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[FAQ]

Evolution strategies are usually recommended when the viscosity law (Bird-Carreau or power laws) involves a low index. However, the selection of an evolution function and its parameters can sometimes enter in conflict with other evolution functions applied on other parameters. What can be done? When the evolution is only required on the power index, – Assign the required value (say n) to the power index – Select f(S) = 1/S – Set initial value of S as the required value n and upper limit of S to 1 When the above evolution scheme enters in conflict with other functions, – Assign the value 1 to the power index – Select f(S) = a + b.S + c.S^2 + d.S^3, with a = 1, b = -2(1-n), c = 1-n – Set initial value of S to 0 and upper limit of S to 1 A quick inspection shows that power index will decrease from 1 to the required value n. Other evolution functions may be created along the same idea and are mainly a matter of algebra. Note: Set evolution function should be such that the value of power index decreases from unity to the desired value.