  Rayleigh-Taylor-instability(RTI) induced flow and mixing are of great
importance in both nature and engineering scenarios. To capture the
underpinning physics, tracers are introduced to make a supplement to discrete
Boltzmann simulation of RTI in compressible flows. Via marking two types of
tracers with different colors, the tracer distribution provides a clear
boundary of two fluids during the RTI evolution. Fine structures of the flow
and thermodynamic nonequilibrium behavior around the interface in a miscible
two-fluid system are delineated. Distribution of tracers in its velocity phase
space makes a charming pattern showing quite dense information on the flow
behavior, which opens a new perspective for analyzing and accessing
significantly deep insights into the flow system. RTI mixing is further
investigated via tracer defined local mixedness. The appearance of
Kelvin-Helmholtz instability is quantitatively captured by mixedness averaged
align the direction of the pressure gradient. The role of compressibility and
viscosity on mixing are investigated separately, both of which show two-stage
effect. The underlying mechanism of the two-stage effect is interpreted as the
development of large structures at the initial stage and the generation of
small structures at the late stage. At the late stage, for a fixed time, a
saturation phenomenon of viscosity is found that further increase of viscosity
cannot see an evident decline in mixedness. The mixing statues of heavy and
light fluids are not synchronous and the mixing of a RTI system is
heterogenous. The results are helpful for understanding the mechanism of flow
and mixing induced by RTI.
