Publication Date:
2019
Description:
〈div data-abstract-type="normal"〉〈p〉Instabilities and flow characteristics in the far wake of a circular cylinder are examined through direct numerical simulations. The transitions to the two-layered and secondary vortex streets are quantified by a new method based on the time-averaged transverse velocity field. Two processes for the transition to the secondary vortex street are observed: (i) the merging of two same-sign vortices over a range of low Reynolds numbers (〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline1.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉) between 200 and 300, and (ii) the pairing of two opposite-sign vortices, followed by the merging of the paired vortices into subsequent vortices, over a range of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline2.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 between 400 and 1000. Single vortices may be generated between the merging cycles due to mismatch of the vortices. The irregular merging process results in flow irregularity and an additional frequency signal 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline3.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 (in addition to the primary vortex shedding frequency 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline4.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉) in the two-layered and secondary vortex streets. In particular, a gradual energy transfer from 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline5.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 to 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline6.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 with distance downstream is observed in the two-layered vortex street prior to the merging. The frequency spectra of 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline7.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 are broad-band for 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline8.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉–300 but become increasingly sharp-peaked with increasing 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline9.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 because the vortex merging process becomes increasingly regular. The ratio of the sharp-peaked frequencies 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline10.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 and 〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190326132244592-0183:S0022112019001678:S0022112019001678_inline11.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉 is equal to the ratio of the numbers of vortices observed after and before the merging. The general conclusions drawn from a circular cylinder are expected to be applicable to other bluff bodies.〈/p〉〈/div〉
Print ISSN:
0022-1120
Electronic ISSN:
1469-7645
Topics:
Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics
,
Physics
Permalink