### Abstract

Direct Numerical Simulations (DNS) are earned out in a turbulent rough-wall pipe at low and medium Reynolds numbers. The rough surface, which is comprised of three-dimensional sinusoidal roughness elements, was viscously scaled from the transitionally rough regime to the fully rough regime. The main aim of this study is to analyse the behaviour of the near-wall cycle as the surface condition changes from smooth through to fully rough. When analysing the streamwise velocity, a triple decomposition is used to distinguish between the fluctuations due to the spatial variation with the actual turbulent fluctuations. For small roughness height (h^{+} < 15), the near-wall cycle streaks occurs above the roughness elements. Although the high and low speed streaks look similar to the smooth wall when visually inspected, subtle differences are observed when the premultiplied energy spectra are analysed. When the flow is fully rough, the near-wall cycle is replaced by the stationary features of the flow which dominate within the roughness elements. We also analyse the contribution of the apparent wall shear stress due to form (pressure) drag (τ^{R}) expressed as a ratio between form and total shear stress (R_{τ} = τ^{R}/τ^{T}. In the fully rough regime, the form drag dominates (R_{τ} > 0.75) and disrupts the near-wall cycle. Townsend's outer layer similarity is observed when the wall normal height normalised by the mean radius of the pipe y/R_{0} is greater than 0.56, where a collapse in the streamwise premultiplied energy spectra is obtained.

Original language | English |
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Title of host publication | 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015 |

Publisher | TSFP-9 |

ISBN (Electronic) | 9780000000002 |

Publication status | Published - 01 Jan 2015 |

Event | 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015 - Melbourne, Australia Duration: 30 Jun 2015 → 03 Jul 2015 |

### Publication series

Name | 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015 |
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Volume | 1 |

### Other

Other | 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015 |
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Country | Australia |

City | Melbourne |

Period | 30/06/15 → 03/07/15 |

### Fingerprint

### All Science Journal Classification (ASJC) codes

- Fluid Flow and Transfer Processes

### Cite this

*9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015*(9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015; Vol. 1). TSFP-9.

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*9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015.*9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, vol. 1, TSFP-9, 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015, Melbourne, Australia, 30/06/15.

**Investigation of a turbulent flow from the transitionally rough regime to the fully rough regime.** / Chan, Leon Zen Hsien; MaCdonald, M.; Hutchins, N.; Chung, D.; Ooi, A.

Research output: Chapter in Book/Report/Conference proceeding › Conference contribution

TY - GEN

T1 - Investigation of a turbulent flow from the transitionally rough regime to the fully rough regime

AU - Chan, Leon Zen Hsien

AU - MaCdonald, M.

AU - Hutchins, N.

AU - Chung, D.

AU - Ooi, A.

PY - 2015/1/1

Y1 - 2015/1/1

N2 - Direct Numerical Simulations (DNS) are earned out in a turbulent rough-wall pipe at low and medium Reynolds numbers. The rough surface, which is comprised of three-dimensional sinusoidal roughness elements, was viscously scaled from the transitionally rough regime to the fully rough regime. The main aim of this study is to analyse the behaviour of the near-wall cycle as the surface condition changes from smooth through to fully rough. When analysing the streamwise velocity, a triple decomposition is used to distinguish between the fluctuations due to the spatial variation with the actual turbulent fluctuations. For small roughness height (h+ < 15), the near-wall cycle streaks occurs above the roughness elements. Although the high and low speed streaks look similar to the smooth wall when visually inspected, subtle differences are observed when the premultiplied energy spectra are analysed. When the flow is fully rough, the near-wall cycle is replaced by the stationary features of the flow which dominate within the roughness elements. We also analyse the contribution of the apparent wall shear stress due to form (pressure) drag (τR) expressed as a ratio between form and total shear stress (Rτ = τR/τT. In the fully rough regime, the form drag dominates (Rτ > 0.75) and disrupts the near-wall cycle. Townsend's outer layer similarity is observed when the wall normal height normalised by the mean radius of the pipe y/R0 is greater than 0.56, where a collapse in the streamwise premultiplied energy spectra is obtained.

AB - Direct Numerical Simulations (DNS) are earned out in a turbulent rough-wall pipe at low and medium Reynolds numbers. The rough surface, which is comprised of three-dimensional sinusoidal roughness elements, was viscously scaled from the transitionally rough regime to the fully rough regime. The main aim of this study is to analyse the behaviour of the near-wall cycle as the surface condition changes from smooth through to fully rough. When analysing the streamwise velocity, a triple decomposition is used to distinguish between the fluctuations due to the spatial variation with the actual turbulent fluctuations. For small roughness height (h+ < 15), the near-wall cycle streaks occurs above the roughness elements. Although the high and low speed streaks look similar to the smooth wall when visually inspected, subtle differences are observed when the premultiplied energy spectra are analysed. When the flow is fully rough, the near-wall cycle is replaced by the stationary features of the flow which dominate within the roughness elements. We also analyse the contribution of the apparent wall shear stress due to form (pressure) drag (τR) expressed as a ratio between form and total shear stress (Rτ = τR/τT. In the fully rough regime, the form drag dominates (Rτ > 0.75) and disrupts the near-wall cycle. Townsend's outer layer similarity is observed when the wall normal height normalised by the mean radius of the pipe y/R0 is greater than 0.56, where a collapse in the streamwise premultiplied energy spectra is obtained.

UR - http://www.scopus.com/inward/record.url?scp=85034445025&partnerID=8YFLogxK

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M3 - Conference contribution

AN - SCOPUS:85034445025

T3 - 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015

BT - 9th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2015

PB - TSFP-9

ER -