CFD Online Discussion Forums - SprayFoam: negative courant number, mesh ok

Hi;

I am running a simulation with sprayFoam (OpenFoam 8) and I receive this error:

Code:

Courant Number mean: -1.373865 max: 2.614777e+04

#0  Foam::error::printStack(Foam::Ostream&) at ??:?

#1  Foam::sigFpe::sigHandler(int) at ??:?

#2  ? in "/lib/x86_64-linux-gnu/libc.so.6"

#3  Foam::nutUWallFunctionFvPatchScalarField::yPlus(Foam::Field<double> const&) const at ??:?

#4  Foam::nutUWallFunctionFvPatchScalarField::nut() const at ??:?

#5  Foam::nutWallFunctionFvPatchScalarField::updateCoeffs() at ??:?

#6  Foam::GeometricField<double, Foam::fvPatchField, Foam::volMesh>::Boundary::evaluate() in "/opt/openfoam8/platforms/linux64GccDPInt32Opt/bin/sprayFoam"

#7  Foam::RASModels::RNGkEpsilon<Foam::CompressibleMomentumTransportModel<Foam::fluidThermo> >::correctNut() at ??:?

#8  ? in "/opt/openfoam8/platforms/linux64GccDPInt32Opt/bin/sprayFoam"

#9  __libc_start_main in "/lib/x86_64-linux-gnu/libc.so.6"

#10  ? in "/opt/openfoam8/platforms/linux64GccDPInt32Opt/bin/sprayFoam"

the checkMesh is ok without any error or warning, the checkMesh -allGeometry gives wellposedness error in some cells.
On this mesh I have already performed a simulation with buoyantSimpleFoam, the mesh can be improved but is not bad at this state.

The sprayCouldProperties has been used in another simulation without generating this error; the source point is on a boundary surface and the direction (of the particles and the surface normal too) is inside the domain.

I know the max courant number should be reduced, but the cells with the highest courant number are not near the particle source.

my question are:
1)the negative courant number is linked with the error below it or are two different errors?
2) do you know how can i resolve this problem?

thanks a lot for your help, and let me know if you need additional informations