Article
Mechanics
Zhicheng Wang, Dixia Fan, Xiaomo Jiang, Michael S. Triantafyllou, George Em Karniadakis
Summary: This paper demonstrates how to accelerate the computationally taxing process of deep reinforcement learning for active control of bluff body flows at high Reynolds number using transfer learning. The results show that transfer learning greatly reduces training episodes and improves stability compared to training from scratch. The wake flow at high Reynolds number is analyzed, revealing an asymmetry in the hydrodynamic forces on the two rotating control cylinders for the first time.
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
Suming Wang, Wenhua Zhang, Xinyi Wang, Xiaobin Li, Hongna Zhang, Fengchen Li
Summary: This paper conducts direct numerical simulations of viscoelastic drag-reducing turbulence (DRT) to investigate the essence of its maximum drag reduction (MDR) state. The results show that the MDR state can be both inertial turbulence (IT) and elasto-inertial turbulence (EIT), with the dominant dynamics shifting from IT-related to EIT-related dynamics as the maximum extension length (L) of polymers increases. These findings provide insights for breaking through the MDR limit. Rating: 8/10.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Engineering, Mechanical
Antoine B. Blanchard, Guy Y. Cornejo Maceda, Dewei Fan, Yiqing Li, Yu Zhou, Bernd R. Noack, Themistoklis P. Sapsis
Summary: This study utilizes Bayesian optimization to design open-loop controllers for fluid flows, achieving good performance in both computational and experimental settings. The research demonstrates that Bayesian optimization can identify optimal controllers at a fraction of the cost of other optimization strategies considered in previous studies. Additionally, Bayesian optimization provides a surrogate model for the latent cost function, assisting in painting a complete picture of the control landscape.
ACTA MECHANICA SINICA
(2021)
Article
Mechanics
Umberto Ciri, Stefano Leonardi
Summary: This study investigates heat transport over super-hydrophobic (SHS) and liquid-infused surfaces (LIS) in turbulent channel flow. It finds that LIS is more robust than SHS and can achieve larger drag reduction. Additionally, heat transfer efficiency scales with the relative magnitude of the thermal and streamwise slip lengths.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Johan Sundin, Umberto Ciri, Stefano Leonardi, Marcus Hultmark, Shervin Bagheri
Summary: Liquid-infused surfaces can reduce friction drag and enhance heat transfer, but research in this area is limited. Numerical simulations show that recirculation in the surface texture can lead to increased heat transfer in both laminar and turbulent flows. The contribution of recirculation to heat transfer can be significant in microchannels but is limited in turbulent flows. Nonetheless, a heat transfer enhancement of around 10% is observed in turbulent channel flow.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Johan Sundin, Stephane Zaleski, Shervin Bagheri
Summary: Liquid-infused surfaces show promise in reducing friction, fouling, and icing in laminar and turbulent flows, but can experience failure due to capillary waves inducing roughness effects that increase drag in turbulent environments. Energy transfer from turbulent flows to the LIS is explained by the presence of negative curvature in the mean flow at the critical layer, with relations provided to determine whether a LIS behaves as a smooth or rough surface in turbunlent flows based on groove width, Weber number, and friction Reynolds number.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Engineering, Chemical
Helvio Mollinedo, Victor X. Mendoza-Escamilla, Gabriela Rivadeneyra-Romero, Israel Gonzalez-Neria, J. Antonio Yanez-Varela, Alejandro Alonzo-Garcia, Jesus Lugo-Hinojosa, Sergio A. . Martinez-Delgadillo
Summary: The effects of axially symmetric control rods on the hydrodynamic performance of a four-blade pitched blade turbine (PBT) were evaluated. The modified PBT with control rods showed improved efficiency and reduced power consumption, as well as a decrease in energy dissipation rate.
INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH
(2022)
Article
Thermodynamics
Qiang Yang, Yongmann M. Chung
Summary: Travelling waves induced by spanwise Lorentz force for skin-friction drag reduction were studied using direct numerical simulations. Both the streamwise and spanwise travelling waves showed regions of drag reduction (DR) and drag increase (DI). The streamwise travelling wave of spanwise Lorentz force had the same DR and DI regions as the streamwise travelling wave of spanwise wall velocity, while the spanwise travelling wave had a DR pattern similar to that of spanwise oscillating Lorentz force. The study also explored oblique travelling waves and found that optimal drag reduction occurred when the wave travelled backward relative to the flow direction.
FLOW TURBULENCE AND COMBUSTION
(2023)
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
Thermodynamics
M. Moataz, N. Reiter, M. Al-Mqbas, A. Metelkin, A. Latifi, J. Jovanovic, A. Delgado
Summary: This study investigates the turbulent drag reduction caused by an extremely diluted aqueous solution of polyacrylamide in a fully developed plane channel flow. The measurement of the Reynolds stress tensor components provides insights into the turbulence anisotropy and independent invariants. The study confirms the chief mechanism relevant to the onset of turbulence and turbulent drag reduction, as well as parametrizes the drag reduction effect based on the polymer-turbulence interaction.
INTERNATIONAL JOURNAL OF HEAT AND FLUID FLOW
(2022)
Article
Mechanics
Maurizio Quadrio, Alessandro Chiarini, Jacopo Banchetti, Davide Gatti, Antonio Memmolo, Sergio Pirozzoli
Summary: In this study, flow control for reducing turbulent skin-friction drag is applied to a transonic airfoil, resulting in improved aerodynamic performance. The control method involves inducing spanwise forcing on a portion of the suction side of the airfoil. This not only reduces friction locally, but also modifies shock waves, leading to increased lift and decreased drag. The findings suggest that skin-friction drag reduction can be used as a tool to improve the overall aerodynamics of complex flows.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Feng Xie, Jose D. Perez-Munoz, Ning Qin, Pierre Ricco
Summary: A turbulent drag-reduction method utilizing synthetic jet sheets was investigated through direct numerical simulations. By adjusting the angle and height of the jet sheets, a drag reduction of 10% to 30% was achieved. The study also found that the global skin-friction drag reduction was a result of a finite counter flow induced by the interaction between the jet-sheet flow and the main flow.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Mohammad Umair, Sedat Tardu
Summary: Direct numerical simulations were conducted to investigate the drag reduction mechanism in turbulent channel flow with spanwise wall oscillations in the form of streamwise travelling waves (STW). The study revealed that the primary effect of the STW forcing is to attenuate the spanwise turbulent enstrophy at the wall, which is linked to the fluctuating wall shear stress. The suppression of the wall-normal turbulent enstrophy is considered to be subordinate.
JOURNAL OF FLUID MECHANICS
(2023)
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)