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
Simon Kneer, Zhengfei Guo, Markus J. Kloker
Summary: This study utilized direct numerical simulations to investigate the effects of various parameters on the laminar-flow-control capabilities of narrowly spaced streaks in a supersonic boundary layer at Mach 2.0. The research found that spectrum-enriching subharmonic modes do not destroy the controlling mechanism, and a complex breakdown scenario triggered by a multi-frequency point source can be effectively controlled.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Anton Pershin, Cedric Beaume, Tom S. Eaves, Steven M. Tobias
Summary: This study examines the nonlinear robustness of laminar plane Couette flow under in-phase spanwise wall oscillations, by computing properties of the edge of chaos and using three measures to quantify the impact of the chosen control strategy on laminar-to-turbulent transition. A novel Bayesian approach is introduced to accurately calculate the laminarization probability, providing global information about the edge and allowing for the evaluation of control effectiveness.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Multidisciplinary Sciences
George H. Choueiri, Jose M. Lopez, Atul Varshney, Sarath Sankar, Bjoern Hof
Summary: Turbulence generally occurs in shear flows when velocities and inertial forces are large, but viscoelastic fluids can exhibit disordered motion even with low inertia. Elastoinertial turbulence (EIT) is observed in a narrow Reynolds number interval, showing weakly chaotic dynamics in experiments and instability at lower Reynolds numbers than predicted by theory. EIT structures can be detected in a wide range of parameters, dominating viscoelastic flows across a significant range of Reynolds numbers.
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
(2021)
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
Romain Paris, Samir Beneddine, Julien Dandois
Summary: This paper proposes a novel method for actuator selection in active flow control using a reinforcement learning agent. The method evaluates the impact of potential actuator removal on the value function to assess agent performance. It is applied to two test cases and demonstrates relevant performances and accurate approximation of the Pareto front.
JOURNAL OF FLUID MECHANICS
(2023)
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
Romain Paris, Samir Beneddine, Julien Dandois
Summary: This research utilizes deep reinforcement learning algorithms to optimize the drag control strategy of a cylinder in laminar flow conditions, achieving closed-loop control through synthetic jets and pressure sensors. The study demonstrates that the proposed control strategy exhibits good robustness under different Reynolds numbers and measurement noise, effectively reducing drag.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Antoine Charles, Francesco Romano, Thierry Ribeiro, Sam Azimi, Vincent Rocher, Jean-Christophe Baudez, S. Amir Bahrani
Summary: We investigated the laminar-to-turbulent transition for non-Newtonian Herschel-Bulkley fluids with shear-thinning or shear-thickening behavior. A reduced-order model considering yield-stress was developed. The study found that increasing yield-stress or decreasing flow index leads to a delayed transition. The model allows for the investigation of flow receptivity to perturbations and their radial position of inception.
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
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
Alessio Roccon, Francesco Zonta, Alfredo Soldati
Summary: This study investigates drag reduction in a lubricated channel using direct numerical simulation, revealing a significant reduction in drag under certain conditions dependent on the viscosity ratio of the two fluids. Through detailed energy budget analysis, two distinct drag reduction mechanisms are identified.
JOURNAL OF FLUID MECHANICS
(2021)
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
Wei Ran, Armin Zare, Mihailo R. Jovanovic
Summary: This paper develops a model-based framework to quantify the effect of streamwise-aligned spanwise-periodic riblets on kinetic energy and skin-friction drag in turbulent channel flow. The study shows that triangular riblets can reliably predict drag-reducing trends, but may lead to performance deterioration for large riblets.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Physics, Fluids & Plasmas
R. C. Chin, R. Vinuesa, R. Orlu, J. Cardesa, A. Noorani, M. S. Chong, P. Schlatter
PHYSICAL REVIEW FLUIDS
(2020)
Article
Mechanics
I. C. Chan, R. Orlu, P. Schlatter, R. C. Chin
Summary: A computational study was conducted to investigate skin friction modification by a large-eddy breakup device in a zero-pressure-gradient turbulent boundary layer. Wake vortices generated by the device were observed to propagate downstream towards the wall, leading to skin friction reduction. Skin friction decomposition revealed three universal regions of different changes in skin friction contributions, with the formation and dissipation of wake vortices playing key roles in the reduction process.
Article
Thermodynamics
Ahmad Zarei, Sadegh Seddighi, Sohail Elahi, Ramis Orlu
Summary: The study investigates the use of bubble injection to improve cold thermal energy storage in a helical coil heat exchanger, leading to significant improvements in cycle performance, heat transfer rate, and exergy destruction. The effectiveness of bubble injection depends on geometry and flow rate, with an optimal flow rate value observed.
APPLIED THERMAL ENGINEERING
(2022)
Article
Thermodynamics
Xiaobo Zheng, Gabriele Bellani, Lucia Mascotelli, Ramis Orlu, Andrea Ianiro, Carlos Sanmiguel Vila, Stefano Discetti, Jacopo Serpieri, Marco Raiola, Alessandro Talamelli, Ye Li, Nan Jiang
Summary: Hot-wire measurements were conducted in the large-scale pipe-flow facility CICLoPE, showing Re-tau-independent modulation features and geometric characteristics. The study revealed the coherence and phase relations of different scale motions, with AM effects and opposite effects observed in different regions of the pipe flow.
EXPERIMENTAL THERMAL AND FLUID SCIENCE
(2022)
Article
Engineering, Mechanical
Lars H. von Deyn, Marius Schmidt, Ramis Oerlue, Alexander Stroh, Jochen Kriegseis, Benjamin Boehm, Bettina Frohnapfel
Summary: Existing engineering tools can predict the impact of homogeneous surface roughness on near-wall turbulent flows to some extent, but they are not reliable for heterogeneous rough surfaces. In this study, we investigate the effect of ridge-type roughness on turbulent flows inside a combustion engine, and find that it induces large-scale secondary motions. This suggests that large scale secondary motions can also be present in technical flows.
EXPERIMENTS IN FLUIDS
(2022)
Correction
Thermodynamics
Narges Tabatabaei, Ricardo Vinuesa, Ramis Orlu, Philipp Schlatter
FLOW TURBULENCE AND COMBUSTION
(2022)
Article
Thermodynamics
Narges Tabatabaei, Ricardo Vinuesa, Ramis Orlu, Philipp Schlatter
Summary: The study investigates the implementation and performance of tripping techniques in turbulence models. Three alternatives for tripping are assessed, including a recent turbulence generation method, a traditional transition model, and a novel formulation combining the two. Validated with DNS, LES and experimental data, the tripping model shows promising results in a variety of test cases, providing more reliable simulation results by excluding the challenging transition region.
FLOW TURBULENCE AND COMBUSTION
(2022)
Article
Engineering, Mechanical
Davide Gatti, Alexander Stroh, Bettina Frohnapfel, Ramis Oerlue
Summary: This study addresses the question of whether spatial resolution effects should be considered in measurements of turbulent wall-bounded flows over rough surfaces in the fully rough regime. By using a direct numerical simulation database, the researchers find that spatial resolution effects are significant in fully rough flows and can be corrected using an appropriate correction scheme.
EXPERIMENTS IN FLUIDS
(2022)
Article
Mechanics
P. Sujar-Garrido, M. Becerra, R. Orlu
Summary: This paper experimentally and numerically investigates the spatial and time-resolved characteristics of a single surface dielectric barrier discharge (sDBD) actuator, and focuses on its efficiency as a flow-control device. The study finds that the commonly used simplified phenomenological model fails to accurately reproduce the induced wall-jet and underestimates the power consumption. A better phenomenological model or a fully coupled physical plasma model is required to improve the accuracy.
Article
Physics, Fluids & Plasmas
C. Chan, R. Orlu, P. Schlatter, R. C. Chin
Summary: The role of streamwise length scales in turbulent skin friction generation is investigated. It is found that both the motions scaled with local boundary layer thickness and the large-scale ejection and sweep events contribute significantly to turbulent skin friction. The reduction of turbulent skin friction can be achieved by modifying both the large- and small-scale quadrant events.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Engineering, Mechanical
Adalberto Perez, Ramis Orlu, Alessandro Talamelli, Philipp Schlatter
Summary: Flush-mounted cavity hot-wire probes are an alternative to classical hot-wire probes for wall-shear stress measurements, as they avoid the heat transfer issue. However, our study shows that the presence of the cavity changes the nature of the flow field, with an increase in mean velocity and turbulence, and a decrease in shear stress.
EXPERIMENTS IN FLUIDS
(2022)
Article
Engineering, Multidisciplinary
Mohammadreza Manami, Sadegh Seddighi, Ramis Orlu
Summary: Measurement of oil and gas two-phase flow with variable flow regimes can be achieved by studying flow patterns and transitions. This study presents a dynamic neural network method (NARX) for flow rate measurement, which can predict oil and gas flow with variable flow regimes more accurately compared to the multilayer perceptron model (MLP).
Article
Mechanics
Andrea Andreolli, Davide Gatti, Ricardo Vinuesa, Ramis Oerlue, Philipp Schlatter
Summary: The presence of very-large-scale motions in wall-bounded turbulent flows is commonly associated with the superposition of large scales at the wall and the modulation of small-scale near-wall turbulence. This study challenges the current understanding by selectively suppressing either superposition or amplitude modulation and studying the remaining phenomenon. The results indicate that there is still a correlation between the amplitude of small scales near the wall and the large-scale signal in the outer flow even when near-wall large-scale motions are suppressed.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Physics, Fluids & Plasmas
Narges Tabatabaei, Majid Hajipour, Fermin Mallor, Ramis Orlu, Ricardo Vinuesa, Philipp Schlatter
Summary: Wake analysis is crucial for wind-farm planning. Computational fluid dynamics (CFD) is an efficient alternative to expensive and time-consuming wind-tunnel tests. This study compares detailed wind-tunnel experiments with CFD results for wake analysis of a NACA4412 wing section profile. The agreement between CFD, experiments, and the literature is fairly good, indicating that CFD is an adequate methodology for characterizing wings and their wakes.
Article
Physics, Fluids & Plasmas
Narges Tabatabaei, Ramis Orlu, Ricardo Vinuesa, Philipp Schlatter
Summary: The article discusses the issue of wall-interference in wind tunnel tests and proposes the use of inclined sidewalls as a simplified wall insert to reduce this interference. By conducting sensitivity analysis and simulations on a NACA4412 wing at various angles of attack, the study evaluates the impact of wall-interference on aerodynamic forces and local pressure coefficients. The approach of modifying wall-alignment as a calibration parameter shows promising results in reducing wall interference and improving agreement between wind tunnel and free-flight data.