Research overview
Identification of transport-dominated large-scale structures in turbulent wall-bounded flows using a Characteristic DMD
Funded by DFG - Priority Program 1881
Coherent structures in turbulent flows are known as organized motions possessing spatial and temporal coherence, having a prominent contribution to the turbulent kinetic energy and mass and momentum diffusion. Therefore, they come with large desirable or undesirable effects in the flow, such as better mixture or more drag. Despite large number of studies carried out in the last decade to understand their physical properties, there is still limited consensus in the scientific community on how to define these structures, what they physically look like, how long they live and how their length scales depend on Reynolds numbers. On the other hand, uncertainties like what they feed on, how their regeneration mechanism works and how they interact with each other, as well as with near wall turbulence, are yet to be unraveled. The main objective of this study is to extract low dimensional subspaces out of highly complex turbulent flow fields, which meet our intuitive understanding of large coherent structures. In particular the structures living in these subspaces shall have a long lifetime, live on large scales and travel with a certain group velocity. To this end, a temporal sequence of state vectors from DNS and time-resolved measurements, will be transformed such that we find persistent dynamical modes on a hypersurface traveling along its normal in space and time on a moving frame of reference. The transformation is in form of a rotation in space and time. A Dynamic Mode Decomposition (DMD) will be carried out on the transformed data to capture the modes possessing small decay or growth rates. Reconstruction of the candidate modes along the normal to the hypersurface and transforming them back to physical space gives the low rank model of the flow. The algorithm which is known as Characteristic DMD (CDMD, Sesterhenn & Shahirpour 2019), will be next applied to the residual of the flow field looking for the next largest group velocity. This gives a hierarchy of structures. In practical terms, we aim to separate large coherent structures, coherent structures, and an uncoherent remaining rest. The method will be applied to two canonical turbulent flows. The resulting structures will be tested against physical evidence, by verifying that the footprints of large-scale coherent structures in premultiplied energy spectra can be reproduced by them. Each group of structures will be studied separately in terms of their lifetimes, spatiotemporal evolution, length scales and turbulent properties.
Contact: Amir Shahirpour, Jörn Sesterhenn
Turbulent jets
In nature and many industrial applications, jets can be found to interact with turbulent environments: fuel injection and electro-coat painting in the automotive industry, jet and rocket engines in the aerospace industry, but also in hydraulic and pneumatic systems, as well as in volcanic eruptions or star jet outflows in astrophysics.Separately, isotropic turbulence and turbulent jets have been already intensively studied (with a significant number of contributions with a numerical approach), but the interaction between a compressible turbulent jet and compressible isotropic turbulence still remains largely unexplored. The technical development of supercomputing centres allows to simulate the interaction of both problems with a massive parallel numerical code since the last decades for a reasonable set of parameters.We aim to know if and how a turbulent jet can sustain a significant level of turbulence in the environment when no other energy input is present.Two research groups from different faculties at the Berlin Institute of Technology will join forces to study this complex, but interesting problem: (i) Müller's group will use their expertise in statistics of magneto-hydrodynamic turbulence to analyse the fundamental properties of turbulence and conduct the numerical simulations of magnetohydrodynamic turbulence and (ii) Sesterhenn's group will bring the know how of compressible flows and will use a proper decomposition of the flow to analyse the effect of the turbulent environment on the jet dynamics.The knowledge of the mechanisms that sustain a turbulent environment induced by a turbulent jet is, on the one hand, a very important step for the scientific community in a wide range of disciplines and can, on the other hand, be applied to optimise the industrial applications where these two elements are present: e.g. how to inject the fuel in an automotive engine to optimise the mixing and the combustion process. The applicants expect this collaboration project to be the first stone of a fruitful long-term relationship that puts both groups at the state of the art in this research topic.
Contact: Gabriele Camerlengo, Jörn Sesterhenn