A "length scale" in a fluid flow does not exist as an independent entity but is associated with the specific flow variable being analyzed. While this may seem obvious, we often discuss the "inertial range" or the "viscous range" of length scales in turbulence as if they exist independently of a flow variable, which in incompressible turbulence is the velocity field. How should we analyze "length scales" in flows with significant density variations, such as across a shock or in multiphase flows? I will discuss different possible decompositions and how only one of them unravels an inertial range of scales in turbulent flows with significant density variations. As an example application, I will describe the energy pathways in Rayleigh-Taylor driven mixing and its implications on implosion modeling at scales of 1—103 micrometers in inertial confinement fusion (ICF). Time permitting, I will preview work we have been pursuing to better understand the coupling between scales of 10—104 km in the global oceanic circulation and its interaction with the winds using satellite observations and global high-resolution simulations.