Hi, all. 

I am really receiving a lot of help from this group whenever I visit here. 

I have a question today. 


I am developing FEM code for analyzing non-newtonian fluids using deal.ii 
library, which is amazing tool. 

In many cases, I have to calculate 'The second invariant of rate-of-strain 
tensor'. 

I wonder there is a easy way to get this value. 

As in step-20 or other fluid problems, we get rate of strain tensor from 
the rank-2 tensor of symmetrize gradient. 


For example....


std::vector<SymmetricTensor<2,dim> > symgrad_phi_u (dofs_per_cell);

.....

symgrad_phi_u[k] = fe_values[velocities].symmetric_gradient (k, q)


>From this symmetric gradient tensor, do we have one easy way to calculate 
the following "the second invariant of rate-of-strain tensor"? 

<https://lh3.googleusercontent.com/-k8SKQzE3aEk/WAQQnorR5lI/AAAAAAAAA60/4XJBD25ZRSAa2IYsdK4NCMOuY3QXJydmwCLcB/s1600/Screen%2BShot%2B2016-10-16%2Bat%2B6.42.40%2BPM.png>



Regards, 

Jaekwang Kim 
 

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