The document summarizes information about gun tunnels, which are a type of hypersonic wind tunnel that can produce flows up to Mach 30-40 by using a piston for isentropic compression. It describes the basic components and operating principles of gun tunnels, including how they use high pressure gases and diaphragms to accelerate test gases to hypersonic speeds over very short durations. Technological challenges of gun tunnels include reproducing equilibrium conditions at very high temperatures and pressures while also requiring fast instrumentation.

1. GUN TUNNELS
BY
SREECHITHRA.AS
16RBTAR008

2. INTRODUCTION
– Gun tunnel is an intermediate altitude hypersonic wind tunnel that can
be configured to produce hypersonic flows at roughly 30 to 40 km
altitude.
– This uses a piston for isentropic compression.
– The hypersonic facility at IISC Bangalore, India has a high enthalpy gun
tunnel, which can produce Schlieren imaging (Schlieren imaging is a
method to visualize density variations in transparent media) and
produce up to 8 mega joules of energy.

3. ⁻ Using a piston can be very tricky due to
reflecting of shocks. At the facility, they use
aluminium diaphragm to produce shocks and
paper diaphragm to avoid shocks and pass
through the hypersonic chamber.
⁻ The pressure used significantly higher like 30
times the atmosphere.

4. HOT SHOT WIND TUNNEL
– One form of HWT is known as a Gun Tunnel or hot shot tunnel (up
to M=27), which can be used for analysis of flows past ballistic missiles,
space vehicles in atmospheric entry, and plasma physics or heat transfer at
high temperatures.
– It runs intermittently, but has a very low running time (less than a second).
– The method of operation is based on a high temperature and pressurized
gas (air or nitrogen) produced in an arc-chamber, and a near-vacuum in the
remaining part of the tunnel.
– The arc-chamber can reach several Mpa, while pressures in the vacuum
chamber can be as low as 0.1 Pa.
– This means that the pressure ratios of these tunnels are in the order of 10
million.

5. ⁻ Also, the temperatures of the hot gas are up to 5000 K.
- The arc chamber is mounted in the gun barrel.
⁻ The high pressure gas is separated by the vacuum by a diaphragm that breaks
down as its resistance is exceeded.
⁻ Prior to a test run commencing, a membrane separates the compressed air
from the gun barrel breech.
⁻ A rifle (or similar) is used to rupture the membrane. Compressed air rushes
into the breech of the gun barrel, forcing a small projectile to accelerate rapidly
down the barrel.
⁻ Although the projectile is prevented from leaving the barrel, the air in front of
the projectile emerges at hypersonic velocity into the working section.
⁻ Naturally the duration of the test is extremely brief, so high speed
instrumentation is required to get any meaningful data.

7. GUN TUNNEL - EXPERIMENT
– Configuration of a gun tunnel is similar to that of a free piston driven
shock tunnel.
– This tunnel is also comprised of a driver gas reservoir, diaphragm,
piston, test gas section or barrel, nozzle and test section cum dump tank.
– During the experiment, diaphragms are put in the respective locations
and desired pressure is attained in the test gas section and dump tank.
– Continuous filling of the piston driver gas in the driver section bursts the
diaphragm and the high pressure driver gas rushes in the barrel or driver
section which sets piston in motion.

8. - Hence piston in the Gun Tunnel performs the same function as that
of the contact surface and avoids the mixing of driver and driven
gas.
- Thus attained motion of the piston compresses the test gas and also
raises its temperature almost adiabatically.
⁻ The main difference in Gun Tunnel and Free piston driven shock
tunnel lies in that the piston compression is attained for the driver
gas in the free piston driven shock tunnel while such is compression
is obtained for the test gas in Gun Tunnel.
⁻ Diaphragm at the nozzle inlet opens up at a particular test gas
pressure and starts the expansion process in the nozzle.
⁻ Thus expanded test gas attains hypersonic conditions in the Mach
number.
⁻ Gun Tunnel is very useful to obtain moderate stagnation enthalpy
and high stagnation pressure hypersonic freestream in the test
section.

9. COMPONENTS OF GUN TUNNEL

10. TECHNOLOGICAL PROBLEMS
– There are several technological problems in designing and constructing a
hyper-velocity wind tunnel:
– Supply of high temperatures and pressures for times long enough to
perform a measurement
– Reproduction of equilibrium conditions
– Structural damage produced by overheating
– Fast instrumentation
– Power requirements to run the tunnel
– Simulations of a flow at 5.5 km/s, 45 km altitude would require tunnel
temperatures of over 9000 K, and a pressure of 3 GPa

11. HYPERSONIC WIND TUNNEL
FACILITY IN INDIA
– ISRO commissioned two major facilities — a Hypersonic Wind Tunnel
and Shock Tunnel — at the Vikram Sarabhai Space Centre here as part of
its continuous and concerted efforts to minimize cost of access to space.
ISRO Chairman A. S. Kiran Kumar said commissioning of such facilities
would provide adequate data for design and development of current and
future space transportation systems in India.

12. MARHY HYPERSONIC LOW DENSITY
WIND TUNNEL
– MARHY, Hypersonic Wind Tunnel Facility in Orleans
FRANCE
– The MARHY Hypersonic low density Wind Tunnel, located
at the ICARE Laboratory in Orleans, France, is a research
facility used extensively for fundamental and applied
research of fluid dynamic phenomena in rarefied
compressible flows, applied to space research.
– Its name is an acronym
for Mach Adaptable Rarefied Hypersonic and the wind
tunnel is recorded under this name under the European
portal MERIL