Discussion

A bit of fluid mechanics from scratch not from scratch

BobbyTables2: Love the thinking but was booing there would be some enlightenment at the end…

barrenko: > in case someone wants to get nerdsniped https://github.com/kamilazdybal/fluid-toolbox

glouwbug: Gases work roughly the same way (until they break the ideal gas law and don't). The ratio of static and dynamic pressures between two chambers can drive nozzle flow [1]. Add in some compression and combustion and you can simulate anything from refrigerators to engines.[1] https://github.com/glouw/ensim4/blob/master/src/chamber_s.h#...

stackghost: Wow, I found the writing style to be extremely irritating, to the point that I can't bear to read it again, but TFAuthor seems to fall into a few common traps that ensnare people new to fluids:>narrator: he’s assuming that the water at different vertical positions, at a given horizontal position, is moving with the same vectorThis assumption is physically impossible, given the first diagram in part #2. Obviously in real life the thing would drain out the side, so obviously there isn't a uniform velocity gradient. This is a common thing where students get so wrapped up in the math with their simplifying assumptions they accidentally/paradoxically "prove" reality wrong.The author does it again with the "top half of an hour glass" shape:>Well, we know that the velocities are always strictly vertical (and downward).No, we don't know this. This is again physically impossible, given the shape of the container. If there was no horizontal component you'd get a tube-shaped void extending up from the spout, and water would magically fail to drain from the sides.>The water at the bottom of the spout cannot be going faster than at the top! [...] In other words, the water does indeed accelerate, and it also gets narrower; the pipe is partly filled with air.The author appears to have forgotten that friction and gravity exist. Generally if the flow is steady (which it will be unless the volume of water in the tank is small compared to the volume of water in the outlet pipe), then the pipe will run full of water at all times. Water does not accelerate in the pipe. If you add length to the pipe then the added height is counteracted by added friction and by the weight of the moving fluid.Grab an undergraduate fluids textbook and do an energy balance.

srean: Prompted by current news I was idly thinking about how caverns and tunnels get impacted by surface level blast overpressured.How much of the energy will get transmitted as opposed to reflected will depend on impedance matching. There are two very important transitions (i) open air to air inside cavern and (ii) cavern air to cavern walls.Regarding (ii) unless the density transition is eased into, (gravel and sand ?) the energy in the air will get reflected right back. Very little will get transmitted into the rock walls.Same principle applies for (i). As a resident of such a cavern you would want high impedance mismatch. It makes sense to have well matched labyrinths to dissipate the energy much like silencers and mufflers. One would want all the energy be converted into turbulence, safely and quickly.I wonder if these considerations inform their design. Probably does.

polishdude20: So for a simple case, do you want a large entrance to your cave or a small one?

barrenko: Oh là là, thank you!

pkaral: This is gold for those of us who dabble in adjacent things but don't want to (or aren't able to) do the math.