Séminaire D2 : Acoustic modes in jet and wake stability

Eduardo Martini, Institut Pprime
Lieu Evenemement: 
SP2MI, bâtiment H2, salle 175/177
Jeudi, 7 Mars, 2019 - 14:00
Jet and wake instabilities are a mature field of research, due both to the wealth of their behaviour and their relevance for many practical applications. With respect to their stability properties, these flow have very different characteristics: isothermal jets typically behave as amplifiers, small disturbances near the nozzle being amplified as they travel downstream;  while wakes typically behave as oscillators, driving the flow at a characteristic frequency, and being somewhat insensitive to external disturbances.

Broadly speaking, these different behaviors can be understood as due to the presence, or lack of, a feedback mechanism between downstream- and upstream-travelling disturbances, and that enables self-sustained disturbance amplification. The presence of such self-sustained dynamics in cold wakes, but which are not typically observed in jets, is well known and has been characterised for many flow configurations. Surprisingly, however, little attention has been paid to the physics underpinning the feedback loop.

We propose a framework in which jets and wakes can be described, in some regions of a parameter space, as acoustic ducts: their shear layers behave as either rigid or pressure-release boundaries. We show that this analogy provides substantial insight where the underlining instability mechanisms are concerned, and explains a large range of previously known but unrelated phenomena, such as the symmetries of dominant instabilities and their dependence on temperature, density and compressibility. The analogy thus provides a more natural and intuitive way to understand and think about these flows.



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