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Unsteady forcing of turbulence by a randomly actuated impeller array

Unsteady forcing of turbulence by a randomly actuated impeller array
Unsteady forcing of turbulence by a randomly actuated impeller array
We investigate the unsteady forcing of turbulent flow in a well-stirred reactor using opposing arrays of pitched-blade impellers which randomly and independently reverse rotation. We systematically explore the dependence of the large-scale motions and the homogeneity and isotropy of the turbulence upon the forcing. We identify three dimensionless control parameters: the source fraction (the fraction of time spent in clockwise motion), the dimensionless forcing period and an impeller Reynolds number. We find the timescale of unsteady motion corresponds to the forcing period T, the average period of impeller reversal, independently of the impeller angular speed Ω and source fraction. As in jet-stirred tanks, unsteady forcing substantially increases the unsteady kinetic energy, energy dissipation, integral length scale and Taylor microscale Reynolds number (Rλ) and improves the homogeneity and isotropy of the flow, provided the source fraction is chosen optimally and the forcing period is sufficiently large (ΩT > 103); impeller Reynolds number has a relatively small influence.
The forcing period must be matched to angular speed: decreasing the forcing period below this threshold results in a less intense, more inhomogeneous turbulent flow. Spectra of two-point velocity increments demonstrate that unsteady energy injection is dominated by axial shear generated across impellers and becomes less prominent at smaller scales.
However, even at Rλ ≈ 354, the signature of this unsteady forcing can still be detected in near-dissipation-range statistics. These observations provide insight into optimisation of forcing and the mechanism of energy transfer when using unsteady forcing to generate turbulence in confined vessels.
0723-4864
Lawson, John
4e0b1895-51c5-41e6-9322-7f79e76e0e4c
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Lawson, John
4e0b1895-51c5-41e6-9322-7f79e76e0e4c
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Lawson, John and Ganapathisubramani, Bharathram (2022) Unsteady forcing of turbulence by a randomly actuated impeller array. Experiments in Fluids, 63 (1), [13]. (doi:10.1007/s00348-021-03364-8).

Record type: Article

Abstract

We investigate the unsteady forcing of turbulent flow in a well-stirred reactor using opposing arrays of pitched-blade impellers which randomly and independently reverse rotation. We systematically explore the dependence of the large-scale motions and the homogeneity and isotropy of the turbulence upon the forcing. We identify three dimensionless control parameters: the source fraction (the fraction of time spent in clockwise motion), the dimensionless forcing period and an impeller Reynolds number. We find the timescale of unsteady motion corresponds to the forcing period T, the average period of impeller reversal, independently of the impeller angular speed Ω and source fraction. As in jet-stirred tanks, unsteady forcing substantially increases the unsteady kinetic energy, energy dissipation, integral length scale and Taylor microscale Reynolds number (Rλ) and improves the homogeneity and isotropy of the flow, provided the source fraction is chosen optimally and the forcing period is sufficiently large (ΩT > 103); impeller Reynolds number has a relatively small influence.
The forcing period must be matched to angular speed: decreasing the forcing period below this threshold results in a less intense, more inhomogeneous turbulent flow. Spectra of two-point velocity increments demonstrate that unsteady energy injection is dominated by axial shear generated across impellers and becomes less prominent at smaller scales.
However, even at Rλ ≈ 354, the signature of this unsteady forcing can still be detected in near-dissipation-range statistics. These observations provide insight into optimisation of forcing and the mechanism of energy transfer when using unsteady forcing to generate turbulence in confined vessels.

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More information

Accepted/In Press date: 3 December 2021
e-pub ahead of print date: 22 December 2021
Published date: January 2022
Additional Information: Funding Information: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 846648.

Identifiers

Local EPrints ID: 453005
URI: http://eprints.soton.ac.uk/id/eprint/453005
ISSN: 0723-4864
PURE UUID: 4d4afa76-bf10-462b-83f5-7e5b06767d58
ORCID for John Lawson: ORCID iD orcid.org/0000-0003-3260-3538
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 07 Jan 2022 12:29
Last modified: 28 Apr 2022 02:25

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