Earlier there was a point made about buoyancy lift from the heated frustum. I did some calculations in an attempt to characterize buoyancy for Shell's EM drive configuration, (frustum, tuning cylinder, and two wave guides) since I do have the needed dimensional data in my meep model.I calculate the volume to be less than 4.490367E-02 m3. Less than because the wave guides are cut at an angle by the frustum but I simply used the length of the long side. So there are two small wedges that are each counted twice. So I found standard atmosphere data in the engineering toolbox and calculated the air density at constant volume and pressure using the Ideal gas law. I used the calculator here:http://www.ajdesigner.com/idealgas/ideal_gas_law_density.php#ajscrollI did this calculation at both standard temperature and 100 degrees C above standard. That is pretty hot, the temperature of boiling water. Oh, out of deference for Shells' high altitude in the mountains, I used 2000 meter data from the standard atmosphere. With the above, I calculated buoyancy force = 0.1181471225 newtons, that's 118 mn and certainly not something that can be neglected. If someone is up to the task of checking my numbers, I'm sure it would be appreciated.
With the above, I calculated buoyancy force = 0.1181471225 newtons, that's 118 mn and certainly not something that can be neglected. If someone is up to the task of checking my numbers, I'm sure it would be appreciated.
Just FYI, there exists OpenFOAM, a computational fluid dynamics (CFD) suite, which appears to be fairly mature. I don't know enough about CFD to comment on whether or not OpenFOAM has the requisite solvers for modeling whichever thermal effects are present in/around a frustum.http://openfoam.org/https://en.wikipedia.org/wiki/OpenFOAMExisting solvers:http://openfoam.org/features/standard-solvers.phpI would be willing to create IGES, STEP, or STL (or a limited number of other, less common formats) files of frustums and/or experimental setups. I the software I use has the capability to "develop" solids for finite element analysis (FEA) and export it as ANSYS PREP7, and so therefore I may be able to help with that (if that is even necessary for CFD). Caveat - I have never "developed" a model for FEA analysis.
Quote from: aero on 12/17/2015 06:28 pmWith the above, I calculated buoyancy force = 0.1181471225 newtons, that's 118 mn and certainly not something that can be neglected. If someone is up to the task of checking my numbers, I'm sure it would be appreciated.That's in the range of comparable calcs done previously for similar designs. Definitely a # to throw into the simulator I have.While you're in a buoyant mood, can you look over HVAC calculators and see what the expected chimney airflow should look like for the same dimensions, depending on orientation, up, down, sideways?The model I'm trying to work out assumes thermal effects in your order of magnitude that would differ slightly depending upon orientation.These thermal effects are not bad. They are to be assumed present. What I'm trying to work out is a model that would show what kind of test protocol is required to statistically demonstrate thrust given that thrust may be a small percentage of thermal effects. i.e. it is possible to create a test protocol where the thrust is 10 mn and the boyancy is 118 mn, and you can at some point declare thrust is real. See attached simulator. If you can help produce initial estimated values and propose other error sources, that's double plus good.
Quote from: glennfish on 12/17/2015 06:39 pmQuote from: aero on 12/17/2015 06:28 pmWith the above, I calculated buoyancy force = 0.1181471225 newtons, that's 118 mn and certainly not something that can be neglected. If someone is up to the task of checking my numbers, I'm sure it would be appreciated.That's in the range of comparable calcs done previously for similar designs. Definitely a # to throw into the simulator I have.While you're in a buoyant mood, can you look over HVAC calculators and see what the expected chimney airflow should look like for the same dimensions, depending on orientation, up, down, sideways?The model I'm trying to work out assumes thermal effects in your order of magnitude that would differ slightly depending upon orientation.These thermal effects are not bad. They are to be assumed present. What I'm trying to work out is a model that would show what kind of test protocol is required to statistically demonstrate thrust given that thrust may be a small percentage of thermal effects. i.e. it is possible to create a test protocol where the thrust is 10 mn and the boyancy is 118 mn, and you can at some point declare thrust is real. See attached simulator. If you can help produce initial estimated values and propose other error sources, that's double plus good.The bigger problem is the transient vortex shedding due to fluid dynamics, interacting with the ON/OFF timing of the magnetron (http://forum.nasaspaceflight.com/index.php?topic=39004.msg1459128#msg1459128 ). The lift force due to natural convection is time-dependent and not a steady constant. The constant bouyancy calculations do not address the timing of the transient natural convection (not yet taken into account), and yet the statistical analysis is predicated on statistical analysis of the ON/OFF timing of the magnetron.
Quote from: Rodal on 12/17/2015 06:47 pmQuote from: glennfish on 12/17/2015 06:39 pmQuote from: aero on 12/17/2015 06:28 pmWith the above, I calculated buoyancy force = 0.1181471225 newtons, that's 118 mn and certainly not something that can be neglected. If someone is up to the task of checking my numbers, I'm sure it would be appreciated.That's in the range of comparable calcs done previously for similar designs. Definitely a # to throw into the simulator I have.While you're in a buoyant mood, can you look over HVAC calculators and see what the expected chimney airflow should look like for the same dimensions, depending on orientation, up, down, sideways?The model I'm trying to work out assumes thermal effects in your order of magnitude that would differ slightly depending upon orientation.These thermal effects are not bad. They are to be assumed present. What I'm trying to work out is a model that would show what kind of test protocol is required to statistically demonstrate thrust given that thrust may be a small percentage of thermal effects. i.e. it is possible to create a test protocol where the thrust is 10 mn and the boyancy is 118 mn, and you can at some point declare thrust is real. See attached simulator. If you can help produce initial estimated values and propose other error sources, that's double plus good.The bigger problem is the transient vortex shedding due to fluid dynamics, interacting with the ON/OFF timing of the magnetron (http://forum.nasaspaceflight.com/index.php?topic=39004.msg1459128#msg1459128 ). The lift force due to natural convection is time-dependent and not a steady constant. The constant bouyancy calculations do not address the timing of the transient natural convection (not yet taken into account), and yet the statistical analysis is predicated on statistical analysis of the ON/OFF timing of the magnetron.The problem of buoyancy lift and natural convection lift (drag) forces are separable. Buoyancy is an effect from inside the EM drive while convection occurs outside the drive.Under the conditions that I assume operation, buoyancy is a function of internal temperature only. Yes, temperature will change as the magnetron (heat source) is turned on and off. Characterizing the internal temperature of the drive as a function of time seems difficult to me, perhaps you have some ideas?Natural convection is also a function of temperature, in this case, the temperature of the copper. Characterizing this temperature would be easier given the internal temperature as a function of time. The problem then arises, given the temperature of the copper as a function of time (power on/off) what do we do with it? I am not ready to solve the Navier-Stokes equations, I had enough of them while in college.
Are we talking millinewtons? micronewtons? How similar is this to the 40 or so micronewtons of horizontal (torsional) Lorentz force? Remember vertical Lorentz force was not measured nor estimated in Mr Li's paper.
Take a dead magnetron, core it out, stuff it with resistors or a electric dryer's heating element, and replace for the active one. If you dissipate the same heat, the same way, you've got a adequate "heat dummy" to do a difference with.
But even a piece of burning paper will experience buoyancy due to the natural convection lift forces on it.
Much more practical, but then what happens is that the magnetics of the magnetron are still in play, and you don't want to start "red herring" theories about the influence of them and the cavity.By having a non microwave source, it's just about heat.
QuoteBut even a piece of burning paper will experience buoyancy due to the natural convection lift forces on it.No, that is not buoyancy. It is a matter of terminology. It is a lift force due to the natural convection currents created by the heat of burning paper. As I wrote above, buoyancy and convection are separable problems.The magnetron itself will experience natural convection lift forces, but not buoyancy, because it is a sealed unit. It does not out-gas hence it's mass is constant. Perhaps there is a very small buoyancy due to the expansion of the metal of the magnetron with temperature but the expansion is small and the mass of the air displaced by that expansion is very, very small, I think negligible, when compared to the natural convection lift forces it experiences.In my post, I was considering SeaShells' test rig and cavity. She has placed the magnetron at the pivot so that lift forces off the hot magnetron itself will be negated, be they buoyancy or natural convection lift forces.I suggest we focus uniquely on a test set-up for our posts because the error sources are different for different test set-ups. Heat from the magnetron was definitely a factor with rfmwguy's test rig, but should not be a factor with Shells' rig. Similarly, buoyancy of the cavity will be a definite factor with Shells' rig but should not have been as much of a factor, if any, with rfmwguy's mesh cavity design.
a : the tendency of a body to float or to rise when submerged in a fluidb : the power of a fluid to exert an upward force on a body placed in it; also : the upward force exerted
In science, buoyancy also known as upthrust) is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. This pressure difference results in a net upwards force on the object. The magnitude of that force exerted is proportional to that pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.For this reason, an object whose density is greater than that of the fluid in which it is submerged tends to sink. If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat. This can occur only in a reference frame which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction (that is, a non-inertial reference frame). In a situation of fluid statics, the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body
Natural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients. In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current; this process transfers heat energy from the bottom of the convection cell to top. The driving force for natural convection is buoyancy, a result of differences in fluid density.
Quote from: aero on 12/17/2015 07:58 pmQuoteBut even a piece of burning paper will experience buoyancy due to the natural convection lift forces on it.No, that is not buoyancy. It is a matter of terminology. It is a lift force due to the natural convection currents created by the heat of burning paper. As I wrote above, buoyancy and convection are separable problems.The magnetron itself will experience natural convection lift forces, but not buoyancy, because it is a sealed unit. It does not out-gas hence it's mass is constant. Perhaps there is a very small buoyancy due to the expansion of the metal of the magnetron with temperature but the expansion is small and the mass of the air displaced by that expansion is very, very small, I think negligible, when compared to the natural convection lift forces it experiences.In my post, I was considering SeaShells' test rig and cavity. She has placed the magnetron at the pivot so that lift forces off the hot magnetron itself will be negated, be they buoyancy or natural convection lift forces.I suggest we focus uniquely on a test set-up for our posts because the error sources are different for different test set-ups. Heat from the magnetron was definitely a factor with rfmwguy's test rig, but should not be a factor with Shells' rig. Similarly, buoyancy of the cavity will be a definite factor with Shells' rig but should not have been as much of a factor, if any, with rfmwguy's mesh cavity design.That may not be buoyancy according to you, but it is according to what I learnt, and also according to:https://en.wikipedia.org/wiki/Buoyancy
QuoteIn science, buoyancy also known as upthrust) is an upward force exerted by a fluid that opposes the weight of an immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the pressure at the bottom of a column of fluid is greater than at the top of the column. Similarly, the pressure at the bottom of an object submerged in a fluid is greater than at the top of the object. This pressure difference results in a net upwards force on the object. The magnitude of that force exerted is proportional to that pressure difference, and (as explained by Archimedes' principle) is equivalent to the weight of the fluid that would otherwise occupy the volume of the object, i.e. the displaced fluid.For this reason, an object whose density is greater than that of the fluid in which it is submerged tends to sink. If the object is either less dense than the liquid or is shaped appropriately (as in a boat), the force can keep the object afloat. This can occur only in a reference frame which either has a gravitational field or is accelerating due to a force other than gravity defining a "downward" direction (that is, a non-inertial reference frame). In a situation of fluid statics, the net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the bodyhttps://en.wikipedia.org/wiki/Natural_convectionQuoteNatural convection is a mechanism, or type of heat transport, in which the fluid motion is not generated by any external source (like a pump, fan, suction device, etc.) but only by density differences in the fluid occurring due to temperature gradients. In natural convection, fluid surrounding a heat source receives heat, becomes less dense and rises. The surrounding, cooler fluid then moves to replace it. This cooler fluid is then heated and the process continues, forming a convection current; this process transfers heat energy from the bottom of the convection cell to top. The driving force for natural convection is buoyancy, a result of differences in fluid density.
Whatever we work out, I'm at a point where DIY experimentation may not be worthwhile unless we can figure a way to characterize thermal dynamics.From my perspective, I knew there was lift, what I was looking for was an interruption of it during mag on conditions. I believe I saw this clearly as either attenuation, reversal or a hold of the lift progression.Remember, the thermal mass of the magnetron assembly does not permit instantaneous heating or cooling. If you look at the thermal videos, you see it is quite gradual...much slower than the instantaneous changes to the thermal lift profile I observed when mag switched between on and off, so my posit is that quick changes to a lift profile CANNOT be due to instantaneous heating or cooling at mag transition.Where am I off base here?