TY - JOUR

T1 - Helical Structures in the Near Field of a Turbulent Pipe Jet

AU - Mullyadzhanov, R.

AU - Abdurakipov, S.

AU - Hanjalić, K.

PY - 2017/3/1

Y1 - 2017/3/1

N2 - We perform a finely resolved Large-eddy simulation to study coherent vortical structures populating the initial (near-nozzle) zone of a pipe jet at the Reynolds number of 5300. In contrast to ‘top-hat’ jets featured by Kelvin-Helmholtz rings with the non-dimensional frequency St≈0.3−0.6, no high-frequency dominant mode is observed in the near field of a jet issuing from a fully-developed pipe flow. Instead, in shear layers we observe a relatively wide peak in the power spectrum within the low-frequency range (St≈0.14) corresponding to the propagating helical waves entering with the pipe flow. This is confirmed by the Fourier transform with respect to the azimuthal angle and the Proper Orthogonal Decomposition complemented with the linear stability analysis revealing that this low-frequency motion is not connected to the Kelvin-Helmholtz instability. We demonstrate that the azimuthal wavenumbers m=1−5 contain the most of the turbulent kinetic energy and that a common form of an eigenmode is a helical vortex rotating around the axis of symmetry. Small and large timescales are identified corresponding to “fast” and “slow” rotating modes. While the “fast” modes correspond to background turbulence and stochastically switch from co- to counter-rotation, the “slow” modes are due to coherent helical structures which are long-lived and have low angular velocities, in agreement with the previously described spectral peak at low St.

AB - We perform a finely resolved Large-eddy simulation to study coherent vortical structures populating the initial (near-nozzle) zone of a pipe jet at the Reynolds number of 5300. In contrast to ‘top-hat’ jets featured by Kelvin-Helmholtz rings with the non-dimensional frequency St≈0.3−0.6, no high-frequency dominant mode is observed in the near field of a jet issuing from a fully-developed pipe flow. Instead, in shear layers we observe a relatively wide peak in the power spectrum within the low-frequency range (St≈0.14) corresponding to the propagating helical waves entering with the pipe flow. This is confirmed by the Fourier transform with respect to the azimuthal angle and the Proper Orthogonal Decomposition complemented with the linear stability analysis revealing that this low-frequency motion is not connected to the Kelvin-Helmholtz instability. We demonstrate that the azimuthal wavenumbers m=1−5 contain the most of the turbulent kinetic energy and that a common form of an eigenmode is a helical vortex rotating around the axis of symmetry. Small and large timescales are identified corresponding to “fast” and “slow” rotating modes. While the “fast” modes correspond to background turbulence and stochastically switch from co- to counter-rotation, the “slow” modes are due to coherent helical structures which are long-lived and have low angular velocities, in agreement with the previously described spectral peak at low St.

KW - Helical structures

KW - Jets

KW - Vortex dynamics

KW - LOW-REYNOLDS-NUMBER

KW - INITIAL CONDITIONS

KW - EIGENFUNCTION DECOMPOSITION

KW - PREFERRED MODE

KW - ROUND FREE JET

KW - AXISYMMETRICAL JET

KW - DIRECT NUMERICAL-SIMULATION

KW - DOWNSTREAM EVOLUTION

KW - ENERGETIC MODES

KW - PROPER ORTHOGONAL DECOMPOSITION

UR - http://www.scopus.com/inward/record.url?scp=84978701047&partnerID=8YFLogxK

U2 - 10.1007/s10494-016-9753-2

DO - 10.1007/s10494-016-9753-2

M3 - Article

AN - SCOPUS:84978701047

VL - 98

SP - 367

EP - 388

JO - Flow, Turbulence and Combustion

JF - Flow, Turbulence and Combustion

SN - 1386-6184

IS - 2

ER -