# Synchronization to big-data: nudging the Navier-Stokes equations for data assimilation of turbulent flows

## TL;DR

This paper systematically evaluates the effectiveness of nudging data assimilation techniques in reconstructing turbulent flows, analyzing different data types and flow complexities, and establishing standards for future applications in complex fluid dynamics scenarios.

## Contribution

It provides a comprehensive assessment of nudging methods for turbulent flow reconstruction, including new insights into performance across data types and flow complexities.

## Key findings

- Nudging effectively reconstructs large- and small-scale flow features.
- Fourier nudging shows superior performance in certain turbulent regimes.
- Complex flows with cyclonic structures can enhance nudging effectiveness.

## Abstract

Nudging is an important data assimilation technique where partial field measurements are used to control the evolution of a dynamical system and/or to reconstruct the entire phase-space configuration of the supplied flow. Here, we apply it to the toughest problem in fluid dynamics: three dimensional homogeneous and isotropic turbulence. By doing numerical experiments we perform a systematic assessment of how well the technique reconstructs large- and small-scales features of the flow with respect to the quantity and the quality/type of data supplied to it. The types of data used are: (i) field values on a fixed number of spatial locations (Eulerian nudging), (ii) Fourier coefficients of the fields on a fixed range of wavenumbers (Fourier nudging), or (iii) field values along a set of moving probes inside the flow (Lagrangian nudging). We present state-of-the-art quantitative measurements of the scale-by-scale {\it transition to synchronization} and a detailed discussion of the probability distribution function of the reconstruction error, by comparing the nudged field and the {\it truth} point-by-point. Furthermore, we show that for more complex flow configurations, like the case of anisotropic rotating turbulence, the presence of cyclonic and anticyclonic structures leads to unexpectedly better performances of the algorithm. We discuss potential further applications of nudging to a series of applied flow configurations, including the problem of field-reconstruction in thermal Rayleigh-B\'enard convection and in magnetohydrodynamics (MHD), and to the determination of optimal parametrisation for small-scale turbulent modeling. Our study fixes the standard requirements for future applications of nudging to complex turbulent flows.