Adding fluctuations to a hydrodynamic simulation to trigger instabilities I'm doing a 2D combustion hydrodynamic simulation and there's a hydrodynamic instability that should be triggered because of the particular physical properties of my system. The key to the instability are temperature gradients perpendicular to the flow direction of my problem. What is a good and "consistent" way to artificially add fluctuations in this perpendicular direction to trigger this instability?
 A: I think the answer would have to depend on the nature of the simulation.  I'm guessing it is some sort of flow simulation in which fluid "enters the system" at some point and exits at some other point.  If so, there must be parameters to do with the fluid at the intake which you have control over.  Is $\rho(\mathbf{x})$ (fluid density as a function of position) at the intake something you can control?  In that case you could create an oscillation where $\rho$ is higher on one side of the intake at one time and higher on the other side of the intake half a period later.
The resulting density gradients (perpendicular to the flow) would result in corresponding temperature gradients as long as the fluid is modelled as being in local thermodynamic equilibrium.  In real life density fluctuations like this occur when fluids enter apertures at high (but subsonic) speed.  It is probably related to whichever instability makes the thumping sound that you hear when you drive on the highway with only one window open in your car.
