Multi-fluid dynamics' simulations require numerical treatments based on the flow characteristics, including flow speed, turbulence, thermodynamic state, and time/length scales. Two distinct problems are investigated: supersonic jet and crossflow interactions; and liquid plug propagation and rupture in lung airways. Supersonic crossflow is the mechanism for fuel injection in SCRAMJET engines. The regime is highly unsteady, involving shocks, turbulent mixing, and large-scale vortical structures. An eddy-viscosity-based multi-scale turbulence model is proposed to resolve turbulent structures...