Low-frequency wake instability of an axisymmetric bluff body in pitch

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Abstract

The backflow instability in the wake past a cylindrical blunt-based body in pitch is investigated at a Reynolds number ReD = 6.7 · 104 based on the cylinder diameter. Time-resolved stereoscopic particle image velocimetry measurements have been performed in a cross-flow plane located 0.3 D downstream of the model base. An increasing displacement of the backflow region from the body centerline with increasing pitch angles is observed in the long-time average of the velocity field, with the emergence of a preferred orientation of the wake. The time history of the backflow centroid position shows a progressive reduction of both amplitude and time scales of the fluctuations, reflecting the transition from large-scale azimuthal meandering to a more stable confinement at an off-center position. Proper Orthogonal Decomposition of the velocity fluctuations reveals a reduction by approximately 80% in the contribution of the first two modes for angles increasing up to 1°, accompanied by a distortion of the dipolar distribution typically associated with backflow meandering, for misalignments from 0.3° and higher. The frequency spectra of the POD time-coefficients display a very-lowfrequency peak near StD ~ 10-3 only within 0.1° deviations from axisymmetric inflow conditions, thus endorsing the hypothesis that the long-term backflow instability only survives within small deviations from axisymmetric inflow conditions.
Original languageEnglish
Title of host publicationProceeedings of the 18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics
Subtitle of host publicationLisbon, Portugal
Number of pages11
ISBN (Electronic)978-989-98777-8-8
Publication statusPublished - 2016
Event18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics - The Calouste Gulbenkian Foundation, Lisbon, Portugal
Duration: 4 Jul 20167 Jul 2016
Conference number: 18
http://ltces.dem.ist.utl.pt/lxlaser/lxlaser2016/

Conference

Conference18th International Symposium on the Application of Laser and Imaging Techniques to Fluid Mechanics
CountryPortugal
CityLisbon
Period4/07/167/07/16
Internet address

Keywords

  • Turbulent axisymmetric wake
  • Azimuthal meandering
  • Stereoscopic PIV

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