Article
Mechanics
T. O. Jelly, A. Ramani, B. Nugroho, N. Hutchins, A. Busse
Summary: The study reveals that ESy can strongly affect the roughness drag penalty, especially for low-ESx surfaces. Additionally, it is observed that specific low-ESy surfaces may result in reduced levels of outer-layer similarity in both mean flow and turbulence statistics, attributed to inadequate scale separation between the outer length scale and the in-plane spanwise roughness wavelength.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Guo-Zhen Ma, Chun-Xiao Xu, Hyung Jin Sung, Wei-Xi Huang
Summary: Direct numerical simulations (DNSs) were conducted to investigate the effects of roughness on statistical properties and large-scale coherent structures in turbulent channel flow over three-dimensional sinusoidal rough walls. The study examined the similarities in mean streamwise velocity and Reynolds stresses in the outer layer by varying the roughness Reynolds number and the ratio of roughness height to the half-channel height. Spectral analysis and scale-energy path display were used to explore the energy transfer mechanism of turbulent motions with different sized roughness elements. The results showed that increasing roughness Reynolds number led to a downward shift of the mean velocity profile and decreased intensity of turbulent Reynolds stresses.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
A. Rouhi, M. K. Fu, D. Chandran, A. Zampiron, A. J. Smits, I. Marusic
Summary: Turbulent drag reduction through streamwise travelling waves is investigated over a wide range of Reynolds numbers. Wall-resolved large-eddy simulations are conducted to examine how the frequency and wavenumber of the travelling wave influence the drag reduction. The study finds that the level of turbulence attenuation, and hence drag reduction, changes with the near-wall Stokes layer protrusion height. A range of frequencies is identified where the Stokes layer attenuates turbulence and increases the drag reduction, while outside this range, the strong Stokes shear strain leads to a drop in drag reduction.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Jonathan Neuhauser, Kay Schaefer, Davide Gatti, Bettina Frohnapfel
Summary: Heterogeneous roughness in the form of streamwise aligned strips can generate large scale secondary motions under turbulent flow conditions. We propose a simple roughness model that can capture the features of turbulent secondary flow without impacting the laminar base flow. The model shows good agreement with experimental data in terms of the secondary flow topology.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
D. Chandran, A. Zampiron, A. Rouhi, M. . K. Fu, D. Wine, B. Holloway, A. J. Smits, I. Marusic
Summary: This paper presents measurements of turbulent drag reduction in boundary layers at high friction Reynolds numbers. The study finds that using streamwise travelling waves of spanwise wall oscillations can effectively reduce turbulence drag, especially with lower frequency actuation that saves energy.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Esther Mateling, Marian Albers, Wolfgang Schroder
Summary: Investigation of the alteration of the near-wall flow field in a turbulent boundary layer flow subjected to spanwise travelling transversal surface waves at a friction Reynolds number Re tau asymptotic to 1525 reveals that this flow control method induces periodic large-scale bursts near the wall and reduces the energetic content of small-scale features. The occurrence of intense large-scale ejections near the wall is crucial for reducing wall-shear stress, as they balance large-scale sweeps from the outer layer, thus attenuating overall fluctuation intensity. Additionally, the periodic secondary flow field interferes with the quasi-streamwise vortices near the wall, deforming their cross-section into an elliptic shape and resulting in vortex disintegration, which, combined with the effect of large-scale ejections, leads to a decrease in wall-normal momentum exchange and the weakening of near-wall streaks.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Mostafa Aghaei-Jouybari, Junlin Yuan, Zhaorui Li, Giles J. Brereton, Farhad A. Jaberi
Summary: Direct numerical simulations were used to investigate the effects of roughness on supersonic turbulent channel flows. Results show that the turbulence is strongly influenced by the roughness topography and the associated shock patterns. Two-dimensional (2-D) roughness generates strong oblique shock waves, while three-dimensional (3-D) roughness results in weaker shocklets. The impingement of shocks on the wall leads to localized augmentations of turbulence shear production. The entropy generation and temperature values are higher in 2-D roughness cases compared to 3-D ones. The effects of roughness extend beyond the near-wall layer due to the presence of shocks.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
M. Gul, B. Ganapathisubramani
Summary: This study investigates the flow characteristics of a turbulent boundary layer over different sand-grain roughness created by different sandpapers. The results show self-similarity in flow properties within a certain range, regardless of transitional or fully rough conditions. Despite differences in turbulence profiles, large-scale structures across all locations remain independent of roughness parameters.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Akanksha Baranwal, Diego A. Donzis, Rodney D. W. Bowersox
Summary: This study investigates the effects of compressibility on the near-wall asymptotic behavior of turbulent fluxes using a large direct numerical simulation (DNS) database, finding that the behavior of compressible turbulent flow near walls differs from incompressible flow even when mean density variations are considered. As Mach number increases, turbulent fluxes containing wall-normal components exhibit a decrease in slope due to increased dilatation effects, with Ity approaching its high Mach number asymptote at lower Mach numbers than other fluxes.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
F. Ghanadi, L. Djenidi
Summary: The behavior of a fully rough-wall turbulent boundary layer under different pressure gradients is studied. It is found that the boundary layer remains in a self-preserving state regardless of the pressure gradient, but different pressure gradients lead to different self-preservation states. The study also reveals that the roughness effect is more significant than the pressure gradient, especially near the wall.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Rong Ma, Karim Alame, Krishnan Mahesh
Summary: The numerical simulation reveals that the influence of a rough bottom wall on flow mainly manifests in the roll-up of the shear layer in the wall-normal direction. There is a strong correlation between the height of random short wave roughness and wall shear stress, depending on the local roughness topography.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Thermodynamics
Guo-Zhen Ma, Chun-Xiao Xu, Hyung Jin Sung, Wei-Xi Huang
Summary: Large-eddy simulations (LESs) were conducted to study the effects of roughness on the secondary motions (SMs) and wall-attached structures in a turbulent open-channel flow. It was found that different roughness arrangements had minor differences in the mean streamwise velocity. The clustered arrangement generated large-scale SMs, while the homogeneous random arrangement maintained outer-layer similarity. Wall-attached structures were also identified and shown to play a significant role in rough-wall turbulence.
INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW
(2023)
Article
Mechanics
Mostafa Aghaei Jouybari, Junlin Yuan, Giles J. Brereton, Michael S. Murillo
Summary: This study investigates the effect of surface roughness on turbulent flow by developing a high-fidelity prediction approach using deep neural network and Gaussian process regression machine learning methods. The predicted equivalent sand-grain height for turbulent flows over various rough surfaces showed high accuracy compared to existing prediction formulas, with surface porosity and roughness slope identified as important factors in drag prediction.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
C. Chen, L. He
Summary: Recent findings on wall-bounded turbulence have led to the development of a new modelling method to capture the influence of large-scale coherent structures and scale-interaction on near-wall turbulence. This paper presents a two-scale approach to simulate this influence and validates its effectiveness.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Davide Modesti, Srikanth Sathyanarayana, Francesco Salvadore, Matteo Bernardini
Summary: This study investigates the characteristics of supersonic turbulent channel flow over cubical roughness elements through direct numerical simulation. The results show that the Mach number has a significant impact on the mean velocity profile, while the thermal field is substantially affected by the roughness.
JOURNAL OF FLUID MECHANICS
(2022)
Review
Mechanics
Daniel Chung, Nicholas Hutchins, Michael P. Schultz, Karen A. Flack
Summary: Reliable full-scale prediction of drag due to rough wall-bounded turbulent fluid flow remains a challenge, with at least 10% uncertainty. Recent advances have lowered barriers and are beginning to impact other multiphysical areas, promising increased predictive reliability.
ANNUAL REVIEW OF FLUID MECHANICS, VOL 53
(2021)
Article
Thermodynamics
S. Endrikat, D. Modesti, M. MacDonald, R. Garcia-Mayoral, N. Hutchins, D. Chung
Summary: The study investigates the breakdown of drag-reduction mechanisms in riblets as their viscous-scaled size becomes large enough for turbulence to approach the wall. Using the minimal-span channel concept, cost-efficient direct numerical simulation of rough-wall flows was conducted to study the inertial-flow mechanisms. Different shapes and sizes of riblets were examined, with findings indicating varying influences on the Kelvin-Helmholtz instability.
FLOW TURBULENCE AND COMBUSTION
(2021)
Article
Mechanics
B. Nugroho, J. P. Monty, I. K. A. P. Utama, B. Ganapathisubramani, N. Hutchins
Summary: The study systematically investigated the influence of the ratio of in-plane roughness wavelength to the boundary layer thickness on wind tunnel test results, finding a demarcation between Δ-type and non-Δ-type behavior under certain conditions.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Endocrinology & Metabolism
Rana S. Dhillon, Lana Nguyen, Wagih Abu Rowin, Ruhi S. Humphries, Kevin Kevin, Jason D. Ward, Andrew Yule, Tuong D. Phan, Yi Chen Zhao, David Wynne, Peter M. McNeill, Nicholas Hutchins, David A. Scott
Summary: Intubation and extubation during endoscopic endonasal pituitary surgery generate high amounts of small particles that remain suspended in air for long durations and disperse throughout the theatre. In contrast, endonasal access and pituitary tumour resection produce lower concentrations of larger particles that are airborne for shorter periods and travel shorter distances.
Article
Mechanics
S. Endrikat, D. Modesti, R. Garcia-Mayoral, N. Hutchins, D. Chung
Summary: Research has shown that only large sharp-triangular and blade riblets increase drag due to the Kelvin-Helmholtz instability, while blunt-triangular and trapezoidal riblets do not exhibit this mechanism. This study also found that the cross-sectional area of the grooves in viscous units serves as a proxy for the necessary wall-normal permeability for the development of Kelvin-Helmholtz rollers. Additionally, the occurrence of the instability is correlated with high momentum absorption at the riblet tips, which can be predicted qualitatively using Stokes flow.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
R. Baidya, J. Philip, N. Hutchins, J. P. Monty, I. Marusic
Summary: The study reveals that the impact of Reynolds number on spanwise velocity generally follows a logarithmic trend, with the presence of intermediate-scale eddies leading to characteristics related to the distance from the wall.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Energy & Fuels
Mathieu Pichault, Claire Vincent, Grant Skidmore, Jason Monty
Summary: This study developed and tested two novel wind power forecasting methods based on LiDAR measurements, showing superior performance compared to traditional benchmark methods. This highlights the potential of remote sensing instruments for short-term wind power forecasting applications.
Article
Mechanics
Davide Modesti, Sebastian Endrikat, Nicholas Hutchins, Daniel Chung
Summary: The study conducted direct numerical simulations on the impact of riblets on turbulent flow, revealing the drag change patterns of different riblet geometries and emphasizing the drag increase associated with dispersive stresses carried by secondary flows.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Mogeng Li, Charitha M. de Silva, Daniel Chung, Dale Pullin, Ivan Marusic, Nicholas Hutchins
Summary: This study presents an experimental dataset documenting the evolution of a turbulent boundary layer downstream of a rough-to-smooth surface transition. The flow in the internal layer is not in equilibrium with the wall-shear stress, leading to a model that blends the rough-wall and smooth-wall profiles to mimic the recovering mean velocity.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Michael Heisel, Charitha M. de Silva, Nicholas Hutchins, Ivan Marusic, Michele Guala
Summary: The statistical properties of prograde spanwise vortex cores and internal shear layers in high-Reynolds-number turbulent boundary layers are evaluated. Results show the importance of the local large-eddy turnover time in determining the strain rate confining the size of the vortex cores and shear layers. The study highlights the relevance of the turnover time and the Taylor microscale in explaining the interaction of coherent velocity structures in the boundary layer flows.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Honglei Bai, Jinlai Gong, Zhenbo Lu
Summary: Time-invariant secondary flows have been observed in turbulent boundary layer flows over spanwise-heterogeneous rough walls, connected with instantaneous large-scale structures. Proper orthogonal decomposition (POD) technique was used to extract dominant energetic structures, revealing significant effects on velocity spatial correlations and Reynolds stress distributions.
Article
Mechanics
D. D. Wangsawijaya, N. Hutchins
Summary: This study examines the turbulent boundary layers developing over surfaces with spanwise heterogeneous roughness. It finds that the heterogeneous cases induce counter-rotating secondary flows, which are compared to the large-scale turbulent structures over smooth walls. The results suggest that the secondary flows are spanwise-locked turbulent structures, with the strength of the structures and locking efficacy determined by the roughness half-wavelength. The study also finds that the secondary flows exhibit maximum unsteadiness when the roughness half-wavelength is approximately 1.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Honglei Bai, Zhenbo Lu, Renke Wei, Yannian Yang, Yu Liu
Summary: The study shows that the sinusoidal wavy cylinder can effectively reduce fluid forces and decrease aeroacoustic noise, resulting in a significant decrease in overall sound pressure levels.
Article
Thermodynamics
Yu Xia, Wagih Abu Rowin, Thomas Jelly, Ivan Marusic, Nicholas Hutchins
Summary: In this study, simulations and experiments are used to investigate the spatial and temporal attenuation of cold wire sensors. The results show that increasing the wire length attenuates the variance of temperature fluctuations but does not affect the mean temperature. The temporal attenuation of temperature fluctuations becomes more significant as the wire aspect ratio decreases below 1000. The attenuation due to insufficient wire aspect ratios affects all energy scales in the boundary layer. Maintaining the viscous-scaled wire length constant and using smaller diameter wires can better resolve temperature fluctuations by sacrificing spatial resolution.
INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW
(2022)
Article
Mechanics
Xuebo Li, Nicholas Hutchins, Xiaojing Zheng, Ivan Marusic, Woutijn J. Baars
Summary: A large-scale array of measuring instruments was used to study the statistical structure of turbulence structures in different stability conditions. The researchers found a self-similar range of turbulence structures and analyzed the scale-dependent inclination angle in the streamwise/wall-normal plane. The results showed that the inclination angle and aspect ratio of the structures were highly sensitive to the stability parameter.
JOURNAL OF FLUID MECHANICS
(2022)
