Chen, Xue (2021) Uniform-momentum zones and coherent structures in Newtonian and non-Newtonian pipe flows. PhD thesis, University of Warwick.

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Abstract

The coherent structures in turbulent pipe flows of Newtonian and non- Newtonian fluids are investigated using direct numerical simulation data for the coherent structure organisation, statistical characteristics, evolution and interplay.

The uniform-momentum zones (UMZ) and UMZ interfaces, namely the internal shear layers (ISL) are investigated. The UMZs are large separate regions travelling at relatively constant streamwise velocities, and are demarcated by the thin high-shear layers clustered with spanwise vortices. The UMZs and ISLs are identified by using four different identification methods, three from the literature and a new method which has no ad-hoc parameters applied for UMZ selection. The UMZs identified from using different methods show qualitatively consistent characteristics. The UMZ characteristics observed in the literature have been successfully captured without the use of ad-hoc parameters. The characteristics and dynamics of the UMZs show similarities to both turbulent boundary layer and channel flows. The hierarchical structural distribution of UMZs matches the hierarchy of multi-scaled eddies in Townsend’s attached eddy hypothesis. Conditional average quantities revealed the abrupt jump of the streamwise velocity when passing the UMZ interfaces, accompanied with rapid decrease in turbulent intensity. The velocity jump across the ISLs is more abrupt across ISLs residing closer to the wall. The level of contortion of the ISLs intensifies when moving away from the wall, and is always more tortuous in the azimuthal direction than the streamwise direction. The local imbalance between sweep and ejection events are quantified by the skewness of the wall-normal location fluctuation of the contorted UMZ interfaces. The UMZ interfaces in the near-wall region show asymmetric modulation on ejections over sweeps, i.e., the ejections are predominantly stronger than sweeps near the wall with frequently observed bursting. In the pipe centre, the flow has local predominantly stronger sweeps than ejections. The location of locally balanced ejections and sweeps is found approximately at half of the pipe radius.

The DNS data of shear-thinning non-Newtonian pipe flows are examined with the power-law rheology model using three power-law indices. The flow starts to show regions of local laminarisation as the flow becomes more shear-thinning. In the shear-thinning fluids, the off-axis fluctuations are critically lowered, particularly in the wall-normal direction. At the highest level of shear-thinning examined, the flow is already transitional, showing turbulent spots and large regions of pre-transition laminar fluctuations. These laminar fluctuations are revealed by proper orthogonal x decomposition as very-large-scale cross-flow instabilities. While the off-axis fluctuations are significantly weakened, the root-mean-square streamwise fluctuation is marginally higher than the Newtonian flow due to the streamwise acceleration in the upstream and downstream of the turbulent spots. The vortical structures are much more organised in the shear-thinning fluids compared with the Newtonian flow. The near-wall streaks are wider in the shear-thinning fluids, and the spanwise streak spacing increases as the flow becomes more shear-thinning. The fine-scale flow topology is interpreted from the joint PDFs of the velocity-gradient tensor invariants. The universal ‘tear drop’ shape of the Q − R distribution is obtained in the most inhomogeneous shear-thinning fluid, even in the pseudo-laminar regions. Hence, the universality of the classical ‘tear drop’-shaped Q − R distribution is not limited to fully-developed inhomogeneous turbulence.

Item Type:	Thesis (PhD)
Subjects:	Q Science > QA Mathematics Q Science > QC Physics T Technology > TA Engineering (General). Civil engineering (General) T Technology > TJ Mechanical engineering and machinery
Library of Congress Subject Headings (LCSH):	Pipe -- Fluid dynamics, Tubes -- Fluid dynamics, Turbulence, Newtonian fluids, Non-Newtonian fluids, Eddies
Official Date:	August 2021
Dates:	Date Event August 2021 UNSPECIFIED
Institution:	University of Warwick
Theses Department:	School of Engineering
Thesis Type:	PhD
Publication Status:	Unpublished
Supervisor(s)/Advisor:	Chung, Yongmann M. ; Wan, Minping
Sponsors:	Southern University of Science and Technology
Format of File:	pdf
Extent:	xviii, 140 leaves : illustrations
Language:	eng

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