Article
Mechanics
XianXu Yuan, YaLu Fu, JianQiang Chen, Ming Yu, PengXin Liu
Summary: In this study, the effects of spanwise-oriented grooves on the kinetic and thermodynamic statistics in a supersonic turbulent channel flow were investigated using direct numerical simulations. The results showed that these grooves induced compressive and expansive waves that traveled across the channel and influenced the distribution of vortices and streaks. These waves enhanced the fluctuations in temperature, density, and pressure, and also altered the correlations between velocity and temperature.
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
Mechanics
Yong Ji, Jie Yao, Fazle Hussain, Xi Chen
Summary: Investigation on vorticity transport in turbulent channels under large-scale active drag control reveals the significant contribution of vorticity fluctuations to frictional drag, with random and coherent components playing different roles in drag reduction. Analysis suggests that suppressing random spanwise-vorticity transport is crucial for more effective drag reduction.
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
Tingting Li, Zhiwen Cui, Xianxu Yuan, Ying Zhang, Qiang Zhou, Lihao Zhao
Summary: In this study, we conducted direct numerical simulations to investigate the behavior of particle suspensions in compressible turbulent vertical channel flows. We found that the compressibility effect amplifies the mean velocity, fluctuations, and slip velocity of particles in the streamwise direction. Additionally, the wall-normal and spanwise fluctuations of particle velocities are augmented by the compressible effect in the channel core region. The quadrant analysis of fluid fluctuation velocities at particle positions revealed preferential distributions in certain quadrants, indicating the influence of compressible flows on particle wall-normal transport.
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)
Review
Mechanics
Zeeshan Saeed, Brian R. Elbing
Summary: This study qualitatively surveys the phenomenon of polymer drag reduction in wall-bounded turbulent flows, focusing on the salient coherent motions in the near-wall region. Turbulence is introduced phenomenologically using the scale separation concept to explain the idea of drag crisis as a reduction in this scale separation. The dynamics of polymer chains are shown to modulate the near-wall structure of turbulent boundary layers and affect drag reduction.
Article
Engineering, Aerospace
Khaled Younes, Jean-Pierre Hickey
Summary: The scaling of near-wall mean velocity profiles in turbulent flows has been traditionally studied using various assumptions. Recent work has shown success with a quasi-equilibrium assumption, but its validity for noncanonical flows is uncertain. This study proposes an alternate transformation method based on the semilocal gradient of the velocity and a modified stress balance. The new method demonstrates improved performance, especially at supercritical pressures.
Article
Mechanics
Manman Wang, Zhiye Zhao, Ankang Gao, Jiaxing Song, Nansheng Liu
Summary: The effects of rotation on turbulence in viscoelastic spanwise-rotating plane Couette flows were studied using direct numerical simulations at Re=1300 and Wi=5. Three regimes of rotation were identified: weak rotation, intermediate rotation, and strong rotation. Polymer-turbulence interactions were found to occur primarily in extensional flows at weak and intermediate rotation, and in the bulk region at high-Ro EIT state.
Article
Engineering, Aerospace
Congcong Chen, Zhuo Wang, Lin Du, Dakun Sun, Xiaofeng Sun
Summary: By using the turbulent wall model and hierarchical Cartesian grid, researchers successfully simulated unsteady flows in turbomachines with the immersed boundary method. Through verification under different conditions, the effectiveness of this method was validated, with results matching well with experimental data. This approach can be effectively applied to simulating unsteady flows in turbomachines.
AEROSPACE SCIENCE AND TECHNOLOGY
(2021)
Article
Physics, Mathematical
M. Petrov, S. Utyuzhnikov, A. Chikitkin, N. Smirnova
Summary: The near-wall domain decomposition method (NDD) has been extended to non-equilibrium regimes with laminar-turbulent transition (LTT) for the first time in this paper. By modifying NDD to efficiently consider LTT and implementing intermittency, the capabilities of NDD to model non-equilibrium turbulent flows with transition have been expanded. The performance of the modified NDD approach demonstrates a significant reduction in computational resources needed while maintaining almost the same accuracy of prediction.
COMMUNICATIONS IN COMPUTATIONAL PHYSICS
(2022)
Article
Mechanics
Rui Ma, Zheng-hong Gao, Shu-sheng Chen, Dong Li
Summary: In this paper, the authors numerically confirm the delay in the transition from laminar to turbulent flow in a compressible boundary layer induced by spanwise wall oscillation. They found that the Stokes boundary layer induced by the oscillation affects the streamwise vortices, weakening their instability and leading to a delayed transition. Furthermore, the oscillation enhances flow compressibility and stabilizes the streamwise vortices, also causing a delayed transition.
Article
Engineering, Aerospace
Tianyi Bai, Kevin P. Griffin, Lin Fu
Summary: This study assessed the performance of several popular velocity profile transformations in noncanonical compressible wall-bounded turbulent flows. It was found that these transformations fail to deliver uniform performance in the logarithmic region and a more sophisticated version is needed. However, data-driven and total-stress-based transformations perform well in the viscous sublayer for all considered flows.
Article
Mechanics
M. Mcdermott, T. A. E. Riou, P. R. Resende, M. C. T. Wilson, A. M. Afonso, G. de Boer
Summary: A novel open-source anisotropic k-epsilon-v(2)-f model is proposed for turbulent viscoelastic duct flow with dilute polymeric solutions. The model incorporates polymeric terms into the governing equations of the turbulence model for channel and square duct flow of Newtonian fluids. The performance of the model is validated by simple closure models and direct numerical simulation data, and it demonstrates good capability in capturing drag reduction features under a wide range of rheological parameters.
Article
Engineering, Chemical
Hui Cai, Guoqing Miao
Summary: This study reported the flow pattern in a vertically vibrated, annular, and granular system, showing either laminar granular flow or turbulent granular flow in different vibration modes. The flow patterns were explained and identified using the Reynolds numbers of the granular system. The results revealed a property peculiar to the granular flow similar to, yet different from, viscous fluids.
Review
Mechanics
Jie Yao, Fazle Hussain
Summary: Reconnection, as a fundamental topology-transforming event, plays a significant role in the dynamics of plasmas, polymers, DNA, and fluids, including both classical viscous and quantum systems. Recent studies have made substantial advances in understanding vortex reconnection in classical viscous flows, including the physical mechanism, its relationship to turbulence cascade, the formation of a finite-time singularity, helicity dynamics, and aeroacoustic noise generation.
ANNUAL REVIEW OF FLUID MECHANICS
(2022)
Article
Mechanics
Xi Chen, Katepalli R. Sreenivasan
Summary: The dominant paradigm in turbulent wall flows, stating that the mean velocity near the wall is independent of the friction Reynolds number, has been challenged in recent research. This study presents a promising perspective supported by data, suggesting that fluctuations displaying non-zero wall values or near-wall peaks are bounded for large friction Reynolds numbers due to the natural constraint of bounded dissipation rate.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Hamid Daryan, Fazle Hussain, Jean-Pierre Hickey
Summary: The study investigates the sound generation mechanism of initially subsonic viscous vortex reconnection at vortex Reynolds number Re = 1500. The Laplacian of kinetic energy, flexion product, enstrophy and deviation from isentropic condition contribute significantly to the acoustic source. The coiling and uncoiling of twisted vortex filaments wrapping around bridges generate significant sound due to rapid strain induced by bridge repulsion. Mutual cancellations among the acoustic source terms and symmetry breaking play important roles in sound generation. Compressibility delays the onset of reconnection events, impacting sound generation.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Jie Yao, Weiyu Shen, Yue Yang, Fazle Hussain
Summary: The dynamics of two slender Hopf-linked vortex rings were studied at different vortex Reynolds numbers, revealing that at high Re, the vortex rings reconnected to form smaller rings that then broke up into turbulent clouds. The global helicity H was not conserved at very high Re, with twist playing a significant role in helicity dynamics at high Re.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
D. J. Simmons, F. O. Thomas, T. C. Corke, F. Hussain
Summary: This experimental study investigates the turbulent boundary layer separation phenomenon on a smooth body under adverse pressure gradient conditions. The research reveals that the separation is highly three-dimensional, whereas the reattachment is spanwise uniform. The study also identifies a surface flow topology, known as the 'fourth kind owl-face pattern', which exhibits high repeatability across multiple experiments. It is demonstrated that the adverse pressure gradient and the secondary flow at the sidewall-ramp juncture are responsible for the formation of surface separation patterns.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Weiyu Shen, Jie Yao, Fazle Hussain, Yue Yang
Summary: This study investigates the topological transition and helicity conversion of vortex torus knots and links using numerical simulations. Three topological transitional routes are found, including merging, reconnection, and transition to turbulence. The transition depends on both the initial topology and geometry of the knots/links. Different transition mechanisms are observed for small, large, and moderate torus aspect ratios.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Jie Yao, Saleh Rezaeiravesh, Philipp Schlatter, Fazle Hussain
Summary: This study conducted direct numerical simulations of flow in a smooth circular pipe and obtained various turbulence statistics. A comparison with other simulations and experiments revealed small but noticeable differences. The friction factor deviated from the Prandtl friction law at low and high Reynolds numbers. The wall shear stress fluctuations and axial turbulence intensity in the pipe were lower than in the channel, but the difference decreased with increasing Reynolds number.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Jie Yao, Xi Chen, Fazle Hussain
Summary: Well-resolved direct numerical simulations of turbulent open channel flows were performed, and it was found that the flow behavior in the near-wall region is similar to closed channel flows, but differs notably in the outer region. The free surface in open channel flows plays a crucial role in various flow phenomena.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Jian-Chao He, Yun Bao, Xi Chen
Summary: Direct numerical simulation (DNS) is conducted for non-slip two dimensional (2D) Rayleigh-Benard convection (RBC) with a wide range of Rayleigh number (Ra up to 10^13), Prandtl number Pr 1/4 0.7, and aspect ratio gamma= 1. The thermal dissipation rate shows an evident scaling transition at Ra asymptotic to 10^9. The mean temperature gradient in the thermal boundary layer is the dominant contribution to the transition. The Nusselt number also exhibits the transition at Ra asymptotic to 10^9. Rating: 7/10.
Article
Multidisciplinary Sciences
Yong Ji, Xi Chen
Summary: The mean velocity distributions of unstably and stably stratified atmospheric surface layers (ASLs) are investigated using the symmetry approach. The study derived an algebraic model for mean velocity profiles in ASL and validated it with observational data.
Article
Mechanics
Xi Chen, Katepalli R. Sreenivasan
Summary: In this article, a self-consistent Reynolds number asymptotics is presented to describe the variations of velocity fluctuation variances and root-mean-square pressure with respect to the wall-normal position in channel flows, pipe flows, and flat-plate boundary layers. The study shows that when normalized by peak values, the Reynolds number dependence and wall-normal variation of these profiles can be decoupled and are in good agreement with experimental and simulation data. Additionally, it is predicted that a finite plateau appears in the outer region for asymptotically high Reynolds numbers.
JOURNAL OF FLUID MECHANICS
(2023)