Large-scale coherent structures in the wake of axisymmetric bodies

Fuchs, Helmut V.; Mercker, E.; Michel, U.

doi:10.1017/s0022112079001841

Cited by 111 publications

(59 citation statements)

References 17 publications

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“…More details on the streamwise development of the mean flow properties can be found in the work of Gentile et al 23 Also the development of time averaged longitudinal velocity along the wake axis agrees well with the results reported in the literature. 3,5 More specifically a minimum value U * = −0.30 is herein observed occurring at x * = 0.70, corresponding to approximately 60% of the rear-stagnation point location (see Fig. 5).…”

Section: A Flow Field Statisticsmentioning

confidence: 63%

“…3 The experiments of Merz et al 4 on the axisymmetric turbulent wake of a truncated cylinder aligned with the freestream show that the recirculating flow produces strong pressure fluctuations at the base. Further studies conducted by Fuchs et al 5 indicate that the most intense fluctuations occur in the low-frequency regime corresponding to a diameter-based Strouhal number, St D ≈ 0.2, and appear in the form of large-scale anti-symmetric (i.e., azimuthal wavelength m = 1) oscillations associated with a vortex shedding phenomenon. Such anti-symmetric oscillations have been later found to be a major feature of the wake of other more complex configurations such as truncated cylinders with afterbody.…”

Section: Introductionmentioning

confidence: 93%

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Low-frequency behavior of the turbulent axisymmetric near-wake

et al. 2016

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The turbulent wake past an axisymmetric body is investigated with time-resolved stereoscopic particle image velocimetry (PIV) at a Reynolds number ReD = 6.7 × 104 based on the object diameter. The azimuthal organization of the near-wake is studied at different locations downstream of the trailing edge. The time-averaged velocity field features a circular shear layer bounding a region of recirculating flow. Inspection of instantaneous PIV snapshots reveals azimuthal meandering of the reverse flow region with a significant radial offset with respect to the time-averaged position. The backflow meandering appears as the major contribution to the near-wake dynamics in proximity of the base, whereas closer to the rear-stagnation point, the shear layer fluctuations become important. For x/D ≤ 0.75, the time-history and probability distributions of the backflow centroid position allow to identify this motion with an irregular precession about the model symmetry axis occurring at time scales in the order of 103 D/U∞ or higher. The first two modes obtained by snapshot proper orthogonal decomposition of the velocity fluctuations can be related to an anti-symmetric mode of azimuthal wave-number m = 1 reflecting a radial displacement of the separated flow region, while the third and fourth proper orthogonal decomposition modes are identified with a second mode pair m = 2 and are representing wake ovalization. Close to the base, a third axisymmetric mode m = 0 is identified, corresponding to a streamwise pulsation of the reverse flow region. Based on the analysis of the spatial eigen-functions and frequency spectra of the time-coefficients, it is concluded that the anti-symmetric mode m = 1 is associated with the backflow instability in the very-low frequency range StD = 10−4/10−3 close to separation, whereas more downstream it reflects the fluctuations related to the shear layer development.

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Section: A Flow Field Statisticsmentioning

confidence: 63%

Section: Introductionmentioning

confidence: 93%

Low-frequency behavior of the turbulent axisymmetric near-wake

et al. 2016

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“…Early experiments in the turbulent wake behind axisymmetric bodies, such as disks and spheres, (Achenbach 1974;Taneda 1978;Fuchs et al 1979) showed that the predominant flow structures in the near wake are coherent antisymmetric modes with |m| = 1, which are shed at a constant frequency. However, and in contrast with the laminar regime, all of the above studies of turbu- …”

Section: Turbulent Regimementioning

confidence: 99%

Low-dimensional dynamics of a turbulent axisymmetric wake

et al. 2014

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The coherent structures of a turbulent wake generated behind a bluff three-dimensional axisymmetric body are investigated experimentally at a diameter based Reynolds number ∼ 2×10 5 . Proper orthogonal decomposition of base pressure measurements indicates that the most energetic coherent structures retain the structure of the symmetry-breaking laminar instabilities and manifest as unsteady vortex shedding with azimuthal wavenumber m = ±1. In a rotating reference frame, the shedding preserves the reflectional symmetry and is linked with a reflectionally symmetric mean pressure distribution on the base. Due to a slow rotation of symmetry plane of the turbulent wake around the axis of the body, statistical axisymmetry is recovered in the time average. The ratio of the timescales associated with the slow rotation of the symmetry plane and the vortex shedding is of order 100.

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“…Similarly, oscillatory bifurcations appearing at low Reynolds numbers (the ones at higher Reynolds numbers breaking reflectional symmetry) also appear in the turbulent wake as axisymmetric "bubble pumping" (Berger et al 1990) and as "vortex shedding" (Achenbach 1974;Taneda 1978;Fuchs et al 1979;Berger et al 1990), a large-scale anti-symmetric oscillation. This has often been referred to as a "helical" mode when, in fact, as shown by Rigas et al (2014b), periodic shedding is perturbed by a random variation of the azimuthal shedding angle on a very long timescale.…”

Section: Introductionmentioning

confidence: 96%

High-frequency forcing of a turbulent axisymmetric wake

Oxlade

Morrison

Qubain³

et al. 2015

J. Fluid Mech.

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A high-frequency periodic jet, issuing immediately below the point of separation, is used to force the turbulent wake of a bluff axisymmetric body, its axis aligned with the free stream. It is shown that the base pressure may be varied more-or-less at will: at forcing frequencies several times that of the shear-layer frequency, the time-averaged areaweighted base pressure increases by as much as 35%. An investigation of the effects of forcing is made using random and phase-locked two-component PIV, and modal decomposition of pressure fluctuations on the base of the model. The forcing does not target specific local or global wake instabilities: rather, the high-frequency jet creates a row of closely spaced vortex rings, immediately adjacent to which are regions of large shear on each side. These shear layers are associated with large dissipation and inhibit the entrainment of fluid. The resulting pressure recovery is proportional to the strength of the vortices and is accompanied by a broadband suppression of base pressure fluctuations associated with all modes. The optimum forcing frequency, at which amplification of the shear layer mode approaches unity gain, is roughly five times the shear-layer frequency.

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Large-scale coherent structures in the wake of axisymmetric bodies

Cited by 111 publications

References 17 publications

Low-frequency behavior of the turbulent axisymmetric near-wake

Low-frequency behavior of the turbulent axisymmetric near-wake

Low-dimensional dynamics of a turbulent axisymmetric wake

High-frequency forcing of a turbulent axisymmetric wake

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