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Jul
30
comment How does a converging-diverging nozzle not violate conservation of energy?
It still doesn't explain why it speeds up in supersonic flow. We know in subsonic flow, it reaches its narrowest point and then as it expands it slows down. Using your car analogy, people floor it through the narrow point and then as the road opens up again, they slow down. But supersonic this isn't the case -- pressure is always decreasing, but why does it always decrease? Why does it only do this for supersonic flow and not subsonic? I'm not trying to give you a hard time, just trying to improve the answer :)
Jul
30
comment How does a converging-diverging nozzle not violate conservation of energy?
This answer doesn't say anything wrong, but I think it's missing the key question -- why does flow accelerate with increasing area while supersonic and how is that not a violation of conservation of energy.
Jul
28
comment What causes this IR emission in UV 365nm LED?
As @dmckee pointed out, green lasers are not as simple as red. Check out this question: physics.stackexchange.com/questions/173677/… It is likely something similar here.
Jul
24
comment Could a Jumbo Jet aircraft fly on paper wings?
And also -- are you referring to a paper skin, ie. the wing surface itself, or the entire wing structure (skin, ribs, spars, etc)?
Jul
24
comment Could a Jumbo Jet aircraft fly on paper wings?
How thick of a paper wing are you talking about? The wings still have to hold the weight of the aircraft while in the air so a single sheet of paper would never work. But paper stacked 4 feet thick like a regular wing? Maybe. The wing spar does a lot more than hold the weight of the things attached to the wing.
Jul
24
comment 9-point stencil “equivalent” for advection equation
I hope you already had all of those equations typed up somewhere and could just copy them over. Otherwise, that's way too much work MathJax'ing an answer!
Jul
23
comment If you compress air to a large enough pressure do new molecules form that have a large activation energy?
What do you mean molecules with a large activation energy? Chemical reactions have an activation energy, but I'm not familiar with that phrase in regards to molecules. Do you mean a large energy requirement to break the new molecules apart again, like pressing carbon into diamonds?
Jul
19
comment No diffusion term in conservation of mass in Navier-Stokes equations?
@csss You can definitely do that. But in a pure fluid, there is no differences in mass. Water molecules all weigh the same, so there's no "layers" of different masses. I left an answer explaining that reasoning.
Jul
19
comment No diffusion term in conservation of mass in Navier-Stokes equations?
@KyleKanos You can do it mathematically, I just left and answer doing it descriptively.
Jul
19
answered No diffusion term in conservation of mass in Navier-Stokes equations?
Jul
19
comment No diffusion term in conservation of mass in Navier-Stokes equations?
@css I'm not sure if you've had the gas dynamics/statistical mechanics background, but if you think about it molecularly where you have exactly identical molecules, as the "diffuse," the new state is indistinguishable from the old state because all of the molecules are the same mass. If the molecules are different masses, then as they diffuse the states will be different and that's why the multi-component equations do include a mass diffusion term.
Jul
19
comment No diffusion term in conservation of mass in Navier-Stokes equations?
All of this is right, but as Kyle pointed out in his answer, is only true for a single-component flow. When you have multi-component (say, hydrogen and air), there is a diffusion term in the species mass transport equations. Although strictly speaking, the total mass density equation can be viewed as redundant because everything is defined by the partial mass equations. So everything is good, but it's important to point out that multi-component mass conservation does have diffusion because you can actually have gradients in components, unlike the pure case where all "air" is indistinguishable.
Jul
17
answered What is the Proper way to determine overall velocity in a pipe?
Jul
16
comment How to understand an equation physically?
As @Alex's answer recommends, it's best to start thinking about the cause-effect in the physical world around you and what those things would look like in equation-form. And like my answer on the other question says, it helps to look at the equations and try to identify what they represent in the physical world. Start simple and small and start building up that logic/intuition!
Jul
16
comment How to understand an equation physically?
So this might be more advanced than the level you are looking for, but check out this question and answer about techniques for understanding equations. Once you start to get a feel for what kinds of terms appear in physics equations (even something like $F = ma$ is a differential equation), you can start to wrap your head around connecting the sometimes-complicated math to the physical world.
Jul
9
comment What granular/colored material suspends itself in an enclosed container filled with water regardless of pressure, gravity and buoyancy?
You might have a hard time with that -- the reason moving through a smoke-filled chamber will create disturbances is because there's entrainment of non-smoke filled air into the chamber. Imagine driving your car through a dense fog -- there isn't any noticeable features behind you because the medium is homogenized. You'll have the same issue in a finite tank of water with a dye or tracer. What are you trying to accomplish by moving the body rather than the fluid?
Jul
9
comment What granular/colored material suspends itself in an enclosed container filled with water regardless of pressure, gravity and buoyancy?
Also, is it submerged or will it run on the surface of the water?
Jul
9
comment What granular/colored material suspends itself in an enclosed container filled with water regardless of pressure, gravity and buoyancy?
What kind of timescales are needed? For instance, if you are only running an experiment over a matter of seconds, is it okay if your tracer would eventually settle down but it took X minutes to do it? Or will your body by moving for long periods? Will you need to wait until the tank is 100% still (taking hours or days to settle)?
Jul
6
comment Fluids Dynamics - Transmission Oil Dilution Problem
It's also much more a math problem than a physics problem. You're just looking at a discrete mixing problem. This could be about a transmission, or about grain in a silo, or milk on a store shelf, etc.
Jul
6
comment Symmetry considerations in Plane Poiseuille Flow
Well, I left it intentionally vague (and as a comment because of that) because I want you to think more about the assumptions that go into the problem. By the time you get to the "Because of the translational symmetry" point of the derivation, several very important assumptions have been made. These assumptions have an influence on what happens next.