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-   -   Importance of Hydraulic Diameter Value for Compressible Flow (http://www.cfd-online.com/Forums/fluent/67186-importance-hydraulic-diameter-value-compressible-flow.html)

RyGuy August 6, 2009 10:58

Importance of Hydraulic Diameter Value for Compressible Flow
 
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:
When I use the default values for the hydraulic diameter I am able to get my solution to converge without a problem and the results seem reasonable (nothing unphysical). But, I would think that since there is an equation defined for the hydraulic diameter I should actually put in the exact value (for precision of results). So when I use the actual hydraulic diameter values and try to get the simulation to converge, it does not.

Therefore, I propose the following questions:

1 - What is the significance of the hydraulic diameter?
2 - How will this value affect my simulation? (Whether it be default or the actual value)
3 - Should I use the actual values defined by the equation instead of the default value?

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

Thanks,
Ryan


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