Article
Mechanics
Shed Nidhan, Oliver T. Schmidt, Sutanu Sarkar
Summary: In this study, spectral proper orthogonal decomposition (SPOD) is used to analyze coherent structures in the turbulent wake of a disk. It is found that the wakes exhibit a low-rank behavior and the contribution of low-rank modes to fluctuation energy increases with downstream distance. Vortex shedding mechanism is active and dominant in both wakes, and its decay is inhibited by buoyancy. Unsteady internal gravity waves (IGWs) are observed at certain downstream distances, and a causal link between IGWs and vortex shedding mechanism is established. The coherence of wakes is prolonged by buoyancy, leading to the formation of horizontal layers and IGWs.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Tomoaki Watanabe, Yulin Zheng, Koji Nagata
Summary: This study investigates the decay of stably stratified turbulence generated by a towed rake of vertical plates through direct numerical simulations. The results are compared with the theory of stably stratified axisymmetric Saffman turbulence. It is found that under certain conditions, the decay of various quantities follows the power laws predicted for low buoyancy Reynolds number Saffman turbulence, but in some cases, the decay process no longer follows power laws.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Nicolaos Petropoulos, Ali Mashayek, Colm-cille P. Caulfield
Summary: This study examines the formation of step-like density staircase distributions induced by stratification and turbulence, and explains it through the Phillips mechanism. However, such density staircases are not often observed in ocean interiors, except in regions where double-diffusion processes are important. Analyzing the evolution of velocity and density gradients using reduced-order models, the study determines the ranges of bulk Richardson numbers and turbulent Prandtl numbers for staircase formation.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mechanics
Mateus C. Guimaraes, Fernando T. Pinho, Carlos B. da Silva
Summary: A new theory is proposed to describe the conformation state of polymer chains in free turbulent shear flows of viscoelastic fluids. The theory shows the existence of minimum and maximum solvent dissipation reduction asymptotes and four different polymer deformation regimes, based on self-similarity arguments and new scaling relations for the turbulent flux of conformation tensor. In addition, analytical solutions for the self-similar transverse profiles of the conformation tensor components are obtained. The theory is validated through excellent agreement with direct numerical simulations employing the FENE-P rheological model.
JOURNAL OF NON-NEWTONIAN FLUID MECHANICS
(2024)
Article
Mechanics
Junye Wang, Kan Wang, Meng Wang
Summary: Large-eddy simulation combined with the Ffowcs Williams-Hawkings equation was used to study noise generation by a rotor ingesting the turbulent wake of a circular cylinder. Results showed good agreement between computed sound pressure levels and experimental measurements across a wide range of frequencies. Rotor in thrusting condition produced stronger sound compared to zero thrust condition, and effects of rotor on wake turbulence were found to be relatively small.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Dario Vincenzi, Takeshi Watanabe, Samriddhi Sankar Ray, Jason R. Picardo
Summary: This paper investigates polymer scission in turbulent flows through stochastic modeling and direct numerical simulations, studying the characteristics of both passive and active polymers. The results show that the reduction of kinetic energy dissipation is maximized by an intermediate polymer relaxation time, where polymers stretch significantly but do not break too quickly.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Adrien Lefauve, P. F. Linden
Summary: We study the experimental properties of exchange flows in a stratified inclined duct. We focus on the 'core' shear layer away from the duct walls and analyze data sets to reveal the permissible regions in the parameter space, momentum forcing and dissipation mechanisms, and the importance of streamwise non-periodicity. The study suggests a tendency towards self-similarity of velocity and density profiles with increasing turbulence.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Astronomy & Astrophysics
C. Wang, S. P. Oh, M. Ruszkowski
Summary: There is strong evidence for subsonic turbulence in galaxy clusters, which plays a crucial role in offsetting radiative losses in the cluster cores. The turbulence in a stratified medium exhibits anisotropy due to buoyancy forces. By studying the analogy between wave-turbulence interactions and MHD turbulence, researchers have found that the cascade times and dissipation rates of turbulence decrease with the Fr number. High-resolution hydrodynamic simulations have validated this finding. The results indicate that turbulent diffusion is more severely suppressed by stratification than turbulent dissipation, leading to higher turbulent velocities required to offset cooling.
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
(2023)
Article
Mechanics
Jose L. L. Ortiz-Tarin, Sheel Nidhan, Sutanu Sarkar
Summary: This study investigates the high-Reynolds-number stratified wake of a slender body using a high-resolution hybrid simulation. The results show that the characteristics of the wake in stratified flow are influenced by the shape of the body.
JOURNAL OF FLUID MECHANICS
(2023)
Article
Mathematics, Interdisciplinary Applications
Tommaso Alberti, Francois Daviaud, Reik V. Donner, Berengere Dubrulle, Davide Faranda, Valerio Lucarini
Summary: Turbulent flows exhibit complex dynamics characterized by non-hyperbolic chaos, randomness, state-dependent persistence, and unpredictability. The concept of universality of turbulent attractors is shown to be invalid as their properties depend on the scale of observation. These changing attractors, which we term chameleon attractors, highlight the dynamic nature of turbulence.
CHAOS SOLITONS & FRACTALS
(2023)
Article
Mechanics
S. F. Lewin, C. P. Caulfield
Summary: Motivated by the variation of local shear produced by internal waves in the ocean, this study investigates the effect of a time-dependent shear forcing on the evolution and mixing of turbulence produced by Kelvin-Helmholtz instability (KHI) through direct numerical simulations. The results demonstrate that turbulence produced by KHI with a decelerating shear mixes in a distinctly different way from the flow with constant background shear, with characteristics more in common with convectively driven flows.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Katherine M. Smith, C. P. Caulfield, J. R. Taylor
Summary: Continuously forced stratified shear flows can enhance mixing rates by driving turbulence, leading to three observed regimes: weakly stratified overturning, strongly stratified scouring, and intermediately stratified intermittent. Each regime exhibits unique physical mechanisms affecting mixing efficiency and the broadening or thinning of buoyancy interfaces.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
Chi-An Yeh, Muralikrishnan Gopalakrishnan Meena, Kunihiko Taira
Summary: This study introduces a network-based modal analysis technique utilizing Katz centrality to identify key dynamical paths for amplifying perturbations on time-varying base flows, with broadcast modes revealing sensitive regions in the fluid-flow network. The application to two-dimensional decaying isotropic turbulence highlights the importance of vortex dipoles in perturbation spreading, demonstrating the effectiveness of modifying turbulent evolution through flow perturbation. This network-inspired approach presents a novel use of network analysis for guiding flow control efforts, particularly for time-varying base flows.
JOURNAL OF FLUID MECHANICS
(2021)
Article
Mechanics
S. L. Tang, R. A. Antonia, L. Djenidi
Summary: In this study, we analyze the approach towards local isotropy in statistically stationary turbulent shear flows using the transport equations for the fourth-order moments of the velocity derivative. It is found that, as the Taylor microscale Reynolds number increases, the large-scale contribution gradually decreases and the small-scale motion becomes more locally isotropic. The rate at which local isotropy is approached depends on the weakening of the large-scale forcing, which is controlled by the magnitude of the non-dimensional velocity shear parameter.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
T-W Lee
Summary: In this study, scaling and structural evolutions of turbulent channel flows are considered from a new perspective. The total integrated turbulence kinetic energy and dissipation are seen as global constraints on turbulence structure, leading to predictable scaling through first and second gradients. This self-similarity allows for profile reconstructions at any Reynolds numbers using simple multiplicative operations, while the use of scaled variables in transport equations provides succinct views of turbulence dynamics and structural characteristics.
Article
Computer Science, Interdisciplinary Applications
H. Cen, Q. Zhou, A. Korobenko
Summary: The study evaluates a turbulence modeling framework applied to stable stratified turbulent channel flow, finding that the framework is able to faithfully capture flow structures, replicate intermittent flow dynamics, and improve numerical accuracy in simulations.
COMPUTERS & FLUIDS
(2021)
Article
Meteorology & Atmospheric Sciences
Theodoros Diamantopoulos, Peter J. Diamessis, Marek Stastna
Summary: Various strategies are proposed for enforcing stress-free boundary conditions on deformed domains using a weak-form-based discretization in a Cartesian frame of reference. The study reveals that for the considered Re values, the pseudo-traction is negligible with respect to the corresponding no-slip tangential shear stress.
Article
Engineering, Chemical
Junwei Guo, Qi Zhou, Ron Chik-Kwong Wong
Summary: Numerical simulations were conducted for particle cloud sedimentation in shear-thinning fluids, revealing that the settling velocity ratio decreases with increasing power-law index and Reynolds number, while showing weak dependence on initial concentration.
CHEMICAL ENGINEERING SCIENCE
(2022)
Article
Computer Science, Interdisciplinary Applications
Haoyang Cen, Qi Zhou, Artem Korobenko
Summary: This study extends the Wall-function-based weak imposition of Dirichlet boundary condition (WFWD) method to stratified flows in wall-bounded turbulent flows. The extended method is validated through numerical experiments and compared with other simulation methods. The results show that WFWD with a smooth wall function offers improved accuracy in predicting turbulent channel flow statistics, and WFWD with a rough wall function successfully predicts the occurrence of super-geostrophic jet.
COMPUTERS & FLUIDS
(2022)
Article
Oceanography
Ming-Huei Chang, Ren-Chieh Lien, Kevin G. Lamb, Peter J. Diamessis
Summary: The study identified shoaling internal solitary waves (ISWs) on the upper continental slope in the northern South China Sea, which exhibit properties that vary fortnightly due to their origination from internal tides. The amplitudes, current speeds, and propagation speeds of ISWs are greater in summer than in winter, with convective breaking mainly occurring between 430 and 600-m depths leading to an increase in vertical velocity variances behind the leading ISWs.
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS
(2021)
Article
Physics, Fluids & Plasmas
Kedar Prashant Shete, David J. Boucher, James J. Riley, Stephen M. De Bruyn Kops
Summary: An assumption in the study of mixed fluids is that the statistics of the scalar field will approach those of the velocity field as the scale separation between flow-specific outer scales and small scales of interest increases. However, research has shown that this assumption is not very accurate at high Reynolds or Schmidt numbers.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Environmental Sciences
Gustavo Rivera-Rosario, Peter J. Diamessis, Ren-Chieh Lien, Kevin G. Lamb, Greg N. Thomsen
Summary: This study investigates the shoaling of an internal solitary wave through three-dimensional simulations. By using field observations and bathymetry data from the South China Sea, the study reveals the formation mechanism of recirculating cores in internal solitary waves and their impacts on turbulence, mixing, and particle transport.
ENVIRONMENTAL FLUID MECHANICS
(2023)
Article
Mechanics
G. D. Portwood, S. M. de Bruyn Kops, C. P. Caulfield
Summary: This study investigates the impact of turbulent dynamic range on active scalar mixing in stably stratified turbulence and proposes a model framework for irreversible diapycnal mixing under high Reynolds number conditions.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Mechanics
Xiaolong Zhang, Rohit Dhariwal, Gavin Portwood, Stephen M. de Bruyn Kops, Andrew D. Bragg
Summary: In this study, budgets of turbulent kinetic energy (TKE) and turbulent potential energy (TPE) at different scales in sheared, stably stratified turbulence are analyzed using a filtering approach. The results show that buoyancy always converts TKE to TPE at larger scales, and the probability of locally convecting regions increases as scale decreases. Additionally, contributions from sub-grid fields are significant in governing the fluxes between scales.
JOURNAL OF FLUID MECHANICS
(2022)
Article
Meteorology & Atmospheric Sciences
Theodoros Diamantopoulos, Sumedh M. Joshi, Greg N. Thomsen, Gustavo Rivera-Rosario, Peter J. Diamessis, Kristopher L. Rowe
Summary: A high-order hybrid continuous-Galerkin numerical method is proposed for simulating non-linear, non-hydrostatic internal waves and turbulence in long computational domains with complex bathymetry. The method utilizes nodal spectral element method for spatial discretization and Fourier-Galerkin discretization for incorporating a third periodic direction. A domain decomposition approach combined with block-Jacobi/deflation-based preconditioning is applied to solve the numerically challenging pressure Poisson problem. The method is demonstrated to be robust through a series of benchmarks and is further applied to simulate a convectively breaking mode-one non-linear internal wave over a realistic South-China-Sea bathymetric transect and background current/stratification profiles.
Article
Mechanics
Junwei Guo, Qi Zhou, Ron Chik-Kwong Wong
Summary: This study investigates the characteristics of Rayleigh-Taylor instabilities at the interface between a suspension of granular particles and a clear fluid. The results show that perturbations at the interface undergo a period of linear growth, which is influenced by the solid fraction of the suspension and the particle shear time. The transition from linear to nonlinear growth occurs at a certain threshold, and the average wave number remains stable at this transition. The growth rate is inversely proportional to the particle shear time and increases with the solid fraction, consistent with theoretical predictions.
Article
Mathematics, Applied
Haoyang Cen, Qi Zhou, Artem Korobenko
Summary: This article introduces a computational framework for numerical modeling of stratified boundary layer over complex terrain. The framework is validated against experiments and shows good agreement with the actual situation, and it is able to handle different degrees of stratified flow. The study also found that the weak imposition of Dirichlet boundary condition has an effect on the performance of the framework.
MATHEMATICAL MODELS & METHODS IN APPLIED SCIENCES
(2022)
Article
Physics, Fluids & Plasmas
Qi Zhou
Summary: This paper investigates the possible self-organization of strongly stratified turbulence in a locally critical state. Numerical simulations of stratified turbulent wakes are analyzed, and the turbulent flow field is decomposed into large-scale and small-scale flows. The local gradient Richardson number is calculated to characterize the large-scale flow, and conditional sampling of various turbulence statistics is performed. The results show that the self-organization of flow structures under strong stratification leads to a nearly constant median gradient Richardson number. Regions with small Richardson numbers are associated with large dissipation and a net transfer of energy to small scales. Back-scatter of kinetic energy to large scales is also observed, with negative values of the local eddy viscosity. The occurrence of both positive and negative values of the eddy viscosity is most frequent around the critical value of the Richardson number, indicating an intense two-way exchange of kinetic energy between the large and small scales. The threshold behavior of the Richardson number highlights its dynamical significance for locally self-sustained turbulence in strongly stratified configurations, similar to self-organized criticality dynamics.
PHYSICAL REVIEW FLUIDS
(2022)
Article
Mechanics
James J. J. Riley, Miles M. P. Couchman, Stephen M. de Bruyn M. Kops
Summary: The effects of varying Prandtl number on turbulence in a stably-stratified fluid were investigated using direct numerical simulation. The study examined the homogeneous decay of turbulence for different initial Reynolds numbers and two Prandtl numbers (1 and 7). Various statistics were analyzed, including energy decay rates, energy dissipation rates, buoyancy fluxes, energy spectra, and statistics conditioned on the local density gradient. The results showed that increasing the Prandtl number led to an increase in kinetic energy dissipation rate, while decreasing the potential energy dissipation rate. Additionally, the conversion of potential energy to kinetic energy was more pronounced for higher Prandtl numbers. Analyses of vertical planes and statistics conditioned on the local density gradient revealed that most irreversible mixing occurred in regions of stronger static stability.
JOURNAL OF TURBULENCE
(2023)
Article
Mechanics
Junwei Guo, Qi Zhou, Ron Chik-Kwong Wong
Summary: The lattice Boltzmann method (LBM) is used to simulate stratified plane Couette flows in their statistically stationary turbulent state. The aim is to assess the suitability of LBM for direct simulation of wall-bounded, sheared turbulence under stable stratification.
