nBodyProblem @ nBodyProblem @lemmy.world

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2 yr. ago

That's not really showing temperature stratification which is a more extreme separation of temperature from surface

I think the definition you are using is far too restrictive, in many contexts temperature stratification simply refers to a situation where you get temperature gradients across a fluid with the warmer fluid gathered near the top of the body. For example, in a factory you will often have “destratification” fans operating because warm air from equipment rising to the ceiling results in a temperature gradient from floor the ceiling.

It is not a phenomena exclusive to surface heating.

That's just showing that the hottest atoms gather to the top, which btw, proves Convection currents.

Yes. My point was not to establish that convection is magically absent from fluids in microwaves, but to establish that it differs significantly from stovetop heating. Convection currents in stovetop heating create a strong stirring action that produces a substantially uniform temperature. Microwaves do not create the same stirring action and this produce a significant nonuniform temperature gradient.

The modified glass is just diverting the hotpots to the bottom to make the convection less "unusual".

Clearly. They make the heating more akin to a stovetop, which is really the point here.

They aren't claiming that convection doesn't accrue, only that it's "unusual convection" resulting in less even heating like that of thermal stratification, not literal thermal stratification where the layers have separate convection currents that prevent mixing all together.

Once again, you are using a definition of thermal stratification that is far too specific. However, arguing over it is really just being pedantic because the core point at issue here is whether or not heating a cup in a microwave or a stovetop produce the same final product. They do not unless you apply some mechanical agitation to mix it up.

I'm well aware of temperature stratification. It doesn't happen in a microwave.

It empirically does. We can argue about the theory all day but the research says microwaves produce stratified temperature gradients when heating liquids. However, I’d point out that, in atmosphere, when we have localized hot spots the warm air can effectively travel in bubbles without significant mixing for quite some distance. There seems to be a similar phenomena at work when microwaving liquids.

See the screenshot below.

I pulled this from “Multiphysics analysis for unusual heat convection in microwave heating liquid” published in 2020 in AIP Advances.

Relevant excerpts:

“ Usually, the fluidity of liquids is considered to make the temperature field uniform, when it is heated, because of the heat convection, but there is something different when microwave heating. The temperature of the top is always the highest in the liquid when heated by microwaves.”

“ The experimental results show that when the modified glass cup with 7 cm metal coating is used to heat water in a microwave oven, the temperature difference between the upper and lower parts of the water is reduced from 7.8 °C to 0.5 °C.”

“According to the feedback from Midea (microwave appliance makers), when users use the microwave oven to heat liquids such as milk or water, the temperature at the top of the liquid will be significantly higher than the temperature at the bottom.”

I mean, it’s not really a matter of debate TBH. There are a number of peer reviewed journal articles documenting the temperature stratification. Here is one source, where the authors attempt to create a special cup to heat the water more evenly.

From a utility side perspective it’s true to say 240v is the standard we just wire our outlets to use one of the two hot wires instead of both of them.

Convection currents don’t stir water in a microwave because the heat source isn’t on the bottom. That’s the difference. You get temperature stratified water where the surface is hotter than the bottom of the cup and they don’t naturally mix.

Of course, here in America, we have this incredible technology called a spoon. Pull that bad boy out, give a little stir, problem solved.

Sure thing!

Honestly I wish we didn’t have to design manned spacecraft to be manually pilotable because you have to make design sacrifices to get there. However, that is unlikely to happen in the lunar program anytime soon

There was a significant amount of manual piloting in the Apollo missions

The guidance gets more difficult in the terminal stages and they didn’t really trust computers to safely control the spacecraft near the surface, so their solution was to have the computer fly 95% of the way down and have the crew take over for the terminal phase.

The Apollo algorithms work fine for non-manned missions as well, but you have to vet the trajectory targets more fully in simulation and add some active retargeting scheme to avoid obstacles near the surface.

Combine the added complexity of a robotic lander with groups like intuitive that have never landed one before, and this sort of thing happens

I find it funny this comment got upvoted more than mine. I am a guidance engineer that worked at NASA on lunar lander programs, it’s literally my job to be an expert in this stuff. Modern computing has reduced propellant usage a bit and improved targeting accuracy, but we have had usable solutions to this problem since the 60s.

Atmosphere makes the problem MUCH harder because the dynamics are far more complicated and the uncertainties are higher.

Yes but a moon landing is fundamentally easier in most respects, while using substantially similar technology, than landing a rocket booster on earth or a rover on mars.

Landing on the moon is, as far as it can be, trivial for a group like NASA. It’s much more challenging for a small private company like Intuitive that has never built a lander before.

Sounds like they should let the kids choose whether they want to you outside or stay in based on whether or not they feel cold

Forcing kids to bring coats is weird to me

Maybe it’s different elsewhere, but I was born into a relatively cold+wet climate and moved to San Diego in elementary school. I didn’t bring a coat because it made me hot, I was acclimated to colder weather, and I didn’t want to carry it around.

They refused to let me go outside for recess for weeks because I didn’t bring a coat and refused to wear one from the lost and found. Finally, one day, they sent me to the principal’s office and called my mom in for a chat to discuss my misbehaving.

My mom’s response was, “You called me in from work for THIS?! If he’s not cold, he’s not cold! He has warm clothing at home. He’s capable of deciding whether or not he would be more comfortable with a jacket on. Let him go outside and leave me alone”

I am 100% the target audience, have worked on multiple teams that did their 6DOF models in Matlab for GNC and orbital dynamics stuff.

I still think simulink is absolutely terrible. It makes certain things a lot easier to implement but the Git implementation is very nearly useless.

It’s a very effective medicine, though. For example, in small doses it’s more effective than adderall or Ritalin for ADHD. It’s less common than either of those drugs because there is a higher abuse potential, but there is nothing really wrong with it either.

The key here is small doses taken orally. Taking it in a medical context is a very different animal than recreational methamphetamine.

Superchargers come with massive parasitic losses, in many cases 10-20%, and there’s a decent handful of cars with clutches on the supercharger pulley. The MR2 is one.

I dunno, I am a engineer in the defense industry and pentagon wars got a lot right in terms of what it’s like

The real LPT is to remove water from cider to make an intensely apple flavored spirit without muting its flavors with heat.

Then sell the liquor to nobles and inflated prices without ever revealing the production method

I agree with everything you said. If the descriptions mostly add flavor and the roll of the dice determines the result, that’s exactly how DnD should be. It is a role playing game, and role playing is half the fun

Unfortunately, it seems like most DMs punish you for roll playing poorly in conversation. In my experience, if you have high charisma it’s often more successful to just say “I want to convince the person of this thing” and roll than to describe your argument to the DM. That discourages roll playing rather than encourages it.

Maybe it’s a role playing game and the entire purpose is to role play as something besides yourself

Sure but that’s still a weird situation when you lack charisma in real life. My 20 CHA character with proficiency in persuasion would have ideas and know which of those options would work best in this situation. It’s absolutely demanding on your real life charisma in a way no other check is.

Nobody asks for a real life strength check before the barbarian can push the boulder out of the way

Nobody asks the player running a rogue to demonstrate “how they’d pick the lock” to determine how high the PC needs to roll in the game

Same. I like the whole engagement ring ritual but I’ll be damned if our marriage is going to hinge on my “proving my love” with some overpriced trinket that costs a couple months’ salary and loses 95% of its value when it leaves the store. If that’s what it takes for us to get married it’s not the type of relationship I want in my life.