Transition to turbulence in particle laden flows

TL;DR

This study experimentally examines how neutrally buoyant particles influence the transition from laminar to turbulent flow in a pipe, revealing concentration-dependent changes in transition behavior and turbulence characteristics.

Contribution

It provides new insights into how particle concentration affects the nature and onset of turbulence in particle-laden pipe flows, including a transition from localized puffs to global fluctuations.

Findings

At low concentrations, turbulence onset resembles single-phase flow with localized puffs.

Higher concentrations lead to a continuous, non-hysteretic transition to a globally fluctuating state.

Very high concentrations suppress Newtonian turbulence signatures, resulting in a different flow regime.

Abstract

Suspended particles can alter the properties of fluids and in particular also affect the transition from laminar to turbulent flow. In the present experimental study, we investigate the impact of neutrally buoyant, spherical inertial particles on transition to turbulence in a pipe flow. At low particle concentrations, like in single phase Newtonian fluids, turbulence only sets in when triggered by sufficiently large perturbations and, as characteristic for this transition localized turbulent regions (puffs) co-exist with laminar flow. In agreement with earlier studies this transition point initially moves to lower Reynolds number (Re) as the particle concentration increases. At higher concentrations however the nature of the transition qualitatively changes: Laminar flow gives way to a globally fluctuating state following a continuous, non-hysteretic transition. A further increase in Re results in a secondary instability where localized puff-like structures arise on top of the uniformly fluctuating background flow. At even higher concentration only the uniformly fluctuating flow is found and signatures of Newtonian type turbulence are no longer observed.