cellocgw wrote:ok, so a standard straw requires V = c/4 . Randall posited this straw has 38.5 mm^2 cross-sectional area. We want to reduce the speed from 73 600 000 m/s to the speed of sound in water, which is 1482 m/s.
That ratio is approximately 49700:1,
so now we need a straw whose diameter is sqrt(49700) * 7 mm , or 1.56 m if I got all my decimal points correct. That shouldn't be too hard to build. We will need to build a funnel the width of the Niagra Falls and which constricts slowly enough not to cause turbulence, but heck, that's just a fun project for some AutoCAD jockey.
I don't think you can prevent turbulence just by decreasing the constricting rate, unless the walls of your straw are ridiculously slippery and smooth. Fluids flowing along walls will pretty much always turn turbulent in their boundary layers after a certain distance Reynolds number is achieved (density times velocity times distance travelled divided by dynamic viscosity), which depends on the roughness and on turbulence levels among other things. If you only very slowly constrict the water you will undoubtedly reach turbulence in the boundary layer simply because of distance travelled. I'm mostly familiar with boundary layers of gases, but liquids are fairly similar at least at subsonic speeds.
Another interesting thing about gas flows through pipes and straws is the Fanno-flow principle, which Randall didn't really touch upon, I assume because straws are generally too short to really make it matter. But an adiabatic (no energy transfer to/from the walls) flow through pipes with constant diameter that has skin friction always tends to go to sonic velocities, supersonic flows slow down by the drag, while subsonic flows speed up (talking about mean velocity over the cross-section). I think it's actually really cool that drag can actually speed up flows even though they apply a force in the opposite direction. If the walls aren't adiabatic, the Rayleigh flow principle gets added, which drives the flow towards sonic in case of heat addition (hot walls, cold gas) and away from sonic flow in case of heat removal (cold walls, hot gas).