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- - **Turbulence Model in interFoam some soul searching**
(*http://www.cfd-online.com/Forums/openfoam-solving/59598-turbulence-model-interfoam-some-soul-searching.html*)

Dear OpenFOAM Users
I am tDear OpenFOAM Users
I am trying to change interFoam for turbulent flows. I am aware of the fact that there exists rasinterFoam as well for that purpose. My questions are based upon the Henrik Rusche's thesis, where he, at length, explains both: 1)two-fluid model -->implemented in bubbleFoam, 2)interface capturing method --> implemented in interFoam Now I have done some reading, and taken apart the basic equations and this is what I understood. Please take a look and comment: according to Rusche thesis (eq 3.23) the Effective Reynolds stress is defined as : R_phi_effective = -nuEff_phi(grad(U_phi) + grad(U_phi) * T( )) - 2.0/3.0 * deltai,j * grad(U_phi) + 2.0/3.0 * deltai,j * k_phi -- (1) In general, the viscous term of momentum equation for compressible flow reads as: div[nu(U i,j + U j,i) - 2.0/3.0 * deltai,j * U k,k)] -- (2) for an incompressible flow the dilatational part can be neglected and that leaves : div[nu(U i,j + U j,i)] --(3) --> in OpenFOAM -->interFoam this in UEqn.H is represented as : - [fvm::laplacian(muf, U) + (fvc::grad(U) & fvc::grad(muf))] --(4) From RANS point of view, for a time averaged momentum equation for incompressible flow the viscous term reads as: div[nu(U i,j + U j,i) - rho*(UiUj) ] --(5) Now according to Boussinesq assumption reynolds stresses are linked to velocity gradients via turbulent viscosity thus replacing rho*(UiUj) = { -nut(U i,j + U j,i) } + { 2.0/3.0 * deltai,j * rho * k } --(6) where second term { 2.0/3.0 * deltai,j * rho * k } is to account for the contraction. inserting for rho*(UiUj) into (5) we get: div[ nu(U i,j + U j,i) - { -nut(U i,j + U j,i) } + { 2.0/3.0 * deltai,j * rho * k } ] --(7) => div[ nu(U i,j + U j,i) + nut(U i,j + U j,i) - { 2.0/3.0 * deltai,j * rho * k } ] => div[ { (nu + nut) (U i,j + U j,i) } - { 2.0/3.0 * deltai,j * rho * k } ] => div[ (nu + nut) (U i,j + U j,i) ] - div [ 2.0/3.0 * deltai,j * rho * k ] --(8) -------------------------------------------------------------------------------- ---------------------------- Now switching to rasInterFoam : surfaceScalarField muf = twoPhaseProperties.muf() + fvc::interpolate(rho*turbulence->nut()); and then UEqn the viscous term is calculated as - [fvm::laplacian(muf, U) + (fvc::grad(U) & fvc::grad(muf))] which is equivalent to the 1st term of Eqn (8). -------------------------------------------------------------------------------- --------- My question are thus : 1) Is it correct to include the 2nd term div [ 2.0/3.0 * deltai,j * rho * k ] into UEqn 2) will that improve the turbulence calculation and thus the calculation of Effective Reynolds stresses I would be grateful if someone could please provide some feedback. With Best Regards Jaswinder |

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