I am simulating the internal flow through a scramjet engine.

Desired analysis type: 2-D steady viscous-laminar

Inlet condition: pressure far field

Pressure: 48Pa

temperature: 67K

mach number: 8

The geometry consists of convergent type inlet followed by constant height combustion chamber and the divergent type nozzle. width is constant throughout. Since the geometry is symmetrical, i tried the simulation with 2-D symmetry condition.

Solver details: coupled, second order upwind discretization implicit scheme.

I meshed with gambit using unstructured (pave scheme) quadrilateral elements. The solution is well converged for inviscid case. the residuals went beyond 10e-12, but the solution didn't converge for viscous case. the X-momentum residuals starts oscillating at 10e-4, then Y-momentum oscillates, then energy and continuity. sometimes it blew up if i continue to simulate.

what could be the possible reasons for it?

If you are interest is in looking for the lift and/or drag coefficients, you might want to keep an eye on them instead of the residuals. Sometimes, it is not possible for the residuals to go below a certain value. Check that the lift/drag or any other coefficient that you are monitoring is at a steady value.

Alternatively, you can probably start with a first order scheme and then switch to higher order after this has reached a desired convergence value. And if you are using, the pressure based solver, you might want to switch to the density based solver and give it a shot.