EHRENSTEIN , Uwe

A separated boundary-layer flow at the rear of a bump is considered. The flow undergoes two-dimensional low-frequency global oscillations known as 'flapping' typical for elongated separation bubbles. Computing equilibrium stationary states of the Navier-Stokes equations and performing a global instability analysis, the onset of the...

A separated boundary layer flow at the rear of a bump is considered. Two-dimensional equilibrium stationary states of the Navier-Stokes equations are determined using a nonlinear continuation procedure varying the bump height as well as the Reynolds number. A global instability analysis of the steady states is performed by computing...

A separated boundary layer flow at the rear of a bump is considered and two-dimensional flow states at increasing Reynolds numbers are computed using a nonlinear continuation procedure for the stationary Navier-Stokes system. The global instability analysis of the steady states is performed by computing two-dimensional temporal modes. The...

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A separated boundary layer flow at the rear of a bump is considered and two-dimensional flow states at increasing Reynolds numbers are computed using a nonlinear continuation procedure for the stationary Navier-Stokes system. The global instability analysis of the steady states is performed by computing two-dimensional temporal modes. The...

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The possibility of model reduction using global modes is readdressed, aiming at the controlling of a globally unstable separation bubble induced by a bump geometry. A combined oblique and orthogonal projection approach is proposed to design an estimator and controller in a Riccati-type feedback setting. An input-output criterion is used to...

The direct numerical simulation of the incompressible Navier-Stokes equations of the flow above a bump shows a stationary longitudinal instability at a Reynolds number of Re=400. A three-dimensional global mode linear analysis is used to interpret these results and shows that the most unstable eigenmode is steady and localized in the...

Une couche limite décollée instable est considérée, le décollement étant induit par une bosse montée sur une plaque plane. L'instabilité de la zone de recirculation fait apparaître un battement à basse fréquence, connu sous le nom de "flapping". Cette instabilité bidimensionnelle peut être décrite par une superposition de modes globaux...

Une couche limite en aval d'une bosse est considérée comme archétype d'un écoulement décollé, afin d'explorer les possibilités d'un contrôle rétroactif de cet écoulement. Des estimateurs de faibles dimensions sont obtenus par projection, utilisant des modes globaux déterminés numériquement. L'estimateur et le contrôleur, obtenus par une...

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We propose a quantitative criterion for the merging of a pair of equal two-dimensional co-rotating vortices. A cross-validation between experimental and theoretical analyses is performed. Experimental vortices are generated by the roll-up of a vortex sheet originating from the identical and impulsive rotation of two plates. The phenomenon is...

The stability of the two-dimensional flat plate boundary-layer is studied by means of global eigenmodes. These eigenmodes depend both on the streamwise and wall-normal coordinate, hence there are no assumptions on the streamwise length scales of the disturbances. Expanding the perturbation velocity field in the basis of eigenmodes yields a...

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