Influence of freestream turbulence on the flow over a wall roughness

Bucci, Michele Alessandro; Cherubini, Stefania; Loiseau, Jean-Christophe; Robinet, Jean-Christophe

doi:10.1103/physrevfluids.6.063903

Cited by 20 publications

(40 citation statements)

References 76 publications

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“…(2015); triangles denote data from Bucci et al. (2021). The symbols in purple denote sinuous modes, in blue denote varicose modes, in green represent the case that transition occurs immediately downstream and global instability has not been applied.…”

Section: Resultsmentioning

confidence: 99%

“…At x = 0, two pairs of streamwise vortices are observed on the lateral sides of the cube in both base and mean flows. The pair very close to the cube are referred to as the symmetry plane (SP) vortices (Iyer & Mahesh 2013) or the rear pair vortices (Ye, Schrijer & Scarano 2016;Bucci et al 2021). They push low-momentum flow upwards, move closer to the symmetry plane, give rise to the low-speed region behind the roughness, and are dissipated farther downstream.…”

Section: Base Flow Versus Mean Flowmentioning

confidence: 99%

“…Bucci et al. (2021) highlighted that the roughness Reynolds number and aspect ratio might not be the only important parameters for flow characteristics; also plays a crucial role in the onset and symmetry of the primary global instability.…”

Section: Introductionmentioning

confidence: 99%

“…Their results suggest that larger elements are able to trip turbulence by either a convective or a global instability in the near-wake region. Bucci et al (2021) highlighted that the roughness Reynolds number Re h = U e h/ν and aspect ratio might not be the only important parameters for flow characteristics; h/δ * also plays a crucial role in the onset and symmetry of the primary global instability.…”

Section: Introductionmentioning

confidence: 99%

“…The wake flow is studied for its importance in both unstable and stable regimes, where past work by showed how in the stable regime, roughness can amplify upstream disturbances and lead to transition. Characterizing unstable modes in terms of their varicose or sinuous nature continues to be useful, as shown by the recent study by Bucci et al (2021), where wake flow containing sinuous instability is seen to be more receptive to disturbance forcing upstream of the roughness elements that results in a larger increase in skin-friction coefficient. We therefore study, for a thin (η 1) roughness element with a large h/δ * , which of the modes is dominant, if the sinuous instability is observed, how the onset of sinuous instability is influenced by Re h , and the resulting nonlinear interaction.…”

Section: Introductionmentioning

confidence: 99%

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Global stability analysis and direct numerical simulation of boundary layers with an isolated roughness element

Mahesh

2022

J. Fluid Mech.

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Global stability analysis and direct numerical simulation (DNS) are used to study boundary layer flows with an isolated roughness element. The aspect ratio of the element ( $\eta$ ) is small, while the ratio of element height to displacement boundary layer thickness ( $h/\delta ^{*}$ ) is large. Both steady base flows and time-averaged mean flows are able to capture the frequencies of the primary vortical structures and mode shapes. Global stability results highlight that although the varicose instability is dominant for large $h/\delta ^{*}$ , sinuous instability becomes more pronounced as $Re_h$ increases for the thin geometry ( $\eta =0.5$ ), due to increased spanwise shear in the near-wake region. Wavemaker results indicate that $\eta$ affects the convective nature of the shear layer more than the type of instability. DNS results show that different instability mechanisms lead to different development and evolution of vortical structures in the transition process. For $\eta =1$ , the varicose instability is associated with the periodic shedding of hairpin vortices, and its stronger spatial transient growth indicated by wavemaker results aids the formation of hairpin vortices farther downstream. In contrast, for $\eta =0.5$ , the interplay between varicose and sinuous instabilities results in a broader-banded energy spectrum and leads to the sinuous wiggling of hairpin vortices in the near wake when $Re_h$ is sufficiently high. A sinuous mode with a lower frequency captured by dynamic mode decomposition analysis, and associated with the ‘wiggling’ of streaks, persists far downstream and promotes transition to turbulence. A new regime map is developed to classify and predict instability mechanisms based on $Re_{hh}^{1/2}$ and $d/\delta ^{*}$ using a logistic regression model. Although the mean skin friction demonstrates different evolutions for the two geometries, both of them efficiently trip the flow to turbulence at $Re_h=1100$ . An earlier location of a fully-developed turbulent state is established for $\eta =1$ at $x \approx 110h$ .

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Section: Resultsmentioning

confidence: 99%

Section: Base Flow Versus Mean Flowmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Global stability analysis and direct numerical simulation of boundary layers with an isolated roughness element

Mahesh

2022

J. Fluid Mech.

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Contribution of Vortices to Laminar-Turbulent Transition in a Boundary Layer Disturbed by a Roughness Element

Römer,

Wenzel,

Rist

2023

Notes on Numerical Fluid Mechanics and Multidisciplinary Design

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Changes in the Transition Process of Roughness-Induced Crossflow Vortices due to Freestream Turbulence

Nakagawa,

Ishida,

Tsukahara

2024

Flow Turbulence Combust

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Laminar-turbulent transitions in boundary layers are one of the major research topics in fluid dynamics. In this study, we focused on a three-dimensional boundary layer formed on a swept flat plate. In this boundary layer, the crossflow instability is dominant, and the instability induces crossflow vortices (CFVs). Many studies have focused on the dependency of the transition process on the intensities of steady or unsteady disturbances, which correspond to a roughness element and freestream turbulence (FST), respectively. On the other hand, the effects of the FST wavelength are still unclear. Moreover, there is a lack of knowledge about the transition processes caused by both steady and unsteady disturbances. We investigated how the transition process of a stationary structure caused by cylindrical roughness changes depending on the FST wavelength using direct numerical simulations. We classified transition processes into two types: processes in which stationary structures grow into CFVs and processes in which hairpin vortices are generated on the stationary structures. The former is further classified into four types depending on the presence or absence of FST and on the FST wavelength. We revealed the contributions of different FST wavelengths to the transition process changes. The short-wavelength FST provides hairpin vortices to the stationary structure at low roughness height conditions because of its high-frequency components. The long-wavelength FST changes the process due to unsteady fluctuations influencing the stationary structure. In summary, the transition processes undergo different mechanisms between short- and long-wavelength FST.

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Influence of freestream turbulence on the flow over a wall roughness

Cited by 20 publications

References 76 publications

Global stability analysis and direct numerical simulation of boundary layers with an isolated roughness element

Global stability analysis and direct numerical simulation of boundary layers with an isolated roughness element

Contribution of Vortices to Laminar-Turbulent Transition in a Boundary Layer Disturbed by a Roughness Element

Changes in the Transition Process of Roughness-Induced Crossflow Vortices due to Freestream Turbulence

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