In this Letter, we report the outcome of a spatial energy budget performed for the linear convective instability of the plane incompressible mixing layer within the inviscid framework. We find that as the critical condition for the onset of absolute instability is approached, the integrated pressure-transport term becomes increasingly more prominent as compared to the integrated production term, and the former dominates the energy budget completely at the critical condition. This implies that, near the threshold of absolute instability, the growth of disturbances is almost entirely due to the pressure-transport mechanism (rather than the more common production mechanism), which is a striking result. The part of the pressure-transport term that represents the work done by the fluctuating pressure forces is seen to be primarily responsible for the observed shift in the energy balance. The profiles of the energy terms reveal a strong coupling between velocity and pressure fluctuations at large cross-stream distances, and a region of negative production whose magnitude increases with increasing counterflow. These results can help us better understand the physical processes causing absolute instability in a mixing layer. In particular, the streamwise redistribution of disturbance energy by fluctuating pressure, which is “nonlocal” in character for incompressible flows, seems to play a key role in this respect.

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• Flow instability
• Transport phenomena