filmViscosityModel::thixotropicViscosity: Added optional Bingham plastic yield stress support

There is now an optional tauy entry in the thixotropicCoeffs to specify the
Bingham yield stress.

src/regionModels/surfaceFilmModels/submodels/thermo/filmViscosityModel/thixotropicViscosity/thixotropicViscosity.C

@@ -69,6 +69,13 @@ thixotropicViscosity::thixotropicViscosity
     c_("c", pow(dimTime, d_.value() - scalar(1)), coeffDict_),
     mu0_("mu0", dimPressure*dimTime, coeffDict_),
     muInf_("muInf", mu0_.dimensions(), coeffDict_),
+    BinghamPlastic_(coeffDict_.found("tauy")),
+    tauy_
+    (
+        BinghamPlastic_
+      ? dimensionedScalar("tauy", dimPressure, coeffDict_)
+      : dimensionedScalar("tauy", dimPressure, 0)
+    ),
     K_(1 - sqrt(muInf_/mu0_)),
     lambda_
     (
@@ -157,6 +164,20 @@ void thixotropicViscosity::correct
     lambda_.max(0);
 
     mu_ = muInf_/(sqr(1 - K_*lambda_) + rootVSmall);
+
+    // Add optional yield stress contribution to the viscosity
+    if (BinghamPlastic_)
+    {
+        dimensionedScalar tauySmall("tauySmall", tauy_.dimensions(), small);
+        dimensionedScalar muMax_("muMax", 100*mu0_);
+
+        mu_ = min
+        (
+            tauy_/(gDot + 1.0e-4*(tauy_ + tauySmall)/mu0_) + mu_,
+            muMax_
+        );
+    }
+
     mu_.correctBoundaryConditions();
 }
 

src/regionModels/surfaceFilmModels/submodels/thermo/filmViscosityModel/thixotropicViscosity/thixotropicViscosity.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     | Website:  https://openfoam.org
-    \\  /    A nd           | Copyright (C) 2013-2019 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2013-2020 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -111,6 +111,10 @@ protected:
         //- Limiting viscosity when lambda = 0
         dimensionedScalar muInf_;
 
+        bool BinghamPlastic_;
+
+        dimensionedScalar tauy_;
+
         //- Model coefficient
         dimensionedScalar K_;