Difference between revisions of "BubbleFoam"
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Latest revision as of 09:54, 2 November 2013
Contents
1 Introduction and applications
The bubbleFoam solver is a twophase solver based on the EulerEuler twoﬂuid methodology [1, 2, 3, 4, 10], suitable to compute dispersed gasliquid and liquidliquid ﬂows. In the EulerEuler twoﬂuid approach, the phases are treated as interpenetrating continua, which are capable of exchanging properties, like momentum, energy and mass one with the other. Typical applications of the twoﬂuid approach, as implemented in bubbleFoam, are:
 bubble columns
 stirred tank reactors
 static mixers
2 bubbleFoam capabilities and limitations
2.1 bubbleFoam capabilities
The bubbleFoam solver implements the twoﬂuid equations derived in [10, 8] for the simulation of gasliquid ﬂows. The model undergoes the following assumptions:
 Phases are incompressible
 The dispersed phase particle diameter is constant
 The ﬂow is isothermal
 Only momentum exchange is accounted for in the momentum transport equations
The main features of the solver are the following:
 Capability to solve for dispersed twophase ﬂows with strong density ratio
 Robust solution algorithm, able to deal with complete ﬂow separation
 Turbulence modelling through model and standard wall functions.
2.2 bubbleFoam limitations
The bubbleFoam solver currently has the following limitations:
 Only one dispersed phase and a continuous phase can be described. It is not possible to account for multiple dispersed phases (i.e. represent a dispersed phase diameter distribution)
 The diameter of the particles1 constituting the dispersed phase is assumed to be constant. Aggragation, breakage and coalescence phenomena are not accounted for
 The drag coeﬃcient is computed as a blend of the drag coeﬃcients evaluated for each phase on the basis of the phase fractions, and no alternative drag models are available
 The interaction between the phases happens only through the momentum exchange term in the corresponding momentum equations:
 It is not possible to model the heat transfer between the phases
 It is not possible to model the mass transfer between the phases
 No chemical reaction model is available.
3 bubbleFoam theory
3.1 Overview
The twoﬂuid equations solved by the bubbleFoam solver are described in this chapter. The general twoﬂuid continuity and momentum equations are introduced, the closure expressions for the phase stress tensor and the momentum transfer term are presented, and the transport equations of the turbulence model are summarized.
3.2 Governing equations
In the twoﬂuid approach, a continuity and a momentum equation are solved for each phase present in the system. These equations can be derived by conditionally averaging the single phase ﬂow equations. The reader interested in the theoretical background is invited to refer to, for example, [1, 2, 3, 10]. The continuity equations for each phase has the form
where represents the phase fraction of phase , is the density of the material constituting the same phase, and is the phase velocity. The phase momentum equation is
in which is the phase laminar stress tensor, assumed to be Newtonian, is the phase Reynolds stress tensor, is the pressure, is the gravitational acceleration vector, and is the momentum exchange term. The laminar stress tensor is deﬁned, for each phase, as
where is the molecular kinematic viscosity of the ﬂuid constituting phase , and is the identity matrix. The phase Reynolds stress tensor is given by
where is the phase turbulent kinetic energy (Note that in the bubbleFoam implementation, it assumed that . In other words, the turbulent kinetic energy is identical for both the phases present in the system), and is the phase turbulent kinematic viscosity, deﬁned as
in which is a constant, and is the phase turbulent dissipation rate. The phase effective viscosity is calculated as the sum of the phase molecular viscosity and of the phase turbulent viscosity, as
The momentum exchange term can be decomposed in a drag contribution, a lift force contribution and a virtual mass contribution
where the terms are modelled according to the mixture model [10]. In particular, indicating with a and b the two phases considered in the twoﬂuid model, where a represents the dispersed phase, the drag term is described by equation
in which and are the phase particle diameters, is the relative velocity vector, and and are the drag coeﬃcients computed with respect to each phase, according to
where .
The lift for term is modelled as (Notice that should actually be . We report for consistency with the code implementation.}
while the virtual mass force term is evaluated as
where
and
.
3.2.1 Turbulence model
The bubbleFoam solver uses a twoequation turbulence model for the continuous phase, and accounts for the influence of the turbulence on the dispersed phase by scaling the dispersed phase turbulent viscosity. The equation for the turbulent kinetic energy of the continuous phase () reads
where is the producton of the turbulent kinetic energy, given by
and is the turbulent Schmidt number. The turbulent dissipation rate is determined by solving the transport equation
where and are constant of the turbulence model. The turbulence viscosity of the continuous phase is calculated from its definition
while the turbulent viscosity of the dispersed phase is evaluated as
being the coefficient and the turbulence response coefficient constants of the model. Standard wall functions are adopted to treat the zone next to the wall.
4 bubbleFoam implementation
The numerical solution of the twophase equations relies on a segregated algorithm based on the PISO procedure extended to twophase ﬂows [5]. The momentum equations are manipulated to stabilize the system of equation at the limits of the range of volume fractions, to avoid singularities, as suggested in [5, 10]. The details of the numerical methodology are brieﬂy summarized in this chapter.
4.1 Numerical methodology
4.1.1 Phase momentum equation
The numerical method used in the bubbleFoam solver relies on the phaseintensive formulation of the phase momentum equations proposed in [10] to overcome the instability arising when the phase volume fraction tends to become zero, which is a frequent situation is fully separated ﬂows. According to this approach, assuming the phase density to be constant, the momentum equation is rewritten in nonconservative form to extract the volume fraction from the transport terms, leading to
By renaming the total stress tensor as
and decomposing it in a diffusive component and a correction term
with
we obtain
Finally, introducing the total phase velocity
introducing
and considering, as an example, phase a, the momentum equation becomes
The momentum equation can be furtherly rewritten by introducing the total convective momentum fluxes
and
where is the surface normal gradient to the surface of the phase fraction, the subscript indicates values computed at cell faces, and is a small number. Once this substitution is performed, the momentum equation can be discretized. In particular
 the unsteady, the convective and diffusive terms are treated fully implicitly
 the stress term correction is handles explicitly as well as the lift terms
 the drag and virtual mass terms are handled semiimplicitly, treating implicitly the part containing the velocity of the phase the equation is written for, and moving it to the pressure equation, and explicitly the other part
 the pressure gradient is not included in the momentum equation directly, but its effect is accounted for only when the phase velocities are corrected
 the same procedure adopted for the pressure gradient is used for the gravity, which does not appear in the momentum equation in the code, but its effect is accounted for in the pressure equation
The discretized equations, can be represented as
where is the diagonal part of the matrix originating from the discretization of the phase momentum equation and is the remaining part. These equations will be used to correct the velocity after the pressure field is updated.
4.1.2 Phase continuity equation
The phase continuity equations have to be solved ensuring that the phase fraction of each phase is kept between zero and one. To obtain this result, the dispersed phase continuity equation is rewritten as a function of the mean and relative velocity. Rewriting the phase velocity in terms of the relative velocity
and the mean velocity weighted on the phase fractions
we find
Substituting into the phase continuity equation, a new expression is obtained
which, if iteratively (Note that the equation is not linear) solved in a fully implicit manner, provides a bounded solution for the phase fraction field.
The volume fraction of the continuous phase can be computed as . However in some cases, this approach could not be satisfactory from a numerical point of view, and the solution of a phase continuity equation also for the continuous phase is recommended, in order to increase the convergence rate. In such a case, after solving a continuity equation for phase b, the boundness of the dispersed phase volume fraction is ensured by evaluating the new value of the dispersed phase fraction as
and recomputing the continuous phase fraction as
.
4.1.3 Pressure equation
The pressure equation is obtained imposing that the divergence of the mixture flux is zero
.
The phase fluxes are obtained by interpolating equations
on cell faces, leading to
The effect of the gravity and of the explicit part of the drag are included in the fluxes as
.
4.2 Solution procedure
The solution procedure adopted in the bubbleFoam solver is summed up as follows:
 Solve the phase continuity equations
 Update lift, drag and virtual mass coeﬃcients
 Construct the momentum equation matrix
 Predict the phase velocity ﬁelds, without considering the pressure gradient at this stage
 Solve the pressure equation
 Correct the velocities with the new pressure ﬁeld, and update the phase fractions
 Solve the transport equations for the turbulence quantities
4.3 Code representation
4.3.1 Implementation of the phase momentum equation
The implementation of the phase momentum equations, in its phaseintensive form is better clariﬁed in the following commented code snippet.
// The fvVectorMatrix for the two phase momentum equations are declared, // setting the correct dimensional units fvVectorMatrix UaEqn(Ua, Ua.dimensions()*dimVol/dimTime); fvVectorMatrix UbEqn(Ub, Ub.dimensions()*dimVol/dimTime); { // The dispersed phase stress tensor is computed volTensorField Rca = nuEffa*(fvc::grad(Ua)().T()); Rca = Rca + (2.0/3.0)*sqr(Ct)*I*k  (2.0/3.0)*I*tr(Rca); surfaceScalarField phiRa =  fvc::interpolate(nuEffa) *mesh.magSf()*fvc::snGrad(alpha)/fvc::interpolate(alpha + scalar(0.001)); // The momentum predictor is set up bringing the virtual mass term // on the LHS for convenience. The drag term is managed semiimplicitly, and the // explicit part is treated at cell faces, moving it to the pressure equation. The implicit // part of the drag is treated as a source term. The buoyancy term is moved to the // pressure equation and managed at faces too UaEqn = ( (scalar(1) + Cvm*rhob*beta/rhoa)* ( fvm::ddt(Ua) + fvm::div(phia, Ua, "div(phia,Ua)")  fvm::Sp(fvc::div(phia), Ua) )  fvm::laplacian(nuEffa, Ua) + fvc::div(Rca) + fvm::div(phiRa, Ua, "div(phia,Ua)")  fvm::Sp(fvc::div(phiRa), Ua) + (fvc::grad(alpha)/(fvc::average(alpha) + scalar(0.001)) & Rca) == // g // Buoyancy term transfered to pequation  fvm::Sp(beta/rhoa*dragCoef, Ua) //+ beta/rhoa*dragCoef*Ub// Explicit drag transfered to pequation  beta/rhoa*(liftCoeff  Cvm*rhob*DDtUb) ); UaEqn.relax(); volTensorField Rcb = nuEffb*fvc::grad(Ub)().T(); Rcb = Rcb + (2.0/3.0)*I*k  (2.0/3.0)*I*tr(Rcb); surfaceScalarField phiRb =  fvc::interpolate(nuEffb) *mesh.magSf()*fvc::snGrad(beta)/fvc::interpolate(beta + scalar(0.001)); UbEqn = ( (scalar(1) + Cvm*rhob*alpha/rhob)* ( fvm::ddt(Ub) + fvm::div(phib, Ub, "div(phib,Ub)")  fvm::Sp(fvc::div(phib), Ub) )  fvm::laplacian(nuEffb, Ub) + fvc::div(Rcb) + fvm::div(phiRb, Ub, "div(phib,Ub)")  fvm::Sp(fvc::div(phiRb), Ub) + (fvc::grad(beta)/(fvc::average(beta) + scalar(0.001)) & Rcb) == // g // Buoyancy term transfered to pequation  fvm::Sp(alpha/rhob*dragCoef, Ub) //+ alpha/rhob*dragCoef*Ua // Explicit drag transfered to peqn. + alpha/rhob*(liftCoeff + Cvm*rhob*DDtUa) ); UbEqn.relax(); }
4.3.2 Implementation of the phase continuity equation
{ // A word is defined to store the discretisation scheme label used to compute the // divergence of the dispersed phase volume fraction. The text between " " is searched // for in the fvSchemes dictionary word scheme("div(phi,alpha)"); // The relative flux is calculated surfaceScalarField phir = phia  phib; // The Courant number is computed on the base of the relative velocity Info<< "Max Ur Courant Number = " << ( max ( mesh.surfaceInterpolation::deltaCoeffs()*mag(phir) /mesh.magSf() )*runTime.deltaT() ).value() << endl; // The phase fraction correction loop is started for (int acorr=0; acorr<nAlphaCorr; acorr++) { // The dipersed phase continuity equation object is created using // implicit discretization for all its terms fvScalarMatrix alphaEqn ( fvm::ddt(alpha) + fvm::div(phi, alpha, scheme) + fvm::div(fvc::flux(phir, beta, scheme), alpha, scheme) ); // Underrelaxation is applied to the dispersed phase continuity equation alphaEqn.relax(); // The dispersed phase continuity equation is solved alphaEqn.solve(); // The continuous phase continuity equation is defined as done for the dispersed // phase. Notice that by default this equation is commented out because only the // equation for the dispersed phase is solved. /* fvScalarMatrix betaEqn ( fvm::ddt(beta) + fvm::div(phi, beta, scheme) + fvm::div(fvc::flux(phir, scalar(1)  beta, scheme), beta, scheme) ); betaEqn.relax(); betaEqn.solve(); alpha = 0.5*(scalar(1) + sqr(scalar(1)  beta)  sqr(scalar(1)  alpha)); */ // The continuous phase volume fraction is computed as the complement to one of // the dispersed phase fraction. beta = scalar(1)  alpha; } Info<< "Dispersed phase volume fraction = " << alpha.weightedAverage(mesh.V()).value() << " Min(alpha) = " << min(alpha).value() << " Max(alpha) = " << max(alpha).value() << endl; } rho = alpha*rhoa + beta*rhob;
4.3.3 Implementation of the pressure equation
The details of the implementation of the pressure equation are commented in the code snippet below, where the implementation of the pseudostaggered algorithm developed by Weller [10, 8] to obtain a stable solution in completely separated ﬂows is adopted.
{ // The phase fraction fields are interpolated at cell faces surfaceScalarField alphaf = fvc::interpolate(alpha); surfaceScalarField betaf = scalar(1)  alphaf; // The central coeffcients are computed volScalarField rUaA = 1.0/UaEqn.A(); volScalarField rUbA = 1.0/UbEqn.A(); // The values of the central coefficients are interpolated at cell faces surfaceScalarField rUaAf = fvc::interpolate(rUaA); surfaceScalarField rUbAf = fvc::interpolate(rUbA); // The phase velocities are predicted Ua = rUaA*UaEqn.H(); Ub = rUbA*UbEqn.H(); // Accounting for the explict drag term and buoyancy effect. Both // these terms are managed at cell faces and their effect is included in the phase flux. // Note that the whole drag term is interpolated and then multiplied by the flux, // instead of reusing the previously computed value of the central coefficient at cell // faces (rUaAf). surfaceScalarField phiDraga = fvc::interpolate(beta/rhoa*dragCoef*rUaA)*phib + rUaAf*(g & mesh.Sf()); surfaceScalarField phiDragb = fvc::interpolate(alpha/rhob*dragCoef*rUbA)*phia + rUbAf*(g & mesh.Sf()); forAll(p.boundaryField(), patchi) { if (isA<zeroGradientFvPatchScalarField>(p.boundaryField()[patchi])) { phiDraga.boundaryField()[patchi] = 0.0; phiDragb.boundaryField()[patchi] = 0.0; } } // The phase fluxes and the total flux are computed phia = (fvc::interpolate(Ua) & mesh.Sf()) + fvc::ddtPhiCorr(rUaA, Ua, phia) + phiDraga; phib = (fvc::interpolate(Ub) & mesh.Sf()) + fvc::ddtPhiCorr(rUbA, Ub, phib) + phiDragb; phi = alphaf*phia + betaf*phib; // The mixture central coefficient is computed surfaceScalarField Dp("(rho*(1A(U)))", alphaf*rUaAf/rhoa + betaf*rUbAf/rhob); // The nonortogonal correction loop is started for(int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++) { // The pressure equation is set up fvScalarMatrix pEqn ( fvm::laplacian(Dp, p) == fvc::div(phi) ); // References for the pressure are set pEqn.setReference(pRefCell, pRefValue); // The pressure equation is solved pEqn.solve(); if (nonOrth == nNonOrthCorr) { // The surface normal pressure gradient is computed surfaceScalarField SfGradp = pEqn.flux()/Dp; // The phase fluxes and the total flux are corrected to account for the updated // pressure field. phia = rUaAf*SfGradp/rhoa; phib = rUbAf*SfGradp/rhob; phi = alphaf*phia + betaf*phib; // Pressure is explicitly relaxed before correcting the phase velocity fields p.relax(); // The pressure surface normal gradient is recomputed. SfGradp = pEqn.flux()/Dp; // Velocities are not updated using directly the pressure, as done conventionally, // but reconstructing the velocity update from the fluxes, which are the primary // variable [8] Ua += (fvc::reconstruct(phiDraga  rUaAf*SfGradp/rhoa)); //Ua += rUaA*(fvc::reconstruct(phiDraga/rUaAf  // SfGradp/rhoa)); Ua.correctBoundaryConditions(); Ub += (fvc::reconstruct(phiDragb  rUbAf*SfGradp/rhob)); //Ub += rUbA*(fvc::reconstruct(phiDragb/rUbAf  // SfGradp/rhob)); Ub.correctBoundaryConditions(); U = alpha*Ua + beta*Ub; } } } #include "continuityErrs.H"
5 bubbleFoam setup and solution strategy
5.1 Case structure
The structure of a typical bubbleFoam case is represented in the following directory tree, which lists the directories and the files contained in the bubbleColumn tutorial.
bubbleColumn
 0
  Ua
  Ub
  alpha
  epsilon
  k
 ` p
 constant
  RASProperties
  g
  polyMesh
   blockMeshDict
  ` boundary
 ` transportProperties
` system
 controlDict
 fvSchemes
` fvSolution
The directory 0
contains six files named as the corresponding variables in the solver:

Ua
dispersed phase velocity 
Ub
continuous phase velocity 
alpha
dispersed phase fraction 
epsilon
turbulent dissipation rate 
k
turbulent kinetic energy 
p
pressure.
Each of these files contain the initial and boundary conditions for the corresponding property field.
Theconstantdirectory contains the physical model settings and the mesh files, in particular

RASProperties
contains the turbulence model settings 
g
contains the gravity vector definition 
polyMesh/blockMeshDict
is the mesh dictionary for blockMesh 
polyMesh/boundary
is the file containing the boundary specifications to construct the mesh 
transportProperties
contains the phase properties.
systemdirectory contains the solver settings:

controlDict
contains information on the time stepping, the desired simulation time, and allows the setup of the flags for data storage 
fvSchemes
contains the definitions of the numerical schemes used during the simulation 
fvSolution
contains the linear solver settings and tolerances, and the definition of the PISO parameters.
Each dictionary specific to the solver is described in detail in the following sections. The reader is invited to refer to the official documentation for further information on how to build computational meshes with blockMesh, and on the numerical schemes for the different terms of the transport equations.
5.2 Initial conditions
Initial conditions are specified following the general rules adopted in OpenFOAM. The reader is invited to refer to the User's Guide, and the setFields utility documentation.
5.3 Solver configuration
The configuration of the physical model of a case to be run with the bubbleFoam solver is set up using the following dictionary files:

environmentalProperties

RASProperties

transportProperties
5.3.1 The environmentalProperties dictionary
The environmentalProperties dictionary only contains the specification of the gravitational acceleration vector:
dimensions [0 1 2 0 0 0 0]; value ( 0 9.81 0 );
The user has to specify the dimensional units, the magnitude and the orientation of the gravity vectory, with respect to the reference frame in which the mesh of the system under consideration was built.
5.3.2 The RASProperties dictionary
The RASProperties dictionary contains all the information regarding the turbulence model. The first option the user has to provide is which turbulence model has to be used. This is selected by specifying the option
RASModel laminar;
The possible choices are

laminar
No turbulence model is used during the simulation, 
kEpsilon}
The mixture model is used, and wall functions are adopted to describe the zone near the wall.
The next option in the dictionary is a switch
turbulence off;
which determines if the turbulence model is used or not during the simulation. If the turbulence model has to be used, the value of the switch has to be set to on, while setting it to off is equivalent to specify the laminar model.
The option
printCoeffs off;
determines if the coefficients of the turbulence model are printed when the simulation is started (on), or not (off).
5.3.3 The transportProperties dictionary
The transportProperties coefficient contains the physical properties of the fluid that constitute the two phases and the coefficients used in the momentum transfer term in the momentum equation, as shown in the example below.
rhoa rhoa [1 3 0 0 0 0 0] 1; rhob rhob [1 3 0 0 0 0 0] 1000; nua nua [0 2 1 0 0 0 0] 1.6e05; nub nub [0 2 1 0 0 0 0] 1e06; da da [0 1 0 0 0 0 0] 0.003; db db [0 1 0 0 0 0 0] 0.0001; Cvm Cvm [0 0 0 0 0 0 0] 0.5; Cl Cl [0 0 0 0 0 0 0] 0; Ct Ct [0 0 0 0 0 0 0] 1;
The keywords of the transportProperties dictionary are

rhoa
material density of phase a 
rhob
material density of phase b 
nua
molecular kinematic viscosity of phase a 
nub
molecular kinematic viscosity of phase b 
da
phase diameter of phase a 
db
phase diameter of phase b 
Cvm
virtual mass coefficient 
Cl
lift force coefficient 
Ct
turbulence response time coefficient
5.4 Solver controls
The bubbleFoam solver is controlled by the standard dictionaries controlDict, fvSchemes and fvSolution. The users is invited to refer to the User's Guide for the syntax and the options generally available in these dictionaries.
However, the fvSolution dictionary requires some clarification, since it contains options specific to the solver. An example of the fvSolution dictionary is reported in the code snipped below.
solvers { p PCG { preconditioner DIC; tolerance 1e10; relTol 0; }; Ua PBiCG { preconditioner DILU; tolerance 1e05; relTol 0; }; Ub PBiCG { preconditioner DILU; tolerance 1e05; relTol 0; }; alpha PBiCG { preconditioner DILU; tolerance 1e10; relTol 0; }; beta PBiCG { preconditioner DILU; tolerance 1e10; relTol 0; }; k PBiCG { preconditioner DILU; tolerance 1e05; relTol 0; }; epsilon PBiCG { preconditioner DILU; tolerance 1e05; relTol 0; }; } PISO { nCorrectors 2; nNonOrthogonalCorrectors 0; nAlphaCorr 2; correctAlpha no; pRefCell 0; pRefValue 0; }
The first part of the dictionary containes the usual settings for the linear solvers of the different equations. The PISO parameters, however, need to be clarified, being some of them specific to the solver:

nCorrectors
specifies the number of PISO loops (recommended values: 2 or 3) 
nNonOrthogonalCorrectors
specified the number of corrector steps to account for nonorthogonality of the mesh 
nAlphaCorr
specifies the number of corrections to execute on the dispersed phase fraction per each PISO corrector step 
correctAlpha
specifies if the dispersed phase correction has to be corrected or not. Possible values are yes and no 
pRefCell
specifies the cell taken as reference for the pressure. The position of the cell defines the reference point for the pressure in the computational domain 
pRefValue
specified the value of the pressure at the reference cell
6 References
 D. A. Drew. Averaged equations for twophase ﬂows. Studies in Applied Mathematics, L(3):205 – 231, 1971.
 D. A. Drew. Continuum modeling of twophase ﬂows. In R. Meyer, editor, Theory of dispersed multiphase ﬂow, pages 173 – 190. Academic Press, 1983.
 H. Enwald, E. Peirano, and A. E. Almstedt. Eulerian twophase ﬂow theory applied to ﬂuidization. International Journal of Multiphase Flow, 22:21 – 66, 1996.
 D. P. Hill. The computer simulation of dispersed twophase ﬂow. PhD thesis, Imperial College of Science, Technology and Medicine, London, U.K., 1998.
 J. P. Oliveira and R. I. Issa. Numerical aspects of an algorithm for Eulerian simulation of twophase ﬂows. International Journal for Numerical Methods in Fluids, 43:1177 – 1198, 2003.
 OpenCFD. OpenFOAM  The Open Source CFD Toolbox  Programmer’s Guide. OpenCFD Ltd., United Kingdom, 1.6 edition, 2009.
 OpenCFD. OpenFOAM  The Open Source CFD Toolbox  User’s Guide. OpenCFD Ltd., United Kingdom, 1.6 edition, 2009.
 H. Rusche. Computational ﬂuid dynamics of dispersed twophase ﬂows at high phase fractions. PhD thesis, Imperial College of Science, Technology and Medicine, London, 2002.
 L. Schiller and A. Naumann. Uber die grundlegenden Berechnungen bei der Schwerkraftaufbereitung. Zeitschrift des Vereins deutscher Ingenieure, 77(12):318, 1933.
 H. G. Weller. Derivation, modelling and solution of the conditionally averaged twophase ﬂow equations. Technical report, OpenCFD Ltd., United Kingdom, 23 February 2005.
Alberto 03:29, 17 February 2010 (UTC)