Contact: Christos VASSILICOS.
Objectives
The problem of turbulence is, ultimately, to massively reduce the number of degrees of freedom and thereby enable quick and easy turbulent flow predictions. This problem is vast because it encompasses a vast range of turbulent flows present widely in nature and engineering. Part of the problem is to determine whether faithful reductions of the number of degrees of freedom are at all possible and up to what level of reduction, for which turbulent flow quantities, and for which class of turbulent flows or perhaps even universally for at least some turbulent flow aspects. Understanding the fundamental physics of turbulence and turbulent flows is an essential steping stone and needs to be done by studying a number of turbulent flows in parallel theoretically, experimentally and computationally. This is indeed the approach of the turbulence research at the LMFL which also includes a permanent eye on applications, turbulence modeling and turbulent flow design and control.
The emphasis of the LMFL’s turbulence research is on fluctuations, inhomogeneity and non-stationarity, which are present in all turbulent flows except in a few average aspects of statistically stationary homogeneous/periodic turbulence. The applications currently pursued concern turbulent flow separation & energy-efficient effective mixing
List of recent and ongoing research projects in the laboratory
1. Chair I-SITE/MEL/Région CoPreFlo (2019-2023)
This project is concerned with non-equilibrium and inhomogeneous turbulence cascades and resulting turbulence dissipation properties in various inhomogeneous turbulent flows. The ways that energy and scalar transfers in scale-space interactss with such transfers in physical spaces is also an important focus of the project. These are fundamental turbulence physics which have a pivotal impact on the evolution of turbulent flows, on the most salient features of these flows such entrainment and turbulent wake/jet/boundary layer growth and on turbulence prediction approaches.
This project combines theory, numerical simulations and laboratory experiments and consists of the following sub-projects (4 PhD students and 2 Post Doctoral Researchers).
- Inter-scale and inter-space energy transfers at the Turbulent/Non-Turbulent Interface (TNTI) of turbulent wakes and jets.
- Entrainment and speed of the TNTI relative to the fluid in various types of turbulent boundary layers and various types of turbulent wakes.
- Inter-scale and inter-space energy tranfers in turbulent channel flows.
- The impact on turbulent fluctuations and skin friction of attached-eddy structures in turbulent boundary layers and turbulent channel flows.
- Helicity and inter-scale and inter-space energy and scalar concentration transfers in closed container mixers stirred by regular/fractal blades, with/without baffles.
- Helicity and coherent structures and their relation to non-equilibrium turbulence dissipation in turbulent wakes.
- Turbulent dissipation scalings in various types of turbulent wakes.
2. «Effect of the adverse pressure gradient in turbulent boundary layer flows » (3 PhD past and present)
The last 15 years several projects and PhD focused on turbulent bouldary layer flows with pressure gradient. A specific ramp was designed in the EUHIT project (2013-2017) to be mounted in the LMFL wind tunnel.The flow was charaterized for several ramp configurations to vary the pressure gradient. In addition, Direct Numerical Simulations with and without pressure gradient were performed to investigate the effect of the pressure gradient on the large scale structures of the flow.
3. ANR EXPLOIT (2018-2022)
EXPLOIT (Etude expérimentale des structures dissipatives en turbulence) is a join project with CEA Saclay. Recent advances in mathematical analysis of the equations governing viscous flow seem to indicate that there is a link between the scaling of dissipation in a turbulent fluid and the development of singularities. However very little is known about the dynamics and statistics of the corresponding dissipative structures. The EXPLOIT project aims at providing a characterization of dissipation structures based on an experimental analysis of a model turbulent flow using multi-scale tools and advanced visualization techniques (4D PTV) in a dedicated meter-size turbulent experiment. Highly resolved numerical simulations are also used to complement the analyses and validate the analyzing tools.
4. ANR DYNEOL (2018-2022)
DYNEOL (DYNamique de la turbulence sur des profils EOLiens et hydroliens) is a joined project with CORIA, LEGI, and PPRIME laboratories which aims at quantifying the dynamical behavior of the flow around a blade as a function of the turbulent upstream flow. This upstream flow may contain different families of coherent structures, or may feature a mean shear due to the presence of a large-scale boundary layer. The blade itself may be fixed or rotating. Such situations are representative of wind and marine turbine farms. However, these phenomena are not presently taken into account in the global performance models. To investigate the interactions of the upstream turbulence with the blade, an original approach will be followed: high-fidelity optical diagnostics and simulations will be applied to the analysis of the conditioned kinetic energy budget. Both fixed-wing and rotating-wing experiments will be investigated. 2D- and 3D-PIV will be compared to Large-Eddy Simulation results to assess the different approaches and bring insight into such interactions. A PIV measurement campaign will be conducted in the LMFL boundary layer wind tunnel.
5. Turbulent flow separation by fractal/multiscale tripping (PhD 2019-2022)
This experimental investigation is concerned with flow separation from a backward-facing ramp and aims to evaluate how a new passive flow control device, the fractal/multiscale trip, can modify the recirculation region, reattachment amplitude and frequency, shear layer flapping, and vortex shedding.
6. Wind Tunnel Simulations of Atmospheric Boundary Layers » (PhD 2020-2023)
The Atmospheric Boundary Layer (ABL) is a complex turbulent flow characterized by complex dynamics due to interactions between turbulence, thermal effects and local topography, directly influencing both natural processes (meteorology, ocean-atmosphere exchanges, heat and water transfers) and human activities (civil engineering, wind turbines, pollutant dispersion, etc). There is a pressing need to reproduce the ABL at smaller scales in the wind tunnel. The devices currently used to reproduce a scaled-down ABL are mostly dedicated to the reproduction of the neutral ABL (no thermal effects) and include active grids and combinations of upstream passive obstacles with roughness elements, mainly designed through trial and error. Various mean velocity profiles have been reproduced but it remains a challenge to reproduce both mean velocity and turbulent intensity profiles and even harder to also reproduce correct integral length scale profiles at the same time. This PhD project addresses these limitations by developing new devices based on the new concept of passive multiscale inhomogeneous grids which have had some intitial success in enabling an independent control of mean flow and turbulent intensity profiles. The final challenge of the project consists in reproducing turbulence profiles representative of ABL in various thermal stratification conditions (stable and unstable), without thermal forcing in the wind tunnel.
