Puff-like instability in laminar to turbulence supercritical transition of round jets

TL;DR

This study identifies a novel puff-like instability in laminar round jets at low Reynolds numbers, revealing a new transition mechanism from laminar to turbulent flow through experimental and theoretical analysis.

Contribution

It introduces a new puff-like instability in round jets at low Re, supported by experimental visualization and linear stability theory, expanding understanding of flow transition mechanisms.

Findings

Discovery of puff-like instability (PFI) in round jets at Re 400-700.

PFI resembles puffs in pipe flow and is linked to supercritical transition.

LST predicts PFI as a superposition of helical modes with azimuthal wavenumbers n=+-1.

Abstract

We explore the laminar to turbulence transition of round jets at low Reynolds number (Re < 1000) using a novel experimental setup and linear stability theory (LST). The setup has a large domain and a low disturbance environment which increases the critical Re to approximately 500, permitting the appearance of a hitherto unknown puff-like instability (PFI). The instability is identified in the self-similar region of the jet through clean flow visualizations (FV) and further corroborated by particle image velocimetry (PIV) measurements. For 400< Re <700, the flow exhibits PFI embedded in a puff-train encapsulated by a laminar flow analogous to 'puffs' in pipe-flow transition; the latter being symbolic of the finite-amplitude disturbance transition scenario. The observation that PFI convects close to the average local velocity with an inflectional velocity profile further strengthens the analogy. LST predicts that PFI is effectively a superposition of the helical mode (HM) pair with azimuthal wavenumbers n=+-1. Hence, PFI can also appear in the infinitesimal-amplitude supercritical route to transition of linearly unstable flows.