I assume you are trying to model the Non-Newtonian fluid using the Ostwald de Waele model. I could be wrong but I don't think you can assign a specific value for dynamic viscosity if the sstrnr goes below a certain value.

If I were in your shoes, I would write an expression for the the dynamic viscosity and bound the lower limit of the shear strain rate such that if sstrnr < 0.001 s^-1, I'd assume it to be 0.001 s^-1. So wherever your shear strain rate would be less than 0.001 s^-1, the dynamic viscosity will be calculated at sstrnr = 0.001 s^-1. The following CEL expressions would bound the lower and upper limits of sstrnr and implement the dynamic viscosity as an expression.

dynVisc = K * ((L * bSSTRNR)^(n-1))

bSSTRNR = if(sstrnr < 0.001 [s^-1], 0.001 [s^-1], cond)

cond = if(sstrnr > 1000 [s^-1], 1000 s^-1, sstrnr)

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K - viscosity consistency; L - time constant; n - power law index; bSSTRNR - bounded sstrnr