rigidBodyDynamics::restraints::linearSpring: Added switch "allowSlack" to allow no restraint when line is shorter than restLength

Based on patch contributed by SeongMo Yeon
Resolved contribution https://bugs.openfoam.org/view.php?id=3352

src/rigidBodyDynamics/restraints/linearSpring/linearSpring.C

@@ -90,7 +90,9 @@ void Foam::RBD::restraints::linearSpring::restrain
     // Force and moment on the master body including optional damping
     vector force
     (
-        (-stiffness_*(magR - restLength_) - damping_*(r & v))*r
+        (allowSlack_ && magR < restLength_)
+      ? -damping_*(r & v)*r
+      : (-stiffness_*(magR - restLength_) - damping_*(r & v))*r
     );
 
     vector moment(attachmentPt ^ force);
@@ -100,6 +102,7 @@ void Foam::RBD::restraints::linearSpring::restrain
         Info<< " attachmentPt " << attachmentPt
             << " attachmentPt - anchor " << r*magR
             << " spring length " << magR
+            << " allow slack " << allowSlack_
             << " force " << force
             << " moment " << moment
             << endl;
@@ -122,6 +125,7 @@ bool Foam::RBD::restraints::linearSpring::read
     coeffs_.lookup("stiffness") >> stiffness_;
     coeffs_.lookup("damping") >> damping_;
     coeffs_.lookup("restLength") >> restLength_;
+    allowSlack_ = coeffs_.lookupOrDefault<Switch>("allowSlack", false);
 
     return true;
 }
@@ -143,6 +147,8 @@ void Foam::RBD::restraints::linearSpring::write
     writeEntry(os, "damping", damping_);
 
     writeEntry(os, "restLength", restLength_);
+
+    writeEntry(os, "allowSlack", allowSlack_);
 }
 
 

src/rigidBodyDynamics/restraints/linearSpring/linearSpring.H

@@ -36,6 +36,7 @@ SourceFiles
 #define RBD_restraints_linearSpring_H
 
 #include "rigidBodyRestraint.H"
+#include "Switch.H"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -72,6 +73,9 @@ class linearSpring
         //- Rest length - length of spring when no forces are applied to it
         scalar restLength_;
 
+        //- Switch to allow no restraint when line is shorter than restLength
+        Switch allowSlack_;
+
 
 public: