  For helical isotropic turbulence, an improved two-term helical subgrid-scale
(SGS) model is proposed and four types of dynamic methods are given to do
large-eddy simulation (LES), which include the standard dynamic procedure, the
least quatratic sum dynamic procedure, the dynamic procedure with single con-
straint of helicity dissipation and the dynamic one with dual constraints of
energy and helicity dissipation. Tested a priori and a posteriori in both
steady and decaying helical isotropic turbulence, the four types of dynamic
helical models and the dynamic Smogorinsky model are compared with results of
direct numerical simulations (DNS) together. Numerical results demonstrate that
the three new types of dynamic helical models predict energy and helicity
evolution better than the standard dynamic helical model, and the two
constrained helical models predict the energy and helicity dissipation rates
better than other models. Fur- thermore, the constrained helical models have
higher correlation to the real SGS stress and have more similar probability
density functions (PDF) to the DNS results. In general, the two constrained
helical models show some more attractive features than other models.
