Coherent Structures in Fluid Mechanics

Dear Colleagues,

Although fluid flows have an infinite number of degrees of freedom, they are quite often organized around characteristic coherent structures, which play a key role in both the dynamics and spectral signature of the flow. For this reason, the identification of coherent structures is decisive in understanding the phenomenology and dynamics of fluid flow. 

A coherent structure is generally defined as a spatiotemporally compact region of a flow, associated with a relevant feature for the description of the dynamics, such as the kinetic energy. For instance, modal decompositions, such as dynamic mode decomposition, can be directly connected to the Koopman analysis inherited from the dynamical system theory. Dynamic modes are therefore informative of the dynamical skeleton of a flow. Modal decompositions capture the connections between the coherent structures—the spatial modes—and their time behaviour, and lead to reduced-order models. Lagrangian coherent structures are another popular family of coherent structures. They are related to topological kernels of the fluid flow, such as material surfaces. Such structures hence govern the flow transport, and, without them, understanding mixing, residence time, or exchange in complex biological flows would be challenging.

There are many other families of coherent structures that are of decisive importance in many applications, ranging from the understanding of the transition to turbulence to predictive models and flow control.

Applications of existing methodologies and development of new methodologies for the identification of coherent structures go hand in hand with the understanding and control of engineering fluid flows. 

This Special Issue will present recent advances in the identification of coherent structures that help us to understand the dynamics of fluid flows and novel methodologies for the identification of such structures.

We invite contributions from academics on the following themes:

• Coherent structures—investigate the dynamics of fluid flows;
• Modal analysis—methodology;
• Lagrangian structures and mixing;
• Dimensionality reduction via coherent structures;
• Edge States—chaos and transition;
• Hairpins, eddies, rollers and streaks—transition to turbulence;
• Machine learning for the identification of coherent structures;
• Markovianity and clustering—links to dynamics.

Dr. Florimond Guéniat
Guest Editor

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