We propose a scaling law for the onset of turbulence in pipe flow of neutrally buoyant suspensions. This scaling law, based on a large set of experimental data, relates the amplitude of the particle-induced perturbations $\epsilon$ to the critical suspension Reynolds number ${\text{Re}}_{s,c}$. Here $\epsilon$ is a function of the particle-to-pipe diameter ratio and the volume fraction of the suspended particles, $\epsilon ={(d/D)}^{1/2}{\varphi }^{1/6}$. ${\text{Re}}_{s,c}$ is found to scale as ${\epsilon }^{-1}$. Furthermore, the perturbation amplitude allows a distinction between classical, intermediate, and particle-induced transitions.

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• Flow instability
• Fluid-particle interactions
• Particle-laden flows
• Sediment transport
• Structure & turbulence of boundary layers
• Transition to turbulence