Units of non - newtonian parameters

raphael_zueger · November 2020

Dear forum

I try to model a viscous debris flow as a herschel - bulkley fluid. The parameters are to be calibrated later with an experiment. For the start values I did a literature research. Thereby I came to the following parameters:

Consistency Index k: 74 (Pa*s^n); Yield Stress: 1341 (Pa); Flow Index n: 1.40 (-); Papanastasiou Index m: 1000 (s) and for the density: 2100 (kg/m^3).

If I enter these values like this, the simulation takes a very long time and the liquid does not flow. Now I noticed that in the configuration XML at Specific yield stress of phase the remark: (Pa m3/kg) is written. But this unit makes no sense for me, because if I enter 1341 * 2100 = 2816100 as value, the calculation will run even longer.

The values for the yield stress in the examples (e.g. DamBreak) are much smaller: 0.0005 in the DamBreak example. The same applies to the consistency index: 0.01 for the DamBreak example.

My question is now: what are the propper units for the parameters in DualSPHysics?

Many thanks in advance

Raphael

Alex · November 2020

@gfourtakas will help you here

gfourtakas · November 2020

Hi @raphael_zueger

In (single) DualSPHysics we use the kinematic molecular viscosity to calculate the viscous forces.

In order to ensure compatibility between single and multiphase_nn versions of the code, we use the kinematic viscosity also in "nn", similarly we use the specific "Specific yield stress of phase". In other word, you need to divide you values by your initial density.

The values you use them currently are using are very very big and I am surprised the simulation runs - numerical error creeps in for such big values.

Best of luck,

George

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Units of non - newtonian parameters

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