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Annual Review of Fluid Mechanics top

► Mixing by Oceanic Lee Waves
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 173-201, January 2021.
► Statistics of Extreme Events in Fluid Flows and Waves
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 85-111, January 2021.
► Layering, Instabilities, and Mixing in Turbulent Stratified Flows
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 113-145, January 2021.
► Statistical Properties of Subgrid-Scale Turbulence Models
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 255-286, January 2021.
► Leonardo da Vinci and Fluid Mechanics
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 1-25, January 2021.
► Introduction
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page v-v, January 2021.
► Turbulence Processes Within Turbidity Currents
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 59-83, January 2021.
► Levitation and Self-Organization of Droplets
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 203-225, January 2021.
► Exact Coherent States and the Nonlinear Dynamics of Wall-Bounded Turbulent Flows
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 227-253, January 2021.
► The Fluid Dynamics of Disease Transmission
    6 Jan, 2021
Annual Review of Fluid Mechanics, Volume 53, Issue 1, Page 473-508, January 2021.

Computers & Fluids top

► A suitability analysis of transient one-dimensional two-fluid numerical models for simulating two-phase gas-liquid flows based on benchmark problems
    

Publication date: Available online 17 July 2021

Source: Computers & Fluids

Author(s): Carina Nogueira Sondermann, Raphael Viggiano Neves de Freitas, Felipe Bastos de Freitas Rachid, Gustavo C.R. Bodstein

► Influence of cylinder breadth and shape on the onset of flow unsteadiness and the aeolian tone level
    

Publication date: Available online 15 July 2021

Source: Computers & Fluids

Author(s): Wagner J. Gonçalves S. Pinto, Florent Margnat

► Anisotropic SST turbulence model for shock-boundary layer interaction
    

Publication date: Available online 15 July 2021

Source: Computers & Fluids

Author(s): Pratikkumar Raje, Krishnendu Sinha

► An efficient, robust and high accuracy framework for direct numerical simulation of 2D and 2D axisymmetric immiscible flow with large property contrast
    

Publication date: Available online 22 July 2021

Source: Computers & Fluids

Author(s): Zhipeng Qin, Amir Riaz

► Applications of a central ENO and AUSM schemes based compressible N-S solver with reconstructed conservative variables
    

Publication date: 15 September 2021

Source: Computers & Fluids, Volume 227

Author(s): Gregorio Gerardo Spinelli, Bayram Celik

► A simple FORCE-type centred scheme accurate for contact discontinuities: Application to compressible Euler flows
    

Publication date: 15 September 2021

Source: Computers & Fluids, Volume 227

Author(s): Lijun Hu, Li Yuan

► On numerical uncertainties in scale-resolving simulations of canonical wall turbulence
    

Publication date: 15 September 2021

Source: Computers & Fluids, Volume 227

Author(s): Saleh Rezaeiravesh, Ricardo Vinuesa, Philipp Schlatter

► HLLC-type methods for compressible two-phase flow in ducts with discontinuous area changes
    

Publication date: 15 September 2021

Source: Computers & Fluids, Volume 227

Author(s): Alexandra Metallinou Log, Svend Tollak Munkejord, Morten Hammer

► Droplet spreading dynamics on hydrophobic textured surfaces: A Lattice Boltzmann study
    

Publication date: Available online 10 July 2021

Source: Computers & Fluids

Author(s): Eslam Ezzatneshan, Ali Asghar, Khosroabadi

► nnnSPH Simulation of Supercooled Large Droplets Impacting Hydrophobic and Superhydrophobic Surfaces
    

Publication date: Available online 6 July 2021

Source: Computers & Fluids

Author(s): Xiangda Cui, Wagdi G. Habashi

International Journal of Computational Fluid Dynamics top

► Development of a Modified Seventh-Order WENO Scheme with New Nonlinear Weights
  15 Jul, 2021
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► Industrial Applications of Smoothed Particle Hydrodynamics
  15 Jul, 2021
Volume 35, Issue 1-2, January-February 2021, Page 1-2
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► Gas-Kinetic Scheme Coupled with Turbulent Kinetic Energy Equation for Computing Hypersonic Turbulent and Transitional Flows
  13 Jul, 2021
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► A Hybrid Targeted Eno-Thinc Scheme With a Modified BVD Algorithm and its Application in Stiff Detonation
    8 Jul, 2021
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► Determining Aerothermodynamic Effects on Very-Low-Earth-Orbit Satellites Using Direct Simulation Monte Carlo Method
  23 Jun, 2021
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► Formatting Elliptic Model From SST k-ω Closure
    9 Jun, 2021
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► Edge-Based Finite Element Formulation of Magnetohydrodynamics at High Mach Numbers
    3 Jun, 2021
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► Large Eddy Simulation of Turbulent Heat Transfer in Pipe Using NEK5000 Based on the Spectral Element Method and Uncertainty Quantification by GCI Estimation
  17 May, 2021
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► A Dynamic Mesh Study of Series and Parallel Bladder Pressure Pulsation Attenuators
  14 May, 2021
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► Modelling Airborne Transmission and Ventilation Impacts of a COVID-19 Outbreak in a Restaurant in Guangzhou, China
    7 Apr, 2021
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International Journal for Numerical Methods in Fluids top

► Coupling fully resolved light particles with the lattice Boltzmann method on adaptively refined grids
  18 Jul, 2021

Abstract

The simulation of geometrically resolved rigid particles in a fluid relies on coupling algorithms to transfer momentum both ways between the particles and the fluid. In this article, the fluid flow is modeled with a parallel lattice Boltzmann method using adaptive grid refinement to improve numerical efficiency. The coupling with the particles is realized with the momentum exchange method. When implemented in plain form, instabilities may arise in the coupling when the particles are lighter than the fluid. The algorithm can be stabilized with a virtual mass correction specifically developed for the lattice Boltzmann method. The method is analyzed for a wide set of physically relevant regimes, varying independently the body-to-fluid density ratio and the relative magnitude of inertial and viscous effects. These studies of a single rising particle exhibit periodic regimes of particle motion as well as chaotic behavior, as previously reported in the literature. The new coupled lattice Boltzmann method is compared with available experimental and numerical results. This serves to validate the presented method and additionally it leads to new physical insight for some of the parameter settings.

► An efficient and simplified Gay‐Lussac approach in secondary variables form for the Non‐Boussinesq simulation of free convection problems
  14 Jul, 2021

Abstract

The Gay-Lussac (GL) approach is an incompressible-based strategy for non-Boussinesq treatment of the governing equations for free convection problems that is established based on extending the density variations beyond the gravity term. Such a strategy leads to emerging the GL parameter as a non-Boussinesq pre-factor of different terms in the governing equations. In this paper, the GL approach is expressed/discussed in terms of the secondary variables, i.e. vorticity and stream-function, for the first time and a simplified version of this approach is proposed by removing density variations from the continuity equation. The difference of results under the simplified and traditional GL approach ranges within a maximum of 1% for different parameters. The lower computational cost of numerical solution of governing equations in the secondary variables formula and the corresponding convergence rate is scrutinized for the simplified GL approach showing around 25% lower computational cost. The performance of this approach is evaluated at high relative temperature differences against the low Mach number scheme and the Boussinesq approximations. In this respect, natural convection in an annulus cavity is numerically simulated using a CVFEM solver under the aforementioned approximations up to Rayleigh number Ra = 105 at Prandtl number Pr = 1 and high relative temperature differences (ε = 0.15 and 0.3). The largest deviations found for either the simplified GL or Boussinesq methods from the low Mach number scheme solution are less than 20% for velocity magnitude, 14% for stream function, 6% for vorticity and 5% for temperature. Results under the three approximations are also analysed in terms of the skin friction and local and average Nusselt number, indicating that the Gay-Lussac approach requires some revisions to act more accurately than the classical Boussinesq approximation at high relative temperature differences in natural convection problems, especially within the convection dominated regime.

This article is protected by copyright. All rights reserved.

► A pressure‐based numerical scheme for compressible–incompressible two‐phase flows
  14 Jul, 2021
A pressure-based numerical scheme for compressible–incompressible two-phase flows

This article presents a numerical scheme for two-phase flows consisting of compressible gas and incompressible liquid. Assuming that the gas density is determined by pressure only and that the liquid density is constant, we develop a new set of governing equations for compressible–incompressible two-phase flows at low Mach numbers. The governing equations are simpler than the previous ones and gives better results for one-dimensional cases and rational results for two-dimensional cases.


Abstract

This article presents a numerical scheme for two-phase flows consisting of compressible gas and incompressible liquid. Assuming that the gas density is determined by pressure only and that the liquid density is constant, we develop a new set of governing equations for compressible–incompressible two-phase flows, which can be seen as a simplification of the previous unified governing equations. The volume fraction of gas, velocity, and pressure are the primary variables. Given the liquid's usual leading role in the flow motion, a pressure-based method is employed to solve the equations. A fifth-order accurate weighted essentially nonoscillatory (WENO) scheme is applied to discretize the advection terms, and the tangent of hyperbola for interface capturing (THINC) scheme is utilized to capture the interface. The numerical scheme proposed is validated against the generalized Bagnold model and the free drop of a water patch in a closed tank. The results show that the scheme can tackle low Mach number compressible–incompressible two-phase flows. For the one-dimensional Bagnold model, the present numerical model achieves first-order convergence and gives better results than that based on the previous unified governing equations with the same numerical methods. In the simulation of the free drop of a water patch in a closed tank, a similar pressure curve with other reported work is given.

► An iterative method to compute conformal mappings and their inverses in the context of water waves over topographies
  14 Jul, 2021
An iterative method to compute conformal mappings and their inverses in the context of water waves over topographies

In this article we present a numerical approach to compute conformal mappings that requires lower computational time than the Schwarz–Christoffel mapping. The method can be easily extended to solve time-dependent water wave problems with large topographic obstacles. Besides, we present an alternative approach to compute the inverse conformal mapping.


Abstract

An iterative numerical method to compute the conformal mapping in the context of propagating water waves over uneven topographies is investigated. The map flattens the fluid domain onto a canonical strip in which computations are performed. The accuracy of the method is tested by using the MATLAB Schwarz–Christoffel toolbox mapping as a benchmark. Besides, we give a numerical alternative to compute the inverse of the conformal map.

► Rotation‐translation collision model for DSMC with restricted energy exchange
  13 Jul, 2021

Summary

In the standard implementation of Borgnakke-Larsen (BL) rotation and translation energy exchange in DSMC not every rotational mode of each molecule is active in every collision. In some collisions, the rotation mode of one molecule is ‘frozen’ (cannot be changed by the collision); in other collisions the rotation mode of both molecules is frozen. Different relaxation rates are obtained by varying the proportion of frozen-rotation collisions. In 1974 Larsen and Borgnakke proposed a ‘restricted exchange’ version of BL in which a portion of the energy of each mode is frozen in each collision; the method seemed physically plausible but did not satisfy the detailed balance condition. In 1978 Pullin proposed a general restricted exchange of BL exchange which did satisfy the detailed balance condition; it was more CPU intensive than the original BL scheme. It was seen as ‘laborius’ and ‘cumbersome’ and it seemed to be incompatible with vibration energy-exchange models. Here a simplified version, a special case, of Pullin's restricted exchange scheme is described. The new method is only 12-17% more CPU-intensive than standard BL methods and can be used with the standard DSMC quantum vibration exchange model. It is shown that the elimination of frozen-rotation collisions changes the high energy distribution of vibration energy in non-equilibrium conditions which may slightly decrease the non-equilibrium dissociation rate when restricted exchange is used with the Quantum-Kinetic method for dissociation reactions.

► A least squares based diamond scheme for anisotropic diffusion problems on polygonal meshes
  11 Jul, 2021

Abstract

We present a new least squares based diamond scheme for anisotropic diffusion problems on polygonal meshes. This scheme introduces both cell-centered unknowns and vertex unknowns. The vertex unknowns are intermediate ones and are expressed as linear combinations with the surrounding cell-centered unknowns by a new vertex interpolation algorithm which is also derived in least squares approach. Both the new scheme and the vertex interpolation algorithm are applicable to diffusion problems with arbitrary diffusion tensors and do not depend on the location of discontinuity. Benefitting from the flexibility of least squares approach, the new scheme and vertex interpolation algorithm can also be extended to 3D cases naturally. The new scheme and vertex interpolation algorithm are linearity-preserving under given assumptions and the numerical results show that they maintain nearly optimal convergence rates for both L 2 error and H 1 error in general cases. More interesting is that a very robust performance of the new vertex interpolation algorithm on random meshes compared with the algorithm LPEW2 in the work of Gao and Wu (2011) is found from the numerical tests.

► Learning how to avoid obstacles: A numerical investigation for maneuvering of self‐propelled fish based on deep reinforcement learning
  10 Jul, 2021
Learning how to avoid obstacles: A numerical investigation for maneuvering of self-propelled fish based on deep reinforcement learning

The self-propelled fish maneuvering for avoiding obstacles under intelligent control is investigated by numerical simulation. Three cases are tested to validate the novel approach, including the fish model maneuvering to avoid a single obstacle and double or multiple obstacles. The results indicate that the fish model can avoid obstacles in a complex environment under intelligent control. This work illustrates the possibility of producing navigation algorithms by DRL and brings potential applications of bionic robotic swarms in engineering.


Abstract

The self-propelled fish maneuvering for avoiding obstacles under intelligent control is investigated by numerical simulation. The NACA0012 airfoil is adopted as the two-dimensional fish model. To achieve autonomous cruising of the fish model in a complex environment with obstacles, a hydrodynamics/kinematics coupling simulation method is developed with artificial intelligence (AI) control based on deep reinforcement learning (DRL). The Navier–Stokes (NS) equations in the arbitrary Lagrangian–Eulerian (ALE) framework are solved by the dual-time stepping approach, which is coupled with the kinematics equations in an implicit strong coupling way. Moreover, the moving mesh based on radial basis function and overset grid technology is taken to achieve a wide range of maneuvering. DRL is introduced into the coupling simulation platform for intelligent control of obstacle avoidance when the self-propelled fish swimming. Three cases are tested to validate the novel approach, including the fish model maneuvering to avoid a single obstacle and double or multiple obstacles. The results indicate that the fish model can avoid obstacles in a complex environment under intelligent control. This work illustrates the possibility of producing navigation algorithms by DRL and brings potential applications of bionic robotic swarms in engineering.

► Economically High‐Order Unstructured‐Grid Methods: Clarification and Efficient FSR Schemes
    7 Jul, 2021

Summary

In this paper, we clarify reconstruction-based discretization schemes for unstructured grids and discuss their economically high-order versions, which can achieve high-order accuracy under certain conditions at little extra cost. The clarification leads to one of the most economical approaches: the flux-and-solution-reconstruction (FSR) approach, where highly economical schemes can be constructed based on an extended κ-scheme combined with economical flux reconstruction formulas, achieving up to fifth-order accuracy (sixth-order with zero dissipation) when a grid is regular. Various economical FSR schemes are presented and their formal orders of accuracy are verified by numerical experiments.

► An efficient p‐multigrid spectral element model for fully nonlinear water waves and fixed bodies
    7 Jul, 2021
An efficient p-multigrid spectral element model for fully nonlinear water waves and fixed bodies

A p-multigrid spectral element method is proposed for water wave simulation. The p-multigrid method is an iterative solver designed to efficiently solve the Laplace problem involved in solving the fully nonlinear potential flow equations. Through numerical test cases it is demonstrated to work well for nonlinear and dispersive water wave simulations addressing both pure wave propagation and a more advanced case for wave-body interaction with a fixed floating production storage and offloading where experimental results are available to validate the simulations.


Abstract

In marine offshore engineering, cost-efficient simulation of unsteady water waves and their nonlinear interaction with bodies are important to address a broad range of engineering applications at increasing fidelity and scale. We consider a fully nonlinear potential flow (FNPF) model discretized using a Galerkin spectral element method to serve as a basis for handling both wave propagation and wave-body interaction with high computational efficiency within a single modeling approach. We design and propose an efficient 𝒪(n)-scalable computational procedure based on geometric p-multigrid for solving the Laplace problem in the numerical scheme. The fluid volume and the geometric features of complex bodies is represented accurately using high-order polynomial basis functions and unstructured meshes with curvilinear prism elements. The new p-multigrid spectral element model can take advantage of the high-order polynomial basis and thereby avoid generating a hierarchy of geometric meshes with changing number of elements as required in geometric h-multigrid approaches. We provide numerical benchmarks for the algorithmic and numerical efficiency of the iterative geometric p-multigrid solver. Results of numerical experiments are presented for wave propagation and for wave-body interaction in an advanced case for focusing design waves interacting with a floating production storage and offloading. Our study shows, that the use of iterative geometric p-multigrid methods for the Laplace problem can significantly improve run-time efficiency of FNPF simulators.

► A class of second‐order schemes with application to chemically reactive radiative natural convection flow in a rectangular enclosure
    6 Jul, 2021

Abstract

A new class of explicit second-order schemes is proposed for solving time-dependent partial differential equations. This class of proposed schemes is constructed on three-time levels. Stability is found for the scalar two-dimensional heat equation and the system of time-dependent partial differential equations. This partial differential equations system comprises a non-dimensional set of equations obtained from the governing equations of natural convection chemically reactive fluid flow in a rectangular enclosure with thermal radiations. Flow is generated by applying the force of pressure. Graphs of streamlines, contours plots of velocity, temperature and concentration profiles, Local Nusselt number, and Local Sherwood number are displayed with the variation of time and parameters in the considered partial differential equations. Results are shown in the form of streamlines and contour plots. It is found that local Nusselt number has dual behavior by enhancing radiation parameter whereas local Sherwood number de-escalates by upraising the reaction rate parameter. It is hoped that the results in this pagination will serve as a valuable resource for future fluid-flow studies in an enclosed industrial environment.

Journal of Computational Physics top

► A fully Eulerian hybrid immersed boundary-phase field model for contact line dynamics on complex geometries
    

Publication date: 15 October 2021

Source: Journal of Computational Physics, Volume 443

Author(s): Armin Shahmardi, Marco Edoardo Rosti, Outi Tammisola, Luca Brandt

► A short note on the accuracy of the discontinuous Galerkin method with reentrant faces
    

Publication date: 15 October 2021

Source: Journal of Computational Physics, Volume 443

Author(s): Will Pazner, Terry Haut

► A study of higher-order reconstruction methods for genuinely two-dimensional Riemann solver
    

Publication date: 15 October 2021

Source: Journal of Computational Physics, Volume 443

Author(s): Boxiao Zhou, Feng Qu, Di Sun, Zirui Wang, Junqiang Bai

► Second Order Linear Decoupled Energy Dissipation Rate Preserving Schemes for the Cahn-Hilliard-Extended-Darcy Model
    

Publication date: Available online 21 July 2021

Source: Journal of Computational Physics

Author(s): Yakun Li, Wenkai Yu, Jia Zhao, Qi Wang

► A conservative level-set/finite-volume method on unstructured grids based on a central interpolation scheme
    

Publication date: Available online 21 July 2021

Source: Journal of Computational Physics

Author(s): Miguel Uh Zapata, Reymundo Itzá Balam

► A new isosurface extraction method on arbitrary grids
    

Publication date: Available online 21 July 2021

Source: Journal of Computational Physics

Author(s): Joaquín López, Adolfo Esteban, Julio Hernández, Pablo Gómez, Rosendo Zamora, Claudio Zanzi, Félix Faura

► A remeshed vortex method for mixed rigid/soft body fluid–structure interaction
    

Publication date: Available online 21 July 2021

Source: Journal of Computational Physics

Author(s): Yashraj Bhosale, Tejaswin Parthasarathy, Mattia Gazzola

► A characteristic-featured shock wave indicator on unstructured grids based on training an artificial neuron
    

Publication date: 15 October 2021

Source: Journal of Computational Physics, Volume 443

Author(s): Yiwei Feng, Tiegang Liu

► A sharp interface Lagrangian-Eulerian method for rigid-body fluid-structure interaction
    

Publication date: 15 October 2021

Source: Journal of Computational Physics, Volume 443

Author(s): E.M. Kolahdouz, A.P.S. Bhalla, L.N. Scotten, B.A. Craven, B.E. Griffith

► Multi-scale two-domain numerical modeling of stationary positive DC corona discharge/drift-region coupling
    

Publication date: 15 October 2021

Source: Journal of Computational Physics, Volume 443

Author(s): Nicolas Monrolin, Franck Plouraboué

Journal of Turbulence top

► Numerical investigation of the effect of rotation on non-premixed hydrogen combustion in developing turbulent mixing layers
  29 Jun, 2021
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► Development of an intermittency transport equation for modeling bypass, natural and separation-induced transition
    3 Jun, 2021
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► Honeycomb-generated Reynolds-number-dependent wake turbulence
  31 May, 2021
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► Estimation of characteristic vortex structures in complex flow
  28 May, 2021
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► Effects of surface roughness topography in transient channel flows
  20 May, 2021
Volume 22, Issue 7, July 2021, Page 434-460
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► Cluster-based probabilistic structure dynamical model of wind turbine wake
  19 May, 2021
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► Transition from axisymmetric to three-dimensional turbulence
    7 May, 2021
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► A study of the influence of coflow on flame dynamics in impinging jet diffusion flames
  26 Apr, 2021
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► Spatial-scaling method and modified large eddy simulation to examine rough-wall turbulence
  26 Apr, 2021
Volume 22, Issue 7, July 2021, Page 413-433
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► Role of vortical structures for enstrophy and scalar transport in flows with and without stable stratification
    4 Mar, 2021
Volume 22, Issue 7, July 2021, Page 393-412
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Physics of Fluids top

► Rock and roll: Incipient aeolian entrainment of coarse particles
  22 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
Aeolian transport of coarse grains is an important topic, finding applications in nature (for infrastructure exposed to wind scour) as well as industry (e.g., considering pneumatic transport). Incipient particle entrainment due to turbulent winds refers to the wind conditions where aeolian transport initiates, and as such, it is at the core of such studies. The research presented herein focuses on identifying and quantifying the dynamical processes responsible for coarse particle entrainment. Specifically designed wind tunnel experiments are conducted for a range of wind conditions near the aeolian transport thresholds. A high-resolution laser distance sensor is employed to provide information for the displacement of an exposed particle ranging from small simple rocking motions to complete entrainments (rolling). Measurements of the exposed particle's angular displacements are acquired, which allow the probabilistic study of incipient motion. The variation of statistical parameters, such as the frequency of entrainments, duration of dislodgements, magnitude of displacements, and time between displacements, is studied for a range of increasing airflow rates. The main findings from these experiments suggest that rocking can be observed only up to a limit angular displacement (equal to 0.41π for the conditions tested herein), which defines the position beyond which the resistance force can be overcome by just the mean aerodynamic forcing. Following this experimental framework to establish aeolian thresholds for a wider range of environments may be useful for the identification of the wind conditions under which aeolian transport may start occurring.
► Elastic instabilities between two cylinders confined in a channel
  22 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
Polymeric flow through porous media is relevant in industrial applications, such as enhanced oil recovery, microbial mining, and groundwater remediation. Biological processes, such as drug delivery and the transport of cells and particles in the body, also depend on the viscoelastic flow through the porous matrix. Large elastic stresses induced due to confined geometries can lead to elastic instability for the viscoelastic fluid flow through porous media. We have numerically studied viscoelastic flow through a channel having two closely placed cylinders to investigate pore scale elastic instabilities. We have discovered three distinct flow states in the region between the cylinders. These flow states are closely coupled with the topology of the polymeric stress field. The transition between the flow states can be identified with two critical Weissenberg numbers ([math] and [math]), where the Weissenberg number (Wi) is the ratio of elastic to viscous forces. At [math], the flow is stable, symmetric, and eddy free. For [math], eddies form in the region between the cylinders. We have measured the area occupied by the eddies for different flow conditions and fluid rheological parameters. At [math], the eddy disappears and the flow around the cylinders becomes asymmetric. We have quantified the flow asymmetry around the cylinders for different flow rates and fluid rheology. We have also studied the effect of the cylinders' diameter and separation on the eddies' size ([math]) and flow asymmetry ([math]). We have also investigated the effect of fluid rheology and cylinders' diameter and separation on the value of critical Weissenberg numbers.
► Experimental investigation of solute transport in variably saturated porous media using x-ray computed tomography
  22 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
Solute transport through variably saturated porous media is ubiquitous in multiple subsurface flows, piquing the geoscience community's interest. This study adopts a novel experimental approach using microfocus x-ray computed tomography for real-time imaging of a three-dimensional NaI tracer plume in a partially saturated packing column. A stabilized two-phase flow field is achievable through continuous co-injection of two-phase fluids: NaCl solvent and pump oil. Thus, the critical role of the NaCl saturation Sw and Péclet number on dispersion can be fully studied by controlling the NaCl fractional flow rate and the total flow rate from the Buckley–Leverett theory. Furthermore, we study solute transport behavior based on statistical moments, the dispersion coefficient, the dilution index, and the mean scalar dissipation rate. Experimental results indicate that the solute transport is Fickian for high Sw ≥ 0.34. In contrast, anomalous transport behavior is found for Sw < 0.34, where the concentration distribution is initially left-tailed and leptokurtic before reaching a well-dispersed regime. The dispersion coefficient is 2–10 times larger for partially saturated cases compared with the fully saturated case and shows a non-monotonical dependency on Sw. Finally, the analysis of the dilution index indicates that the overall mixing strength increases when Sw decreases, whereas the mean scalar dissipation rate reveals that the time scaling of transverse mixing is the largest at an intermediate Sw. The results can be used to elucidate the solute transport behavior in a two-phase system.
► Chaotic streaklines in new exact solutions to the Navier–Stokes equations
  22 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
Exact solutions to the Navier–Stokes equations with time-dependent viscosity [math] are constructed. The fluid velocities [math] for these solutions are eigenvector fields for the operator curl. New exact solutions with [math] for [math], which describe dynamics of viscous fluid inside a ball and a spherical shell, have chaotic streaklines. For the derived solutions with [math] or with [math], where [math] and [math], the length of each fluid streakline is finite and, therefore, its dynamics is not chaotic. As a consequence, it is shown that all Trkalian flows with constant viscosity [math] are not chaotic.
► Effects of syringe pump fluctuations on cell-free layer in hydrodynamic separation microfluidic devices
  22 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
Syringe pumps are widely used biomedical equipment, which offer low-cost solutions to drive and control flow through microfluidic chips. However, they have been shown to transmit mechanical oscillations resulting from their stepper motors into the flow, perturbing device performance. These detrimental effects have mostly been reported on microdroplet production, but have never been reported on hydrodynamic two-phase separation, such as in microdevices making use of cell-free layer phenomena. While various mechanisms can be used to circumvent syringe pump oscillations, it is of interest to study the oscillation effects in naïve systems, which are common in research settings. Previous fluctuation studies focused on relatively low flow rates, typically below 5 ml/h, and showed a linear decay of the relative pressure fluctuations as a function of the flow rate. In this work, we have uncovered that the relative pressure fluctuations reach a plateau at higher flow rates, typically above 5 ml/h. Using a novel low-cost coded compressive rotating mirror camera, we investigated the effect of fluctuations in a hydrodynamic microfluidic separation device based on a cell-free layer concept. We demonstrated that cell-free zone width fluctuations have the same frequency and amplitude than the syringe pump-induced pressure oscillations and illustrated the subsequent degradation of particle separation. This work provides an insight into the effect of syringe pump fluctuations on microfluidic separation, which will inform the design of microfluidic systems and improve their resilience to pulsating or fluctuating flow conditions without the use of ancillary equipment.
► Double masking protection vs. comfort—A quantitative assessment
  21 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
COVID-19 has forced humankind to adopt face masks as an integral part of everyday life. This preventive measure is an effective source control technique to curb the spread of COVID-19 and other similar diseases. The virus responsible for causing COVID-19 has undergone several mutations in the recent past, including B.1.1.7, B.1.351, P.1, and N501Y, B.1.617, with a higher infectious rate. These viruses' variants are mainly responsible for the recent spike in COVID-19 cases and associated steep rise in mortality rate worldwide. Under these circumstances, the Center for Disease Control (CDC) and health experts recommend double masking, which mainly includes a surgical mask and a cotton mask for the general public. This combination provides an additional layer of protection and masks fitment to minimize the leakage of droplets expelled during coughing, sneezing, talking, and breathing. This leakage may cause airborne transmission of the virus. In the present study, we report a systematic quantitative unsteady pressure measurement supplement with flow visualization to quantify the effectiveness of a single and double mask. We have also evaluated double masking consisting of a surgical mask and an N-95 mask used by medical professionals. A simple knot improves the surgical mask fitment significantly, and hence, the leakage of droplets is minimized. The leakage of the droplets was reduced to a large extent by using a double mask combination of a two-layer cotton mask over the surgical mask with a knot. The double mask combination of surgical + N-95 and two-layer cotton + N-95 masks showed the most promising results, and no leakage of the droplets is observed in the forward direction. A double mask combination of surgical and N-95 mask offers 8.6% and 5.6% lower mean and peak pressures compared to surgical, and cotton mask. The best results are observed with cotton and N-95 masks with 54.6% and 23% lower mean and peak pressures than surgical and cotton masks; hence, this combination will offer more comfort to the wearer.
► Fully implicit spectral boundary integral computation of red blood cell flow
  21 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
This paper is on an implicit time integration scheme for simulation of red blood cell (RBC) flow in an ambient fluid. The intra- and extracellular plasmas are modeled as Stokes flows and represented by boundary integral equations (BIE) written in a weakly singular form. The cell membrane is modeled as a thin elastic shell. Expressed in this way, the RBC flow model constitutes an implicit ordinary differential equation (IODE) in the cell shape. The cell shape and velocity field are discretized spatially by a spectral approach using spherical harmonic basis functions. It is then convenient to express the BIE in the Galerkin form with the spherical harmonics themselves as test functions. The key aspect in this paper is the recognition of the IODE structure of the RBC flow model and consequent application of a multi-step implicit solver for time integration. As with any implicit solver, a nonlinear equation in the cell shape is solved at each time step, for which Newton's method is applied. This requires the Jacobian of the IODE, or equivalently computation of Jacobian-vector products. An important contribution is the formulation of such Jacobian-vector products as evaluating a second BIE. The original weakly singular form is crucial in facilitating this formulation. The implicit solver employs variable order and adaptive time stepping controlled by truncation error and convergence of Newton iterations. Numerical examples show that larger time steps are possible and that the number of matrix-vector products is comparable to explicit methods. Source code is provided in the online supplementary material.
► Eddy viscosity modeling around curved boundaries through bifurcation approach and theory of rotating turbulence
  21 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
A novel approach to curvature effects based on the bifurcation theory and rotation turbulence energy spectrum is implemented to improve the sensitivity of the [math] two-equation turbulence model (Jones–Launder form) to curved surfaces. This is done by accounting for the vorticity tensor, which becomes more significant in curved flows, something that the standard [math] model does not originally consider. This new eddy viscosity model is based on the energy spectrum for a turbulent flow undergoing rotation and is then modeled on the bifurcation diagram in [math] phase space. The approach is demonstrated on three different test cases, 30° two-dimensional curved channel, 90° three-dimensional bend duct, and flow past cylinder, to test for the effects of convex and concave curvatures on turbulence. The results from these test cases are then contrasted against other existing eddy viscosity models as well as experimental data. The proposed approach provides better turbulence predictions along convex or concave surfaces, better memory effects, and are closer to the experimental results. For flow past cylinder, the new eddy viscosity model predicts drag coefficient that is closer to experiments with 8% difference, against 30% difference predicted by standard [math] and Pettersson models.
► Weakly nonlinear broadband and multi-directional surface waves on an arbitrary depth: A framework, Stokes drift, and particle trajectories
  21 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
Surface gravity waves in coastal waters are broadband and multi-directional, whose quadratic properties are of considerable engineering and scientific interest. Based on a Stokes expansion and an envelope-type framework, a new semi-analytical approach is proposed in this paper for the description of weakly nonlinear broadband and multi-directional surface waves. This approach proposes solving for the second-order wave fields through the separation of harmonics, by using a Fast Fourier transform and a time integration method. Different from some other methods, e.g., the High-Order Spectral method, the approach introduces a spectral shift for the superharmonic waves, leading to computationally efficient and accurate spectral predictions. The approach has been validated through comparisons with the results based on Dalzell [“A note on finite depth second-order wave–wave interactions,” Appl. Ocean Res. 21, 105–111 (1999)]. An envelope-type framework for the fast prediction of particle trajectories and Stokes drifts up to the second order in wave steepness is also derived in this paper, based on the semi-analytical approach. This paper shows that the results based on a narrowband assumption lead to underestimates of Stokes drift velocities driven by broadband unidirectional focused wave groups. The cases, examined for particle trajectories below broadband unidirectional focused wave groups, show that a larger bandwidth and water depth can enhance the differences in the net mean horizontal displacement of particles at water surface relative to these at seabed.
► Competition between shear and biaxial extensional viscous dissipation in the expansion dynamics of Newtonian and rheo-thinning liquid sheets
  21 Jul, 2021
Physics of Fluids, Volume 33, Issue 7, July 2021.
When a drop of fluid hits a small solid target of comparable size, it expands radially until reaching a maximum diameter and subsequently recedes. In this work, we show that the expansion process of liquid sheets is controlled by a combination of shear (on the target) and biaxial extensional (in the air) deformations. We propose an approach toward a rational description of the phenomenon for Newtonian and viscoelastic fluids by evaluating the viscous dissipation due to shear and extensional deformations, yielding a prediction of the maximum expansion factor of the sheet as a function of the relevant viscosity. For Newtonian systems, biaxial extensional and shear viscous dissipation are of the same order of magnitude. On the contrary, for thinning solutions of supramolecular polymers, shear dissipation is negligible compared to biaxial extensional dissipation and the biaxial thinning extensional viscosity is the appropriate quantity to describe the maximum expansion of the sheets. Moreover, we show that the rate-dependent biaxial extensional viscosities deduced from drop impact experiments are in good quantitative agreement with previous experimental data and theoretical predictions for various viscoelastic liquids.

Theoretical and Computational Fluid Dynamics top

► Numerical study of the mechanisms of enhanced oil recovery using nanosuspensions
    1 Aug, 2021

Abstract

The results of the numerical study of oil recovery enhancement using nanosuspension are presented. The research was carried out using the volume of fluid method (VOF) for a 2D microporous core model. Experimentally measured values of interfacial tension (IFT) and the contact angle (CA) were used for numerical modeling. An aqueous suspension with silicon oxide nanoparticles (5 nm) is used. It was shown that when 1 wt% of nanoparticles are added to the displacing liquid, its density increases by about 1%, the viscosity increases by 10%, the IFT decreases by 10%, and the contact angle increases from 70 to 145 \(^\circ \) . The results of numerical study showed that the injection of nanoparticles has a significant effect on the displacement front movement in the microporous model. It has been shown that the nanosuspension can increase the oil recovery factor (ORF) almost twice as compared to water. To clarify the mechanisms of increasing the oil recovery during the reservoir flooding with nanofluid, a systematic study of factors affecting the displacement process efficiency was carried out. Viscosity, interfacial tension and the wetting angle of the displacing fluid were considered as such factors. As a result of systematic research, it has been shown that the main factor affecting the increase in the oil recovery flooding of nanosuspensions is a variation of wettability.

► An empirical correlation between lift and the properties of leading-edge vortices
    1 Aug, 2021

Abstract

Using data from numerical simulations, we show that the lift experienced by both impulsively started and surging airfoils correlates well with the sum of the circulation of the leading-edge vortices truncated at the trailing edge. Therefore, we suggest that reasonable estimates of the lift can be obtained using only two vortex parameters, i.e., its circulation and its position. In addition to being convenient for non-intrusive estimation of forces from PIV measurements, we show that this approach can be used to derive low-order models for the analysis of vortex-lift configurations. In particular, we apply this correlation to model high-amplitude surging, which allows us to quantify the effect of wake-capture mechanisms and to determine the flow parameters that drive optimal lift.

► Settling of two-way momentum and energy coupled particles subject to Boussinesq and non-Boussinesq heating
    1 Aug, 2021

Abstract

This work establishes a procedure to accurately compute heat transfer between an Eulerian fluid and Lagrangian point-particles. Recent work has focused on accurately computing momentum transfer between fluid and particles. The coupling term for momentum involves the undisturbed fluid velocity at the particle location which is not directly accessible in the simulation. Analogously, in the context of thermal coupling, the undisturbed fluid temperature at the particle location is not directly accessible in simulations and must be estimated. In this paper, we develop a scheme to accurately estimate the undisturbed fluid temperature of a point-particle exchanging thermal energy with a surrounding fluid. The temperature disturbance is correlated with the enhanced temperature curvature in the vicinity of the particle and is formally valid in the low heating, low convection limit. We conduct extensive verification of the correction procedure for a settling particle subject to radiation. This setup allows the simultaneous testing of thermal and momentum corrections. By considering equations of drag and Nusselt number extended to finite Péclet and Boussinesq numbers, we establish a large range over which the correction procedure can be applied.

► The effect of incoming boundary layer thickness and Mach number on linear and nonlinear Rossiter modes in open cavity flows
    1 Aug, 2021

Abstract

The Rossiter modes of an open cavity were studied using bi-global linear analysis, local instability analysis and nonlinear numerical simulations. Rossiter modes are normally seen only for short cavities; hence, in the study, the length over depth ratio was two. We focus on the critical region; hence, the Reynolds numbers based on cavity depth were close to 1000. We investigated the effect of the ratio boundary layer thickness to cavity depth, a parameter often overlooked in the literature. Increasing this ratio is destabilizing and increases the number of unstable Rossiter modes. Local instability analysis revealed that the hierarchy of unstable modes was governed by the mixing in the cavity opening. The effect of Mach number was also studied for thin and thick boundary layers. Compressibility had a very destabilizing effect at low Mach numbers. Analysis of the Rossiter mode eigenfunctions indicated that the acoustic feedback scaled to \(\mathrm{Ma}^3\) and explained the strong destabilizing effect of compressibility at low Mach numbers. At moderate Mach numbers, the instability either saturated with Mach number or had an irregular dependence on it. This was associated with resonances between Rossiter modes and acoustic cavity modes. The analysis explained why this irregular dependence occurred only for higher-order Rossiter modes. In this parameter region, three-dimensional modes are either stable or marginally unstable. Two-dimensional simulations were performed to evaluate how much of the nonlinear regime could be captured by the linear stability results. The instability was triggered by the \(10^{-13}\) flow solver noise floor. The simulations initially agreed with linear theory and later became nonlinearly saturated. The simulations showed that, as the flow becomes more unstable, an increasingly more complex final stage is reached. Yet, the spectra present distinct tones that are not far from linear predictions, with the thin boundary layer cases being closer to empirical predictions. The final stage, in general, was dominated by first Rossiter mode, even though the second one was the most unstable linearly. It seems this may be associated with nonlinear boundary layer thickening, which favors lower frequency in the mixing layer, or vortex pairing of the second Rossiter mode. The spectra in the final stages are well described by the mode R1 and a cascade of nonlinearly generated harmonics, with little reminiscence of the linear instability.

► Modified formulation of the interfacial boundary condition for the coupled Stokes–Darcy problem
    1 Aug, 2021

Abstract

A modified formulation of the interfacial boundary condition for the coupling of the Stokes and Darcy models describing the incompressible fluid flow in the free space and porous medium domains is proposed using the dimension analysis procedure. The case is considered for the porous media formed by circular or square cylinders located in the centers of rectangular cells. The vorticity is derived as a linear combination of the tangential velocity components in the free space and porous medium. The proposed condition is potentially directly applicable for a class of 2D problems with an arbitrary shaped boundary for the boundary element method. The fluid flow problems are solved numerically using the Stokes flow model and analytically for the Stokes–Darcy flow model to determine the coefficients in the introduced linear dependence for the vorticity. As a result, the corresponding coefficients in the boundary condition are found as a porosity function for two types of the porous medium configuration. The approximate analytical estimation of the coefficients confirms the numerical dependencies. The verifications of the found coefficients were made by solving two 2D fluid flow problems. It is shown that the fluid flow calculated on the basis of the Stokes–Darcy flow model with modified boundary condition agrees well with the results of the microscopic Stokes flow model. The advantages of the proposed boundary condition are discussed.

► Hydroelastic interaction of nonlinear waves with floating sheets
    1 Aug, 2021

Abstract

Hydroelastic responses of floating elastic surfaces to incident nonlinear waves of solitary and cnoidal type are studied. There are N number of the deformable surfaces, and these are represented by thin elastic plates of variable properties and different sizes and rigidity. The coupled motion of the elastic surfaces and the fluid are solved simultaneously within the framework of linear beam theory for the structures and the nonlinear Level I Green–Naghdi theory for the fluid. The water surface elevation, deformations of the elastic surfaces, velocity and pressure fields, wave reflection and transmission coefficients are calculated and presented. Results of the model are compared with existing laboratory measurements and other numerical solutions. In the absence of any restriction on the nonlinearity of the wave field, number of surfaces, their sizes and rigidities, a wide range of wave–structure conditions are considered. It is found that wave reflection from an elastic surface changes significantly with the rigidity, and the highest reflection is observed when the plate is rigid (not elastic). It is also found that due to the wave–structure interaction, local wave fields with different length and celerity are formed under the plates. In the case of multiple floating surfaces, it is observed that the spacing between plates has more significant effect on the wave field than their lengths. Also, the presence of relatively smaller floating plates upwave modifies remarkably the deformation and response of the downwave floating surface.

► Mixing and combustion at low heat release in large eddy simulations of a reacting shear layer
    1 Aug, 2021

Abstract

In this paper, the mixing and combustion at low-heat release in a turbulent mixing layer are studied numerically using large eddy simulation. The primary aim of this paper is to successfully replicate the flow physics observed in experiments of low-heat release reacting mixing layers, where a duty cycle of hot structures and cool braid regions was observed. The nature of the imposed inflow condition shows a dramatic influence on the mechanisms governing entrainment, and mixing, in the shear layer. An inflow condition perturbed by Gaussian white noise produces a shear layer which entrains fluid through a nibbling mechanism, which has a marching scalar probability density function where the most probable scalar value varies across the layer, and where the mean-temperature rise is substantially over-predicted. A more sophisticated inflow condition produced by a recycling and rescaling method results in a shear layer which entrains fluid through an engulfment mechanism, which has a non-marching scalar probability density function where a preferred scalar concentration is present across the thickness of the layer, and where the mean-temperature rise is predicted to a good degree of accuracy. The latter simulation type replicates all of the flow physics observed in the experiment. Extensive testing of subgrid-scale models, and simple combustion models, shows that the WALE model coupled with the Steady Laminar Flamelet model produces reliable predictions of mixing layer diffusion flames undergoing with fast chemistry.

► Evaluation of near-singular integrals with application to vortex sheet flow
  13 Jul, 2021

Abstract

This paper presents a method to evaluate the near-singular line integrals that solve elliptic boundary value problems in planar and axisymmetric geometries. The integrals are near-singular for target points not on, but near the boundary, and standard quadratures lose accuracy as the distance d to the boundary decreases. The method is based on Taylor series approximations of the integrands that capture the near-singular behaviour and can be integrated in closed form. It amounts to applying the trapezoid rule with meshsize h, and adding a correction for each of the basis functions in the Taylor series. The corrections are computed at a cost of \(O(n_w)\) per target point, where typically, \(n_w\) =10–40. Any desired order of accuracy can be achieved using the appropriate number of terms in the Taylor series expansions. Two explicit versions of order \(O(h^2)\) and \(O(h^3)\) are listed, with errors that decrease as \(d\rightarrow 0\) . The method is applied to compute planar potential flow past a plate and past two cylinders, as well as long-time vortex sheet separation in flow past an inclined plate. These flows illustrate the significant difficulties introduced by inaccurate evaluation of the near-singular integrals and their resolution by the proposed method. The corrected results converge at the analytically predicted rates.

► Investigation of Görtler vortices in high-speed boundary layers via an efficient numerical solution to the non-linear boundary region equations
    3 Jul, 2021

Abstract

Streamwise vortices and the associated streaks evolve in boundary layers over flat or concave surfaces due to disturbances initiated upstream or triggered by the wall surface. Following the transient growth phase, the fully developed vortex structures become susceptible to inviscid secondary instabilities resulting in early transition to turbulence via ‘bursting’ processes. In high-speed boundary layers, more complications arise due to compressibility and thermal effects, which become more significant for higher Mach numbers. In this paper, we study Görtler vortices developing in high-speed boundary layers using the boundary region equations (BRE) formalism, which we solve using an efficient numerical algorithm. Streaks are excited using a small transpiration velocity at the wall. Our BRE-based algorithm is found to be superior to direct numerical simulation (DNS) and ad hoc nonlinear parabolized stability equation (PSE) models. BRE solutions are less computationally costly than a full DNS and have a more rigorous theoretical foundation than PSE-based models. For example, the full development of a Görtler vortex system in high-speed boundary layers can be predicted in a matter of minutes using a single processor via the BRE approach. This substantial reduction in calculation time is one of the major achievements of this work. We show, among other things, that it allows investigation into feedback control in reasonable total computational times. We investigate the development of the Görtler vortex system via the BRE solution with feedback control parametrically at various freestream Mach numbers \(M_\infty \) and spanwise separations \(\lambda \) of the inflow disturbances.

► The Mack’s amplitude method revisited
  27 Jun, 2021

Abstract

Mack (1977) criticized methods referring to a single frequency perturbation for correlation of transition prediction because the external disturbance source (like free stream turbulence) should have a broadband spectrum. Delta-correlated perturbations are characterized by the mean square of physical amplitude, which is expressed as a double integral of the power spectral density in frequency and the spanwise wave number. It is suggested to evaluate this integral asymptotically. The results obtained using the asymptotic method and direct numerical integration are compared with ad hoc approaches for high speed and moderate supersonic boundary layers. This allows us to suggest recommendations on rational usage of the amplitude method with avoiding unconfirmed simplifications while reducing the computational effort to the level affordable for engineering practice.


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