Shock Waves (1997) 7: 205 209
Rupture-disk-less shock-tube with compression tube
driven by free piston
T. Abe, E. Ogura, S. Sato, K. Funabiki
Institute of Space and Astronautical Science,Yoshinodai 3-1-1, Sagamihara, Kanagawa 229, Japan
Received 6 June 1996 / Accepted 6 October 1996
Abstract. A new technique is proposed for a shock tube driven complex factors, it is rather difficult to manufacture the rup-
by a freely moving piston. In a conventional free-piston-driven ture disk with the same specific performance. Hence, in the
shock tube, a rupture disk is employed between the compres- conventional Stalker tube, it is rather difficult to make a shock
sion tube and the shock generation tube. In the present method, wave or a hypersonic flow condition with good reproducibil-
however, the conventional rupture disk is replaced by a newly ity. Because of the same reason, it is also difficult to design in
developed fast action valve which is activated by the com- advance the rupture disk with a specific performance. Hence
pressed gas generated in the compression tube. The present the proper rupture disk must be selected on trial-and-error ba-
method enables us to generate high Mach number shock waves sis. Furthermore we must point out another drawback of the
of arbitrary strength with good reproducibility. The perfor- rupture disk; the burst of the rupture disk causes a scattering
mance of the new method is demonstrated experimentally. This of fragments. These fragments may cause damages to the fa-
also enables us to be carefree to scattering of fragments of the cility or undesirable effects on the experiment. In the present
rupture disk. paper, we propose a new method in which a conventional rup-
ture disk is replaced by a newly-developed fast action valve.
The performance of the fast action valve is demonstrated by
Key words: Rupture-disk-shock tube, Free piston, Stalker
a shock tube equipped with such a valve. Because of the fea-
tube, Fast action valve
ture of the fast-action valve, the present method enables us to
produce shock waves with arbitrary strength in good repro-
1 Introduction
ducibility. This also enables us to be carefree to scattering of
the fragments of the rupture disk.
Recently an interest for hypersonic flow research is revital-
ized aiming to develop new type recoverable space vehicles
which conduct hypersonic flight through the atmosphere. For
the development of such vehicle, ground facilities to realize hy- 2 Experimental set-up and a fast action valve
personic flow conditions are inevitable. To realize hypersonic
The schematical view of the experimental set-up is shown in
flow conditions on ground, the shock tunnel or shock tube is
Fig. 1. The facility is composed of 4 main parts; 1) a high-
useful even though only a short running time can be provided
pressure vessel to drive the free piston, 2) a compression tube
by such facilities. In the shock tunnel or the shock tube, a high
through which the free piston runs and compresses the gas, 3)
pressure gas source is inevitable to generate a shock wave.
a shock generation tube, and 4) a fast action valve between the
In the free-piston-driven shock tunnel known as Stalker tube,
compression tube and the shock generation tube.
the high pressure gas source is generated by compressing a
The high-pressure vessel to drive the free piston has a vol-
gas in the compression tube by means of the piston moving
freely through the tube (Stalker 1961, 1965). The compres- ume of 0.0867 m3 and is equipped with a fast action valve
actuated by an electromagnetic valve (Abe et al. 1992). Thus
sion tube is connected to the shock generation tube, in which
the start of the facility can be initiated by an external electrical
the shock is generated by using a rupture disk between them
trigger to open the electromagnetic valve. The compression
which breaks at a proper pressure level in the compression
tube is 5.3 m in length and is 82 mm in diameter. As shown in
tube. The strength of the shock wave thus generated depends
Fig. 2, inside the compression tube, is loaded the free piston
on the pressure level at which the rupture disk breaks. Hence
the reproducibility of the shock wave depends on the repro- which is a circular cylinder made of aluminum and is 3.72 kg
in weight. The weight of the free piston is adjustable by in-
ducibility of the burst phenomenon of the rupture disk. Since
serting a balance weight into an inner cavity of the free piston.
the burst phenomenon of the rupture disk is dominated by
On the fore and aft part of the piston, respectively, a pair of
Correspondence to: T. Abe the Teflon O-rings is mounted for the purpose of sealing the
206
Fig. 1. Experimental set-up
gap between the piston and the inner surface of the compres- by the pressure in the cavities, and the cylinders are moved
sion tube. The free piston is driven by the high pressure gas in aside; the fast action valve opens. The moving cylinders open
the high-pressure vessel, runs towards the end of the compres- the through-hole in the casing to connect the compression tube
sion tube and compresses the gas ahead of the free piston. The and the shock generation tube, and enables the compressed gas
motion of the free piston inside the compression tube is moni- to rush into the shock generation tube. Eventually the highly
tored by the optical measurement system the main component compressed gas entering into the shock generation tube gen-
of which is the paired optical fibers (see Fig. 2). A set of the op- erates the shock wave in the tube.
tical fibers is mounted in the wall of the compression tube at a
Since the cavity behind the cylinder in the casing reduces
location of 160 mm apart from the end of the compression tube.
its volume due to the backward motion of the cylinder, the
One optical fiber of the pair is used for transmission of light
pressure in the cavity increases with the opening motion of the
from an external light source into the compression tube and the
fast action valve. Even though the pressure behind the cylinder
other one is used for transmission of the scattered light from
rises by this backward motion of the cylinders and it strength-
the piston surface to an external sensor, a photo-multiplier. To
ens the force to push back the cylinder, the fast action valve
measure the temporal variation of the piston velocity during
does not open instantaneously but only sufficiently after the
its passage at the optical measurement system, a circular stripe
highly compressed gas enters into the shock generation tube,
pattern is carved on the piston surface (Komuro et al. 1995).
because 1) the pressure in the compression tube keeps rising
The stripe pattern modulates the scattered light, which enables
even after the initial opening of the fast action valve because
us to identify the stripe at the location of the optical measure- of the inertial motion of the free piston and 2) there is a mo-
ment system. This enable us to measure both the location and
tion delay of the cylinders due to the mass of the cylinders. The
the instant velocity of the free piston during its passage along
rapid opening motion of the fast action valve, however, still re-
the optical measurement system. At the end of the compression
quires cylinders of light weight. Hence, in the actual cylinders
tube, a ring-shaped obstacle made of Nylon is mounted inside
for the fast action valve, cavities are drilled in the cylinders
the tube in order to reduce the damage on the compression
from behind in order to reduce the mass of the cylinders as
tube in case of an emergency in which the free piston hits the
shown in Fig. 3. The additional cavities in the cylinders can
end of the compression tube. A pressure monitor is mounted
also reduce the pressure rise in the cavity in the casing behind
at the end of the compression tube.
the cylinders because of the increase of the cavity volume. To
Between the compression tube and the shock generation monitor the motion of the cylinders in the casing, the optical
tube, the newly developed fast action valve is inserted instead measurement system similar to the system for the free piston in
of a conventional rupture disk. A conventional rupture disk can the compression tube is equipped in the fast action valve. Also
also replace the newly developed fast action valve for compar- the pressure gauges are equipped to monitor the pressure rise
ison of the performance. The sketch of the fast action valve is in the cavity behind the cylinders. Between the compression
presented in Fig. 3. The fast action valve is composed of a pair tube and the fast action valve, a thin aluminum foil is inserted
of cylinders and the casing for them. The mass of a cylinder is for the purpose to tighten a seal between the compression tube
0.300 kg. The pair of cylinders are pressed against each other and the shock generation tube before the operation of the facil-
by pressurized gas charged into the respective cavities behind ity. As for the shock generation tube, we employed an exiting
the cylinders inside the casing. A through-hole is drilled at tube of 82 mm in diameter. Since the diameter is larger than
the center of the casing to directly connect the compression the through-hole in the fast action valve, we cannot expect an
tube and the shock generation tube when the pair of cylinders efficient generation of the shock wave. But the tube is suffi-
are separated and the fast action valve opens. The pressur- ciently useful for the purpose only to monitor the shock wave
ized gas in the compression tube is able to generate a force generated by the highly compressed gas through the fast ac-
to move the pair of cylinders against the adverse force due to tion valve. The speed of the shock wave is measured by using
the pressure in the cavities. When the pressure at the compres- a pair of pressure gauges located at 2.5 m downstream of the
sion tube rises and reaches a certain value, its opening force fast action valve with a separation distance of 100 mm between
acting on the cylinders overcomes the pressing force supplied them along the tube.
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Fig. 2. Sketch of the free piston and the optical measurement system
to monitor the free piston motion in the compression tube
Fig. 3. Sketch of the fast action valve
Fig. 4. Typical signals to monitor the operation
3 Results
we can see that the free piston, reaching a speed of 83.3 m/sec
at the measuring point, is decelerated continuously and halts
Air was used as working gas for the facility. To demonstrate
at a location of around 45 mm from the end of the compression
a performance of the present facility, we select the following
tube. The pressure of the compressed gas reaches a value of
variables as free parameters of the operation. One is the pres-
17.6 MPa at the peak. Just before the compressed gas pressure
sure for the high-pressure vessel and another is the pressure
reaches its peak value, the fast action valve is initiated to open.
behind the cylinders in the fast action valve. The pressures of
The pressure of the compressed gas keeps rising even after the
the compression tube and the shock generation tube are fixed
initial opening of the fast action valve. Parallel to this, the fast
at 0.101 MPa and 8.0 kPa, respectively. The monitor signals
action valve keeps increasing its opening width. Since, once
are displayed in Fig. 4 for a typical run of the facility. In this
the fast action valve opens widely, the highly compressed gas
run (the experiment number 21507 in Table 1), the pressure of
in the compression tube rushes into the shock generation tube,
the high-pressure vessel is 1.5 MPa and the pressure behind
the pressure of the compressed gas goes down sharply after it
the cylinders in the fast action valve is 3.0 MPa. In the mon-
reaches the peak value.
itor signals of the free piston and the cylinder motions in the
fast action valve, we can see modulated signals composed of As described above, the cylinders inside the fast action
pulses of various duration which is generated by the scattering valve begin to move just before the compressed gas reaches
light modulated due to the circular stripe patterns on the free its peak pressure. After 1.79 ms from the initial valve opening
piston or the cylinders in the fast action valve. From the signal, motion, the valve reaches an almost full-open status. After al-
208
most all the compressed gas escapes into the shock generation requirement for the high pressure behind the cylinders, how-
tube, the valve starts to close again. The opening duration of ever, violates the second requirement. That is, due to the high
the valve is sustained during about 3.20 ms. The opening mo- pressure behind the cylinders, the opening width and open-
tion of both the cylinders are well synchronized to each other ing duration of the fast action valve is reduced. In the results
since the cavities are connected by a pipe so that the pres- listed in Table 1, we can see that the higher back pressure
sures in both the cavities keep balance. The pressure behind gives rise to the higher compression because of the fulfillment
the cylinders rises simultaneously with the opening motion of of the first requirement. Within the present experimental con-
the cylinders. The peak value for the back pressure behind the ditions, the higher back pressure gives rise to a faster shock
cylinder is 8.31 MPa which is expected from the volume ra- speed, even though it causes a narrower opening width and a
tio between the cavity volumes in the casing allocated for the shorter opening duration. This is because, within the present
completely closed and opened configuration. When the valve experimental conditions, the higher compression attained in
closes again after opening, both heads of the cylinders collide the compression tube compensates the drawback due to the
with each other and the cylinders stick to each other as seen narrower opening width. However, it is expected that a back
from the monitor signals for the back pressure and the motion pressure higher than in the present experimental conditions
of the cylinders. The visual inspection of the valve, however, will cause, sooner or later, a slower shock speed because of
exhibits no damage on it. Also the shutting of the valve does the narrower opening width and the shorter opening duration.
not have any impact on the shock generation performance be- In this sense, there must be an optimal pressure behind the
cause it occurs only after almost all the highly compressed cylinder in the fast action valve corresponding to a given pres-
gas has escaped into the shock generation tube. Nevertheless sure value in the high-pressure vessel. The higher pressure in
we can see that some gas still remains in the compression the high-pressure vessel can give rise to a stronger compres-
tube as can be seen from the slight pressure rise in the com- sion of the gas in the compression tube. Hence, when we select
pression tube after the valve closed. The highly compressed an appropriate pressure behind the cylinders in the fast action
gas generated by the free piston rushes into the shock gener- valve against the pressure in the high-pressure vessel, this en-
ation tube when the fast action valve is open, and generates ables us to obtain a faster shock speed. As can be seen from
the shock wave there. As shown in Fig. 4, the pressure mea- Table 1, the appropriate pressure behind the cylinders in the
surement shows that the shock wave is generated properly and fast action valve shifts to higher values with higher pressures
there is an arrival time difference at the pressure ports sepa- in the high-pressure vessel. This is because the higher back
rated by 100 mm along the tube. This gives a shock speed of pressure is required in order to keep the valve shut until the
1.31 km/sec for this run, which coincides with the shock speed higher compression is attained in the compression tube due to
estimated from the pressure rise behind the shock wave. The the higher pressure in the high-pressure vessel. It is expected
summary of the experimental results is collected in Table 1. that the higher pressure in the high-pressure vessel gives rise to
As can be seen from the results for the same operational con- the more rapid motion of the cylinders in the fast action valve,
ditions, the reproducibility of the shock speed is reasonable. which may cause damage to them and may shorten their life
As demonstrated in the successful operation conditions listed time. Under the present experimental conditions, however, no
in Table 1, the various shock speeds can be attained by vary- significant damage on the fast action valve can be observed.
ing the pressure of the high-pressure vessel and the pressure Therefore, it can be expected that the harder operational con-
behind the cylinders in the fast action valve. ditions may be applicable to the present fast action valve. The
results obtained from the facility equipped with a conventional
In Table 1 PD is Drive pressure in the high-pressure vessel,
rupture disk are collected in Table 2. An iron plate of 1.5 mm
PV : Valve back pressure (the pressure behind the cylinder in
in width is used as rupture disk with a cross groove on its
the fast action valve), Pmax: Peak pressure in the compression
surface. The performance of the rupture disk depends on the
tube, Pi: Pressure at which the fast action valve initiates its
depth of the groove. Within the experimental conditions, the
motion, tp: Mean half duration of the pressure development
rupture disk with the shallower groove gives rise to higher
in the compression tube, tv: Duration of the open status of the
compression and faster shock speed. The shock speed avail-
fast action valve, W : Opening width of the valve, tD: Delay
able, under corresponding experimental conditions, is rather
time from the initial motion of the valve to the peak pressure
faster than the value obtained from the facility equipped with
in the compression tube, Vs: Speed of the shock wave.
the present fast action valve. This suggests that, in the present
There are two major requirements on the fast action valve
fast action valve, there still exists a slight loss in the procedure
in order to effectively generate a shock wave in the shock
of generating the compressed gas and its release into the shock
generation tube; 1) to be closed until a compression as high
generation tube, in comparison with the facility equipped with
as possible is attained in the compression tube, 2) to open as
the conventional rupture disk.
widely, speedily, and long as possible once the necessary high
compression is attained. The former is necessary to generate a In Table 2 PD is Drive pressure in the high-pressure vessel,
shock wave as strong as possible while the latter is necessary Wt: Thickness of the rupture disk, Ws: Groove depth on the
to make use of the highly compressed gas as effectively as pos- rupture disk, Pmax: Peak pressure in the compression tube,
sible. Both requirements, however, are contradictory; in order PR: Pressure at which the rupture disk breaks, tP : mean half
to attain the first requirement, the pressure behind the cylin- duration of the pressure development in the compression tube,
ders in the fast action valve must be as high as possible. The Vs: Speed of the shock wave.
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Table 1. Operational conditions and test results in case of the fast action valve
PD PV Pmax Pi tp tv W tD Vs Experiment No.
[MPa] [MPa] [MPa] [MPa] [ms] [ms] [mm] [ms] [km/sec]
1.7 8.0 35.3 29.0 1.98 2.14 8<"10 0.598 1.62 22602
1.7 8.0 36.9 29.4 1.86 2.14 8<"10 0.652 1.60 22604
1.7 7.0 33.3 22.3 1.90 2.42 10<"12 0.628 1.57 22601
1.7 7.0 33.7 23.9 1.97 2.36 10<"12 0.630 1.54 22603
1.7 6.0 33.7 24.3 2.01 2.46 10<"12 0.646 1.58 22701
1.7 6.0 32.9 23.1 2.00 2.52 10<"12 0.646 1.59 22702
1.5 5.0 25.3 18.0 2.31 2.86 10<"12 0.818 1.56 21503
1.5 5.0 24.7 16.5 2.35 3.01 10<"12 0.732 1.45 21505
1.5 4.0 21.6 12.4 2.63 3.54 10<"12 0.818 1.42 21502
1.5 4.0 20.8 12.4 2.63 3.68 10<"12 0.778 1.38 21506
1.5 3.0 18.2 9.80 2.56 3.92 14<"15 0.906 1.31 21507
1.5 3.0 17.6 9.21 2.64 4.02 14<"15 0.858 1.29 21504
1.3 4.0 17.2 12.2 3.31 3.81 8<"10 0.942 1.32 21601
1.3 4.0 17.6 15.1 3.31 3.61 10<"12 0.9 1.38 21603
1.3 3.0 14.5 11.6 3.49 10<"12 1.02 1.21 21605
1.3 3.0 14.9 11.8 3.48 10<"12 1.05 1.21 21606
1.3 2.0 11.2 7.64 3.98 14<"15 1.12 1.20 21602
1.3 2.0 10.3 6.08 4.19 4<"15 0.906 1.12 21604
Table 2. Operational conditions and test results in case of the rupture disk
PD Wt Ws Pmax PR tp Vs Experiment no.
[MPa] [mm] [mm] [MPa] [MPa] [ms] [km/s]
1.5 1.5 0.6 31.8 31.8 1.86 1.89 22703
1.5 1.5 0.8 22.3 22.0 2.27 1.73 22801
1.5 1.5 1.0 17.7 16.7 2.55 1.70 22802
1.5 1.5 1.1 14.9 12.7 3.66 1.56 22803
1.5 1.5 1.1 15.2 14.9 3.87 1.51 22901
1.5 1.5 1.1 14.9 14.5 3.68 1.53 22902
1.5 1.5 1.1 14.9 13.7 3.68 1.54 22903
4 Conclusion References
Abe T, Funabiki K, Oguchi H (1992) A combined facility of ballistic
In the present paper, we proposed a new method utilizing a
range and shock tunnel using a fast action valve. in: Takayama
fast action valve instead of a conventional rupture disk. The
(ed) Shock Waves 1025 1030
performance of the fast action valve is demonstrated in the
Komuro T, Sato K, Tanno H, Ueda S and Itoh K (1995) Pilot free
free-piston-driven shock tube. The present method enables us
piston shock tunnel. Proc 27th Fluid Dynamics Conference, at
to produce a shock wave with good reproducibility and also to
Kagamihara, Japan.
easily produce a shock wave with arbitrary strength. This also
Stalker RJ (1961) An investigation of free piston compression of
enables us to be carefree regarding scattering of fragments of
shock tube driver gas. National Research Council of Canada,
the rupture disk. The shock speed available from the present
MT-44
method is slightly slower in comparison with the case using the
Stalker RJ (1965) The free-piston shock tube, Aeron Quart 17:351
rupture disk instead of the present fast action valve. However,
the drawback in the present method can be compensated by
its merits, such as good reproducibility and easy generation of
This article was processed by the author using Springer-Verlag TEX
shock waves of arbitrary strength. PJour2g macro package version 1.1