Author Topic: EM Drive Developments - related to space flight applications - Thread 3  (Read 1879642 times)

Offline frobnicat

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FYI

Roughly 8 X 10^8 Nm/kg is the minimum needed from Earth orbit to Pluto's ..

Just passing thru so maybe not relevant ?

Which amounts to a deltaV of 40km/s, the claimed short transit time (unadulterated 1N/kW => 12.4 month) amounts to deltaV of 360km/s, square the ratio for energy density comparison... at those levels trajectories becomes quite straight and Sun's gravity well (departure from LEO) becomes a marginal gentle slope.
« Last Edit: 07/18/2015 03:28 PM by frobnicat »

Offline Ricvil

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https://en.m.wikipedia.org/wiki/Maxwell_stress_tensor




EM DRIVE FORCE vs. TIME



The stress component σxx  is compressive at both bases.  Therefore the force obviously points in different directions:  at the big base it points in the direction from the small base to the big base while at the small base it points in the direction from the big base to the small base.  From the interior to the surface in both cases. 



I think Is the opposite Rodal.

See in wikipedia, the force density f=div(sigma) +Poynting vector term, and the expressions "negative pressure" & "negative direction"
If one want to calculate the force on a object ( the big end for example), one must enclose the object by a surface of outward normal orientation ,to use gauss divergence theorem, and to obtains a surface integral.
For the big end, the normal of the enclosing surface, where the sigmas are non zero ( inside cavity), is pointing to -x direction, so when integrating the force on the big end , we have dfx=sigmaxx.(-ds).
Then the force acting on big end is positive and pointing to positive x direction, because sigmaxx is negative.
For the small end is the inverse.
This is natural. The big end and the small and are both reflecting the waves backward to cavity.

« Last Edit: 07/18/2015 03:55 PM by Ricvil »

Offline TheTraveller

FYI

Roughly 8 X 10^8 Nm/kg is the minimum needed from Earth orbit to Pluto's ..

Just passing thru so maybe not relevant ?

Which amounts to a deltaV of 40km/s, the claimed short transit time (unadulterated 1N/kW => 12.4 month) amounts to deltaV of 360km/s, square the ratio for energy density comparison... at those levels trajectories becomes quite straight and Sun's gravity well (departure from LEO) becomes a marginal gentle slope.

Eagleworks Dr. White's Pluto data is attached.

Enjoy.
« Last Edit: 07/18/2015 03:47 PM by TheTraveller »
"As for me, I am tormented with an everlasting itch for things remote. I love to sail forbidden seas.
Herman Melville, Moby Dick

Offline CW

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Proper scientific study is small steps with valid results. Hyperbole like heavy lifters, mass production and pluto missions are, well...hypobole. Yes, I am a sales and marketing expert with a very technical background. I would humbly suggest we leave sales & marketing/blue sky plans to another forum. Not that we can't deam, however...
The big difference between Internet Forums and sales and marketing in the real commercial world is that in an Internet Forum sales and marketing hyperbole can be done posting anonymously in various forums, so when the dreams of honest followers and believers that have invested their time in such hopes are eventually dashed, there will be no loss of reputation of the anonymous poster, who can then quietly disappear and assume another moniker.

In the stock market it is well known that financial message boards are full of anonymous posters that use financial message board forums to "pump and dump" stocks playing on the hope and fear of honest stock market investors:  https://en.wikipedia.org/wiki/Pump_and_dump

I find it laughable to even talk about spaceships and missions without even having an established and widely accepted, repeatable science behind it. It baffles me to read time and again that Mr. Shawyer is sitting on the holy grail of propulsion, but doesn't seem to be willing to publicly present a current device that produces Newtons of thrust, as claimed by him since quite a long time now.

Show it, or shut it.

@Rodal
Great work so far, good doctor :)
Reality is weirder than fiction

Offline rq3

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SeeShells: notice

<< to my specs ....and N2 filled >> chirality and nitrongen inversion at room temperature

Pure N2 in my cavity can convert to chiral molecules???????????
Are you using polymer O Rings ?

Was planning on using space qualified (no outgassing) silicon O rings.

I've never heard of a silicon "O" ring. If there is such an animal, it would be fantastically brittle.

If you actually mean "silicone", they may be low outgassing and "space qualified" but will be VERY gas permeable. Perhaps almost as bad as polytetrafluoroethylene.

You might want to contact a company like Chomerics, who specializes in conductive "O" rings for applications like this.

Good luck.

So quick to attach! Apologies, dropped the e.

Sometime my big fingers on a small mobile phone screen gens typos or my old eyes miss errors.

Will probably be a gasket. Have a few mates in the aerospace industry that are working on the best material to provide good electrical conductivity and good sealing, with min outgassing using N2 inside the frustum at 1/2 atmo pressure.

You have any suggestions?

Not an attach (attack?), just pointing out that you might have issues with the silicon "O" ring. There are indeed metal seals used for ulta-high vacuum work, and brittle seals made of all manner of strange materials are used for diamond cells at ultra-high pressures.

Then assuming that you actually meant "silicone", I merely point out that silicone tends to be quite gas permeable. Nor are silicones inherently electrically conductive.

I already provided a suggestion, in the form of just one reference to just one firm that does this kind of thing for a living.

If you'd like another suggestion, here's one. Do not use a gasket, as in a compressed sheet of conductive material between the walls and the end plate(s). You want the end plate(s) to be in close physical AND electrical contact with the wall, ideally with a knife-edge compression seal on the interior wall of the "O" ring groove. This is why microwave assemblies have multiple screws/bolts spaced MUCH less than a wavelength apart. They provide multiple, sub-wavelength, electrical paths AND provide even compression to the seal.

Personally, I would fabricate a template out of machineable wax, and then use the result to electroform the required frustum. This would allow the fabrication of integrated "O" ring grooves, and if done properly would yield a frustum of any required wall thickness with an interior surface already silver plated (as part of the electroforming process) and a surface finish of 2 uinch or better. It would also provide dimensional tolerances much tighter than fabricating the frustum out of OFHC copper sheet or plate, and would be potentially much less expensive as an added benefit.
« Last Edit: 07/18/2015 04:17 PM by rq3 »

Offline Rodal

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https://en.m.wikipedia.org/wiki/Maxwell_stress_tensor




EM DRIVE FORCE vs. TIME



The stress component σxx  is compressive at both bases.  Therefore the force obviously points in different directions:  at the big base it points in the direction from the small base to the big base while at the small base it points in the direction from the big base to the small base.  From the interior to the surface in both cases. 



I think Is the opposite Rodal.

See in wikipedia, the force density f=div(sigma) +Poynting vector term, and the expressions "negative pressure" & "negative direction"
If one want to calculate the force on a object ( the big end for example), one must enclose the object by a surface of outward normal orientation ,to use gauss divergence theorem, and to obtains a surface integral.
For the big end, the normal of the enclosing surface, where the sigmas are non zero ( inside cavity), is pointing to -x direction, so when integrating the force on the big end , we have dfx=sigmaxx.(-ds).
Then the force acting on big end is positive and pointing to positive x direction, because sigmaxx is negative.
For the small end is the inverse.
This is natural. The big end and the small and are both reflecting the waves backward to cavity.

For a second order tensor, one defines the unit cube as follows:



Stress component sign conventions are as follows.
For a normal stress: positive (negative) if it produces tension (compression) in the material.
For a shear stress: positive if, when acting on the + face identified by the first index, it points
in the + direction identified by the second index. Example: τxy is + if on the +x face it points
in the +y direction

It is the same in Continuum Mechanics, comprising solid and fluid mechanics, electromagnetism and General Relativity.

By definition, equilibrium is embedded in the definition of the stress tensor unit cube: hence its symmetry. 
For example: on one face of the unit cube above you have σ11.  On the opposite side of the cube you have σ11 acting in the OPPOSITE direction.  If one face σ11 is applying pressure on the surface, on the opposite side you hve σ11 applying pressure on the other surface.   There is no need to refer to the equilibrium equations involving the flux (Poynting vector), divergence etc., to ascertain the meaning of the stress tensor components.  When analyzing a solid body, or a fluid problem, if you are told that the normal component of stress is negative you know that means COMPRESSION.


As in Continuum Mechanics, it is easy:

a negative stress tensor diagonal component means a compressive pressure on the unit cube (inward presssure on the surface)

a positive stress tensor diagonal component means a tension on the unit cube  (outward tension on the surface)

Hence a negative value of the stress tensor means that the unit cube is in compression.

Both the big and small bases are in a state of compression normal to the surface.

The direction of the force is a compressive pressure pushing against the surface.

It is the same situation as a compressive force acting on a solid surface, or a fluid.  Negative stress means forces pressing on the surface, NOT pulling on them. 

"This is natural" and well understood:  the electromagnetic fields produce a pressure on the surfaces, certainly they are not pulling on the surfaces, they are applying pressure on them.  Hence the direction of the forces were correctly defined.




« Last Edit: 07/18/2015 05:12 PM by Rodal »

Online RonM

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FYI

Roughly 8 X 10^8 Nm/kg is the minimum needed from Earth orbit to Pluto's ..

Just passing thru so maybe not relevant ?

Which amounts to a deltaV of 40km/s, the claimed short transit time (unadulterated 1N/kW => 12.4 month) amounts to deltaV of 360km/s, square the ratio for energy density comparison... at those levels trajectories becomes quite straight and Sun's gravity well (departure from LEO) becomes a marginal gentle slope.

The rocket equation and delta-v do not apply in this situation. There is no propellent being expelled. This is simple kinematics.

x=1/2*a*t2, solve for time.

Calculate for half the distance to account for acceleration and double the time to account for deceleration.

A drive that produces 1 N for every 1000 W pumped into it is not a perpetual motion machine. It's not even efficient. The advantage is not having to carry and spew propellent out the back.

Whether or not this is real or science fiction is the question.

Offline aero

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Hello,

I've been tinkering around with meep on Debian, and I have a procedure for installing meep 1.3 on Debian 8, which packages the older meep 1.2 by default:

# apt-get install h5utils openmpi-bin
# apt-get build-dep meep-openmpi
This installs everything we'll need for dependencies, but by default ./configure won't find libhdf5, so we need to do this.
# cd /usr/lib/x86_64-linux-gnu/
# ln -s libhdf5_openmpi.so libhdf5.so
# export CPPFLAGS="-I/usr/include/hdf5/openmpi"
Now we can head back to our source-building directory and get on with it.
# wget http://ab-initio.mit.edu/meep/meep-1.3.tar.gz
# cd meep-1.3
# ./configure --with-mpi --prefix=$HOME
Obviously this assumes $HOME is in your path, you can install it wherever.
# make
# make install


If anyone has meep files that they lack the time to process on their own hardware, I have a reasonably powerful system that can crank through a 12-thread run of NSF-1701.ctl in about 40 minutes, as well as a web server (nginx) running so I can package the output files (h5, csv, png, whatever) and provide a link.  I'm a sysadmin by trade, and the last physics I took was Newton, so I don't yet know enough to write my own Scheme scripts for meep and get any meaningful output.

Yes, for those wondering, I'm also tidux on /r/emdrive.

40 minutes hmm. That means you could change line 230 of the control file to (define cc 64) instead of (define cc 32), and make the run in less than an hour and a half. Running for 64 cycles would give you .h5 files containing 33 + complete cycles of data beyond anything Dr. Rodal has had to work with to date. That is, if you don't change line 251, which gives the start of output of time slices. Now, he may not want to work with 330+ time slices, so need to get him to make the request, but I for one would like to see the continuation of the net force graph he posted above. I think he has or is developing a standardized data request form/file which explains what he wants and should be included with the output data when you post it, but I could be mistaken.

You might want to run a few little timing tests to identify the correct number of threads to use. There is a trade in meep between the logic of the lattice bunch interfaces (read that processor interfaces, but in meep code) and the straight number crunch of propagating the numbers within the bunches. Each model is optimized with a different trade unless the computational model problem is so very large that it just generally needs more machine. The NSF-1701 model is not that large.
Retired, working interesting problems

Offline deltaMass

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FYI

Roughly 8 X 10^8 Nm/kg is the minimum needed from Earth orbit to Pluto's ..

Just passing thru so maybe not relevant ?

Which amounts to a deltaV of 40km/s, the claimed short transit time (unadulterated 1N/kW => 12.4 month) amounts to deltaV of 360km/s, square the ratio for energy density comparison... at those levels trajectories becomes quite straight and Sun's gravity well (departure from LEO) becomes a marginal gentle slope.

The rocket equation and delta-v do not apply in this situation. There is no propellent being expelled. This is simple kinematics.

x=1/2*a*t2, solve for time.

Calculate for half the distance to account for acceleration and double the time to account for deceleration.

A drive that produces 1 N for every 1000 W pumped into it is not a perpetual motion machine. It's not even efficient. The advantage is not having to carry and spew propellent out the back.

Whether or not this is real or science fiction is the question.
I'm sorry, but this is not even wrong. Do a simple calculation of energy in vs. energy out over time and you'll see the problem. What you mean by "is not efficient" is unclear.

Offline aero

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https://en.m.wikipedia.org/wiki/Maxwell_stress_tensor




EM DRIVE FORCE vs. TIME



The stress component σxx  is compressive at both bases.  Therefore the force obviously points in different directions:  at the big base it points in the direction from the small base to the big base while at the small base it points in the direction from the big base to the small base.  From the interior to the surface in both cases. 



I think Is the opposite Rodal.

See in wikipedia, the force density f=div(sigma) +Poynting vector term, and the expressions "negative pressure" & "negative direction"
If one want to calculate the force on a object ( the big end for example), one must enclose the object by a surface of outward normal orientation ,to use gauss divergence theorem, and to obtains a surface integral.
For the big end, the normal of the enclosing surface, where the sigmas are non zero ( inside cavity), is pointing to -x direction, so when integrating the force on the big end , we have dfx=sigmaxx.(-ds).
Then the force acting on big end is positive and pointing to positive x direction, because sigmaxx is negative.
For the small end is the inverse.
This is natural. The big end and the small and are both reflecting the waves backward to cavity.

While the solar sail analogy is attractive, the antenna standing in for the sun and augmented by resonant reflections, that analogy breaks down in the face of experiment. EW data shows the thrust from large end to small end, as I understand it. That is not consistent with the solar sail analogy.
Retired, working interesting problems

Offline apoc2021

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I find it laughable to even talk about spaceships and missions without even having an established and widely accepted, repeatable science behind it. It baffles me to read time and again that Mr. Shawyer is sitting on the holy grail of propulsion, but doesn't seem to be willing to publicly present a current device that produces Newtons of thrust, as claimed by him since quite a long time now.

Show it, or shut it.

@Rodal
Great work so far, good doctor :)

Couldn't agree with you more. Between 2006 and 2015 we have nine full years, during which any number of videos, live streams, supplementary demonstrations, etc etc could have been published and discussed. Videos of higher quality than the 2006 video could be made with a run of the mill cell phone. Opportunities to engage with an interested community of potential supporters are abundant. Many areas of experimental inquiry (such as running the demonstrator engine in a vacuum) still exist. The fact that none of these have happened suggests ... nothing good?

I also feel compelled to state what I think is fairly obvious to anyone familiar with business: SPR is not a robust commercial enterprise working diligently towards refining and licensing its technology. SPR is simply a legal entity - a corporation created to represent the efforts and house the expected IP of what is in effect a one-man operation. Any of us could do the same with a few hundred dollars and a few hours of time filling out web forms. So we are best reminded when discussing the volition of SPR Ltd that we are in fact discussing the volition of Roger Shawyer.

I have nothing against Roger Shawyer, the man. It seems he came across this phenomenon, tried to refine, explain and commercialize it, and for whatever reasons financial or intellectual has been unable to do so effectively. Where I take issue, however, is in the disconnect between his claims and his actions. He claims to hold in his hands the power to change and improve the world - to fundamentally alter the human experience for the better - and yet he seems to feel no moral imperative to do so. If it was me, I would shout from every mountaintop until the world heard. Including this one here, full of interested and capable theorists, experimentalists and supporters. I find it telling that this isn't happening.

Offline aero

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Dr. Rodal - Looking at your net force curves and trying to see a graphic straight line average, it looks to me like the average force for that cycle is ~ -0.00045x10-12 N and changing (going more negative) at a rate of about 0.0002 x 10-12 N/cycle. Plug in period in seconds for cycle and you see that that is not to shabby. Well, maybe not great at about -0.5 micro-Newton/second. We need to see a larger number of cycles, either all together or intermittently from a longer simulation run, But You Need to Request Such a Run, clearly and unambiguously. If you would like to ask it of me, I will interface for you to make sure that someone running my model understands your request.
« Last Edit: 07/18/2015 05:38 PM by aero »
Retired, working interesting problems

Offline Flyby

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I have nothing against Roger Shawyer, the man. It seems he came across this phenomenon, tried to refine, explain and commercialize it, and for whatever reasons financial or intellectual has been unable to do so effectively. Where I take issue, however, is in the disconnect between his claims and his actions. He claims to hold in his hands the power to change and improve the world - to fundamentally alter the human experience for the better - and yet he seems to feel no moral imperative to do so. If it was me, I would shout from every mountaintop until the world heard. Including this one here, full of interested and capable theorists, experimentalists and supporters. I find it telling that this isn't happening.

I could not have said it better...

There is nothing wrong with having dreams and hopes, as long they are in the domain of the achievable.
Once you go beyond that borderline you wander into the land of illusions and air castles...

Dreaming about interplanetary travel only makes sense, once we have the general established evidence that the EMdrive really works.
Once that confirmed, then it make sense of dreaming up the needed improvements to make interplanetary or even interstellar voyages...

In the current situation - with only shallow indications that something might be going on - it actually hurts the credibility of the EMdrive, instead of helping it....

so.. focus, focus and focus on getting down to earth results instead of fabricating an illusive future....


BTW, we're well over the 200 pages.... hmmm... time to swap to thread #4?
« Last Edit: 07/18/2015 05:39 PM by Flyby »

Online RonM

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FYI

Roughly 8 X 10^8 Nm/kg is the minimum needed from Earth orbit to Pluto's ..

Just passing thru so maybe not relevant ?

Which amounts to a deltaV of 40km/s, the claimed short transit time (unadulterated 1N/kW => 12.4 month) amounts to deltaV of 360km/s, square the ratio for energy density comparison... at those levels trajectories becomes quite straight and Sun's gravity well (departure from LEO) becomes a marginal gentle slope.

The rocket equation and delta-v do not apply in this situation. There is no propellent being expelled. This is simple kinematics.

x=1/2*a*t2, solve for time.

Calculate for half the distance to account for acceleration and double the time to account for deceleration.

A drive that produces 1 N for every 1000 W pumped into it is not a perpetual motion machine. It's not even efficient. The advantage is not having to carry and spew propellent out the back.

Whether or not this is real or science fiction is the question.
I'm sorry, but this is not even wrong. Do a simple calculation of energy in vs. energy out over time and you'll see the problem. What you mean by "is not efficient" is unclear.

Oh, I see what you mean. That is about 180 times too high. The upper theoretical limit would be about 0.0056N/kW. An actual drive would not be that good. This puts the EM drive at about an order of magnitude less thrust than ion propulsion.

I really don't see the point of the posts describing again and again how shorter travel times would be with a working EMDrive tech. Everyone knows wonderful things could be achieved. There is no scientific value in such "projections".

Offline Rodal

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FYI

Roughly 8 X 10^8 Nm/kg is the minimum needed from Earth orbit to Pluto's ..

Just passing thru so maybe not relevant ?

Which amounts to a deltaV of 40km/s, the claimed short transit time (unadulterated 1N/kW => 12.4 month) amounts to deltaV of 360km/s, square the ratio for energy density comparison... at those levels trajectories becomes quite straight and Sun's gravity well (departure from LEO) becomes a marginal gentle slope.

The rocket equation and delta-v do not apply in this situation. There is no propellent being expelled. This is simple kinematics.

x=1/2*a*t2, solve for time.

Calculate for half the distance to account for acceleration and double the time to account for deceleration.

A drive that produces 1 N for every 1000 W pumped into it is not a perpetual motion machine. It's not even efficient. The advantage is not having to carry and spew propellent out the back.

Whether or not this is real or science fiction is the question.
I'm sorry, but this is not even wrong. Do a simple calculation of energy in vs. energy out over time and you'll see the problem. What you mean by "is not efficient" is unclear.

Oh, I see what you mean. That is about 180 times too high. The upper theoretical limit would be about 0.0056N/kW. An actual drive would not be that good. This puts the EM drive at about an order of magnitude less thrust than ion propulsion.
Yes, to bring this back to reality, NASA reported 0.0011 N/kW in vacuum, or about 20% of that " upper theoretical limit ".

Shawyer's first EM Drive patent was in the late 1980's but he has never reported any thrust figures for a test in vacuum.  Neither has Yang reported any thrust figures for a test in vacuum.

Offline SeeShells

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Ready for a cold drink and a break before I head back out the the shop. Look what happens while I'm away, someone is claiming the moon while sitting on this ball of mud looking at it.

I was asked what I dream about and I said looking up at the sky and seeing that same twinkling light I saw on a cold night  in October of 1957. Others dream of other things not quite as noble but holding up the seedier side of humanity to see. It makes me very sad to see such a noble deed of trying to take man to the stars commercialized with blubbering rubbish.

I will say this before I head out to the shop again. I choose to dream, for humanity. Just like I've said here and on other sites. 

We should put aside the Ronco toy salesmanship claims, (like putting the mute on the TV during a bad commercial) letting them postulate that they do own the moon, while we know better.

I have work to do... >pressing mute.

Shell


Offline Rodal

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Dr. Rodal - Looking at your net force curves and trying to see a graphic straight line average, it looks to me like the average force for that cycle is ~ -0.00045x10-12 N and changing (going more negative) at a rate of about 0.0002 x 10-12 N/cycle. Plug in period in seconds for cycle and you see that that is not to shabby. Well, maybe not great at about -0.5 micro-Newton/second. We need to see a larger number of cycles, either all together or intermittently from a longer simulation run, But You Need to Request Such a Run, clearly and unambiguously. If you would like to ask it of me, I will interface for you to make sure that someone running my model understands your request.

This was my analysis:

<<6) A fitted model of the time variation of the force (with excellent R^2 = 0.999981), shows that the present Finite Difference model (from which the force has been computed at the last two cycles ending at 0.013 microseconds from the time at which the Microwave feed was turned on), would have to be marched forward for 1,000 times longer, to a total of 10 microseconds, for the force to be magnified by the calculated exponential growth to a value of 10 microNewtons (for an inputPower of 43 watts).   Given the fact that the present Meep model takes an hour to run on a good PC modern computer, 1,000 hours of computer time represents over 41 days of computing time.  Thus running the Meep model to steady state is impractical.  Rather than using a supercomputer to perform such a computation, I suggest to use an implicit (unconditionally stable) Finite Difference model in time (rather than the explicit time difference model presently used that is subject to stability problems that limit the maximum finite difference time step).  Such implicit finite difference models are well known (I developed a version of them in my PhD thesis 35 years ago) and can be run much faster than explicit FD models.  There are also numerous alternative numerical schemes that are more accurate than Finite Differences.>>

At the present number of time steps, the solution at 0.013 microseconds has only achieved 10^(-15) Newton, that is 9 orders of magnitude (1 billion times) smaller than 1 microNewton (10^(-6) Newtons).  It will take a lot of cycles even with an exponential rate of increase to grow by 1 billion times.  I calculate that the model would have to reach about 10 microseconds, in other words, that is 1,000 times greater, as discussed above, to reach 10 microNewtons.

Ten microseconds is not that much time to achieve steady state.  Before arriving at this conclusion someone else in this forum, based on experience, had predicted it would take over 1 microsecond.  So that is consistent.
Ten microseconds is not even appreciable in the NASA time response plots, which have a duration of ~40 seconds.

« Last Edit: 07/18/2015 08:50 PM by Rodal »

Offline Ricvil

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https://en.m.wikipedia.org/wiki/Maxwell_stress_tensor




EM DRIVE FORCE vs. TIME



The stress component σxx  is compressive at both bases.  Therefore the force obviously points in different directions:  at the big base it points in the direction from the small base to the big base while at the small base it points in the direction from the big base to the small base.  From the interior to the surface in both cases. 



I think Is the opposite Rodal.

See in wikipedia, the force density f=div(sigma) +Poynting vector term, and the expressions "negative pressure" & "negative direction"
If one want to calculate the force on a object ( the big end for example), one must enclose the object by a surface of outward normal orientation ,to use gauss divergence theorem, and to obtains a surface integral.
For the big end, the normal of the enclosing surface, where the sigmas are non zero ( inside cavity), is pointing to -x direction, so when integrating the force on the big end , we have dfx=sigmaxx.(-ds).
Then the force acting on big end is positive and pointing to positive x direction, because sigmaxx is negative.
For the small end is the inverse.
This is natural. The big end and the small and are both reflecting the waves backward to cavity.

For a second order tensor, one defines the unit cube as follows:



Stress component sign conventions are as follows.
For a normal stress: positive (negative) if it produces tension (compression) in the material.
For a shear stress: positive if, when acting on the + face identified by the first index, it points
in the + direction identified by the second index. Example: τxy is + if on the +x face it points
in the +y direction

It is the same in Continuum Mechanics, comprising solid and fluid mechanics, electromagnetism and General Relativity.

By definition, equilibrium is embedded in the definition of the stress tensor unit cube: hence its symmetry. 
For example: on one face of the unit cube above you have σ11.  On the opposite side of the cube you have σ11 acting in the OPPOSITE direction.  If one face σ11 is applying pressure on the surface, on the opposite side you hve σ11 applying pressure on the other surface.   There is no need to refer to the equilibrium equations involving the flux (Poynting vector), divergence etc., to ascertain the meaning of the stress tensor components.  When analyzing a solid body, or a fluid problem, if you are told that the normal component of stress is negative you know that means COMPRESSION.


As in Continuum Mechanics, it is easy:

a negative stress tensor diagonal component means a compressive pressure on the unit cube (inward presssure on the surface)

a positive stress tensor diagonal component means a tension on the unit cube  (outward tension on the surface)

Hence a negative value of the stress tensor means that the unit cube is in compression.

Both the big and small bases are in a state of compression normal to the surface.

The direction of the force is a compressive pressure pushing against the surface.

It is the same situation as a compressive force acting on a solid surface, or a fluid.  Negative stress means forces pressing on the surface, NOT pulling on them. 

"This is natural" and well understood:  the electromagnetic fields produce a pressure on the surfaces, certainly they are not pulling on the surfaces, they are applying pressure on them.  Hence the direction of the forces were correctly defined.



Ok.

Then using the  notation /convention above, that is what I'm trying to say.

Offline Rodal

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Yes, that drawing agrees with the force in my plots, positive at the small base, and negative at the big base.  Forces are directed from the interior to the bases, acting as pressure.  (Aero chose x as the longitudinal axis, oriented positive pointing from the big base to the small base).


EDIT: Evidently the discrepancy was because you had the x axis oriented posiive from the small base to the big base.

Aero chose the x axis oriented in the opposite direction: oriented from the big base to the small base.

Aero has the big base at the left, and the small base at the right
« Last Edit: 07/18/2015 08:56 PM by Rodal »

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