Experimental investigation of the turbulent wake of partially submerged horizontal circular cylinders

Time-resolved particle image velocimetry was used to study the wake characteristics of partially submerged horizontal circular cylinders. Two submergence levels, namely 50% and 75% of the cylinder, and blockage ratios of 5% and 9% were examined at a Reynolds number of 10 000. The results from the partially submerged cases are compared to a reference case of a cylinder fully immersed deep in the uniform flow. The wake characteristics are analyzed in terms of instantaneous and mean flow, shear layer growth, Reynolds stresses, and turbulence transport while the unsteady characteristics are examined using frequency spectra and temporal autocorrelation coefficients of fluctuating velocities, proper orthogonal decomposition (POD), and reverse flow areas. The results show that distinct recirculation bubbles form in the wake of the cylinders. The size of the recirculation bubble of the partially submerged cylinder increases linearly with submergence level. The turbulence levels in the wake of the partially submerged cylinder are enhanced when blockage ratio is increased but are not significantly affected by changes in submergence level. Regardless of submergence level, the size of the recirculation bubble in the partially submerged cases is substantially larger, while the turbulence levels are significantly lower compared to the uniform case. In the uniform case, the first two POD modes make similar energy contributions mainly from vertical velocity fluctuations but in the submerged cases, most of the energy comes from streamwise velocity fluctuations and the contribution from mode 1 is two to three times as large as that from mode 2. The degree of correlation, assessed through the cross-correlation coefficient, between the first POD mode and the flapping motion of the shear layer is higher for the submerged cases compared to the uniform case but varies with submergence level and blockage ratio. The dominant dimensionless flapping frequencies (based on recirculation length) in both flow configurations are consistent with reported values for separated flows induced by a wide range of bluff body geometries.