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Annual Review of Fluid Mechanics top

► Dynamic Mode Decomposition and Its Variants
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 225-254, January 2022.
► FLEET Velocimetry for Aerodynamics
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 525-553, January 2022.
► Fluid Dynamics of Axial Turbomachinery: Blade- and Stage-Level Simulations and Models
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 255-285, January 2022.
► Continuum and Molecular Dynamics Studies of the Hydrodynamics of Colloids Straddling a Fluid Interface
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 495-523, January 2022.
► Flow and Drop Transport Along Liquid-Infused Surfaces
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 83-104, January 2022.
► Flood Inundation Prediction
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 287-315, January 2022.
► Physics and Modeling of Large Flow Disturbances: Discrete Gust Encounters for Modern Air Vehicles
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 469-493, January 2022.
► The Influence of Boundaries on Gravity Currents and Thin Films: Drainage, Confinement, Convergence, and Deformation Effects
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 27-56, January 2022.
► Vortex Reconnection and Turbulence Cascade
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 317-347, January 2022.
► Drop Impact Dynamics: Impact Force and Stress Distributions
    5 Jan, 2022
Annual Review of Fluid Mechanics, Volume 54, Issue 1, Page 57-81, January 2022.

Computers & Fluids top

► A sixth order entropy condition scheme for compressible flow
    

Publication date: Available online 26 May 2022

Source: Computers & Fluids

Author(s): Tong Zhou, Haitao Dong

► Stochastic boundary condition effects on supersonic leading edge blowing
    

Publication date: Available online 25 May 2022

Source: Computers & Fluids

Author(s): Francisco Lozano, Iman Rahbari, Guang Lin, Guillermo Paniagua

► Flow modulation and heat transport of radiatively heated particles settling in Rayleigh–Bénard convection
    

Publication date: 15 June 2022

Source: Computers & Fluids, Volume 241

Author(s): Ming Pan, Yuhong Dong, Quan Zhou, Lian Shen

► CFD-based shape optimization under uncertainties using the Adjoint-assisted Polynomial Chaos Expansion and projected derivatives
    

Publication date: 15 June 2022

Source: Computers & Fluids, Volume 241

Author(s): Th. Skamagkis, E.M. Papoutsis-Kiachagias, K.C. Giannakoglou

► An implementation of MPI and hybrid OpenMP/MPI parallelization strategies for an implicit 3D DDG solver
    

Publication date: 15 June 2022

Source: Computers & Fluids, Volume 241

Author(s): Xiaofeng He, Kun Wang, Yiwei Feng, Lili Lv, Tiegang Liu

► A Novel Method for Bounding the Phase Fractions at Both Ends in Eulerian multi-fluid Model
    

Publication date: Available online 25 May 2022

Source: Computers & Fluids

Author(s): Jijian Lian, Xiuwei Yang, Bin Ma, Wenjuan Gou

► Editorial Computers and Fluids, Special Issue NAHOMCon ’19
    

Publication date: Available online 24 May 2022

Source: Computers & Fluids

Author(s): Gustaaf B Jacobs, Jose Castillo

► Theoretical advances and applications of high-fidelity computation and modelling in fluid dynamics
    

Publication date: Available online 4 May 2022

Source: Computers & Fluids

Author(s): Hui Xu, Guohua Tu, Spencer J. Sherwin

► High-resolution ILW outflow boundary treatment for the Navier–Stokes equations
    

Publication date: Available online 21 May 2022

Source: Computers & Fluids

Author(s): Luciano K. Araki, Rafael B. de R. Borges, Nicholas Dicati P. da Silva, Chi-Wang Shu

► Three-dimensional discontinuous Galerkin based high-order gas-kinetic scheme and GPU implementation
    

Publication date: Available online 21 May 2022

Source: Computers & Fluids

Author(s): Yuhang Wang, Liang Pan

International Journal of Computational Fluid Dynamics top

► Bayesian Uncertainty Reduction of Generalised k-ω Turbulence Model for Prediction of Film-Cooling Effectiveness
  19 May, 2022
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► Reynolds Stress Turbulence Modelling with γ Transition Model
  16 May, 2022
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► Perturbation of Wall Boundary Condition to Trigger Vortex Shedding Over a Circular Cylinder
    5 Apr, 2022
Volume 35, Issue 10, December 2021, Page 872-892
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► Coupling Contraction-expansion Arrays with Spiral Microchannels to Enhance Microfluidic-Based Particle/Cell Separation
    1 Apr, 2022
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► A Conceptual Alternative Machine Learning-Based Method for Mesh Sensitivities Calculation in a Turbomachinery Blades Optimisation Framework
  28 Mar, 2022
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► A Divergence-Free High-Order Spectral Difference Method with Constrained Transport for Ideal Compressible Magnetohydrodynamics
    2 Mar, 2022
Volume 35, Issue 10, December 2021, Page 826-849
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► Three-Dimensional Lattice Boltzmann Model for Acoustic Waves Emitted by a Source
  29 Dec, 2021
Volume 35, Issue 10, December 2021, Page 850-871
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► A Finite Volume Chimera Method for Fast Transient Dynamics in Compressible Flow Problems
  13 Dec, 2021
Volume 35, Issue 10, December 2021, Page 799-825
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► Erratum
  18 Aug, 2014
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International Journal for Numerical Methods in Fluids top

► A time‐staggered second order conservative time scheme for variable density flow
  24 May, 2022

Abstract

In this study, we present a robust conservative time-staggered scheme for variable density flow. This pressure correction scheme uses the compressible Navier-Stokes equations and is implemented in the collocated finite-volume open-source computational fluid dynamics solver code_saturne. The Helmholtz equation is solved for the pressure increment, taking the thermodynamic pressure into account and avoiding the acoustic time step limitation. The internal energy equation is used and completed by a source term derived from the discrete kinetic energy equation, thus enforcing total energy conservation and consistency for irregular solutions. A numerical analysis providing conditions ensuring the positivity of the thermodynamic variables is proposed. The scheme is verified and validated against analytical and experimental test cases. Its ability to reproduce the pressure variation while conserving the mass is demonstrated. Its conservative property and time convergence order are also verified. An irregular shock solution is studied, emphasising the importance of the source term in the internal energy equation. Finally, the scheme is validated against reference numerical results on a two-dimensional natural convection cavity and experimental data on a three-dimensional ventilation test case. The comparison against experimental data is made using first-and second-order turbulent simulations.

► Model order reduction for bifurcating phenomena in Fluid‐Structure Interaction problems
  24 May, 2022

Abstract

This work explores the development and the analysis of an efficient reduced order model for the study of a bifurcating phenomenon, known as the Coandă effect, in a multi-physics setting involving fluid and solid media. The latter is governed by the Navier-Stokes equations for an incompressible, steady and viscous fluid and by the elasticity constitutive relations modelling the behaviour of the solid. Taking into consideration a Fluid-Structure Interaction problem, we aim at generalizing previous works towards a more reliable description of the physics involved. In particular, we provide several insights on how the introduction of an elastic structure influences the bifurcating behaviour. We have addressed the computational burden by developing a reduced order branch-wise algorithm based on a monolithic Proper Orthogonal Decomposition. We compared different constitutive relations for the solid, and we observed that a nonlinear hyper-elastic law delays the bifurcation w.r.t. the standard model, while the same effect is even magnified when considering linear elastic solid.

► Improved Implicit Potential Method for Incompressible Flows on a Fully Collocated Mesh
  21 May, 2022

Abstract

In the present work, we present a new version of the pressure-based Implicit Potential (IPOT) method for incompressible flows, which can be applied on a fully collocated mesh. The new version combines the IPOT algorithm with the Rhie and Chow (RC) technique, to produce solutions on collocated grids that are free of spurious pressure modes. The IPOT-RC method retains all the benefits of the original algorithm, i.e. explicit velocity-pressure coupling, easy implementation and reduced iteration time, without requiring a special grid topology. The presentation of the IPOT-RC method, is accompanied by an extensive discussion on the cause of the spurious oscillations in zero-div problems in general, and a possible cure that is linked to the Rhie and Chow technique. The IPOT-RC method is validated through several benchmark problems including the lid-driven cavity flow, flow over a backward facing step and Direct Numerical Simulation (DNS) of turbulent channel flow.

► An efficient and accurate implicit DG solver for the incompressible Navier–Stokes equations
  19 May, 2022
An efficient and accurate implicit DG solver for the incompressible Navier–Stokes equations

We propose an accurate and robust solver for the incompressible Navier–Stokes equations. The method is based on a DG spatial discretization and on the TR-BDF2 method for time discretization and its superior efficiency with respect to other widely used implicit approaches has been shown in a number of classical benchmarks. Good scaling properties of the parallel implementation in the framework of the deal.II software package, as well as accurate simulations in complex geometries, are presented, making the proposed solver attractive also for large scale industrial applications.


Abstract

We propose an efficient, accurate, and robust implicit solver for the incompressible Navier–Stokes equations, based on a DG spatial discretization and on the TR-BDF2 method for time discretization. The effectiveness of the method is demonstrated in a number of classical benchmarks, which highlight its superior efficiency with respect to other widely used implicit approaches. The parallel implementation of the proposed method in the framework of the deal.II software package allows for accurate and efficient adaptive simulations in complex geometries, which makes the proposed solver attractive for large scale industrial applications.

► A hybrid volume of fluid and level set interface capturing scheme with quartic surface representation for unstructured meshes
  17 May, 2022

Abstract

In this paper, we developed a hybrid volume of fluid (VOF) and level set interface capturing scheme with quartic surface representation for unstructured meshes. The so-called {THINC-scaling scheme with quartic surface representation (THINC-scaling/QSR)} preserves the merits of the mass/volume conservativeness from the VOF scheme along with the geometric faithfulness from the level set scheme. Compared with linear/quadratic surface representation used in many other interface capturing schemes, the fluid interface is defined more precisely as a quartic polynomial surface of THINC function which synchronize the interface between the VOF and level set function in a straightforward solution procedure. We update the VOF function using a finite volume formulation. The level set function is first locally updated via a semi-Lagrangian method for interface cells and then reinitialized within each time step. Two-dimensional algorithm has been developed and verified by some benchmark tests on unstructured meshes. Convincing evidence suggests that the present scheme, with quartic surface representation, can provide high-fidelity solution with more pronounced sub-grid interface capturing capability than other most advanced methods.

► Mesh‐free model for Hopf's bifurcation points in incompressible fluid flows problems
  17 May, 2022
Mesh-free model for Hopf's bifurcation points in incompressible fluid flows problems

In this article, a new numerical model is proposed to detect numerically the Hopf bifurcation point of incompressible fluid flows problems. The concept of this model consists to propose a Hopf bifurcation indicator which is solved by a high order mesh free algorithm (HO-MFA) using the moving least squares (MLS) approximation. This numerical model is based on a strong formulation of Navier–Stokes equations. This procedure allowed us to recover the results of the literature in 3D using only a 2D modelization.


Abstract

In this article, a new numerical model is proposed to detect numerically the Hopf bifurcation point of incompressible fluid flows problems. The concept of this model consists to propose a Hopf bifurcation indicator which is solved by a high order mesh free algorithm (HO-MFA) using the moving least squares (MLS) approximation. This numerical model is based on a strong formulation of Navier–Stokes equations. This procedure allowed us to recover the results of the literature in 3D$$ 3D $$ using only a 2D$$ 2D $$ modelization for low Reynolds number. This new model is tested on the classical flow around a cylindrical obstacle and the backward-facing step flow to show the advantage and ability of the proposed model to detect the bifurcation points and to determinate the flow periodicity.

► A novel dynamic adaptive unstructured mesh algorithm for simulating multi‐object relative motion in incompressible fluid
  11 May, 2022

Abstract

A novel dynamic adaptive unstructured mesh (DAUM) algorithm is proposed to solve incompressible multi-object relative motion (MORM). The DAUM algorithm, consisting of exponential function deformation, adaptive edge swapping, and area Laplace smoothing, can greatly improve dynamic mesh robustness and perfectly overcome mesh skewness. The core of DAUM is the adaptive edge swapping inspired by Delaunay triangulation, which is distinguished from traditional edge swapping. The adaptive edge swapping can fully consider the relationship of neighbor elements only using Delaunay triangulation. Meanwhile, the implementation and reliability of adaptive edge swapping are better than the traditional edge swapping method due to eliminating the interference of the non-convex polygon, so more code remedies can be avoided. Using the DAUM, none of the vertices is inserted or deleted so that the manipulation of the dynamic mesh is easily implemented and maintains computational efficiency in the process of mesh motion. Three representative geometries are used to assess the performance of the DAUM in MORM. To systematically analyze the advantages of DAUM, two relatively moving cylinders have been numerically investigated in incompressible flow. The inline force and lift force profiles on two cylinders are obtained and analyzed by using the flow field information. Three interaction stages are divided based on the parameter G and the interactional intensity of two inner anticlockwise vortices is considered as the division criteria. At the running process of the DAUM algorithm, the dynamic mesh quality is well controlled and remains in the high-quality range based on the aspect ratio (AR) criterion. The results indicate that the proposed DAUM algorithm can properly solve the difficulties caused by MORM, especially for period oscillation motion.

► On the DLM/FD methods for simulating neutrally buoyant swimmers moving in non‐Newtonian shear thinning fluids
    9 May, 2022
On the DLM/FD methods for simulating neutrally buoyant swimmers moving in non-Newtonian shear thinning fluids

We have generalized a Lagrange multiplier based fictitious domain (DLM/FD) method to simulating the motion of neutrally buoyant particles of non-symmetric shape in non-Newtonian shear-thinning fluids. For a self-propelled swimmer formed by two different size disks, the effect of shear-thinning makes the swimmer moving faster in the direction of larger disk and decreases the critical Reynolds number (for the moving direction changing to the opposite one) when decreasing the value of the power index n in the Carreau-Bird model.


Abstract

In this article we discuss the generalization of a Lagrange multiplier based fictitious domain (DLM/FD) method to simulating the motion of neutrally buoyant particles of non-symmetric shape in non-Newtonian shear-thinning fluids. Numerical solutions of steady Poiseuille flow of non-Newtonian shear-thinning fluids are compared with the exact solutions in a two-dimensional channel. Concerning a self-propelled swimmer formed by two disks, the effect of shear-thinning makes the swimmer moving faster and decreases the critical Reynolds number (for the moving direction changing to the opposite one) when decreasing the value of the power index n in the Carreau-Bird model.

► An incompressible smoothed particle hydrodynamics‐finite volume method coupling algorithm for interface tracking of two‐phase fluid flows
    9 May, 2022
An incompressible smoothed particle hydrodynamics-finite volume method coupling algorithm for interface tracking of two-phase fluid flows

(i) The ISPH-FVM method has higher computational efficiency than the pure SPH particle method. (ii) Two-phase interface can be well captured by the ISPH-FVM coupling algorithm. (iii) The ISPH-FVM coupling method can accurately calculate the flow field variables, while overcoming the complexity of pure particle method to deal with the inflow/outflow boundary.


Abstract

Two-phase flow involves complex interface evolution process such as the formation, development, pulsation, and rupture of phase interfaces. Numerical simulation is one of the important means to study two-phase flow. The tracking and reconstruction of phase interface is the focus of two-phase flow simulation. A two-phase flow simulation algorithm based on coupled incompressible smoothed particle hydrodynamics (ISPH) method and finite volume method (FVM) is developed in this article. In present ISPH-FVM coupling algorithm, one phase which has smaller volume is represented by SPH particles, while the other phase is defined on the FVM grids. The coupling of ISPH and FVM is achieved through the transfer and interaction of physical parameters at the overlapping area of the SPH particles and FVM grids. The continuous medium surface force model is also introduced into the ISPH-FVM coupling algorithm to study the effect of surface tension on the two-phase flow. Several numerical examples of two-phase flow are adopted to verify the effectiveness of the ISPH-FVM coupling method.

► The physics informed neural networks for the unsteady Stokes problems
    9 May, 2022
The physics informed neural networks for the unsteady Stokes problems

We solve the unsteady Stokes problems by using the physics informed neural networks coupled with small sample learning, and give the proofs to guarantee the convergence of the neural networks as well as the convergence of the loss function. Moreover, we combine the model-driven (the differential forms of the neural networks) and the data-driven (the numerical data) to construct the loss function and enhance the efficiency as well as the accuracy of the method. Furthermore, this method is meshfree and can simultaneously solve each variable of the equations separately in parallel framework.


Abstract

In this article, we develop the physics informed neural networks (PINNs) coupled with small sample learning for solving the transient Stokes equations. Specifically, the governing equations are encoded into the networks to construct the loss function, which involves the residual of differential equations, the initial/boundary conditions, and the residual of a handful of observations. The approximate solution was obtained by optimizing the loss function. Few sample data can rectify the network effectively and improve predictive accuracy. Moreover, the method can simultaneously solve each variable of the equations separately in a parallel framework. The information of the numerical data is compiled into the networks to enhance efficiency and accuracy in practice. Therefore, this method is a meshfree and fusion method that combined data-driven with model-driven. Inspired by the Galerkin method, the paper proves the convergence of the loss function and the capability of neural networks. Furthermore, numerical experiments are performed and discussed to demonstrate the performance of the method.

Journal of Computational Physics top

► A well-balanced weighted compact nonlinear scheme for shallow water equations on curvilinear grids
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Mingyang Cheng, Lingyan Tang, Yaming Chen, Songhe Song

► A linear and nonlinear analysis of the shallow water equations and its impact on boundary conditions
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Jan Nordström, Andrew R. Winters

► Bound-preserving discontinuous Galerkin methods with second-order implicit pressure explicit concentration time marching for compressible miscible displacements in porous media
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Wenjing Feng, Hui Guo, Yue Kang, Yang Yang

► Iterated Kalman methodology for inverse problems
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Daniel Zhengyu Huang, Tapio Schneider, Andrew M. Stuart

► A positivity-preserving scheme for fluctuating hydrodynamics
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Francesco Magaletti, Mirko Gallo, Sergio P. Perez, José A. Carrillo, Serafim Kalliadasis

► Validation of the low dissipation computational algorithm CABARET-MFSH for multilayer hydrostatic flows with a free surface on the lock-release experiments
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): V.M. Goloviznin, Pavel A. Maiorov, Petr A. Maiorov, A.V. Solovjev

► VecDualSPHysics: A vectorized implementation of Smoothed Particle Hydrodynamics method for simulating fluid flows on multi-core processors
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Sifan Long, Xiaokang Fan, Chao Li, Yi Liu, Sijiang Fan, Xiao-Wei Guo, Canqun Yang

► Sobolev gradient type iterative solution methods for a nonlinear 4th order elastic plate equation
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): J. Karátson

► Nonintrusive manufactured solutions for non-decomposing ablation in two dimensions
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Brian A. Freno, Brian R. Carnes, Victor E. Brunini, Neil R. Matula

► High order well-balanced asymptotic preserving finite difference WENO schemes for the shallow water equations in all Froude numbers
    

Publication date: 15 August 2022

Source: Journal of Computational Physics, Volume 463

Author(s): Guanlan Huang, Yulong Xing, Tao Xiong

Journal of Turbulence top

► Convection of multi-scale motions in turbulent boundary layer by temporal resolution particle image velocimetry
  24 May, 2022
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► Dependence of wall jet phenomenology on inlet conditions and near-field flow development
    6 May, 2022
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► Build up of yield stress fluids via chaotic emulsification
    6 May, 2022
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► Dimension reduced turbulent flow data from deep vector quantisers
  12 Apr, 2022
Volume 23, Issue 4-5, April - May 2022, Page 232-264
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► Large eddy simulations of single and multiple turbulent round jets
    1 Apr, 2022
Volume 23, Issue 4-5, April - May 2022, Page 173-213
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► Effect of the Froude number on a stratified turbulence under two shear orientations using coupled SSG and SL models
  29 Mar, 2022
Volume 23, Issue 4-5, April - May 2022, Page 214-231
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Physics of Fluids top

► Thermal deformity and thermolysis of magnetized and fractional Newtonian fluid with rheological investigation
  26 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
Thermolysis and its deformities can occur at every developmental stage at all temperatures during the process of heating, pyrolysis mechanism, and thermodynamical decompositions. This manuscript investigates thermal-fluid flow of a magnetized Newtonian fluid under the influence of porosity via modern fractional differential approaches. The mathematical modeling of thermal-fluid flow of the magnetized Newtonian fluid is developed for temperature distribution and velocity profile under the Mittag–Leffler function as an imposed boundary condition. The governing equations of thermal-fluid flow of the magnetized Newtonian fluid are non-dimensional and fractionalized through modern non-integer differentiations. The mathematical model of thermal-fluid flow for the magnetized Newtonian fluid is tackled via integral transforms for generating exact analytical solutions. For the sake of comparative analysis of thermodynamical aspects, the Nusselt number from the thermal fractional model and skin friction from the momentum fractional model have been compared graphically via two types of modern non-integer differentiations and statistical observations. The results indicate and suggest the significant impacts in realistic hypothesis.
► Simulating liquid–gas interfaces and moving contact lines with the immersed boundary method
  26 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
In this work, we use the immersed boundary method with four extensions to simulate a moving liquid–gas interface on a solid surface. We first define a moving contact line model and implements a static-dynamic friction condition at the immersed solid boundary. The dynamic contact angle is endogenous instead of prescribed, and the solid boundary can be non-stationary with respect to time. Second, we simulate both a surface tension force and a Young's force with one general equation that does not involve estimating local curvature. In the third extension, we splice liquid–gas interfaces to handle topological changes, such as the coalescence and separation of liquid droplets or gas bubbles. Finally, we re-sample liquid–gas interface markers to ensure a near-uniform distribution without exerting artificial forces. We demonstrate empirical convergence of our methods on non-trivial examples and apply them to several benchmark cases, including a slipping droplet on a wall and a rising bubble.
► Alcove formation in dissolving cliffs driven by density inversion instability
  25 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
We demonstrate conditions that give rise to cave-like features commonly found in dissolving cliffsides with a minimal two-phase physical model. Alcoves that are wider at the top and tapered at the bottom, with sharp-edged ceilings and sloping floors, are shown to develop on vertical solid surfaces dissolving in aqueous solvents. As evident from descending plumes, sufficiently large indentations evolve into alcoves as a result of the faster dissolution of the ceiling due to a solutal Rayleigh–Bénard density inversion instability. In contrast, defects of size below the boundary layer thickness set by the critical Rayleigh number smooth out, leading to stable planar interfaces. The ceiling recession rate and the alcove opening area evolution are shown to be given to first-order by the critical Rayleigh number. By tracking passive tracers in the fluid phase, we show that the alcoves are shaped by the detachment of the boundary layer flow and the appearance of a pinned vortex at the leading edge of the indentations. The attached boundary layer past the developing alcove is then found to lead to rounding of the other sides and the gradual sloping of the floor.
► Physics-informed data based neural networks for two-dimensional turbulence
  25 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
Turbulence remains a problem that is yet to be fully understood, with experimental and numerical studies aiming to fully characterize the statistical properties of turbulent flows. Such studies require huge amount of resources to capture, simulate, store, and analyze the data. In this work, we present physics-informed neural network (PINN) based methods to predict flow quantities and features of two-dimensional turbulence with the help of sparse data in a rectangular domain with periodic boundaries. While the PINN model can reproduce all the statistics at large scales, the small scale properties are not captured properly. We introduce a new PINN model that can effectively capture the energy distribution at small scales performing better than the standard PINN based approach. It relies on the training of the low and high wavenumber behavior separately leading to a better estimate of the full turbulent flow. With 0.1% training data, we observe that the new PINN model captures the turbulent field at inertial scales leading to a general agreement of the kinetic energy spectra up to eight to nine decades as compared with the solutions from direct numerical simulation. We further apply these techniques to successfully capture the statistical behavior of large scale modes in the turbulent flow. We believe such methods to have significant applications in enhancing the retrieval of existing turbulent data sets at even shorter time intervals.
► Explosive boiling induced fast transportation of Leidenfrost droplet to target location
  25 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
Leidenfrost droplet possesses ultra-low flow resistance, but it is challenging to obtain large thrust force for fast transportation and regulate the direction of droplet motion. Here, for the first time, we demonstrate a novel mechanism for the control of droplet dynamics by explosive boiling. Our system consists of two surfaces that have different functions: a smooth surface running in the Leidenfrost state for droplet levitation and a skirt ring edge surface (SRES) as an explosive boiling trigger. For droplet-wall collision with SRES, micro/nanoscale roughness not only enhances energy harvesting from the skirt ring to the droplet due to increased radiation heat transfer but also provides nucleation sites to trigger explosive boiling. The symmetry breaking of explosive boiling creates a thrust force that is sufficient to propel the droplet. The suppression of the thrust force relative to the inertia force regulates the droplet trajectory as it passes through a target location. We show orbit lines passing through a focusing spot that is ∼1% of the Leidenfrost surface area around its center with a maximum traveling speed of ∼85 cm/s, which is ∼2 times of that reported in the literature. The scale law analysis explains the droplet size effect on the self-propelling droplet dynamics. Our work is attractive for applications under the conditions of the required traveling speed and direction of the droplet.
► Vertical confinement effects on a fully developed turbulent shear layer
  25 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
The effects of vertical confinement on a turbulent shear layer are investigated with large-eddy simulations of a freely developing shear layer (FSL) and a wall-confined shear layer (WSL) that develops between two horizontal walls. In the case of the WSL, the growth of the shear layer is inhibited by the walls. Once the walls prevent the development of the shear layer, highly anisotropic velocity fluctuations become prominent in the flow. These anisotropic velocity fluctuations are recognized as elongated large-scale structures (ELSS), whose streamwise length is much larger than the length scales in the other directions. Spectral analysis confirms that the turbulent kinetic energy is dominated by the ELSS, whose streamwise length grows continuously. A proper orthogonal decomposition can effectively extract a velocity component associated with the ELSS. The isotropy of the Reynolds stress tensor is changed by the presence of the ELSS. These changes in flow characteristics due to the ELSS are not observed in the FSL, where the shear layer thickness increases continuously. These behaviors of the WSL are consistent with those of stably stratified shear layers (SSSLs), where flow structures similar to ELSS also develop when the vertical flow development is confined by the stable stratification. The vertical confinement by the walls or stable stratification strengthens mean shear effects. The flow behavior at large scales in the WSL and SSSL is consistent with rapid distortion theory for turbulence subject to mean shear, suggesting that the development of ELSS is caused by the mean shear.
► Turbulent displacement flows of viscoplastic fluids in obstructed eccentric annuli: Experiments
  24 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
We present an experimental study of turbulent displacement flows in eccentric annuli, where the displacing fluids are water, 0.1% and 0.2% xanthan gum solutions, and the in situ fluid is a viscoplastic, 0.15% Carbopol solution. We focus on the effect of a solid obstruction on the narrow side of the annular gap, analogous to a consolidated cuttings bed in well cementing operations. For comparison we include experiments with an unobstructed test section with eccentricity e set at ∼0.5. While the displacement flow is successful without the obstruction regardless of the displacing fluid, we find that the obstruction at [math] is mostly detrimental to removal of the yield stress fluid stuck downstream of it, and a decrease in Reynolds numbers via formulation of xanthan gum solutions contribute to a less effective displacement in all situations due to the decrease in fluctuating stresses. Upstream of the obstruction, we observe complete removal of the Carbopol, likely due to enhanced vorticity effects. However, at high eccentricity values of [math], the effect of the obstruction on the displacement of Carbopol appears to be negligible, especially in the less turbulent situations with the xanthan gum solutions where the stationary Carbopol layer covers the entirety of the solid blockage, both upstream and downstream. Thus, in a highly eccentric obstructed annulus, the eccentricity remains the dominant factor to hinder the displacement.
► Stochastic disturbances, induced by plasma actuator in a flat plate boundary layer
  24 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
Mechanism of hydrodynamic noise generation in a subsonic flat plate boundary layer by a barrier discharge plasma actuator is described. The origin of the pulsations is an inscintric unsteadiness of the discharge structure caused by wandering of the microdischarges. Statistics of discharge wandering is obtained from discharge light emission. Propagation of the disturbances in a slightly unstable Blasius boundary layer is studied both experimentally and numerically. It is demonstrated that the discharge-induced noise can be modeled as a sum of delta-correlated localized boundary layer forcing events, with each event represented by the region of longitudinal and transversal force. Discharge-induced disturbances in the boundary layer undergo three main stages as they move downstream: streak-like structures in the near field, oblique wave fans, and eventually a plane Tollmien–Shllichting wave. A simple statistical model, describing the dependency of the pulsations power on actuator driving frequency and voltage, is proposed.
► Airborne lifetime of respiratory droplets
  24 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
We formulate a model for the dynamics of respiratory droplets and use it to study their airborne lifetime in turbulent air representative of indoor settings. This lifetime is a common metric to assess the risk of respiratory transmission of infectious diseases, with a longer lifetime correlating with higher risk. We consider a simple momentum balance to calculate the droplets' spread, accounting for their size evolution as they undergo vaporization via mass and energy balances. The model shows how an increase in the relative humidity leads to higher droplet settling velocity, which shortens the lifetime of droplets and can, therefore, reduce the risk of transmission. Emulating indoor air turbulence using a stochastic process, we numerically calculate probability distributions for the lifetime of droplets, showing how an increase in the air turbulent velocity significantly enhances the range of lifetimes. The distributions reveal non-negligible probabilities for very long lifetimes, which potentially increase the risk of transmission.
► Computational analysis of compressibility effect on flow field and aerodynamics at low Reynolds numbers
  24 May, 2022
Physics of Fluids, Volume 34, Issue 5, May 2022.
The flow around the wing of exploration aircraft flying in the very dilute Martian atmosphere experiences very specific low-Reynolds-number compressible flows. A fundamental understanding of compressibility effects, especially on a separation bubble, is an important issue in the aerodynamic wing design. We investigated the effect of compressibility on the laminar separation bubbles and aerodynamic properties of a blunt flat plate with a thickness of 5% of the chord length in unsteady two-dimensional laminar and large-eddy simulations at chord-based Reynolds numbers (Re) of 6100 and 11 000 in the Mach number (M) range from M = 0.1 to 0.8. In the incompressible flow below M = 0.2 at Re = 11 000, the separated shear layer rolled up due to the Kelvin–Helmholtz (KH) instability reattaches to the surface with turbulent transition and then forms a laminar separation bubble. In contrast, with increasing Mach number, the transition and reattachment are delayed, and the separation bubble is stretched. In particular, at M = 0.8, the rolled-up vortex is not distorted in the span direction and advects downstream with a strong spanwise two-dimensionality. This is due to the compressibility effect that suppresses the KH instability. On the other hand, at Re = 6100, the separated shear layer is stable without spanwise distortion even in the incompressible flow due to the stronger influence of viscous forces. With increasing Mach number, such stable separated shear layer reattaches without rolling up further downstream, and the Strouhal number of the vortex also gradually decreases. This leads that the large lift oscillation owing to periodic and two-dimensional spanwise vortex shedding is significantly suppressed with an increasing Mach number. This is also due to the weakening of the KH instability due to the compressibility effects. However, at Re = 11 000, the reattachment points move downstream more significantly with the increasing Mach number, indicating that the suppression of the KH instability due to the compressibility effect is more pronounced at Re = 11 000 than at Re = 6100. Consequently, the negative skin friction shear stress owing to the reverse flow inside the expanded separation bubble becomes relatively large, decreasing the total drag with the increasing Mach number.

Theoretical and Computational Fluid Dynamics top

► Matched asymptotic shock-layer analysis of the interaction between a planar viscous-hypersonic boundary layer and a thin inviscid layer
  26 May, 2022

Abstract

A compressible boundary-layer flow over a flat plate with sharp leading edge is studied in the hypersonic limit. The interaction between the shock wave and boundary layer is characterized using the hypersonic interaction parameter \(\chi = M_{\infty }^3\sqrt{C/\mathrm{Re}_{\infty }}\) where \(M_{\infty }\) and \(\mathrm{Re}_{\infty }\) are the free-stream Mach and Reynolds numbers, respectively, and C is the Chapman–Rubesin constant. The flow is studied for Prandtl number \(\mathrm{Pr}=1\) using a shock-layer analysis of the equations of motion governing high-speed compressible flow. In the strong interaction limit the value for \(\chi \) approaches infinity, \(\chi \rightarrow \infty \) , and there exists coupling between the shock wave and boundary layer that extends the plate length. For finite interaction, \(1< \chi < \infty \) , there is coupling between the shock wave and boundary layer that can extend well-beyond the plate’s leading edge. To study this transition, from \(\chi \sim O\) (1) to \(\chi \gg 1\) , we solved the Prandtl boundary-layer equations that represent the viscous-layer flow (Region I) at non-adiabatic wall conditions using a standard line relaxation method. For the inviscid layer (Region II), we reduced the governing thin inviscid-layer equations to ordinary differential equations by using the method of characteristics. We then matched values for flow variables of similar order computed in the viscous-(Region I) and inviscid-(Region II) layers at the boundary-layer’s edge by using a minimization algorithm. Thus, solutions produced using the technique, denoted asymptotic matching Technique A, required only a single streamwise sweep to achieve convergence between these flow variables computed in the viscous and inviscid layers and matched at the boundary-layer edge. Solutions for flow variables found using Technique A are then compared with solutions for similar \(\chi \) and wall enthalpy values found using a separate shock-layer analysis, denoted matching Technique B, that utilized a tangent-wedge approximation for the inviscid layer. Technique B required successive streamwise sweeps such that initial pressure conditions upstream and downstream are both satisfied at each sweep. The converged solution was obtained during the final sweep based on a preset convergence criteria. Shock-wave and boundary-layer profiles; wall pressure and shear stress computed using both Techniques A and B are compared with values computed using computational fluid dynamics (CFD). The results show good agreement.

Graphical abstract

► Towards robust data-driven reduced-order modelling for turbulent flows: application to vortex-induced vibrations
  23 May, 2022

Abstract

This work presents a robust method that minimises the impact of user-selected parameter on the identification of generic models to study the coherent dynamics in turbulent flows. The objective is to gain insight into the flow dynamics from a data-driven reduced order model (ROM) that is developed from measurement data of the respective flow. For an efficient separation of the coherent dynamics, spectral proper orthogonal decomposition (SPOD) is used, projecting the flow field onto a low-dimensional subspace, so that the dominating dynamics can be represented with a minimal number of modes. A function library is defined using polynomial combinations of the temporal modal coefficients to describe the flow dynamics with a system of nonlinear ordinary differential equations. The most important library functions are identified in a two-stage cross-validation procedure (conservative and restrictive sparsification) and combined in the final model. In the first stage, the process uses a simple approximation of the derivative to match the model with the data. This stage delivers a reduced set of possible library function candidates for the model. In the second, more complex stage, the model of the entire flow is integrated over a short time and compared with the progression of the measured data. This restrictive stage allows a robust identification of nonlinearities and modal interactions in the data and their representation in the model. The method is demonstrated using data from particle image velocimetry (PIV) measurements of a circular cylinder undergoing vortex-induced vibration (VIV) at \(\mathrm{Re}=4000\) . It delivers a reduced order model that reproduces the average dynamics of the flow and reveals the interaction of coexisting flow dynamics by the model structure.

► Numerical study of a water droplet impacting on a moving hydrophobic wall using a 3D lattice Boltzmann method
    7 May, 2022

Abstract

Understanding the dynamics of a water droplet after impacting on a moving wall is significant for many applications such as repelling rain droplets from a vehicle. In this paper, a water droplet impacting on a moving hydrophobic wall is studied numerically using a 3D lattice Boltzmann method (LBM). The accuracy of the present model is validated by comparing with existing correlation equations for the maximum spread factor and the contact time. It is found that the droplet spreads into an asymmetric shape after impacting on the moving wall owing to the momentum transfer from the wall to the droplet. The droplet deformation increases with the increasing of the wall velocity. Because of different bouncing behaviors of the droplet, the effect of the wall velocity on the droplet contact time varies with contact angles: the droplet contact time decreases with the increasing of the wall velocity for θ = 156°, while the droplet contact time increases with the increasing of the wall velocity for θ = 130°. It is also found that the droplet bouncing motion will be suppressed at a high wall velocity for θ = 130°. Finally, a map in terms of the Weber (We) number versus the contact angle (θ) is obtained, showing that a larger critical contact angle is required for droplet rebounding from a moving wall. This work provides a guidance that a moving wall needs to be more hydrophobic than a stationary wall to repel water droplets.

Graphical abstract

► Resolvent-based approach for $$\pmb {H_2}$$ H 2 -optimal estimation and control: an application to the cylinder flow
  23 Apr, 2022

Abstract

We consider estimation and control of the cylinder wake at low Reynolds numbers. A particular focus is on the development of efficient numerical algorithms to design optimal linear feedback controllers when there are many inputs (disturbances applied everywhere) and many outputs (perturbations measured everywhere). We propose a resolvent-based iterative algorithm to perform (i) optimal estimation of the flow using a limited number of sensors, and (ii) optimal control of the flow when the entire flow is known but only a limited number of actuators are available for control. The method takes advantage of the low-rank characteristics of the cylinder wake and provides full-dimensional solutions by implementing a terminal reduction technique based on resolvent analysis. Optimal feedback controllers are also obtained by combining the solutions of the estimation and control problems. We show that the performance of the estimators and controllers converges to the true global optima, indicating that the important physical mechanisms for estimation and control are of low rank.

Graphic abstract

► Electrophoretic motion of a porous polyelectrolyte microcapsule
  23 Apr, 2022

Abstract

This paper investigates the problem of electrophoretic motion of a polyelectrolyte capsule with a porous arbitrary charged conducting shell in an electrolyte (of the same type as the one inside the capsule’s cavity) under the action of an external electric field. The corresponding boundary value problem for the velocity components and pressure in the case of small electrical potentials is analytically solved in quadratures. The solution is analyzed numerically for different values of the specific permeability of the capsule, and the thickness of the porous and the electric double layers. The minimum of electrophoretic velocity dependence on the inverse permeability of the porous layer has been found. It is shown that the electrophoretic mobility decreases upon decrease in the conductivity of the material constituting the porous layer. This means that a dielectric capsule can be used for electrophoresis as well. Moreover, its velocity will be even greater than that of a conducting capsule, all other conditions being equal.

Graphic abstract

► Evolution of high-frequency instabilities in the presence of azimuthally compact crossflow vortex pattern over a yawed cone
    1 Apr, 2022

Abstract

Hypersonic boundary-layer flows over a circular cone at a moderate yaw angle can support strong crossflow instability away from the windward and leeward rays on the plane of symmetry. Due to the more efficient excitation of stationary crossflow vortices by surface roughness, a possible path to transition in such flows corresponds to rapid amplification of the high-frequency instabilities sustained in the presence of finite amplitude stationary crossflow vortices. This paper presents a computational analysis of crossflow instability over a 7-degree half-angle, yawed circular cone in a Mach 6 free stream. Specifically, the nonlinear evolution of an azimuthally localized crossflow vortex pattern and the linear amplification characteristics of high-frequency instabilities evolving in the presence of that pattern are described for the first time. Focusing on the azimuthally compact vortex pattern allows us to overcome significant limitations of the prior secondary instability analyses of azimuthally inhomogeneous boundary layer flows. A comparison between plane-marching parabolized stability equations and direct numerical simulations (DNS) reveals favorable agreement in regard to mode shapes, most amplified disturbance frequencies, and the N-factor evolution. In contrast, the quasiparallel predictions are found to result in a severe underprediction of the N-factors. The most amplified high-frequency instabilities are found to originate from Mack’s second mode waves sustained within the upstream region of nearly unperturbed, quasi-homogeneous boundary layer.

► Secondary instability of Görtler vortices in hypersonic boundary layer over an axisymmetric configuration
    1 Apr, 2022

Abstract

The nonlinear development of Görtler instability over a concave surface gives rise to a highly distorted inflectional flow field in the boundary layer that exhibits strong velocity gradients in the spanwise direction as well as in the wall-normal direction. Such a flow field is susceptible to strong, high frequency secondary instability that can lead to the onset of transition. The present numerical study uses direct numerical simulations and linear secondary instability theory to investigate finite amplitude Görtler vortices and their secondary instability characteristics, respectively, in a hypersonic flow over an axisymmetric cone with a concave aft body. The Görtler modes are excited via azimuthally periodic deformations of the surface geometry and, hence, are fully realizable. For sufficiently small initial amplitudes, the computed growth of the roughness- induced Görtler vortices is shown to agree with the predictions of optimal growth theory. Earlier work on nonlinear Görtler vortices had focused on vortex structures with intermediate amplitudes that resembled bell shaped structures, unlike the mushroom structures with thin stems encountered in lower speed flows. The present results corroborate the findings of other recent studies that fully developed mushroom structures can also exist in the hypersonic regime when the Görtler vortex amplitude is sufficiently large. Computations also reveal that the dominant modes of secondary instability correspond to an antisymmetric “stem” mode associated with the strong, nearly wall-normal shear layers bounding the stem underneath the mushroom structure. The dominant stem modes have supersonic phase velocities, resulting in acoustic radiation to the flow just outside of the boundary layer. To our knowledge, this is the first work documenting the existence of supersonic secondary instabilities in the context of stationary Görtler modes.

► Multi-scale study of the transitional shock-wave boundary layer interaction in hypersonic flow
    1 Apr, 2022

Abstract

A high-fidelity simulation of the massively separated shock/transitional boundary layer interaction caused by a 15-degrees axisymmetrical compression ramp is performed at a free stream Mach number of 6 and a transitional Reynolds number. The chosen configuration yields a strongly multiscale dynamics of the flow as the separated region oscillates at low-frequency, and high-frequency transitional instabilities are triggered by the injection of a generic noise at the inlet of the simulation. The simulation is post-processed using Proper Orthogonal Decomposition to extract the large scale low-frequency dynamics of the recirculation region. The bubble dynamics from the simulation is then compared to the results of a global linear stability analysis about the mean flow. A critical interpretation of the eigenspectrum of the linearized Navier–Stokes operator is presented. The recirculation region dynamics is found to be dominated by two coexisting modes, a quasi-steady one that expresses itself mainly in the reattachment region and that is caused by the interaction of two self-sustained instabilities, and an unsteady one linked with the separation shock-wave and the mixing layer. The unsteady mode is driven by a feedback loop in the recirculation region, which may also be relevant for other unsteady shock-motion already documented for shock-wave/turbulent boundary layer interaction. The impact of the large-scale dynamics on the transitional one is then assessed through the numerical filtering of those low wavenumber modes; they are found to have no impact on the transitional dynamics.

► Prediction of aerothermal characteristics of a generic hypersonic inlet flow
    1 Apr, 2022

Abstract

Accurate prediction of aerothermal surface loading is of paramount importance for the design of high-speed flight vehicles. In this work, we consider the numerical solution of hypersonic flow over a double-finned geometry, representative of the inlet of an air-breathing flight vehicle, characterized by three-dimensional intersecting shock-wave/turbulent boundary layer interaction at Mach 8.3. High Reynolds numbers ( \(Re_L \approx 11.6 \times 10^6\) based on free-stream conditions) and the presence of cold walls ( \(T_w/T_\circ \approx 0.26\) ) leading to large near-wall temperature gradients necessitate the use of wall-modeled large eddy simulation (WMLES) in order to make calculations computationally tractable. The comparison of the WMLES results with experimental measurements shows good agreement in the time-averaged surface heat flux and wall pressure distributions, and the WMLES predictions show reduced errors with respect to the experimental measurements than prior RANS calculations. The favorable comparisons are obtained using a standard LES wall model based on equilibrium boundary layer approximations despite the presence of numerous non-equilibrium conditions including three-dimensionality in the mean, shock/boundary layer interactions, and flow separation. We demonstrate that the use of semi-local eddy viscosity scaling (in lieu of the commonly used van Driest scaling) in the LES wall model is necessary to accurately predict the surface pressure loading and heat fluxes.

► Computational modeling and stability analysis of BOLT hypersonic geometry including off-nominal conditions
    1 Apr, 2022

Abstract

With an interest in developing and studying the stability of laminar undisturbed basic-state solutions, this work is focused on accurately modeling the laminar flowfield of the boundary layer transition (BOLT) geometry under nominal and off-nominal conditions (i.e., nonzero angles of pitch and yaw). The BOLT flowfield is studied using the DPLR flow solver with MUSCL Steger–Warming fluxes using a set of five grids at different resolutions and identical grid topologies. A total of three different sets of conditions are studied: two flight conditions and one wind-tunnel-scale (33%) condition. (1) For the two sets of nominal flight operating conditions, it is found that the flow structures in the centerline region of BOLT are similar to those found in prior studies including in shape, location, and extent both vertically and spanwise, but a detailed comparison of velocity contours shows that further quantitative convergence studies are warranted. The centerline region, however, extends to at most 4 cm in semi-span at the aft end of the geometry (20% of the semi-span). Away from the centerline and where wind-tunnel-scale results have observed regions of possibly transitional behavior, the laminar flowfield converges with high accuracy. (2) For nominal wind-tunnel operating conditions, all grid resolutions simulated show good agreement in most regions as compared with prior results, with any differences falling within the scatter of existing experimental and DNS results. Aside from this focus, boundary-layer stability is examined outboard of the centerline region at nonzero pitch and yaw for a flight case, and second mode and stationary crossflow instabilities are considered. Second-mode instability is found to be locally significant at certain pitch and yaw angles particularly downstream of the swept leading edges. In addition, stationary crossflow is found to become highly amplified in significant wedges extending to the aft end of the BOLT geometry, with N-factors consistent with those found for HIFiRE-5b associated with transitional flow. The reasons for amplification of these different instabilities are also investigated from a physics-based perspective.


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