Bug with the boundary::outflow

The boundaries are set as:
boundaryCondition->setPressureConditionOnBlockBoundaries ( *lattice, outlet, boundary::outflow );
boundaryCondition->setPressureConditionOnBlockBoundaries ( *lattice, inlet, boundary::dirichlet );
setBoundaryVelocity(*lattice, inletCenter, Array<T,3>((T) 0, (T) 0, vx));

The “inletCenter” is about 2/15 of the boundary inlet. The vx is about 2m/s in physical units. When the fluid kinematic viscosity in physical units is set as 1e-4, the program goes well. But when the fluid kinematic viscosity is set as 1e-6, the program goes wrong after some steps. I’d like to know the reason and how to solve the problem.

Thanks in advance.