How the Herschel-Bulkley model works in CFX?

[pitney1]

I am interested in how the Herschel-Bulkley model works in CFX. When I do the calculations by hand, I have to explicitly find the yield surface in the fluid then divide my region into two pieces for the velocity calculations. How does CFX calculate the yield surface, if at all? The Herschel-Bulkley equation gives an infinite viscosity for parts of the flow. Having an infinite viscosity would give numerical problems, so what modifications to the Herschel-Bulkley equations have been made to make the numerical solution tractable? How are the material parameters, yield stress and min/max shear strain rate values, used?

[william]

The shear rates are clipped using the minimum and maximum shear rate parameters. The solver then calculates a viscosity based on these clipped values. This ensures that the viscosity does not become infinite at zero shear rate - it will just approach the limiting value at the minimum shear rate. The same thing will happen at the high end of the range. In practice, most polymers approach Newtonian behavior at low and high shear rates, which this implementation should model properly.

The yield stress will be used as the equations show. Since the shear rate is clipped to a minimum value, there¿s no testing required to see if the shear stress lies above a certain value for deformation - a minimum shear rate will always be used in the calculation.