# Increasing run time as flow behavior index (n) decreases in Power Law fluids

Hi there

I am simulating the Poiseulle flow of Power low fluids by DualSPHysics.

I am using HBP model to stimulate non-Newtonian behavior of Power low fluids by setting the appropriate value of m and n parameters in HBP model. When n ≠ 1 and m = 0 the HBP model reduces to a Power Law model.

I am observing an issue that with the same number of particles, with the same IDP by decreasing flow behavior index (n) the run time increases.

I will be appreciated if someone can explain to me what is the reason behind increasing run time with the same IDP.

Happy new year in advance

Best regards

## Comments

Let us ask @gfourtakas

Regards

Hi,

This is to be expected with the fully explicit time integration nature of the code and the viscous CFL condition.

Let me explain: when the flow behaviour index reduces and the your obtain shear thinning characteristics at the higher end of your strain rate the slope is getting flat i.e. low viscosity. At the other end, low strain rates, your slope is quite sharp (the more you decrease the flow index, the higher the slope), which results in a large viscosity. Plot apparent viscosity vs stress to understand this better.

As we require to use a very severe CFL condition h^2/ν the timestep suffers. Please use the relaxation parameter (with extreme caution) to try and speed it up a bit :)

Regards,

George

Dear Dr Fourtakas

Thank you so much for your response.

I will try to understand that.

Merry Christmas

Best regards