Measurement of flow around a cactus-analogue grooved cylinder at : Wall-pressure fluctuations and flow pattern

The flow characteristics around a cactus-analogue grooved cylinder and a smooth cylinder were extensively compared at Re D ==5.4×104 based on the free-stream velocity and cylinder diameter (D). Wall-pressure fluctuations on the cylinder surfaces and the fluid flows around the cylinders were measured using a microphone and planar particle image velocimetry, respectively. The influence of the cactus-analogue grooved cavities on the wake characteristics was assessed in terms of the circumferential distributions of the wall-pressure fluctuation intensity, the wall-pressure spectra, and the auto-correlation coefficient of the wall-pressure fluctuations. The global distributions of the time-averaged velocity field, reverse-flow intermittency, streamwise velocity fluctuation intensity, Reynolds shear stress, and spatial velocity correlation were also considered. The circumferential distributions of the wall-pressure fluctuation intensity showed that the wall-pressure fluctuations on the cactus-analogue cylinder were remarkably reduced by around 50% at most stations. On the grooved cylinder, the magnitude of the wall-pressure spectra at the characteristic frequency of the shedding from the large-scale vortical structures was considerably reduced by around 7 dB. This convincingly demonstrated that the unsteady aerodynamic forces on the cactus-analogue cylinder were significantly weakened. In contrast, the auto-correlation analysis of the wall-pressure fluctuations showed that the consecutive motion of the shedding from the large-scale vortical structures close to the grooved cylinder was much more organized. Inspection of the time-averaged velocity field around the cylinders disclosed that the velocity of the near-surface fluid around the grooved cylinder was increased by the small recirculation flows immersed inside the cavities. Near the grooved cylinder, the trace of the peaked streamwise velocity fluctuation intensity was highly localized at θ = 105 ° while in the smooth cylinder a very long, slender peaked region began at θ = 60 ° and extended to the leeside of the cylinder. Finally, a snapshot proper orthogonal decomposition analysis was performed on the velocity fields. The correlation map of the first two POD coefficients convincingly indicated that the vortex shedding process in the grooved cylinder system was more coherent but weaker than that in the smooth cylinder system, which is further confirmed by the phase-dependent contour plots of the swirling strength and its standard deviation.