Pressure coherence and flow structure of a twisted elliptical cylinder at moderate Reynolds number

In this study, we performed an experimental study to explore the surface pressures and flow structures around twisted elliptic (TE) cylinders at a Reynolds number of 3.8 x 104. Two TE cylinders, denoted as TE11 and TE12, with aspect ratios of 1.1 and 1.2 for elliptical cross-sections, respectively, were selected to compare the performance of vortex shedding suppression. The distribution characteristics of the fluctuating pressure at different spanwise positions and the phase difference and coherence function between the pressure signals were analyzed. The results showed that TE12 cylinder was not dominated by antiphase shedding vortices, as in the baseline and TE11 cylinders, but rather by low-frequency components in the flow field, yielding a Gaussian distribution of the fluctuating pressure. The pressure pulsations were significantly asymmetric for the TE12 cylinder due to the earlier flow separation on the upper surface, but this asymmetry was not apparent for the TE11 cylinder. The Reynolds shear stress distribution revealed that the transition to turbulence in the separated layer was significantly delayed by the TE surface. Moreover, for TE12 cylinder, zero turbulence fluctuations in the near wake explained the low-frequency domination of the pressure fluctuations, resulting in 93.8% attenuation of the lift fluctuations.