[Beowulf] Moores Law is dying

Lux, James P james.p.lux at jpl.nasa.gov
Fri Apr 10 00:58:08 PDT 2009

From: beowulf-bounces at beowulf.org [beowulf-bounces at beowulf.org] On Behalf Of Robert G. Brown [rgb at phy.duke.edu]
Sent: Thursday, April 09, 2009 2:31 PM
To: Prentice Bisbal
Cc: Beowulf Mailing List
Subject: Re: [Beowulf] Moores Law is dying

...On Thu, 9 Apr 2009, Prentice Bisbal wrote:

> I disagree with the sonic barrier wall analaogy. Is it that clearly
> technical barrier the slowed down jet research, or did the nuisance of
> sonic booms to people on the ground just make supersonic R&D less
> convenient? I've heard that supersonic travel over land is restricted in
> the US.

.....Yes, indeed, supersonic flight over land in the US is restricted. Exceptions for military training flights in designated areas (e.g. desert wasteland) and things like shuttle landings (characteristic double sonic boom from leading and trailing shock waves.. the shuttle is big and fast).  Kind of put paid to useful Concorde flights except from New York and DC (not to mention that Concorde was just plain old LOUD on the ground)

....Sonic booms are also used as a tactical weapon of sorts in, e.g., Gaza.

Actually, historically, it was absolutely the technical barrier, which
was profound.  Pilots in WWII not infrequently went into a dive, and of
course diving one can approach the sound barrier quite easily.

They died.  With very few exceptions, and they were lucky ones.  One of
two things killed them.  At near-supersonic speeds, the equations that
govern airflow and lift completely and nonlinearly change form.  All of
a sudden, the pilots discovered that they were unable to actually move
the yoke of their aircraft against the enormous forces that locked them
in, and they discovered that the lift they were counting on to pull them
out of the dive (in particular the lift generated by the aircraft tail)
suddenly disappeared.  A few clever pilots thought to put on their
airbrakes, slowed to subsonic speeds, and managed to pull out.  The rest
didn't.  The other problem that plagued the deliberate attempts to break
the sound barrier were harmonics that appeared and were nonlinearly
amplified as the aircraft approached the barrier.\

...I don't know that they're harmonics, per se, but just the phenomenon of aerodynamic flutter, which was very poorly understood and is basically a resonance thing. (the nonlinearities in the state equation for air doesn't help with understanding, of course).  Flutter can occur at lower speeds, too (it's what sets Vne on some small planes), but at lower speeds, how to deal with it is easier to solve empirically AND it's testable by gradually creeping up on it and waiting for the evil moment.  When you get close to sonic speed (and actually, it's localized sonic speed over part of the airframe that's troubling), then you get very fast changes.  One needs to be a manly man test pilot.

BTW, demonstrating sonic flow is easy.  Your run of the mill air nozzle is probably in "choked flow" with a shock wave in the throat.  If you have a usual shop air compressor, and the air is damp (bad for the tank, but good for the demo), and vent it through something like an 1/8" hole, you'll probably see a nice Mach cone, as the rapid change in pressure causes the water to condense.  An old style non-OSHA safe air blowing nozzle does nicely, so does a 1/4" or 1/2" ball valve feeding a pipe cap with a hole drilled in it.

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