From:      skunk-works-digest-owner@mail.orst.edu
To:        skunk-works-digest@mail.orst.edu
Subject:   Skunk Works Digest V5 #586
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Skunk Works Digest        Sunday, 31 December 1995      Volume 05 : Number 586

In this issue:

	Re: diamonds

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From: Mary Shafer <shafer@ursa-major.spdcc.com>
Date: Sat, 30 Dec 1995 22:11:30 -0500 (EST)
Subject: Re: diamonds

We call them shock diamonds and they're caused by the reflection of the
shockwave in the exhaust plume at the edge of the plume.  Pretty, aren't
they?  Any good aerodynamics book will explain the phenomenon.  They're
sort of standing shocks.  I suppose that you can tell how fast the exhaust
plume is by counting the diamonds, but I've never heard anyone actually
say that this is possible or how to do it.

If you look at a really good photo or up close in real life (standing at
the center taxiway for the SR-71, for example), you can see that there's
actually quite a bit of structure in the plume--little diamonds inside the
big diamonds, etc.  Usually by the time you see a photo of a moving plume,
scanned in, you get blobs instead of beautiful sharp-edged diamonds.

The phenomenon is not limited to the SR-71, by the way--I've seen it on
every plane with an afterburner.

Regards,
Mary

Mary Shafer  DoD #0362 KotFR  shafer@ursa-major.spdcc.com
URL http://www.dfrc.nasa.gov/People/Shafer/mary.html
Some days it don't come easy/And some days it don't come hard
Some days it don't come at all/And these are the days that never end....


On Thu, 28 Dec 1995, Greg Fieser wrote:

> > 
> > Can someone please tell me what creates/created the "diamonds" in the
> > exhaust of the SR-71?  I have often seen pictures of this but I have
> > never read a real explanation of how this happens.  THANKS!  :)
> > 
> 
> These "diamonds" are visible in many augmented (afterburner-equipped) jet 
> powered aircraft, including the SR-71.  I have heard several myths, but I do 
> not know the true explanation...here are some I've heard:
> 
> 1) Sometimes called Mach Diamonds, related to the (relative) speed of the
>     exhaust gases;
> 
> 2) Claims that the number of visible diamonds represent the number of reheat
>     (afterburner) stages in the engine;
> 
> 3) Function of the internal engine geometry, i.e. length/diameter of AB
>     section, nozzle area, etc.
> 
> 4) Phase of the moon, hair color of the pilot, etc.
> 
> I too would be interested in an informed explanation of this phenomenon...
> 
> 
> 	Greg Fieser




------------------------------

End of Skunk Works Digest V5 #586
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