The turbulent flow in a slug

In seminal experiments reported in the 1970s, Wygnanski and coworkers made detailed measurements of transitional pipe flow, an intermittently laminar and fluctuating flow. Their findings led to the sharp distinction between ``puffs”, a fluctuating domain of fixed length, and ``slugs”, an ever-growing chaotic region. Puffs showed apparent differences with fully-developed turbulent flows, for which there is no laminar-turbulent intermittency. When Wygnanski compared slugs and fully-developed turbulence, however, he concluded that ``The structure of the flow in the interior of a slug is identical to that in a fully developed turbulent pipe flow.” This conclusion has become an unquestioned cornerstone in our understanding of transitional pipe flow. We have repeated Wygnanski’s measurements using modern experimental techniques and computer simulations and find that his conclusion comes with important caveats, which are missing in the original work, and that his evidence requires a thorough re-evaluation. Our results reveal a richer picture of equivalence between slugs and turbulence that also unveils the crucial role of the Reynolds number.