  Most of the turbulence models in practice are based on the assumption of a
linear relation between Reynolds stresses and mean flow strain rates which
generally provides a good approximation in case of attached and fully turbulent
flows. A two dimensional numerical study has been carried out over NACA 0021
with k-\omega SST model with non-linear correction at Re = 120,000 for various
angles of attack which experiences the formation of a laminar separation bubble
(LSB). A correct prediction of LSB requires an accurate resolution of
anisotropy in Reynolds stresses. For comparison with other linear models, the
simulations are also performed with k-kl-\omega, k-\omega SST and Spalart
Allmaras. The performance of these models is assessed through aerodynamic lift,
drag, pressure and friction coefficients. It is found that the non-linear
k-\omega SST and k-kl-\omega transition model provide comparable quality of
prediction in lift and drag coefficients (in spite of the fact that non-linear
k-\omega SST involves solving less number of transport equation than the
transition model) as observed in the experiments whereas k-\omega SST and SA
models under predict the drag coefficient value at low angle of attack due to
inability to capture the separation induced transition. It is also observed
that the location of laminar separation bubble is captured accurately when
nonlinear or transition model is used as opposed to the SA or linear SST
models, which lack in the ability to predict the same.
