Making Reynolds-number reasoning visible: complementary classroom models for plume organization and geometry–flow coupling

Reynolds number is central to cardiovascular physiology, yet students often memorize the equation without understanding the competing inertial and viscous effects that it represents. We developed two complementary, low-cost classroom demonstrations intended to make selected relationships relevant to Reynolds-number reasoning visible without treating either apparatus as a quantitative Reynolds experiment. In the first, an adjustable aquarium air pump drives bubbles through submerged diffusers in transparent chambers. Changes in nominal pump drive and fluid composition produce qualitative changes in bubble rise, plume width, lateral motion, and plume organization. Because this is buoyant two-phase flow, these observations are not classifications of laminar or turbulent internal flow. In the second, aerosol generated by ultrasonic foggers is carried through nominal 5-mL and 50-mL serological pipettes and illuminated at the outlets. Changing pipette geometry changes flow resistance and can alter volumetric flow, internal and exit velocity, and jet momentum flux, but these quantities were not measured. The visible output is the external free jet; therefore, plume speed and coherence are described as apparent, qualitative observations rather than direct measures of internal velocity or Reynolds number. The activities use a predict-observe-explain sequence and connect cautiously to clinical examples as mechanistic analogies. This report describes qualitative instructional demonstrations, not a quantitative fluid-mechanics study or a formal educational-outcomes study.