"D'Yakov-Kontorovich Instability in Planar Reactive Shocks" & "Interaction of Oblique Shocks and Laminar Shear Layers"

8 mars 2019 à 11h30 / amphi 2 - plot 4 - Centrale Marseille

Double séminaire

César Huete

D'Yakov-Kontorovich Instability in Planar Reactive Shocks

The standard D'yakov and Kontorovich (DK) instability refers to planar shock waves that, once perturbed, oscillate with constant amplitude in the long-time regime. As a direct result, pressure perturbations generated right behind the shock propagate downstream as non-evanescent sound waves, an effect known as Spontaneous Acoustic Emission (SAE). For the DK-regime to be achieved, the slope of the Rankine-Hugoniot curve in the post-shock state must meet certain conditions, which have been usually related to non-ideal equations of state. It has been found that DK-instability, or SAE, can also occur in shocks moving in perfect gases when exothermic effects take place. In particular, a planar detonation, initially perturbed with a wavelength much larger than the detonation thickness, may exhibit constant-amplitude oscillations when the amount of heat release is positively correlated with the shock strength.

Daniel Martínez Ruiz

Interaction of Oblique Shocks and Laminar Shear Layers

The impingement of oblique shocks on surfaces of discontinuity (shear-mixing layers) is a matter of interest in numerous high-speed flow applications. In particular, the canonical laminar problem of a coflow with two different Mach numbers separated by an infinitely-thin shear layer under the effect of a traversing shock can be algebraically solved as an external flow. The study of the laminar problem, upon calculation of the transmitted shock inclination and post-shock values of the fluid variables, is key to the understanding of more intricate flows. However, the determination of the shock-front shape across the thin inner-layer region poses a complicated free-boundary problem. The influence of the postshock flow on the shock shape must be considered through the Mach lines reaching the shock from behind. Therefore, one must renounce to classical ad hoc simplifications, that neglect waves resulting from internal reflection to circumvent this complicating feature of the calculation, providing finally the matching of internal and external solutions. Otherwise, conservation of certain physical magnitudes may be compromised.