Extreme events in turbulence: the case of stratified flows

P.D. MININNI

Punta del Este

Congreso; Dynamics Days Latin America and the Caribbean 2018; 2018

Universidad de Montevideo

The definition of what an extreme event is can be different depending on the physical problem considered. In fluid dynamics, the phenomenon of intermittency is the sine qua non of turbulence, and calls for a precise definition of extreme events and for the development of specific theoretical tools to study them. Indeed, the complexity of a turbulent flow is often associated with the fact that turbulence comes in intermittent ?gusts?. These intermittent gusts (or extreme events) are associated with non-Gaussian statistics of some physical quantity in the system. As such, intermittency is a highly spatially- and temporally-localized phenomenon, which thus requires high-resolution instrumentation, be it in the laboratory, in atmospheric observations, or in numerical simulations. In this talk I will discuss some tools from statistics and from dynamical systems that are regularly used to study intermittency and extreme events, including probability density functions of field increments, structure functions, and scaling exponents. As a particular example, I will focus on the problem of vertical drafts and mixing observed in stratified flows. In this problem, for values in parameter space relevant for the atmosphere and the oceans, I will show that non-stationary very large fluctuations of the vertical velocity take place spontaneously in the flow. This behavior can be captured by a simple model representing the competition between gravity waves on a fast time-scale, and non-linear steepening on a slower time-scale. The existence in these flows of a resonant regime characterized by enhanced large-scale intermittency, can then linked to the emergence of specific structures in the velocity and potential temperature fields.