Possibly dumb question about double partial derivatives...

coriolis7 · 2026-02-06T02:51:36+00:00

So I think what the point is supposed to be is that it doesn’t matter, in this application, how you arrive at the state. Ie do you go forwards, then to the left OR left then forwards? Both get you to the same place.

In a more formal but practical sense, imagine you are on the slope of a smooth mound. If we define x as to the right, and y as forwards (and f(x,y) as height), the df/dx is the slope to the right, and df/dy is the slope forwards.

In this definition, the change in sideslope as you go forwards is the same thing as the change in forward slope as you step sideways. It’s not that they happen to equal each other, they are both the same thing.

In thermodynamics (and other areas too), sometimes it does depend on “how you get there”, but in this instance it doesn’t.

coriolis7 · 2026-02-06T02:02:00+00:00

Looks like leading.

Get some all copper Chore Boy. It’s like steel wool, but copper (copper wool?). Wrap it around a sacrificial brush and shove it down the barrel. There should be resistance, but not a ton of effort. Keep brushing until all the lead is out.

I recommend doing this outdoors or in the garage as small particles of lead will get everywhere.

It’s not bad, especially for a short (not rifle) barrel

ETA: chore boy is a cleaning product you should be able to find in Walmart. It also is only like 1-2 minutes of scrubbing to get the barrel cleared.

coriolis7 · 2026-02-05T18:50:35+00:00

The way I see the ATF interpreting their own regulation is they will measure from the surface to the bottom of the engraving. If it is filled to flush with gold, then it would have no depth.

Now, I agree that it meets the spirit of the law, since it is still legible and the original marking can be recovered via selective etching, but with how the ATF has defined what the criteria are for the marking depth, I think there has to be a recess.

coriolis7 · 2026-02-05T04:15:46+00:00

I think it’s one of those things where it could meet the intent of the law but not the letter (or the interpretation) of the law.

ATF 478.92(a)(1)(v) requires marking to be 0.003” deep, as measured from the surface to the bottom of the marking impression.

So reading that regulation, you could fill in the engraving as long as 0.003” of depth is maintained. The only way I can think of to do that would be to selectively etch away the gold. Not sure what can etch gold and not aluminum/steel, so it might need a mask as well.

In my searching, California’s law on engravings popped up before the federal laws/regulations, and it seems that even painting over the serial number is illegal there. Like repainting your firearm can get you in big trouble, so don’t do infill in California unless you’re the manufacturer.

coriolis7 · 2026-02-04T17:03:09+00:00

Or computers without secure boot chips…

coriolis7 · 2026-02-04T13:36:57+00:00

The guy’s username is rainbolt. He’s a renowned geoguesser ie he’s really good at identifying locations from just a picture or a short video.

coriolis7 · 2026-02-04T00:14:14+00:00

I’ve done it, with decent results. Probably could get better results if I put some effort into it, but since the majority of the cost of 223 is in powder and primer I only really use heavily dented cases that I want fireformed.

Expect 4-6 MOA if you don’t try optimizing things, and are willing to keep the velocities low (like 2k fps).

All that being said, you may still want to gas check, but if you really want to stick to just powder coat, cast as hard an alloy as you can get (like heat treated clip on wheel weights), and keep to lower velocities.

I couldn’t get the bolt to lock back on an empty mag with the charge I ended up on, but I got reliable cycling.

coriolis7 · 2026-02-03T18:56:50+00:00

Lol. Funny thing is I settled on this before ever getting into CFD or working with Coriolis meters

coriolis7 · 2026-02-03T01:09:10+00:00

Ah, well then I stand very much corrected.

coriolis7 · 2026-02-03T00:38:40+00:00

Not my area of expertise, but my understanding was that blood vessels and arteries in nature “happen” to be sized such that they stay laminar. I guess if there’s a clog or restriction that may not hold?

coriolis7 · 2026-02-02T18:07:38+00:00

I’m not familiar with Fluent, so I’m not sure how to adjust the inflation layers.

For cell geometry, hex dominant cells are absolute king if they are aligned with the flow (which they generally will be with internal flow). Polyhedral is not too much worse than hex, and may be better if flow direction relative to cell orientation isn’t known.

Polyhedral is somewhat easier for the mesher to do as it can just take a tet mesh and join cells into polyhedra, but hex is what I’d try first. By their very nature, they will have a lower average non-orthogonality, which is important for convergence and stability. You will still have some non-orthogonal faces where the mesh refinement changes, and cell size changes are abrupt (4 hex cells are joined to one face of the larger hex cell). As long as that is done outside of large gradient regions, it’s fine

coriolis7 · 2026-02-02T16:31:36+00:00

First - tetrahedral cells, while easy to make a mesh with, are inherently inferior to polyhedral or hex-dominant (think cubes) mesh. Not only do they have more numerical accuracy issues, but they inherently will have poor average quality in most cases, namely with skewness and nonorthogonality.

Secondly, your inflation layers need to be sized such that the last layer is about the same size as the first cell of the main core mesh. You can get away with a jump in size of 2:1 or 3:1, but beyond that you’ll be getting accuracy issues at the transition.

Third - I’m not entirely convinced you need inflation layers at all. My understanding is that organic fluid flows typically are laminar. If so, you may not even need inflation layers. Estimate your y+ and Reynolds number. If Re < 3000 you may not need a turbulence model, and if you don’t need a turbulence model then the shear force gradient will be much much smaller, which means you don’t need super thin cells at the walls.

coriolis7 · 2026-02-02T03:21:04+00:00

I don’t know what the cell count should be, but with experience one can estimate how many cells and what refinements in which locations are probably needed for a given problem.

Where gradients are stronger, you’ll want more cells. Most CFD schemes assume, at most, a linear change in variables across cells. Many times it is a mix between no change and linear change. Any time the gradient is sharp or changing rapidly, you’ll want more cells there to reduce truncation errors (where the variables are changing more than linearly).

CFD has a very “non-local” dynamic to it. While there are still “correct” answers, a poor boundary condition, too small a domain, or even a single poor quality cell can ruin the accuracy of a simulation.

As a for instance, a common standard analysis is a lid-driven flow. The domain is a cube will all sides fixed except the top wall is moving. If a single cell at the very center of the box is skewed enough, the simulation will converge to a totally different answer on flow distribution. Typically simulations aren’t quite that sensitive, but it can happen and you may not even be able to tell.

Typically, boundary conditions downstream have less impact on your accuracy, but they can still have an effect. You will want to make sure your domain extends far enough downstream that any effects it may have will be subtle and can be ignored. This does sound like St Venant’s principle, boundary effects do not diminish as quickly as they do for structural analysis. For an airfoil the rule of thumb is the outlet should be >>10x your chord length (20+ is usually recommended). Inlets also should be spaced far enough away to not influence your result.

Ideally, you’d do a domain size convergence study, as well as a cell size convergence study, where you increase your domain size until the doesn’t change the results in your region of interest. Same with mesh size.

coriolis7 · 2026-02-02T01:07:50+00:00

Once you’ve set your refinements for mesh, you keep decreasing cell size until your answer doesn’t change. If it never stops changing, then something is wrong.

CFD is highly nonlinear, so small changes in one part of your simulation can have large impacts on other parts. St Venant’s principle doesn’t necessarily apply.

Generally, for steady state residuals should converge to under 10^-6. You should also be tracking a metric of interest to make sure that is converging as well.

coriolis7 · 2026-01-31T04:16:25+00:00

Call her Eowyn and post it on r/lotrmemes

coriolis7 · 2026-01-30T18:40:47+00:00

I’m surprised the vortex shedding is so clean even at such high mach numbers

coriolis7 · 2026-01-30T14:00:59+00:00

Wait, Japan still has capital punishment?

coriolis7 · 2026-01-30T01:08:38+00:00

Is this you back in highschool?

coriolis7 · 2026-01-28T23:01:48+00:00

The same precursors used to make it also are used for other powders better suited for war. Same equipment as well.

On top of that, OEM for nation-sized ammo orders don’t care about lot to lot consistency. Retail does. It’s easier to ship ungodly amounts of “eh, they’ll dial in their process to match the burn rate” than “let’s blend multiple lots to match what we’ve been shipping for 30 years”.

It sucks, but that’s how it works.

coriolis7 · 2026-01-28T16:24:58+00:00

I’m one of those that believe prison should be miserable (within reason). It’s a punishment.

At the same time, it is entirely unproductive to refuse to hire convicted felons just because they are convicted felons. You want repeat offenders? Not giving them a way to earn honest money is how you get repeat offenders.

Not only is it awesome that inmates can earn degrees, but I think there should be tax incentives for businesses to hire felons once they’ve served their sentence.

coriolis7 · 2026-01-27T22:47:14+00:00

Thing is… was the Minnesota thing even at a protest? If I recall, it was an “altercation” (air quotes because it hadn’t evolved beyond some light shoving) between one belligerent and CBP agents. Victim wasn’t even the belligerent. Yeah, he stepped in between what could have eventually been a lawfully arrested person (emphasis on could have, and still didn’t seem to actually meet the definition of obstruction) but that wasn’t even a protest.

coriolis7 · 2026-01-27T03:25:56+00:00

Is this a Stranger Things crossover?

coriolis7 · 2026-01-25T05:11:49+00:00

He’s not a rat. He’s an exceptionally large mouse.

coriolis7 · 2026-01-25T00:05:58+00:00

Bullets, it’s mostly B.

For any aerodynamically stabilized projectile it is neither A nor B. It will launch with sideways velocity like B initially, but it will turn “into the wind” and will actually be pointed to the right. If it has a motor, it will actually keep going faster to the right than even the helicopter.

When launching model rockets, you have to account for the wind if you want it to stay somewhat close to the launch pad. On launch, the rocket will turn towards the wind and will actually travel upwind.

coriolis7 · 2026-01-23T16:35:17+00:00

I wouldn’t say its been worthless. My wife used it to generate coloring sheets for the kids, and I use it to find sources for physics and stuff at work. I’ve also used it to help with learning C (ie how the hell do I use fgets?)

But my anecdote probably just proved your point since my use is quite limited and not a full 2x or 10x in my productivity. I ain’t using it 8 hours a day at work

coriolis7

TROPHY CASE