ENH: lagrangian injection - exposed minimum number of particles per parcel. See #728

The minimum number of particles per parcel can now be set in the
injection model input, e.g.:

    model1
    {
        type            ...;
        massTotal       ...;
        parcelBasisType ...;
        minParticlesPerParcel 1; <-- new optional entry
        SOI             ...;
        ...

Uses a value of 1 by default if the entry is not present.  The value of
1 is generally recommended and beneficial for coupled cases where small
time steps can lead to the injection of too many parcels and subsequently
greatly over-predict the particle source contributions (momentum, heat,
mass transfer etc)

src/lagrangian/intermediate/submodels/Kinematic/InjectionModel/InjectionModel/InjectionModel.C

@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2011-2017 OpenFOAM Foundation
-     \\/     M anipulation  | Copyright (C) 2016 OpenCFD Ltd.
+     \\/     M anipulation  | Copyright (C) 2016-2018 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -275,6 +275,7 @@ Foam::InjectionModel<CloudType>::InjectionModel(CloudType& owner)
     nParticleFixed_(0.0),
     time0_(0.0),
     timeStep0_(this->template getModelProperty<scalar>("timeStep0")),
+    minParticlesPerParcel_(1),
     delayedVolume_(0.0),
     injectorID_(-1)
 {}
@@ -304,6 +305,11 @@ Foam::InjectionModel<CloudType>::InjectionModel
     nParticleFixed_(0.0),
     time0_(owner.db().time().value()),
     timeStep0_(this->template getModelProperty<scalar>("timeStep0")),
+    minParticlesPerParcel_
+    (
+        this->coeffDict().template
+            lookupOrDefault<scalar>("minParticlesPerParcel", 1)
+    ),
     delayedVolume_(0.0),
     injectorID_(this->coeffDict().lookupOrDefault("injectorID", -1))
 {
@@ -382,6 +388,7 @@ Foam::InjectionModel<CloudType>::InjectionModel
     nParticleFixed_(im.nParticleFixed_),
     time0_(im.time0_),
     timeStep0_(im.timeStep0_),
+    minParticlesPerParcel_(im.minParticlesPerParcel_),
     delayedVolume_(im.delayedVolume_),
     injectorID_(im.injectorID_)
 {}
@@ -517,7 +524,7 @@ void Foam::InjectionModel<CloudType>::inject
                             pPtr->rho()
                         );
 
-                    if (pPtr->nParticle() >= 1.0)
+                    if (pPtr->nParticle() >= minParticlesPerParcel_)
                     {
                         parcelsAdded++;
                         massAdded += pPtr->nParticle()*pPtr->mass();

src/lagrangian/intermediate/submodels/Kinematic/InjectionModel/InjectionModel/InjectionModel.H

@@ -3,7 +3,7 @@
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
     \\  /    A nd           | Copyright (C) 2011-2017 OpenFOAM Foundation
-     \\/     M anipulation  |
+     \\/     M anipulation  | Copyright (C) 2018 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -81,7 +81,7 @@ public:
     // Enumerations
 
         //- Parcel basis representation options
-        //  i.e constant number of particles OR constant mass per parcel
+        //- i.e constant number of particles OR constant mass per parcel
         enum parcelBasis
         {
             pbNumber,
@@ -100,7 +100,7 @@ protected:
             scalar SOI_;
 
             //- Total volume of particles introduced by this injector [m^3]
-            //  - scaled to ensure massTotal is achieved
+            //- Note: scaled to ensure massTotal is achieved
             scalar volumeTotal_;
 
             //- Total mass to inject [kg]
@@ -128,7 +128,7 @@ protected:
             parcelBasis parcelBasis_;
 
             //- nParticle to assign to parcels when the 'fixed' basis
-            //  is selected
+            //- is selected
             scalar nParticleFixed_;
 
             //- Continuous phase time at start of injection time step [s]
@@ -137,8 +137,12 @@ protected:
             //- Time at start of injection time step [s]
             scalar timeStep0_;
 
+            //- Minimum number of particles used to represent each parcel
+            //- default = 1
+            scalar minParticlesPerParcel_;
+
             //- Volume that should have been injected, but would lead to
-            //  less than 1 particle per parcel
+            //- less than minParticlesPerParcel_ particle per parcel
             scalar delayedVolume_;
 
             //- Optional injector ID