to stream. When the vortex frequency and stream natural frequency become equal resonance occurs and the maximum amplitude resulting from transverse waves may cause destruction of the structure. These waves can also affect suspended sediment transportation. This study was carried out to determine the vortex frequency of obstacles at various arrangements through stream during wave resonance and the effect of flow variations on vortex frequency. Therefore, some experiments were conducted with a rectangular flume at constant discharge using cylindrical and prismatic obstacles with triangular cross section and with water flow directed at the edge or face. The frequency of waves for in-line and staggered arrangements was determined for the two oscillation modes. The obstacles frequency and flow natural frequency were compared at different arrangements and various obstacles with collecting flow characteristics at two modes. Non-submerged obstacles produced larger wave amplitude than submerged obstacles, with the frequency of stream being close to the vortex frequency. Finally, some equations were developed for determining the Strouhal Number of different obstacles using statistical methods and data analysis.