= Counterpropagating wave instability
Waves travelling in opposite directions relative to their local background currents can share a laboratory <phase velocity>. Their interaction can then lock their phases and extract energy from the mean shear, producing growing disturbances. In a layered flow, the coupling is often exponentially weak when the interface separation exceeds the waves' decay length. A <dispersion relation for two density interfaces in uniform shear> gives a simple explicit example.
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