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Divergence Only with Second Order Discretization

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Old   August 6, 2009, 11:03
Question Divergence Only with Second Order Discretization
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Ryan
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Hello All,

I am simulating a single phase supersonic gas flow in a convergent-divergent de-Laval type nozzle.

I am using the density based implicit formulation (2D, steady state), along with the standard k-epsilon viscous model. I have defined the ideal gas law for compressible flow in the Density field for the Gas (in the materials panel) along with the Sutherland viscosity law. I defined the operating pressure to be atmospheric. As well, I have enabled the Intensity & Hydraulic Diameter option in the drop down menu for the boundary conditions (due to the internal flow of my simulation)

Boundary Conditions are as follows:
Main Inlet - pressure of 6.15 atm and temperature of 500 K, intensity ratio of 3% and the default hydraulic diameter of 1m.
Outlet - atmospheric gauge pressure and 300 K, intensity ratio of 3% and the default hydraulic diameter of 1m.
Walls - enabled specified shear and temperature.

My Problem:
After the solution has converged using first order discretization, I change to second order discretization for improved accuracy and iterate again. To my dismay, the solution diverges quickly even when Fluent reduces the Courant number as low as 0.05.

Therefore, I propose the following questions:

1 - Why does the solution not converge using second order?
2 - Is there something wrong with my set-up that would cause the solution to converge with first order but diverge with second order?
3 - How do I fix this?

If anyone can help me with this topic it would be greatly appreciated...

Thanks,
Ryan
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compressible, courant, divergence, second order

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