Oh. Oh my. This is a great thread but it's all over the place.
lyuc said:
As speed increases, the energy transferred from air friction must start to heat the aircraft. Eventually the plane will glow with radiating heat, such as a re-entering space craft.
Presumably a plane can do something about this with cooling systems?
Presumably there must be a heating limit to practical atmospheric flight?
The short answer to your original question is "it depends." The glow of a surface depends on its temperature, which depends on numerous flow properties and material properties. There is no one answer. Also, at a certain temperature the air around a hypersonic object will ionize and the resulting plasma itself will glow.
Baluncore said:
Not quite. There is no "air friction". The air is compressed against the front edges of the aircraft and heated by compression. That hot air radiates heat to the skin of the aircraft.
It's compressed away from surfaces, too, by shock waves and other compression waves in the flow field. Also, the majority of the heating comes from conduction/convection into the surface from the surrounding gas except at very high Mach numbers, where radiation can become important.
Baluncore said:
Cold air is a good way to cool the surface. Why do you think supersonic jets have sharply pointed noses ?
Why do
you think supersonic jets have pointed noses? It isn't for cooling. In fact, a pointy noise makes the heating problem worse and will be one of the first parts of a vehicle to experience thermal failure at high speeds. It's pointy because it reduces wave drag. One side effect is that the tip gets extremely hot and concentrated in a small are where it is hard to cool.
lyuc said:
Looking at front views of modern military jets, I can draw little detailed knowledge about modern "streamlining". In this photo of the Rafale I have no idea what's going on in the small gap between the engine air intake and the fuselage.
The gaps are primarily there to swallow the boundary layer. The inlets are designed with an assumption that any shock wave will be attached to the leading edge of the compression ramp at supersonic speeds, and the presence of a thick boundary layer will violate that assumption (as well as cause greater unpredictability when maneuvering). By swallowing the boundary layer into that gap, it means shocks can remain attached to the inlet leading edge more easily.
Baluncore said:
A shock wave travels at greater than the speed of sound because it is compressed and so is hotter than the surrounding air. To reduce the heat of the shock wave near the aircraft it must be spilled into the slipstream. The pointed nose does not support a permanant pool of shock wave heated air in front of the aircraft.
The shock itself is not really hotter in the sense that you seem to be implying. They are usually only a few microns thick, and so the temperature inside the shock itself is of very little consequence. What
is important is the amount of compression that occurs
across the shock. The air is cooler upstream and hotter downstream, often considerably so.
I don't know what you mean by saying "it must be spilled into the slipstream." That's not a concept I've every encountered.
A pointed noise absolutely does support a "pool" of heated air. In fact, for a pointier nose, the temperature at the stagnation point will be identical to that at the stagnation point of a blunt body. The major difference is that this temperature will be located next to a much smaller chunk of material which will therefore heat up much faster than a blunt body would and be much harder to actively cool if desired.
Baluncore said:
The air intake on a supersonic jet is used to slow the air down as the first stage before compression. I suspect the wall and duct on that Rafale are designed to capture the hot skin layer to keep it out of the air compressor inlet. Maybe it bypasses the first stage and enters the second.
In certain cases, that air that passes into that gap may be used in some way, but it's unclear how you could bypass the first stage of the compressor. That compressed air would be at a higher pressure, so if there were a flow path leading from the second stage into that gap, air would flow
out of the engine, not into it.
Baluncore said:
The air above 100,000 feet, where the re-entry occurs, is very thin and the re-entry vehicle is traveling at over M20. The glow is not the metal structure but the ceramic or ablative thermal protection.
The glow is also often the air itself, which has become ionized and formed a plasma.
Baluncore said:
Aircraft cannot fly much above 85,000 feet. The re-entry vehicle has stopped glowing by the time it reaches that altitude.
This is absolutely not true. The peak heating rate actually usually occurs at a much lower altitude than that in the lower atmosphere. The reason is that the air at high altitudes, even if compressed to much higher temperatures, is simply so thin that the convection coefficient is still very small. In the lower atmosphere, where the Mach number is closer to the middle single digits, the atmosphere is much thicker and transfers heat much more efficiently. The surface temperature therefore continues increasing until you get pretty low in the atmosphere.
Baluncore said:
The aircraft speed limit is NOT decided by the “glowing” material problem. Long before that, at about M3.5, the jet engine compressors fail to function efficiently. The afterburner is insufficient to maintain M3.5 without over-heating the tail structure of the aircraft to the point of collapse. That is long before the skin glows.
To go faster than M3.5 requires the jet engine be replaced with a rocket.
Also false. Around Mach 3, you can switch over to ramjet operation. This could be a separate engine or a combined cycle engine whereby the compressor can be bypassed and the stagnation against the inlet itself produces enough compression to sustain the Brayton cycle. Once you get to Mach 5 or 6, you start to have issues where you can no longer efficiently compress the flow with shock waves without losing too much total pressure, so you have to move toward scramjets (supersonic combustion ramjets).
Scramjets are not yet a fully-solved problem, but experimental versions have been able to reach up to nearly Mach 10 on the X-43. That engine used hydrogen as a fuel. The record with hydrocarbons, which are far more practical, is about Mach 5 on the X-51.
snorkack said:
Where the air flows normal to surface, it is compressed and heated up where it approaches the surface, and expanded and cooled where it retreats from the surface. Since air around the objects is conserved, it is reversible... unless the heat at hot spots gets transferred out of the air.
It's actually generally not reversible in supersonic flows. If a shock forms, that is a dissipative phenomenon and compression across a shock is not isentropic. It
is still adiabatic, though. The amount of dissipation (and therefore total pressure loss) across a shock increases with Mach number.
snorkack said:
Where the air flows tangential to surface, there IS air friction, and work done against friction gets converted to heat, irreversibly.
Viscous dissipation contributes a minute amount of heat to the overall heating problem associated with high-speed flight.
lyuc said:
Not counting things that go into space, non-ballistic missiles can match or exceed this these speeds (e.g. R-37M air-to-air, and certain ABMs) but much less is written about them. Also they're considerably smaller than aircraft and I'm unsure how this changes the heating they undergo.
The size (importantly, the mass) is important. If you have two vehicles of identical material composition and shape traveling at the same speed but with different sizes, the smaller one will heat faster. There is simply less material to heat.
Baluncore said:
In laminar flow the contact layer is very thin and is attached to the surface of the vehicle.
Neither of these are necessarily true. Laminar boundary layers need not be attached to the surface of the vehicle. In fact, laminar boundary layers are
less likely to remain attached to the surface than turbulent ones.
Baluncore said:
Outside that, shear within the laminar flow is NOT friction, it is viscous flow. Where viscous flow heats the air, that air will radiate and so heat the nearby passing vehicle surface.
Baluncore said:
At the front, where the air is being displaced by the increasing sectional area of the vehicle, the air will be heated by compression. The vehicle surface will then be heated by radiation as the vehicle section passes through that heated air. There will be some heating of the air by viscous drag, but all heating of the vehicle will be by radiation alone.
Radiation is a very, very tiny component of heat transfer until you reach very high Mach numbers. Standard convection processes dominate the process in almost all other cases and it's really not even close. Viscous dissipation (not drag) plays an incredibly small role at all conditions.
Baluncore said:
There is absolutely NO contact friction. It is the viscosity of the air, NOT the coefficient of friction that matters in this case. If you don't believe me then you should try to calculate the friction with a mythical coefficient of friction between air and air. Maybe you should be thinking in terms of drag, rather than friction.
In a lot of ways this is a distinction without a difference. Viscosity leads to the no slip condition against a surface moving relative to an adjacent region of fluid. This is, in some ways, akin to static friction in that the fluid does not move relative to the surface. The primary difference is that the force required to move the fluid against the wall is not proportional to the weight of the fluid contacting the wall. This is a fluid, after all, not a solid.
So generally I agree with your assertion that this is not standard friction. But friction is still a somewhat useful (if also somewhat misleading) analogy for the wall shear in a viscous flow.
Baluncore said:
Behind the greatest vehicle section, where the air begins to return and fill the wake, the air will be cooled by depression, and warmed slightly by wake turbulence, but by then the vehicle will have shed that air and left the scene.
In reality, the air cools rapidly well before maximum thickness. At the tip of a typical supersonic aircraft (say, the ogive nose cone on a fighter jet), the hottest, slowest air will be right at the tip at the stagnation point. The free stream will be pointed along the direction tangential to the surface, so the flow is "opening up" relative to the free stream. This is an expansion. The air rapidly accelerates and cools. The heat is really only a problem right at the tip.
There are also other extremely hot regions that come after the point of maximum thickness. Basically, anywhere the flow instead turns "into itself," there will be a compression (generally via shock) that re-heats the air. These are common around things like control surfaces.
Further complicating matters are issues like shock/boundary-layer interactions and other vortex-forming phenomena, which can cause intense local heating. Laminar-turbulent transition in the boundary layer can also lead to localized heating spikes.
Baluncore said:
That process of wake formation is driven by atmospheric pressure
I have no idea what this means.
snorkack said:
Air is 99,95% dinitrogen, dioxygen and argon. Diatomic homonuclear molecules and lone atoms. No legal transitions to absorb or emit in infrared or visible. If heating of the vehicle were by radiation, hot air should be harmless.
Hot air burns by direct conduction, not radiation.
Unless the gas dissociates at high Mach number.
