Understanding Air Motion Transformers - Page 3 - diyAudio
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 4th March 2010, 10:14 PM #21 thadman   diyAudio Member   Join Date: Dec 2005 Location: west lafayette I'm not sure how to apply adiabatic or isothermal compression due to the boundary conditions. We might reach further resolution by defining the state of the flow, whether it be compressible or incompressible. If we wished to model compressible flow, we'd have to do a direct numerical simulation of the Navier-Stokes equations (massive computational requirements). However, I'm not sure how valuable such an analysis would be. If we admit compressible flow, we'd be assuming significant non-linearity as the speed of sound will vary throughout the volume due to the pressure gradients. I believe the optimum design would restrict the state space to incompressible flow, where the velocity of the flow is insignificant with respect to the speed of sound. We might reach further resolution on the state of the flow by defining the Reynolds number (ratio of viscous to inertial forces). I believe a Reynolds number <2000 would be desirable. However, I am having difficulty defining the hydraulic radius and the velocity of the fluid since the duct geometry varies with displacement of the membrane. Wikipedia suggests that incompressible can be assumed if the Mach number is below 0.3. However, Wikipedia is often erroneous. Can anybody offer further insight on this? __________________ "It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
 5th March 2010, 12:38 AM #22 thadman   diyAudio Member   Join Date: Dec 2005 Location: west lafayette With regards to the total system, we might reach further resolution by normalizing the objectives. If the structure is allowed to cover the front and rear apertures, the induction may be very uniform throughout the volume. We might achieve maximum coherence in the membrane response. However, we may admit a compromise in the state of the flow. If the apertures are unrestricted, we will minimize the Reynolds number and maximum the upper output limit. For a given surface area, there will be an upper limit for output related to the transition from incompressible to compressible flow and transition from laminar to turbulent flow. However, the induction may vary significantly throughout the volume and we may admit a compromise in the coherence of the membrane response. Any thoughts? __________________ "It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
Arick
diyAudio Member

Join Date: Dec 2008
Location: Montréal
Quote:
 Originally Posted by thadman I'm not sure how to apply adiabatic or isothermal compression due to the boundary conditions. We might reach further resolution by defining the state of the flow, whether it be compressible or incompressible. If we wished to model compressible flow, we'd have to do a direct numerical simulation of the Navier-Stokes equations (massive computational requirements). However, I'm not sure how valuable such an analysis would be. If we admit compressible flow, we'd be assuming significant non-linearity as the speed of sound will vary throughout the volume due to the pressure gradients. I believe the optimum design would restrict the state space to incompressible flow, where the velocity of the flow is insignificant with respect to the speed of sound. We might reach further resolution on the state of the flow by defining the Reynolds number (ratio of viscous to inertial forces). I believe a Reynolds number <2000 would be desirable. However, I am having difficulty defining the hydraulic radius and the velocity of the fluid since the duct geometry varies with displacement of the membrane. Wikipedia suggests that incompressible can be assumed if the Mach number is below 0.3. However, Wikipedia is often erroneous. Can anybody offer further insight on this?
Hi, the best info my friend and I found for now is that the flow can be considered incompressible bellow 228 mi/h (102m/s) at 68 F (20C). I also have another source (Fundamentals of Hydro and Aeromechanics) wich states: The error introduced by the gas to be incompressible in the equation of continuity is 4% at speeds of about 300ft/sec and 1% at 160ft/sec.
Hope it helps.
Eric

 5th March 2010, 02:15 AM #24 Arick   diyAudio Member     Join Date: Dec 2008 Location: Montréal I almost forgot, the second reference is at 59F (15C) 29.92 inches of mercury (1013.25mb) at sea level (std. atmosphere) Eric
diyAudio Member

Join Date: Dec 2005
Location: west lafayette
Quote:
 Originally Posted by Arick Hi, the best info my friend and I found for now is that the flow can be considered incompressible bellow 228 mi/h (102m/s) at 68 F (20C). I also have another source (Fundamentals of Hydro and Aeromechanics) wich states: The error introduced by the gas to be incompressible in the equation of continuity is 4% at speeds of about 300ft/sec and 1% at 160ft/sec. Hope it helps. Eric
Interesting. Thanks
__________________
"It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."

 7th March 2010, 02:48 AM #26 thadman   diyAudio Member   Join Date: Dec 2005 Location: west lafayette I'd be interested in building a prototype AMT. Motor construction would appear straightforward. However, I am having issues determining possible Membrane (film) and Conductor (foil) geometries. How can I determine if its possible to produce a given geometry? How can I determine what effect the geometry has on material wear (high cycle fatigue)? How can I determine if the membrane will support that geometry for a given length of time? What techniques are used to produce the pleats? I have access to a large number of academic journals through my University, however I am not sure what keywords to use when searching. Could anybody offer further insight on this? Thanks, Thadman __________________ "It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
 7th March 2010, 02:10 PM #27 Keith Taylor   diyAudio Member   Join Date: Nov 2007 Location: Adelaide www.diyaudio.com/forums/planars-exotics/119625-heil-amt-prototypes-anyone-ever-seen-anything-like-these-units-2.html Thadman, follow the link, and the one given in the fourth post on this page for DIY diaphragm construction. Keith Last edited by Keith Taylor; 7th March 2010 at 02:14 PM.
 9th March 2010, 04:55 PM #28 thadman   diyAudio Member   Join Date: Dec 2005 Location: west lafayette If we allow the driver to operate dipole, could we assume Qts = Qtc? If the driver is operated below f0 (fundamental resonance), could we assume uniform pressure over the surface of the membrane? Could we deduce output from mass flow? __________________ "It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
 9th March 2010, 05:20 PM #29 thadman   diyAudio Member   Join Date: Dec 2005 Location: west lafayette Have users experimented with constant current sources? What electrical impedance would be realistic for 100w current drive? What electric / magnetic interactions should we attempt to quantify? I would assume the magnetic field produced by the conductor would tend to self cancel. What mechanical / acoustic losses should we attempt to quantify? If we wished to define the mechanical behavior of the system, could we define the volume as a helmholtz chamber and couple it to the membrane in series? In such a way, we might be able to apply the concept of adiabatic compression / expansion. If the volume was operated below the fundamental resonance could we treat it as a lumped mass? How could we define the total mass / volume of the chamber? I assume it will couple to a mass of air beyond the mouth. Could we define the area ratio (driven area / mouth area) as a transformer? In such a way, could we define the optimum bandwidth for a given set of parameters? __________________ "It is a profound and necessary truth that the deep things in science are not found because they are useful; they are found because it was possible to find them."
 11th February 2012, 01:17 PM #30 Myjamoffice   diyAudio Member   Join Date: Feb 2012 Location: Baltimore Has any one, finnished the project? do you have any reviews of the end product? Insite into how it works? I'm hooked, but have no skill to DYI this type of project so i like though others on this one.

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