· Hullo A couple of weeks ago I made a video about the development of the space shuttle and
its aerodynamics, and in the process I found this amazing photo, which I thought looked like the result of a
computer simulation. But IN fact it’s a 1970’s photograph of a test of a shuttle model in a Mach 20
helium wind tunnel, using electron beam fluorescence. And there were a lot of questions about this - how do
you make a hypersonic wind tunnel, why would you use helium and what’s the electron beam fluorescence all about.
So… let’s start with How you make a hypersonic wind tunnel? If you’re only casually familiar with wind tunnels
you might know enough that you think of them as a tunnel with big fans pushing air through them and
a location to mount test objects so you can observe aerodynamic flow around them. The tunnel might be open to
the world OR it could be arranged in a loop with the air circulating to reduce power needs, and there’ll
be all sorts of vanes and grid type devices to direct and control airflow and try to make it as
smooth as possible. Either way if you know anything about aircraft you’ll know that aircraft with propellers aren’t going
supersonic, The Republic XF-84H Thunderscreech was an attempt to get a prop driven plane close to mach 1, but it
was much more successful at converting aviation fuel into noise. Propellers are good at adding a small amount of speed
to a large amount of air and getting supersonic flows might be possible but prohibitive. Anyway, a wind
tunnel isn’t just about fans pushing air around, there’s a lot of other engineering required to make air flow the
way you need it to, and a big part of wind tunnel design is the shape of those tunnels that
the wind flows through. If you’re even casually aware of physics you’ve probably heard of bernouli’s laws, these are
a set of relationships that explain how pressure and velocity of a fluid changes as it moved through pipes with
differing cross sections. If a fluid flows from a wide pipe into a narrow pipe then conservation of mass requires
that the velocity of the fluid increases so that the same amount of mass per second flows through the fat
section and the thin section of the tube. And wind tunnels can use use this, after all a tunnel
can just be considered like a pipe from the point of view of Bernoulli’s laws. So you’ll see wind tunnels
with wide sections that narrow down just before the test section with the models, increasing the speed of the air
where it’s tested. So you might think that’s the problem solved, just shrink the tube down until you get
supersonic flows and shrink it more to get hypersonic flows. But this doesn’t work, the commonly cited version of bernoullis
law is for incompressible fluids, and you can only get away with approximating gasses as incompressible up to speeds of
about Mach 0.3. You can still make the air go faster, but it’s not longer a simple linear relationship
between cross section and flow rates. The air gets compressed, the density rises and so you don’t get the same
increase in speed that you’d expect in the incompressible regime. But if you push hard enough the flow reaches
Mach 1, and pushing it even harder doesn’t increase the flow speed, it just increases the density, pressure and temperature.
This is a condition called choked flow. So the next step is then to allow this gas flow
to expand out of the choked flow condition through a widening tunnel section, and what happens is the stored energy
in the form of pressure pushes the gas to higher and higher speeds, past mach 1 and getting faster as
the cross section of the tunnel expands. So for supersonic gas flows the intuitive form of bernoullis law is
reversed - increasing the area accelerates the flow and decreasing the area slows the flow. Now, many of my
long time viewers might be thinking they’ve seen me explain this before, and you’re right, because it’s exactly the same
phenomena used in rocket engine nozzles to maximize the exhaust speed. You have the high pressure combustion chamber, a throat
to generate the choked flow and an expanding nozzle section to accelerate the gas. And then there is the
example of the thermal structure test tunnel at Langley that wanted to simulate reentry conditions, and they did this by
effectively building a hypersonic wind tunnel that was a methane fueled rocket engine, generating high temperature flow conditions at Mach
7. One difference is that the throat and expansion geometry in rockets has a circular cross section to reduce
hot spots, but it’s common to see wind tunnels only contract and expand in one dimension, some of them even
make the throat and nozzle adjustable in real time, others can vary it between tests by opening the tunnel up
and replacing blocks that construct the flow. Anyway, we’re getting ahead of ourselves, with this supersonic expansion phenomenon you
can now see how it’s possible to turn high pressure subsonic flows into supersonic, or even hypersonic flows.
But in driving this you want high pressures, and fans aren’t good at that, so those get replaces by pumps
to compress the gas more effectively. Sustaining large tunnels at high flow rates can require huge amounts of power -
hundreds of megawatts, but with rapid response instrumentation you can perform all sorts of tests quickly. So many small
tunnels are driven by compressed gas where there’s a high pressure reservoir that gets pumped up to pressure and then
a valve is opened and the gas flows for a short time, sometimes minutes, sometimes fractions of a second.
In this image these large spheres are actually vacuum spheres that the high pressure gas exhausts into, to get the
best pressure gradient. So, now we’ve explained how you can make a hypersonic tunnel, why then is this tunnel
using Helium instead of air? Well there is a downside to this expansion process, the expanding gas cools down,
this is just conservation of energy as heat and pressure are converted into kinetic energy. The faster you go the
cooler the gas gets. If there’s moisture in the air it’ll condense out, this is a similar phenomena to
the condensation cones seen around transonic aircraft and rockets. Air in wind tunnels need to be dried to avoid this,
and that can be a serious technical challenge if the tunnel has to continually draw in fresh air. But
as you go faster the temperature can drop to the point where air itself liquifies, and that sets the limit
on how fast a tunnel can simulate, for air this is between mach 4 and 5. One way around this
problem is heating the air before the throat so it remains above the liquefaction point when expanded, it only takes
modest heating to 50 Celsius to make a mach 5 tunnel work, but for mach 10 the air temperature needs
to be about 900 Celsius and mach 20 would demand temperatures of over 4000 Celsius, necessitating some exotic systems for
heating air and stopping it from destroying the tunnel. But another alternative is to use a gas with a
lower boiling point, and Helium is about as good as you can get in that department. So you can
push a helium wind tunnel up to mach 20 easily without spending all your time solving thermal management problems.
Helium obviously isn’t a perfect stand-in for air, it’s monatomic which alters its physical properties compared to diatomic gasses like
Oxygen and Nitrogen., but also realize that heating air to 5000 Celsius changes its chemistry compared to regular air so
a mach 20 air tunnel wouldn’t be realistic either. Its one of many compromises engineers have to accept for
tests like this. This isn’t the only way to solve these problems - shock tunnels, expansion tunnels, light gas
guns, arcjets all offer different techniques, some of which address some parts of the problem better than others, there’s no
one best technique short of actually flying a heavily instrumented test article in the test conditions. So anyway the
final part is about the electron beam fluorescence which gives the image its beautiful appearance. Wind tunnels are used
to analyse the aerodynamic qualities of models, and there are many techniques for measuring how the airflows are disturbed in
the vicinity of the model. You might have seen some schlieren imaging which captures density changes in the flows by
exploiting the changes in refractive index of the gas, under the right conditions I’ve even seen transonic shocks on top
of wings on passenger planes. However in the case of these hypersonic helium tunnels this doesn’t work well because
helium has a much lower refractive index than air, and, the process of expanding the gas to get hypersonic flows
means the gas density is lower, and the low density lowers the refractive index even more. So instead
an electron beam is fired through the helium and this excites the atoms into a higher energy state, then when
the atoms drop back to their base state they emit light. Now in a gas the rate at which the
atoms do this is higher when the gas is higher density. So, the result is a brighter glow in higher
density regions. Depending on the test it’s also common to include small amounts of nitrogen which does a much
better job at emitting light and illustrating the changes in air density near an object. So, now you know
what’s going on in this 50 year old photo - and hopefully you‘rave learned a bit about hypersonic wind tunnels
in the process. I'm Scott Manley, Fly Safe