Elena Vyazmina

Ecole polytechnique palaiseau, Laboratoire d'Hydrodynamique, Graduate Student

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Sensitivity Analysis for the Stability and Control of Spiral Vortex Breakdown

The physical origin of spiral vortex breakdown is investigated using the direct and adjoint Navie... more The physical origin of spiral vortex breakdown is investigated using the direct and adjoint Navier-Stokes equations linearized around axisymmetric vortex breakdown. The axisymmetric solution is computed using a Newton solver for the steady nonlinear Navier-Stokes equations. As a result of the so-called convective non-normality the direct and adjoint global modes for helical perturbations are located downstream and upstream, respectively. In particular, the adjoint mode is dominant in the recirculation bubble where the flow is thus most sensitive to periodic forcing. The wave modes region, defined as the overlap region between the adjoint and direct global modes, allows us to determine whether the wake of the recirculation region or the recirculation region itself causes the spiral vortex breakdown.

The dynamics of vortex breakdown

Swirling flows with jet-like axial velocities are studied by means of direct numerical simulation... more Swirling flows with jet-like axial velocities are studied by means of direct numerical simulations for various control parameters. The numerical code solves the low-Mach number approximation of the Navier-Stokes equations in cylindrical coordinates which allows the incorporation of compressible and variable-density effects without the adverse effect of acoustic waves on the numerical time-step. The governing parameters, such as swirl and coflow, are varied, and their effect on the breakdown type and breakdown location is studied. The nonlinear direct numerical simulation traces the steady states of the swirling flow, and the structure of the bifurcation is described as a function of the swirl parameter, the Reynolds number and the coflow parameter. In particular, unstable branches are computed and their physical relevance is discussed. The analysis gives new insight into the prevalence of coherent states and their controllability.

Effect of Axial Pressure Gradient on the Bifurcation Structure of Viscous Vortex Breakdown

Incompressible open swirling flows are studied by means of direct numerical simulation (DNS) and ... more Incompressible open swirling flows are studied by means of direct numerical simulation (DNS) and linear stability analysis. The bifurcation structure is obtained by varying control parameters including: the swirl parameter S, the Reynolds number Re, and the nondimensional external pressure gradientbeta. Nonlinear steady states are traced by pseudo-arclength continuation using the Recursive Projection Method (RPM) applied to the fully nonlinear DNS. For zero pressure gradient and large Re, the bifurcation curve shows a characteristic fold representing the existence of multiple solutions associated with vortex breakdown. Large favorable pressure gradients prevent vortex breakdown giving access to new stable or unstable branches corresponding to high swirl number, breakdown-free states. These branches are traced back to the case with zero pressure gradient by applying continuation into the pressure gradient parameter.

The bifurcation structure of viscous steady axisymmetric vortex breakdown with open lateral boundaries

Physics of Fluids, 2009

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Un-ignited and ignited high pressure hydrogen releases: concentration - turbulence mapping and overpressure effects

by Jérôme Daubech and Elena Vyazmina

Journal of Loss Prevention in the Process Industries, 2015

ABSTRACT Safety studies for production and use of hydrogen reveal the importance of accurate pred... more ABSTRACT Safety studies for production and use of hydrogen reveal the importance of accurate prediction of the overpressure effects generated by delayed explosions of accidental high pressure hydrogen releases. Analysis of previous experimental work demonstrates the lack of measurements of turbulent intensities and lengthscales in the flammable envelope as well as the scarceness of accurate experimental data for explosion overpressures and flame speeds. AIR LIQUIDE, AREVA STOCKAGE ENERGIE and INERIS join in a collaborative project to study un-ignited and ignited high pressure releases of hydrogen.

Elena Vyazmina

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