When you model fluid flow through a pipe with wavy walls, you have to decide what stays constant as the wave amplitude increases. Option one: hold the mean radius fixed. Option two: hold the interior volume fixed. Most researchers pick one without thinking about it. The difference seems cosmetic — a bookkeeping choice.
Guo and Thomas (arXiv:2603.04362) show it is anything but.
At 20% wave amplitude (relative to mean radius), the two conventions produce flow rates and hydraulic resistances that differ by 10%. At larger amplitudes, the discrepancy reaches 50%. Half the answer. From a choice that appears in a single line of the problem setup.
The mechanism is straightforward once you see it. Holding mean radius constant while increasing wave amplitude means the interior volume grows — the bulges outward contribute more volume than the constrictions remove. Holding volume constant requires shrinking the mean radius to compensate, which changes the baseline flow even before the waviness matters. The two conventions describe physically different pipes.
What makes this structural rather than pedantic is that the error is invisible inside either convention. If you work consistently within mean-radius-constant, all your calculations are self-consistent. Same for volume-constant. The problem appears only when you compare results across conventions, or when you try to match your model to a real pipe — which has a definite volume and a definite geometry, not a convention.
This has immediate implications for decades of work on peristaltic pumping (biological and engineered), where moving-wave analyses routinely use one convention or the other without stating which one or why. Published flow rates may carry systematic errors on the order of tens of percent, not from any computational mistake but from a framing choice made before computation began.
The through-claim: the measurement frame is as load-bearing as the data. Change the convention and you change the answer without changing the physics. The error hides in the step you don't think of as a step.