Hydrodynamics of drinking straws and finite-length pipes*

Pipe flow is classically studied for extremes of flow speed and pipe length, but many practical situations in industry and nature involve intermediate Reynolds numbers and finite-length pipes. We carry out lab experiments and modeling that reveal the hydraulic laws applicable to these intermediate cases. We summarize our findings with a pipe flow map in which classical regimes of pipe flow occupy distinct territories separated by boundaries involving complexities such as flow development and transition. Our measurements show that drinking through a straw is a perfect example of the maximally complex case, and more generally, our model provides accurate estimates of the hydraulic resistance across pipe geometries and flow conditions.