Article
Mechanics
Alan H. Duong, Thomas C. Corke, Flint O. Thomas
Summary: Experiments using active flow control successfully reduced the viscous drag in turbulent boundary layers and decreased the wall-normal vorticity component, improving turbulence characteristics.
JOURNAL OF FLUID MECHANICS
(2021)
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
Y. X. Wang, K-S Choi, M. Gaster, C. Atkin, V Borodulin, Y. Kachanov
Summary: This study conducted opposition control of artificially initiated turbulent spots in a low-turbulence wind tunnel in order to delay the transition to turbulence by modifying the turbulent structure within the spots. Through wall-normal jets, the high-speed region of the turbulent spots was cancelled and replaced by a carpet of low-speed fluid. The variable-interval time-averaging technique showed a decrease in burst duration and intensity within the spots, but an increase in burst frequency.
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
Rahul Deshpande, Dileep Chandran, Alexander J. Smits, Ivan Marusic
Summary: This study investigates the role of inter-scale interactions in the high-Reynolds-number skin-friction drag reduction strategy. It is found that imposing low-frequency streamwise travelling waves at the wall to actuate the drag generating outer scales is more energy efficient than targeting the drag producing inner scales. The study shows that increased drag reduction is associated with increased coupling between the inner and outer scales through manipulation of the phase relationships. It is also found that this enhancement of nonlinear coupling occurs with increasing Reynolds number, indicating improved efficacy of the energy-efficient drag reduction strategy at very high Reynolds numbers.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Paolo Olivucci, Daniel J. Wise, Pierre Ricco
Summary: A study on turbulent channel flow modified by the motion of free rotating discs under the influence of wall turbulence reveals that half discs rotating with finite angular velocity can significantly reduce turbulence activity and skin-friction drag. The maximum drag reduction over the entire walls is 5.6%, and simplified results show excellent agreement with nonlinear results for large disc diameters.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Engineering, Marine
Yucel Ozmen, Bendiks Jan Boersma
Summary: An experimental study was conducted to investigate the reduction of friction in fully developed turbulent pipe flow using different types of polyacrylamides as friction reducing polymers. The study examined the effect of polymer concentration, Reynolds number, and polymer type on friction reduction. It was found that higher molecular weight polymers were more effective at reducing friction, and the percentage of friction reduction increased with increasing polymer concentration up to 100 wppm before decreasing.
Article
Mechanics
Takahiro Sonoda, Zhuchen Liu, Toshitaka Itoh, Yosuke Hasegawa
Summary: In this study, reinforcement learning is applied to develop control strategies for reducing skin friction drag in a fully developed turbulent channel flow at a low Reynolds number. A deep neural network is used to express the nonlinear relationship between sensing information and control input, and the network is trained to maximize drag reduction as a long-term reward. The results demonstrate that reinforcement learning can be a novel framework for systematically learning and developing effective control strategies.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Multidisciplinary Sciences
Giulio Foggi Rota, Alessandro Monti, Marco E. Rosti, Maurizio Quadrio
Summary: Viscous dissipation in fluid flows causes significant energy losses. Laminar flows in ducts have minimum resistance, while turbulence increases friction and energy requirements for pumping. A novel technique of intermittently pumping the flow accelerates it to a quasi-laminar state, saving energy compared to constant pumping and reducing harmful emissions.
SCIENTIFIC REPORTS
(2023)
Article
Engineering, Marine
Cong Wang, Morteza Gharib
Summary: The dynamic free-slip surface method can manipulate the turbulent boundary layer (TBL) and achieve a significant drag reduction effect on water vehicles. The optimal setting of control parameters, particularly a large Weber number, is crucial for maximizing the drag reduction effect.
JOURNAL OF MARINE SCIENCE AND ENGINEERING
(2022)
Article
Mechanics
Azadeh Jafari, Beverley J. McKeon, Maziar Arjomandi
Summary: The potential of frequency-tuned surfaces as a passive control strategy for reducing drag in wall-bounded turbulent flows is investigated using resolvent analysis. It is shown that wall impedance can suppress the modes resembling the near-wall cycle and the very-large-scale motions and the Reynolds stress contribution of these modes. Furthermore, a wall with only shear-driven impedance is found to suppress turbulent structures over a wider range in spectral space, leading to an overall turbulent drag reduction.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Mechanical
Brian R. Elbing
Summary: This study reexamines past studies of how drag-reducing polymer solutions modify the log-region of a developing turbulent boundary layer. Recent work has shown the inaccuracies of the classical view, especially at high drag reduction levels. The scatter of the von Karman coefficient between studies is related to the inner variable based Weissenberg number.
JOURNAL OF FLUIDS ENGINEERING-TRANSACTIONS OF THE ASME
(2021)
Article
Mechanics
Kushal U. Kempaiah, Fulvio Scarano
Summary: This study quantifies the distortions of turbulent structures induced by spanwise wall oscillations through feature analysis and compares the results with statistical analysis to understand the mechanism of drag reduction. The findings suggest that the rear region close to the wall is primarily affected by the wall motion, inhibiting hairpin auto-generation and resulting in different organizations of turbulent structures in the near-wall region. The reduction of low-speed streaks and ejections further supports the hypothesis of rapid lateral distortion being responsible for drag reduction.
Article
Engineering, Mechanical
Xinwei Wang, Yufei Wang, Haiping Tian, Nan Jiang
Summary: The study demonstrates that the anisotropy superhydrophobic surface has a significant impact on the turbulent boundary layer, resulting in well-fitted wall friction velocity, drag reduction rate, slip velocity, among other effects. Large-scale structures in motion on the superhydrophobic surface show upright wave packet structures and changes in convection velocity, structure morphology, and shear layer distortion.
ACTA MECHANICA SINICA
(2021)
Article
Engineering, Mechanical
Jianing Yu, Dewei Fan, Bernd R. Noack, Yu Zhou
Summary: An AI open-loop control system was developed to manipulate a turbulent boundary layer over a flat plate in order to reduce friction drag. Experiments showed that under AI control, the local drag reduction rate increased and the reduction area was extended.
ACTA MECHANICA SINICA
(2021)
Article
Mechanics
R. Garcia-Mayoral, G. Gomez-de-Segura, C. T. Fairhall
FLUID DYNAMICS RESEARCH
(2019)
Article
Mechanics
C. T. Fairhall, N. Abderrahaman-Elena, R. Garcia-Mayoral
JOURNAL OF FLUID MECHANICS
(2019)
Article
Mechanics
Nabil Abderrahaman-Elena, Chris T. Fairhall, Ricardo Garcia-Mayoral
JOURNAL OF FLUID MECHANICS
(2019)
Article
Mechanics
Akshath Sharma, Ricardo Garcia-Mayoral
JOURNAL OF FLUID MECHANICS
(2020)
Article
Mechanics
Akshath Sharma, Ricardo Garcia-Mayoral
JOURNAL OF FLUID MECHANICS
(2020)
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
Thermodynamics
Garazi Gomez-de-Segura, Ricardo Garcia-Mayoral
INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW
(2020)
Article
Mechanics
A. Chavarin, G. Gomez-de-Segura, R. Garcia-Mayoral, M. Luhar
Summary: Utilizing the resolvent framework, this study explores the impact of anisotropic permeable substrates on turbulent channel flow, revealing the potential of porous materials to reduce drag in wall-bounded turbulent flows. The findings suggest that substrates with high streamwise permeability and low spanwise permeability can suppress the gain of the corresponding resolvent mode, leading to drag reduction.
JOURNAL OF FLUID MECHANICS
(2021)
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
Joseph I. Ibrahim, Garazi Gomez-de-Segura, Daniel Chung, Ricardo Garcia-Mayoral
Summary: The virtual origin shift between the mean flow and turbulence is a key parameter for near-wall turbulence. The use of slip-like boundary conditions affects the virtual origins for mean flow and turbulence, determined by the streamwise and wall-normal/spanwise slip lengths. The virtual origin perceived by quasi-streamwise vortices plays a key role in determining the virtual origin for turbulence.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
S. Endrikat, R. Newton, D. Modesti, R. Garcia-Mayoral, N. Hutchins, D. Chung
Summary: This study investigates flow characteristics above non-optimal riblets through experimental and numerical simulation data protocols. It reveals that large-scale riblets can affect the flow dynamics and enhance secondary flows as riblet size increases. Additionally, imperfectly repeated riblets can impede turbulence in grooves, impacting their performance.
JOURNAL OF FLUID MECHANICS
(2022)
Proceedings Paper
Physics, Applied
Akshath Sharma, Ricardo Garcia-Mayoral
THIRD MADRID SUMMER SCHOOL ON TURBULENCE
(2018)
Proceedings Paper
Physics, Applied
G. Gomez-de-Segura, C. T. Fairhall, M. MacDonald, D. Chung, R. Garcia-Mayoral
THIRD MADRID SUMMER SCHOOL ON TURBULENCE
(2018)
Article
Mechanics
Jongmin Seo, Ricardo Garcia-Mayoral, Ali Mani
JOURNAL OF FLUID MECHANICS
(2018)