rigidBodyMeshMotion: Added optional force damping ramp function

to provide smoother behavior on start-up when an acceleration impulse is
applied, e.g. if the body is suddenly released.  e.g.

dynamicFvMesh       dynamicMotionSolverFvMesh;

motionSolverLibs   ("librigidBodyMeshMotion.so");

solver             rigidBodyMotion;

rigidBodyMotionCoeffs
{
    report          on;

    solver
    {
        type    Newmark;
    }

    ramp
    {
        type     quadratic;
        start    0;
        duration 10;
    }
.
.
.

will quadratically ramp the forces from 0 to their full values over the first
10s of the run starting from 0.  If the 'ramp' entry is omitted no force ramping
is applied.

src/rigidBodyMeshMotion/rigidBodyMeshMotion/rigidBodyMeshMotion.C

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2016 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2016-2017 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -30,6 +30,7 @@ License
 #include "pointConstraints.H"
 #include "uniformDimensionedFields.H"
 #include "forces.H"
+#include "OneConstant.H"
 #include "mathematicalConstants.H"
 
 // * * * * * * * * * * * * * * Static Data Members * * * * * * * * * * * * * //
@@ -115,6 +116,7 @@ Foam::rigidBodyMeshMotion::rigidBodyMeshMotion
     test_(coeffDict().lookupOrDefault<Switch>("test", false)),
     rhoInf_(1.0),
     rhoName_(coeffDict().lookupOrDefault<word>("rho", "rho")),
+    ramp_(nullptr),
     curTimeIndex_(-1)
 {
     if (rhoName_ == "rhoInf")
@@ -122,6 +124,15 @@ Foam::rigidBodyMeshMotion::rigidBodyMeshMotion
         rhoInf_ = readScalar(coeffDict().lookup("rhoInf"));
     }
 
+    if (coeffDict().found("ramp"))
+    {
+        ramp_ = Function1<scalar>::New("ramp", coeffDict());
+    }
+    else
+    {
+        ramp_ = new Function1Types::OneConstant<scalar>("ramp");
+    }
+
     const dictionary& bodiesDict = coeffDict().subDict("bodies");
 
     forAllConstIter(IDLList<entry>, bodiesDict, iter)
@@ -232,10 +243,12 @@ void Foam::rigidBodyMeshMotion::solve()
         curTimeIndex_ = this->db().time().timeIndex();
     }
 
+    const scalar ramp = ramp_->value(t.value());
+
     if (db().foundObject<uniformDimensionedVectorField>("g"))
     {
         model_.g() =
-            db().lookupObject<uniformDimensionedVectorField>("g").value();
+            ramp*db().lookupObject<uniformDimensionedVectorField>("g").value();
     }
 
     if (test_)
@@ -270,7 +283,7 @@ void Foam::rigidBodyMeshMotion::solve()
             functionObjects::forces f("forces", db(), forcesDict);
             f.calcForcesMoment();
 
-            fx[bodyID] = spatialVector(f.momentEff(), f.forceEff());
+            fx[bodyID] = ramp*spatialVector(f.momentEff(), f.forceEff());
         }
 
         model_.solve

src/rigidBodyMeshMotion/rigidBodyMeshMotion/rigidBodyMeshMotion.H

@@ -2,7 +2,7 @@
   =========                 |
   \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
    \\    /   O peration     |
-    \\  /    A nd           | Copyright (C) 2016 OpenFOAM Foundation
+    \\  /    A nd           | Copyright (C) 2016-2017 OpenFOAM Foundation
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
 License
@@ -40,6 +40,7 @@ SourceFiles
 
 #include "displacementMotionSolver.H"
 #include "rigidBodyMotion.H"
+#include "ramp.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -116,6 +117,9 @@ class rigidBodyMeshMotion
         //  as rhoInf
         word rhoName_;
 
+        //- Ramp the forces according to the specified function and period
+        autoPtr<Function1<scalar>> ramp_;
+
         //- Current time index (used for updating)
         label curTimeIndex_;