Typologie d'événements: Seminars

Despite the intrinsic complexity of turbulent flows, large-scale coherent structures are present in turbulence, and are related to aerodynamic drag and sound radiation, for example. This presentation shows methods to model such coherent structures, and approaches to control flows in order to reduce drag and noise radiation. Linear models will be considered first, with coherent structures modelled as dominant responses from the linearised Navier-Stokes system. Such dominant modes compare favourably with coherent structures educed from experimental or numerical data, and allow the proposition of changes to the system aiming at drag or noise reduction; moreover, such linearised models allow the formulation of estimation and control problems. Non-linear reduced-order models (ROMs) are then obtained by the Galerkin projection of the full governing equations onto a basis of modes obtained from the linearised system. Quantitative agreement with reference statistics is obtained for a number of canonical flow configurations. Finally, a ROM for turbulent Couette flow is used to devise a control method aiming at turbulence suppresion. The obtained strategy is seen to relaminarise turbulence in direct numerical simulations, showing that ROMs so obtained have promising applications in optimisation and control.

Wake-induced laminar-to-turbulent bypass transition in a separated laminar boundary layer (SLB) is investigated downstream of a cylinder of diameter D mounted in a constant free stream near a smooth flat plate. Direct numerical simulation (DNS) is conducted for a moderate gap of 0.9D with _ = 3900 and _ = 150, based on the momentum thickness. The transition process is driven by coherent vortex dynamics along the wall synchronized with Kármán shedding behind the cylinder. The transition process is identified from the mean velocity and Reynolds stress fields and then characterized from the spatial evolution o the velocity field spectra. Turbulence emerges at such remarkably low _ because the transition process follows a hybrid pathway combining SLB instability with wake-driven near-wall Λ-vortex formation and their interaction with periodically shed vortices. The transition unfolds in distinct stages: linear disturbance amplification; nonlinear saturation via super-harmonic resonance; and turbulent breakdown. Unlike classical SLB transitions, the wake’s periodicity imposes unique spectral signatures on the transition dynamics. Based on weakly nonlinear theory, an approach for analyzing the nonlinear interactions and spectral energy transfer between scales is proposed. The findings characterize wake-boundary interactions inherent to turbomachinery cascades or slotted-wing aerodynamic systems, offering insights for potential flow control strategies.

Detonation waves in gases have a three-dimensional reaction zone structure.  Their structure departs from the laminar, one-dimensional reaction zone structure postulated by Zel’dovich, Von Neumann and Doering.  We review our recent experiments that established how the detonation cellular structure enhances the detonability of detonations. 

Three main mechanisms were established.  The first mechanism is the reflection of triple points, which amplifies the lead shock by shock focusing. The stronger shock leads to higher temperatures and faster rate of energy release.  For a sufficiently rapid energy release rate, frontal and transverse detonation waves are created with embedded sonic surfaces. 

The second mechanism involves the convection assisted ignition by forward jets.  When the lead shock ignition becomes inefficient, remaining hotspots created by triple shock reflection are convected by jets. The origin of the jets is the triple shock reflections. The third mechanism is the deflagrative burning along layers where hydrodynamic instabilities are created. This mechanism burns out large portions of non-reacted pockets, which have escaped ignition by the first two mechanisms. Parameters controlling each mechanism are reviewed and ongoing modeling approaches are discussed.

Many rivers worldwide, and specifically in densely populated countries such as Switzerland, have been confined for engineering purposes and consequently exhibit morphological and ecological degradation. The revised Swiss Water Protection Act demands the restoration of 4,000 eco-morphologically impaired river kilometers by 2090. To contribute to this effort, it is crucial to investigate the physical and ecological interactions for habitat creation in fluvial systems. The use of wood placements as an example of nature-based solutions for habitat creation has increased significantly in recent years. However, there is a lack of planning tools for the design of these measures, for example, to quantify the wake flow or footprint of suspended particle deposition to create targeted habitats.
This talk explores the influence of wood placements, such as engineered logjams, on river restoration efforts, focusing on their dual role in sediment management and microplastic deposition. We will discuss how different placements of wood affect flow dynamics, leading to beneficial sediment declogging, which restores fish habitats, as well as varying patterns of microplastic accumulation. By examining the effects of wood on both organic and inorganic matter, this presentation provides insights into optimizing river restoration practices to enhance ecological health while mitigating pollution.

Weakly dispersive, fully nonlinear waves in channels of arbitrary cross-section are considered from a variational viewpoint. A set of general equations is derived, that resemble the Serre-Green-Naghdi equation. Traveling wave solutions in prismatic channels are derived in the form of pseudo-elliptic integrals. The case of solitary waves in trapezoidal channels is addressed more deeply, with comparison to experiments as regards the celerity of the leading wave in an undular bore.