[sua]

Hello everyone,

I am simulating the post-printing behavior( the objact has been already printed that mean now i wanted to recover the viscosity upto its intial value) of a 3D concrete printing (3DCP) model using the Herschel-Bulkley model for a shear-thinning (with power law index = 0.3) material in ANSYS CFX. I have defined the following parameters:

Shear Strain Rate Range: 1e-03 to 1e+03 s^-1


Yield Stress = 20 Pa
Power Law Index (n): 0.3
Thixotropic Viscosity Recovery Model si being defined with viscosity consistency with expression: Kt = Kinf + (K0 - Kinf) * exp(-Time/ Lamda)

where:
𝐾0 = minimum viscosity consistency after shearing.

Kinf = initial viscosity consistency (before shearing, or should the viscosity recover to this value , or thixotropy).

λ is the recovery time constant.
This time-dependent consistency index

K(t) is used to model thixotropic viscosity recovery.

Problem Statement:
Even though I have explicitly defined the shear strain rate range from 1e-03 to 1e+03 s, inside the Herschel-Bulkley Model for non newtonian fluid in the material properties, the simulation results show an unexpected shear strain rate range of 1e-13 to 1e-15 s^-1. And i am

I would like to understand:

Why does ANSYS CFX not consider the shear strain rate range I have defined?
Is the solver automatically determining strain rates based on velocity gradients, or is there a setting I need to modify to enforce my defined range?
How can I ensure that the solver respects the intended range of strain rates?
Any insights or suggestions would be highly appreciated!

Thanks in advance for your help.

Kind Regards, Sua

[ghorrocks]

You have misunderstood the model. Shear strain rates are the result of the model, and they are not something the user can directly set. Just like the user does not set the velocity and pressure, the user does not set the shear strain rate either. For instance, if the model result shows the fluid just sits there like a stationary lump of toothpaste then the shear strain rates will be zero.

The maximum and minimum shear strain rate parameters in the H-B model refer to the max and min SSR values the model uses to calculate the viscosity. In other words, if the SSR is below the minimum value, it will use the minimum SSR value to calculate the viscosity. This is required as when the SSR is very high or close to zero the viscosity model falls apart and gives unphysical results. So you put a simple max and min on the inputs to keep them within physically realistic bounds.