# Reynolds No. & Downsizing

I just watched “Downsizing”. They have this method of shrinking people down to sizes of around 5 inches.

Now, suppose you downsize a person of say, 5 feet to 5 inches, which turns out to be about (1/12) times his original height. Then, the Reynolds number for such a person would also be 12 times smaller. This much I understand.

What I don’t understand and really want to know is, how would such a person “feel” in a medium having a Reynolds number 12 times smaller ?

I believe I understand Reynolds no. to be a helpful mathematical tool in analysing different flow regimes in fluid dynamics. I, however, don’t have a clear understanding of how change in sizes alone affects the perception of the flow around that body ( the “feel”), keeping the density and dynamic viscosity the same.

• I don't know specifics; but they would definitely experience flows a lot differently than a regular sized person. When testing scale models in wind-tunnels for example; you need to actually adjust flow rates to compensate for non-linear scaling effects. – JMac Feb 6 '18 at 14:59
• you should read this absolutely fascinating lecture by E. M. Purcell: Life at low Reynolds number, Am. J. Phys. v.45, No.1, Jan 1977. – hyportnex Feb 6 '18 at 15:09
• -1 Not clear what you are asking. Questions about perception (the "feel" of something) are subjective, hence outside of physics. Please re-frame your question using physical concepts. – sammy gerbil Feb 6 '18 at 15:31
• @sammygerbil what a person feels or perceives depends upon the density and viscosity of the medium he is in. The question was aimed at knowing how this perception can change when only the size is factored in the Reynolds no. keeping the density and viscosity the same. Does the perception of surroundings change, as both the density and viscosity are the same, or does the change in Reynolds no. Become the decisive factor in altering that perception. It’s always fun to know how Downsizing someday in near future might affect those aspects of our lives which we take for granted. – RedHelmet Feb 6 '18 at 15:46
• @sammygerbil I think you're being overly pedantic here. It's pretty clear (to me at least) what is meant and I was able to answer it accordingly. Changing the ratios of forces does change the relative impact those forces have a body, and so that body would "feel" something different. No need to go after common-use English for a conceptual, entry-level question. – tpg2114 Feb 6 '18 at 17:39

Think of the Reynolds number as the ratio of inertial forces ($u L$) to viscous forces ($\nu$). Using that process, shrinking the length by 12 is equivalent to multiplying the viscosity by 12. Does that help you figure out what it would feel like?