Disentangling coherent structures and the origin of swirl-switching

TL;DR

This paper introduces a novel modal decomposition method, FHPOD, to better identify coherent structures in turbulent pipe flow and investigates the physical mechanisms behind swirl-switching phenomena.

Contribution

The proposed FHPOD method effectively separates flow modes, revealing that swirl-switching is an intrinsic flow instability rather than caused by incoming turbulence.

Findings

FHPOD produces four distinct mode families, including a swirl-switching mode.

Swirl-switching is linked to flow instability, not external turbulence.

Downstream modes relate to local shear layers of the base flow.

Abstract

Modal decomposition of turbulent flows using classical proper orthogonal decomposition (POD) often suffers from mode mixing, in which a distinct coherent structure may be distributed over several POD modes. We propose a decomposition method based on the Hilbert transform and band-pass filtering to address this issue (filtered Hilbert POD -- FHPOD). We apply this approach to the turbulent flow through a 180 bent pipe at $Re_D=10,000$ (based on bulk velocity ($U_b$) and pipe diameter ($D$)) and curvature $\gamma=0.2$, simulated using direct numerical simulation. The FHPOD results in four distinct mode families, including a swirl-switching mode at Strouhal number of 0.13 localised in the curved section. Our novel modal decomposition shows that the modes observed in the bend and downstream correspond to distinct physical mechanisms rather than to a single universal swirl-switching instability throughout the pipe, as previous work implied. To further examine the origin of the swirl-switching mode, we perform a local stability analysis of the cross-sectional mean flow along the bend. We find unstable eigenmodes at the same streamwise wavenumber and within the same range of Strouhal numbers as the swirl-switching mode found in the modal decomposition. The result supports the interpretation that the swirl-switching phenomenon is an intrinsic instability of the curved-pipe flow that can be excited and potentially enhanced by incoming turbulent structures, but is ultimately not caused by them. Finally, we also establish a link of the downstream modes to the local shear layers of the modified base flow, highlighting the different nature of these modes.