Merge branch 'fix/IncNS_Stability_ImagShift_OnlyWithHomoSingleMode' into 'master'

Add a test in an Arpack driver and a comment in documentation. Minor documentation fix

See merge request nektar/nektar!928

CHANGELOG.md

@@ -133,6 +133,9 @@ v4.4.2
 - Updated PETSc to 3.7.7 (!916)
 - Fix typcase to an integer which set Lz < 1 to zero when postprocess hdf5 output (!9922)
 
+**IncNavierStokesSolver**
+- Add a test for imaginary shift to be only used with Homogenous and SingleMode on. (!928)
+
 **NekMesh**
 - Fix missing periodic boundary meshing and boundary layer mesh adjustment
   configurations in 2D (!859)
@@ -142,6 +145,7 @@ v4.4.2
 - Fix sign of the viscous term in the velocity correction scheme equations in
   the user guide (!856)
 - Fixed anonymous clone URL (!909)
+- Add information on the limitations of Imaginary Shift for stability. (!928)
 
 **FieldConvert**
 - Allow passing input name with trailing separator (!879)

docs/user-guide/solvers/incompressible-ns.tex

@@ -1165,8 +1165,9 @@ The following parameters can be specified in the \texttt{PARAMETERS} section of
 \item \inltt{Re}: sets the Reynolds number
 \item \inltt{Kinvis}: sets the kinematic viscosity $\nu$.
 \item \inltt{kdim}: sets the dimension of the Krylov subspace $\kappa$. Can be used in: \inltt{ModifiedArnoldi} and \inltt{Arpack}. Default value: 16.
-\item \inltt{evtol}: sets the tolerance of the eigenvalues. Can be used in: \item \inltt{imagShift}: provide an imaginary shift to the direct sovler eigenvalue problem by the specified value. 
-ltt{ModifiedArnoldi} and \inltt{Arpack}. Default value: $10^{-6}$.
+\item \inltt{evtol}: sets the tolerance of the eigenvalues. Can be used in: \item \inltt{imagShift}:
+provide an imaginary shift to the direct solver eigenvalue problem by the specified value.
+\inltt{ModifiedArnoldi} and \inltt{Arpack}. Default value: $0.0$. Works only with \inltt{Homogeneous} set to \inltt{1D} and inltt{ModeType} set to \inltt{SingleMode}.
 \item \inltt{nits}: sets the maximum number of iterations. Can be used in: \inltt{ModifiedArnoldi} and \inltt{Arpack}. Default value: 500.
 \item \inltt{LZ}:  sets the length in the spanswise direction $L_z$. Can be used in \inltt{Homogeneous} set to \inltt{1D}. Default value: 1.
 \item \inltt{HomModesZ}: sets the number of planes in the homogeneous directions. Can be used in \inltt{Homogeneous} set to \inltt{1D} and \inltt{ModeType} set to \inltt{MultipleModes}.

library/SolverUtils/DriverArnoldi.cpp

@@ -109,7 +109,21 @@ void DriverArnoldi::v_InitObject(ostream &out)
     m_session->LoadParameter("realShift", m_realShift, 0.0);
     m_equ[0]->SetLambda(m_realShift);
 
-    m_session->LoadParameter("imagShift", m_imagShift, 0.0);
+    m_session->LoadParameter("imagShift",m_imagShift,0.0);
+
+    // The imaginary shift is applied at system assembly
+    // Only if using HOMOGENEOUS expansion and ModeType set to SingleMode
+    if(m_imagShift != 0.0)
+    {
+        if(!m_session->DefinesSolverInfo("HOMOGENEOUS")&&!m_session->DefinesSolverInfo("ModeType"))
+        {
+            NEKERROR(ErrorUtil::efatal, "Imaginary shift only supported with HOMOGENEOUS expansion and ModeType set to SingleMode");
+        }
+        else if(!boost::iequals(m_session->GetSolverInfo("ModeType"),"SingleMode"))
+        {
+            NEKERROR(ErrorUtil::efatal, "Imaginary shift only supported with HOMOGENEOUS expansion and ModeType set to SingleMode");
+        }
+    }
 
 }