Abstract
Large particles disturb a flow more than small particles. Using Magnetic Resonance Imaging (MRI) and pressure drop measurements we find that large spherical and cubic particles in pipe flow promote subcritical turbulent-like flow disturbances (as seen in the radial mean velocity and rms profiles at Re=700). These disturbances lead to significant changes in the laminar-turbulent transition at low particle concentrations ($l5$% per volume). We suggest these observations can be explained by the relative magnitude of (i) particle interactions, (ii) flow disturbances introduced by the particles and (iii) viscous dissipation in the system.
Highlights
The classical transition from laminar to turbulent flow is affected if solid particles are added
In this work we show that, for particle-laden pipe flows with large particle-to-pipe diameter ratios d/D, the φ threshold for altering the transition is much lower than previously reported for smaller particles
Magnetic resonance velocimetry reveals that particles introduce turbulent-like fluid velocity fluctuations in laminar flow
Summary
If the Reynolds number is large enough, the puffs grow into faster traveling slugs that spread and lead to fully developed turbulence. This process is rather sudden and the change from a laminar to a turbulent state is distinct [7,8,9]. One way to determine the flow state in pipe flow is from the friction factor f. It is a dimensionless measure of the fluid friction, or drag, and is defined as f = D P/(2ρU 2 L) where P/ L is the pressure drop per unit length of pipe needed to overcome wall friction.
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