Direct numerical simulation of single gas bubbles in pure and contaminated liquids

Disperse gas bubbles play an important role in many industrial applications. Knowing the rising velocity, the interfacial area, or the critical size for break–up or coalescence in different systems can be crucial for the process design. Hence, knowing the fundamental behaviour of a single bubble appears mandatory for the examination of bubble swarms and for the Euler–Lagrange or Euler–Euler modelling of disperse systems. In the present work a level–set–based volume–tracking method is implemented into the CFD–code OpenFOAM to follow the free interface of a single bubble. The volume–tracking method is coupled with a transport model for surfactants on the interface, including adsorption and desorption processes. The dependency of surface tension on the local surfactant concentration on the interface is modelled by a non–linear (Langmuir) equation of state. Marangoni forces, resulting from surface tension gradients, are included. The rise of a single air bubble (i) in pure water and (ii) in the presence of surfactants of different strengths is simulated. The results show good agreement with available correlations from literature. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of the three-dimensional Kolmogorov flow in a shear layer

Numerical simulation is used to study the Kolmogorov flow in a shear layer of a compressible inviscid medium. A periodic permanent force applied to the flow gives rise to a vortex cascade of instabilities. The influence exerted by the size of the computational domain, the initial conditions, and the amplitude of the force on the formation of an instability cascade and the transition to turbulence is studied. It is shown that the mechanism of the onset of turbulence has an essentially three-dimensional nature. For the turbulent flows computed, the classical Kolmogorov ?5/3 power law holds in the inertial range.     

Numerical simulation study on gas–solid two-phase flow in pre-calciner

A three-dimensional numerical simulation of DD (dual combustion and denitratior process) pre-calciner for cement production was conducted in this paper. In Euler coordinate system, the fluid phase is expressed with RNG k–ε two-equation model and the solid phase is expressed with particle stochastic trajectory model in Lagrange coordinate system. Four mixture fractions are deduced in this article to simulate the gas compositions. The results of numerical simulation predicted the burn-out ratio of coal and the decomposition ratio of limestone particles along with particle trajectories. It also supplied theoretical foundation for industrial analysis of the coupling relation between coal combustion and calcium carbonate decomposition.     

Numerical simulation of shear and normal forces in standard and shear flow sequencing batch reactor (SF-SBR)

The purpose of this contribution is to compare the shear and the normal stresses in two different types of bioreactors. In the first one (SBR), the granules are generated by liquid and bubbles flow, whereas in the second one (SF-SBR), the shear rate is achieved by installing a rotating cylinder inside the reactor. Such shear flow can be applied for anaerobic process in the wastewater treatment. The results demonstrate the effective role of the process parameters and the reactor geometry on the shear and normal stresses and consequently on the granulation process. Hereby, the different tendencies of the velocity fields, the particle sedimentation as well as Taylor vortices in (SF-SBR), are observed. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical study of dynamics of single bubbles and bubble swarms

A systematic computational study of the dynamics of gas bubbles rising in a viscous liquid is presented. Two-dimensional simulations are carried out. Both the dynamics of single bubbles and small groups of bubbles (bubble swarms) are considered. This is a continuation of our previous studies on the two-bubble coalescence and vortex shedding [A. Smolianski, H. Haario, P. Luukka, Vortex shedding behind a rising bubble and two-bubble coalescence: a numerical approach, Appl. Math. Model. 29 (2005) 615–632]. The proposed numerical method allows us to simulate a wide range of flow regimes, accurately capturing the shape of the deforming interface of the bubble and the surface tension effect, while maintaining the mass conservation. The computed time-evolution of bubble’s position and rise velocity shows a good agreement with the available experimental data. At the same time, the results on the dynamics of bubble interface area, which are, up to our knowledge, presented for the first time, show how much the overall mass transfer would be affected by the interface deformation in the case of the bubble dissolution. Another set of experiments that are of interest for chemical engineers modelling bubbly flows concerns the bubble swarms and their behavior in different bubble-shape regimes. The ellipsoidal and spherical shape regimes are considered to represent, respectively, the coalescing and non-coalescing bubble swarms. The average rise velocities of the bubble swarms are computed and analyzed for both regimes.     

Computational fluid dynamics simulation of aerodynamic performance and flow separation by single element and slatted airfoils under rainfall conditions

This study investigated the effects of rainfall on flow separation and the aerodynamic performance of single element and slatted NACA 0012 airfoils by using a mathematical model developed with the commercial computational fluid dynamics solver ANSYS FLUENT 18.2. A two-way momentum coupled Eulerian–Lagrangian multiphase approach was used to simulate the formation of the water film layer on the airfoil's surface. According to the results, very low values of the lift-to-drag ratio at low angles of attack reflected severe degradation of the aerodynamic performance of the airfoil in the presence of water accumulated on its surface. The impact of rain droplets on the leading-edge slat surface led to less water accumulating on the main section of the airfoil. In particular, the maximum water film mass concentrated on the airfoil surface decreased from 15 g to 1 g compared with the single element airfoil. Hence, the thickness of the water film layer was not sufficiently large to significantly affect the aerodynamic coefficients of the slatted airfoil, especially the maximum lift coefficient, compared with the thicker water film layer on the single element airfoil. In addition, the use of slats clearly enhanced the aerodynamic coefficients and increased the stall angle from 13° to 22° in dry conditions, and from 16° to 24° in rainy conditions. Slats also significantly decreased the boundary layer thickness and delayed the separation at higher angles of attack.     

Slow viscous flow near a superhydrophobic surface with trapped gas bubbles

The Boundary Element Method (BEM) is used to solve the problem of Stokes flow of a viscous fluid over a periodic striped texture of a superhydrophobic surface (SHS), partially filled with frictionless gas bubbles. The shape of the bubble surfaces and the position of the meniscus pinning points relative to the cavity walls are taken into account in the study. Two kinds of flows important for practical applications are considered: a pressure-driven flow in a thin channel with a bottom superhydrophobic wall and a shear-driven flow over a periodic texture. We study the flow pattern in the fluid over a single cavity containing a bubble with a curved phase interface shifted into the cavity. A parametric numerical study of the averaged slip length of the SHS is performed as a function of the geometric parameters of the texture. It is shown that the curvature of the phase interface and/or its shift into the cavity both result in the decrease in the average slip length. It is demonstrated that the BEM can be an efficient tool for studying Stokes flows over textured superhydrophobic surfaces with different geometries of microcavities and phase interfaces. (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of flow of viscoelastic fluids

The paper deals with the numerical calculation of highly viscous, non-newtonian fluid flows in apparatus of mechanical engineering. We use a differential constitutive equation to approximate the real behaviour of technical fluids like polymer melts. By means of calculated flow examples in two and three dimensional geometries we demonstrate the influence of the non-newtonian behaviour. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Relaxation time simulation method with internal energy exchange for perfect gas flow at near-continuum conditions

This paper presents an internal energy exchange scheme for the relaxation time simulation method (RTSM) which solves the BGK equation for the perfect gas flow at near-continuum region discrete rotational energies are introduced to model the relaxation of internal energy modes. This development improved the agreements between RTSM and DSMC with little additional computational cost. The result shows a possibility of an improved hybrid RTSM/DSMC code for the continuum/rarefied gas flow.     

Numerical simulation of building evacuation in emergency conditions

In the present work, pedestrian behaviour in crowds is assumed to be a coupled phenomena. Therefore, the use of continuousand individual-models in different regions according to its local density is suggested. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of airfoil vibrations induced by turbulent flow

The subject of this paper is the numerical simulation of the interaction between two-dimensional incompressible viscous flow and a vibrating airfoil. A solid elastically supported airfoil with two degrees of freedom, which can rotate around the elastic axis and oscillate in the vertical direction, is considered. The numerical simulation consists of the stabilized finite element solution of the Reynolds averaged Navier–Stokes equations with algebraic models of turbulence, coupled with the system of ordinary differential equations describing the airfoil motion. Since the computational domain is time dependent and the grid is moving, the Arbitrary Lagrangian–Eulerian (ALE) method is used. The developed method was applied to the simulation of flow-induced airfoil vibrations.     

The modeling and numerical simulation of gas flow networks

Summary. A network formulation is introduced for the modeling and numerical simulation of complex gas transmission systems like a multi-cylinder internal combustion engine. Several simulation levels are discussed which result in different network representations of a specific system. Basic elements of a network are chambers of finite volume, straight pipes and connections like valves or nozzles. The pipe flow is modeled by the unsteady, one-dimensional Euler equations of gas dynamics. Semi-empirical approaches for the chambers and the connections yield differential-algebraic equations (DAEs) in time. The numerical solution is based on a TVD scheme for the pipe equations and a predictor-corrector method for the DAE-system. Simulation results for an internal combustion engine demonstrate the practical interest of the new approach. Received May 12, 1994 / Revised version received August 26, 1994     

Numerical simulation of laminar flow past a circular cylinder

The present paper focuses on the analysis of two- and three-dimensional flow past a circular cylinder in different laminar flow regimes. In this simulation, an implicit pressure-based finite volume method is used for time-accurate computation of incompressible flow using second order accurate convective flux discretisation schemes. The computation results are validated against measurement data for mean surface pressure, skin friction coefficients, the size and strength of the recirculating wake for the steady flow regime and also for the Strouhal frequency of vortex shedding and the mean and RMS amplitude of the fluctuating aerodynamic coefficients for the unsteady periodic flow regime. The complex three dimensional flow structure of the cylinder wake is also reasonably captured by the present prediction procedure.     

Numerical simulation of non-Newtonian blood flow in bypass models

The article presents the numerical investigation of non–Newtonian effects of steady blood flow in complete idealized 3–D bypass models, whose native artery is either coronary or femoral with average physiological parameters. Considering the blood to be a generalized Newtonian fluid, the shear–dependent viscosity is described by two well–known macroscopic non–Newtonian models (the Carreau–Yasuda model and the modified Cross model). The results were obtained by own developed computational software based on the pseudo–compressibility approach and on the cell–centred finite volume method defined on unstructured hexahedral grids. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of glottal flow in surgically changed larynx

According to clinical experience the shape of the larynx seems to have a significant impact on the airflow degree and breathing pattern. In this study we perform a set of numerical experiments simulating the glottal flow in surgically changed larynx, where the vocal folds or false vocal are modified. We are aiming to find an optimal geometry of the larynx in terms of easiness for breathing. To this goal we numerically solve the system for weakly compressible Navier–Stokes equations using the finite element method and we compare the airflow resistance and the volumetric flow rate for the set of geometries for inspiration as well as expiration. We discuss the optimal geometry with respect to the quality of breathing. (© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of shear band formation in hyperelastic material by energy relaxation

A theorical framework for the analysis of localized failure in hyperealstic material is presented based on the energy minimization principles associated with micro-structure developments. The theory is predicated upon the assumptions that the thickness of shear band represented by its volume fraction tends to zero as well as that the energy inside shear band is a function of the norm of the deformation gradient. Shear bands are treated as laminates of first order. The existence of shear bands in the structure leads to an ill-posed problem which can be solved by means of energy relaxation. An application of the proposed formulation to Neo-Hookean material is presented. Numerical simulation is shown in order to evaluate the performance of the proposed energy relaxation. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Numerical simulation of shear band formation in elastic material by energy relaxation

Following up the previous work, [1], a new approach to the problem of shear localization is proposed, based on energy minimization principles associated with micro-structure developments. In this approach, two different material models are included. They represent the behaviour of material at very small strain and very large strain, respectively. Herein, shear bands are treated as the micro-shearing of rank-one laminates. The thickness of a shear band represented by its volume fraction is assumed to tend to zero while the strain inside the shear band tends to infinity. The existence of shear bands in the structure leads to an ill-posed problem which can be solved by means of energy relaxation. The performance of the proposed energy relaxation is demonstrated through numerical simulation of a tension test under plane strain conditions. The presented numerical simulation shows that there is no mesh-dependence. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Using laser-cantilever anemometry under various flow conditions

We present velocity measurements executed with the new laser-cantilever anemometer (LCA) under various flow conditions. Previously, the basic principles and characteristics of the LCA were investigated. Measurements led to results comparable to common measurement techniques for turbulent flows, such as hot-wire anemometry for air and hot-film anemometry for water [1]. Here we present further experiments where the LCA was used in a snow wind tunnel to investigate the behavior of the cantilever under particle impact. In comparison to data collected with a hot-film anemometer under same conditions, the times series of the LCA showed less pronounced impact characteristics than that of the hot-film, i.e. a recovery time that is shorter and easier to identify. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)