Momentum analysis of complex time-periodic flows

TL;DR

This paper introduces Fourier-Averaged Navier-Stokes (FANS) equations, a novel framework for analyzing complex time-periodic flows, providing clearer insights into momentum transfer and nonlinear interactions in turbulent wakes.

Contribution

The paper presents FANS equations as a new method to interpret flow physics by simplifying the analysis of momentum transport and nonlinear interactions in complex cyclic flows.

Findings

FANS effectively characterizes momentum transfer in turbulent wakes.

The method simplifies flow physics interpretation through Fourier analysis.

FANS is applicable to broadband spectral energy distributions.

Abstract

Several methods have been proposed to characterize the complex interactions in turbulent wakes, especially for flows with strong cyclic dynamics. This paper introduces the concept of Fourier-Averaged Navier-Stokes (FANS) equations as a framework to obtain direct insights into the dynamics of complex coherent wake interactions. The method simplifies the interpretations of flow physics by identifying terms contributing to momentum transport at different timescales. The method also allows for direct interpretation of non-linear interactions of the terms in the Navier-Stokes equations. By analysing well-known cases, the characteristics of FANS are evaluated. Particularly, we focus on physical interpretation of the terms as they relate to the interactions between modes at different timescales. Through comparison with established physics and other methods, FANS is shown to provide insight into the transfer of momentum between modes by extracting information about the contributing pressure, convective, and diffusive forces. FANS provides a simply calculated and easily interpreted set of equations to analyse flow physics by leveraging momentum conservation principles and Fourier analysis. The method is applicable to flows with complex cyclic waveforms, including broadband spectral energy distributions.