A Problem of Smagorinsky models


I have a problem of reading Smagorinsky models
I just can’t understand how to deduce the following equation
S = (-nu + sqrt( nunu + 18.0CsqrQo ))/ (6.0Csqr);
where Qo equal to the square root of the local stress tensor,Csqr is the Smagorinsky constants.
Can anybody tell me how or any other relative documents?