Turbulent stress measurements of fibre suspensions in a straight pipe

Abstract

The focus of the present work is an experimental study of the behaviour of semi-dilute, opaque fibre suspensions in fully developed cylindrical pipe flows. Measurements of the normal and turbulent shear stress components and the mean flow were acquired using phase-contrast magnetic resonance velocimetry. Two fibre types, namely, pulp fibre and nylon fibre, were considered in this work and are known to differ in elastic modulus. In total, three different mass concentrations and seven Reynolds numbers were tested to investigate the effects of fibre interactions during the transition from the plug flow to fully turbulent flow. It was found that in fully turbulent flows of nylon fibres, the normal, ⟨uzuz⟩+, and shear, ⟨uzur⟩+ (note that ⟨·⟩ is the temporal average, u is the fluctuating velocity, z is the axial or streamwise component, and r is the radial direction), turbulent stresses increased with Reynolds number regardless of the crowding number (a concentration measure). For pulp fibre, the turbulent stresses increased with Reynolds number when a fibre plug was present in the flow and were spatially similar in magnitude when no fibre plug was present. Pressure spectra revealed that the stiff, nylon fibre reduced the energy in the inertial-subrange with an increasing Reynolds and crowding number, whereas the less stiff pulp fibre effectively cuts the energy cascade prematurely when the network was fully dispersed.

Experimental data for solid–liquid flows at intermediate and high Stokes numbers

“…Two-phase flows are abundant in nature and in industrial applications, where they exhibit many interesting behaviours arising from the interaction between the phases and the different length scales at play (Sundaresan 2000;Tenneti & Subramaniam 2014;Stone 2017). The presence of the second phase can augment or diminish the turbulence characteristics (De Marchis & Milici 2016;Eaton & Longmire 2017;MacKenzie et al 2018) and change the rheological properties of the fluid (Wylie, Koch & Ladd 2003;Brennen 2005). The phase interactions can also lead to an uneven distribution of particles in the flow (Troutt 2006;Lau & Nathan 2016) and erosion or dissolution processes (Claudin, Duran & Andreotti 2017;Ristroph 2018).…”

Universal flapping states of elastic fibers in modulated turbulence

“…The interaction between elastic objects and fluid flows plays an essential role in a variety of both fundamental and applied problems, ranging from biological (e.g., mucus-cilia or red blood cells hydrodynamics) [1][2][3] and environmental processes (e.g., flow in flexible canopies and plant reconfiguration) [4][5][6][7] to engineering applications (e.g., energy harvesting and flow control). [8][9][10][11][12][13][14] Despite the great advancement of research in these fields, the available knowledge on such nontrivial phenomena mostly concerns the situation where the flow is laminar, [15][16][17][18] whereas the interaction between elastic objects and turbulence has started to be understood only more recently. [19][20][21][22][23] In this framework, the following naturally arising questions are still open: Does a universal scenario hold in the interaction between an ensemble of elastic objects and a turbulent flow?…”

Universal flapping states of elastic fibers in modulated turbulence

“…The interaction between elastic objects and fluid flows plays an essential role in a variety of both fundamental and applied problems, ranging from biological (e.g., mucus-cilia or red blood cells hydrodynamics) 1-3 and environmental processes (e.g., flow in flexible canopies and plant reconfiguration) [4][5][6][7] to engineering applications (e.g., energy harvesting and flow control) [8][9][10][11][12][13][14] . Despite the great advancement of research in these fields, the available knowledge on such nontrivial phenomena mostly concerns the situation where the flow is laminar [15][16][17][18] , whereas the interaction between elastic objects and turbulence has started to be understood only more recently [19][20][21][22][23] .…”