Modal and transient dynamics of jet flows

X. Garnaud, L. Lesshafft, P. J. Schmid, and P. Huerre

The linear stability dynamics of incompressible and compressible isothermal jets are investigated by means of their optimal initial perturbations and of their temporal eigenmodes. The transient growth analysis of optimal perturbations is robust and allows physical interpretation of the salient instability mechanisms. In contrast, the modal representation appears to be inadequate, as neither the computed eigenvalue spectrum nor the eigenmode shapes allow a characterization of the flow dynamics in these settings. 
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Phys. Fluids 25, 044103 (2013)

Geostrophic turbulence near rapid changes in stratification

K. S. Smith and E. Bernard

Geostrophic turbulence near horizontal surfaces on which the vertical velocity vanishes exhibits a forward cascade of buoyancy variance, characterized by a shallow energy spectrum, secondary roll-up of filaments, and a fat-tailed vorticity probability distribution. Such surfaces occur at rigid boundaries, but also at discontinuous jumps in stratification. Here we relax this mathematical idealization and investigate geostrophic turbulence near a rapid but smooth jump in stratification, modeled by N(z) = N₀[1 + αtanh (z/h)]. (Click on the link to read more)

Phys. Fluids 25, 046601 (2013)

Numerical simulation of the dynamics of freely falling discs

Marcin Chrust, Gilles Bouchet, and Jan Dušek

We present a comprehensive parametric study of the transition scenario of freely falling discs. The motion of the discs is investigated by a direct numerical simulation of the solid-fluid interaction. The discs are assumed to be homogeneous and infinitely thin. The problem is shown to depend on two independent parameters, the Galileo number expressing the ratio between effects of gravity and viscosity and the non-dimensionalized mass characterizing the inertia of the disc. (Click on the link to read more)

Phys. Fluids 25, 044102 (2013)