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Entrainment effects in periodic forcing of the flow over a backward-facing step

Entrainment effects in periodic forcing of the flow over a backward-facing step
Entrainment effects in periodic forcing of the flow over a backward-facing step
The effect of the Strouhal number on periodic forcing of the flow over a backward-facing step (height, H) is investigated experimentally. Forcing is applied by a synthetic jet at the edge of the step at Strouhal numbers ranging from 0.21 < StH < 1.98 (StH = f H/U∞) at a Reynolds number of ReH = HU∞/ν = 41000. In the literature, the effect of Strouhal number on the reattachment length is often divided into low- and high frequency actuation, referring to specific frequencies present in the unforced flow. In the present study, variations with Strouhal number are explained based on a continuous variation of entrainment of momentum into the recirculation region rather than on specific frequencies. The reattachment length is shown to decrease linearly with entrainment of momentum. Vertical momentum flux is driven by vortices generated by the forcing and locally vertical momentum flux is shown to be qualitatively similar to circulation for all cases considered. Total circulation (and therewith entrainment of momentum and the effect on the reattachment length) is shown to decrease with Strouhal number whereas this is not predicted by models based on specific low- and high frequencies. An empirical model for the (decay of) circulation is derived by tracking vortices in phase-locked data. This model is used to decipher relevant scaling parameters that explain the variations in circulation, entrainment of momentum and reattachment length. Three regimes of Strouhal number are identified. A low-Strouhal-number regime is observed for which vortices are formed at a late stage relative to the recirculation region, causing a decrease in effectiveness. For high Strouhal numbers vortices are being re-ingested into the actuator or are packed so close together that they cancel each other, both decreasing the effectiveness of forcing. In the intermediate regime a vortex train is formed of which the decay of circulation increases for increasing Strouhal number. The scaling of this decay fully explains the observed variation in reattachment length. The observations on entrainment of momentum made in this study are expected to also hold for periodic forcing of other bluff-body flows.
2469-990X
Berk, Tim, Alexander
6d080fa4-abf6-4a81-9e35-178e8454e755
Medjnoun, Takfarinas
878ec432-7360-48ad-bce1-c7a1dc2191cb
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052
Berk, Tim, Alexander
6d080fa4-abf6-4a81-9e35-178e8454e755
Medjnoun, Takfarinas
878ec432-7360-48ad-bce1-c7a1dc2191cb
Ganapathisubramani, Bharathram
5e69099f-2f39-4fdd-8a85-3ac906827052

Berk, Tim, Alexander, Medjnoun, Takfarinas and Ganapathisubramani, Bharathram (2017) Entrainment effects in periodic forcing of the flow over a backward-facing step. Physical Review Fluids. (doi:10.1103/PhysRevFluids.2.074605).

Record type: Article

Abstract

The effect of the Strouhal number on periodic forcing of the flow over a backward-facing step (height, H) is investigated experimentally. Forcing is applied by a synthetic jet at the edge of the step at Strouhal numbers ranging from 0.21 < StH < 1.98 (StH = f H/U∞) at a Reynolds number of ReH = HU∞/ν = 41000. In the literature, the effect of Strouhal number on the reattachment length is often divided into low- and high frequency actuation, referring to specific frequencies present in the unforced flow. In the present study, variations with Strouhal number are explained based on a continuous variation of entrainment of momentum into the recirculation region rather than on specific frequencies. The reattachment length is shown to decrease linearly with entrainment of momentum. Vertical momentum flux is driven by vortices generated by the forcing and locally vertical momentum flux is shown to be qualitatively similar to circulation for all cases considered. Total circulation (and therewith entrainment of momentum and the effect on the reattachment length) is shown to decrease with Strouhal number whereas this is not predicted by models based on specific low- and high frequencies. An empirical model for the (decay of) circulation is derived by tracking vortices in phase-locked data. This model is used to decipher relevant scaling parameters that explain the variations in circulation, entrainment of momentum and reattachment length. Three regimes of Strouhal number are identified. A low-Strouhal-number regime is observed for which vortices are formed at a late stage relative to the recirculation region, causing a decrease in effectiveness. For high Strouhal numbers vortices are being re-ingested into the actuator or are packed so close together that they cancel each other, both decreasing the effectiveness of forcing. In the intermediate regime a vortex train is formed of which the decay of circulation increases for increasing Strouhal number. The scaling of this decay fully explains the observed variation in reattachment length. The observations on entrainment of momentum made in this study are expected to also hold for periodic forcing of other bluff-body flows.

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Berk_PRF_2017_2 - Accepted Manuscript
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Accepted/In Press date: 5 July 2017
e-pub ahead of print date: 31 July 2017

Identifiers

Local EPrints ID: 412218
URI: https://eprints.soton.ac.uk/id/eprint/412218
ISSN: 2469-990X
PURE UUID: 01b134e8-8c99-406d-a392-1b4bd1c069c7
ORCID for Bharathram Ganapathisubramani: ORCID iD orcid.org/0000-0001-9817-0486

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Date deposited: 14 Jul 2017 16:30
Last modified: 20 Jul 2019 04:59

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