# Rayleigh-Taylor Instability dependence on acceleration direction

I'm trying to bolster my understanding of the Rayleigh-Taylor instability, and I've gotten stuck on the point of which fluid (more or less dense) is being accelerated into the other. Cases of uniform acceleration make sense (e.g. plane-parallel fluids under gravity; or being accelerated by a piston), but my understanding falters in more exotic configurations. Consider the following:

A wind tunnel(-like) setup---without gravity---in which one fluid (density $\rho_1$) is at rest, while another ($\rho_2$) is blown into the first with a constant velocity. Because the two substances are colliding --- they'll both feel an acceleration. But because there is no acceleration between the reference frames of $\rho_1$ and $\rho_2$, how do you tell which configuration is stable, and which is not?

• Also, what do you mean by "configuration is stable." My understanding of colliding flows is that Vishniac instabilities will appear. – Kyle Kanos Dec 13 '13 at 3:34
• @KyleKanos I had never heard of that effect, it seems to be exclusively relevant to radiative transfer, specifically anisotropy formation by scattering... – DilithiumMatrix Dec 13 '13 at 3:50
• It's related to the thermal pressure of a shock matching the ram pressure of an ambient medium, see this short paper for a review. As soon as an inhomogeneity arises in the ambient (which happens in colliding flows), the Vishniac instability kicks in. – Kyle Kanos Dec 13 '13 at 3:58
• @KyleKanos Interesting; I don't see how that's any different from RT however. – DilithiumMatrix Dec 13 '13 at 4:03
• What you are describing sounds closer to an incompressible Richtmyer–Meshkov instability. In the absence of surface tension and gravity, I believe both situations, heavy-light and light-heavy, are linearly unstable. – SimpleLikeAnEgg Jan 8 '14 at 1:28