  Accurate prediction of mixing transition induced by interfacial instabilities
is vital for engineering applications, but has remained a great challenge for
decades. For engineering practices, Reynolds-averaged Navier-Stokes simulation
(RANS) is the most viable method. However, existing RANS models for mixing
problems are mostly designed for fully developed turbulence, failing to depict
the locally spatio-temporal-dependent characteristic of transition. In the
present study, the idea of the intermittent factor (denoted as $\gamma$), which
has been widely used in boundary layer transition in aerospace engineering, is
extended to the mixing problems. Specifically, a transport equation for
$\gamma$ is built based on local flow variables, which is used to describe the
locally spatio-temporal-dependent characteristic of transition. Furthermore,
$\gamma$ is coupled into the widely used K-L turbulent mixing model to
constrain the two key product sources terms that dominate the evolution of
mixing, i.e. the Reynolds stress and the buoyancy effect. Subsequently, the
simulations of two reshocked Richtmyer-Meshkov mixing cases with remarkable
transition effects confirm that the proposed model has a good performance for
predicting mixing transition. To the best of our knowledge, it is the first
study that an extra transport equation for intermittent factor has been
proposed for a RANS mixing transition model. More importantly, the present
modeling framework is flexible and has the potential to be applied to other
RANS models. It provides a promising strategy for more advanced modeling for
mixing transition.
