Analyses on flow structures behind a wavy square cylinder based on continuous wavelet transform and dynamic mode decomposition. (15th November 2020)

• Record Type: Journal Article
• Authors: Zheng, Yan; Rinoshika, Hiroka; Zhang, Dan; Rinoshika, Akira
• Is Part Of: Ocean engineering. Volume 216(2020)
• Publication Date: 2020-11-15
• Subjects: PIV -- Vortex -- Square cylinder wake -- Wavy-square cylinder -- Wavelet transform -- Dynamic mode decomposition; Ocean engineering -- Periodicals; Ocean engineering; Periodicals; 620.4162
• DOI: 10.1016/j.oceaneng.2020.108117
• Languages: English
• ISSNs: 0029-8018

Abstract

Abstract: A comparative study on the flow structures behind square- and wavy-square were performed to understand the wake characteristics under spanwise periodic perturbation. The high-speed PIV measurements were carried out at Reynolds number of 4075. The time-averaged flow field suggested that wavy leading edge triggered the alternative secondary vortices along the span. The instantaneous flow field indicated that the regular asymmetric vortex shedding mode was changed to symmetric mode by wavy leading edge. It was found that periodical large-scale flows were suppressed, while the motions of small-scale vortical structures were enhanced by the wavy leading edge based on the continuous wavelet transform. The modal energy distribution of the dynamic mode decomposition suggested that the wavy leading edge weakened the dominance of large-scale vortical structure and promoted the transition from large-scale to small-scale vortical structure. The spatial patterns of representative dynamic modes DM1 and DM2 in streamwise plane indicated that the wavy leading edge changed the wake flow by the distortion of Karman vortex and its corresponding secondary vortical structure, which made the vortical structures more unstable and dissipates more quickly. Furthermore, the phase averaging process based on temporal coefficients of DM1 and DM2 revealed that the separated shear layer of wavy square cylinder extended to further downstream and the separated vortex was broken into several relatively small vortices during the shedding process. Highlights: A comparative study on the flow structures behind square- and wavy square-cylinders were performed. Continuous wavelet transform and dynamic mode decomposition were used. The oscillation of large-scale flow was suppressed and the small-scale structures were enhanced by the wavy leading edge. The distortion and the breakdown process of shedding vortex were observed in the phase averaging process.