Publikationsdatum:
2019
Beschreibung:
〈h3〉Abstract〈/h3〉
〈p〉In this paper, the wake modes behind a circular cylinder under streamwisely forcing oscillating motion are studied at Reynolds number 〈em〉Re〈/em〉 = 360–460 which are observed by laser-induced fluorescence flow visualization technique.
The forcing frequency 〈span〉〈span〉\(f_{\mathrm{e}}\)〈/span〉〈/span〉 ranges from 0 to 6.85 〈span〉〈span〉\(f_{\mathrm{s}}\)〈/span〉〈/span〉 , where 〈span〉〈span〉\(f_{\mathrm{s}}\)〈/span〉〈/span〉 is the vortex shedding frequency behind a stationary cylinder, and the forcing amplitude 〈span〉〈span〉\(A/d = 0.2, 0.5, 1.0\)〈/span〉〈/span〉, where 〈em〉d〈/em〉 is the cylinder diameter. Both time-invariant and linearly ramping 〈span〉〈span〉\(f_{\mathrm{e}}\)〈/span〉〈/span〉 are investigated. Following our previous modal notation, the following conclusions can be drawn: firstly, three rarely reported modes in numerical studies, C-I, C-II and S-III, are now confirmed in experiments (though with differentiable appearance in their far wake behaviour) at higher 〈em〉A〈/em〉 and/or 〈span〉〈span〉\(f_{\mathrm{e}}\)〈/span〉〈/span〉 ranges and the envelope lines of S modes and C modes, yielded from a vortex circulation model, are shown to be dependent on the peak relative velocity of the free stream to the cylinder surface. So is the occurrence of the S-II mode. Secondly, near the demarcation of A modes and S-I mode, wake mode undergoes constant transition in a stochastic manner at fixed 〈em〉Re〈/em〉, 〈em〉A〈/em〉 and 〈span〉〈span〉\(f_{\mathrm{e}}\)〈/span〉〈/span〉. Thirdly, a typical hysteretic effect can be observed when the oscillation frequency of the cylinder ramps up and down in a linear way, and the extent of delay is dependent on the ramping rate 〈em〉k〈/em〉. Finally, mode switching during frequency ramping obeys a unidirectional order. During 〈span〉〈span〉\(k〈0\)〈/span〉〈/span〉 (ramp-down), when S-I (Type-II) mode switches to A-IV mode, or A-IV to A-III, the flow structure downstream is affected by the upstream and the entire wake flow eventually switched, which is classified as slow switches. In contrast, during 〈span〉〈span〉\(k〉0\)〈/span〉〈/span〉 (ramp-up), a clear and abrupt switch can be observed in the wake when A-IV or A-III switch to S-I (Type-II) modes, which are jump switches.〈/p〉
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〈h3〉Graphical abstract〈/h3〉
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〈img alt="" src="https://static-content.springer.com/image/MediaObjects/12650_2019_554_Figa_HTML.jpg"〉
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Print ISSN:
1343-8875
Digitale ISSN:
1875-8975
Thema:
Informatik
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