Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
FLUID STRUCTURE INTERACTION OF FUEL TANK IMPACT
Caroline M. SEDDON, Moji MOATAMEDI
*
L Stress Analysis Research Group, Institute for Materials Research, The University of Salford, Greater
Manchester, M5 4WT
[email protected]
1 Introduction
The High speed projectile impact upon structures can lead to severe structural damage
resulting in catastrophic failure, often causing huge loss of life. Aircraft structures are particularly
vulnerable to high speed projectile impacts, resulting from uncontained engine debris following
engine failure, objects encountered during flight such as bird strike or hail and impacts with runway
debris during take off or landing. One particular area of concern is the impact of such high speed
projectiles upon the aircraft fuel tank.
Several catastrophic accidents have occurred in which high speed projectiles have directly
impacted the fuel tank of the aircraft. In 1985 an accident occurred in which uncontained engine
debris impacted the fuel tank of a Boeing 737 during take off at Manchester International Airport
[1]. The impact caused rupture of the fuel tank causing a fire within the cabin. In 1995
uncontained engine debris impacted the fuselage of a McDonnell Douglas DC-9 causing failure of
the fuel line [2]. This failure led to a cabin fire causing serious injuries to the passengers and crew.
The most recent accident involving fuel tank impact was the incident involving the Air France
Concorde at Paris Charles De Gaulle Airport in July 2000. This particular accident appears to have
resulted from a fuel tank rupture caused by an impact from a piece of burst tyre. The subsequent
accident investigations have suggested that the projectile did not penetrate the fuel tank upon
impact causing rupture. It was however, suggested that the initial impact occurred between two
adjacent wing ribs resulting in the fuel tank skin initially deflecting inwards and causing the
adjacent panel to deflect outwards. At the same instant a strong hemispherical shock wave
resulting from the impact was transmitted through the fuel, its movement combining with the
outwardly deflecting panel inducing stresses sufficient enough to cause a localized failure of the
tank.
In the mid 1990’s research concerning the impact of aircraft fuel tanks was undertaken by the
Federal Aviation Authority [3]. This study highlighted the poor recognition of the fuel tank failure
modes in aircraft accidents for which fuel tank rupture had been identified. The subsequent
investigations, however, assumed that the projectile impacts the fuel tank with sufficient kinetic
energy to puncture and penetrate the tank causing hydrodynamic ram, an overpressure produced by
the motion of the projectile within the fuel. In 1999 further work was undertaken by Anderson et al
[4] which considered the numerical modelling of hydrodynamic ram, again assuming that the
projectile fully penetrates the tank causing overpressure. Recent research undertaken specific to
aircraft structures has mainly been concerned with the numerical modelling and validation of the
failure of flat skin panels upon high speed impact [5-7]. It is understood that the probable failure
mechanism discovered in the Concorde accident has yet to be fully confirmed both experimentally
and numerically.
The suggested failure mechanism of transient fluid structure interaction has relevance to many
other industries such as rail, chemical, nuclear and offshore industries, in which risks are posed to
safety critical structures. It is therefore becoming increasingly important to be able to model and
predict such phenomenon. Analytical solutions can provide only a limited understanding of the
nature of the behaviour; therefore it is necessary to investigate the phenomenon experimentally and
to assess both the suitability and predictive capabilities of numerical modelling techniques such as
finite element analysis for the analysis of transient fluid structure interaction.
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
The work reported here describes the development of an experimental test rig and test
procedures loosely modelling the Concorde accident. Next a numerical modelling technique
utilising full fluid structure interaction is described and is applied in the modelling of the
experiment. Three cases were examined in which a full tank was impacted at 20 m/s and a partially
full tank was impacted at 14 m/s and 21m/s. The experimental and numerical results are compared
in order to assess the validity and accuracy of the numerical modelling methodology.
2 Expriment
Test Facility
The Health and Safety Laboratory experimental set-up consisted of a gas gun facility, the main
parts of which can be seen in Fig. 1. The gas gun was 6m long with a smooth bore 75mm internal
diameter barrel. The barrel was manufactured from stainless steel tube having a theoretical burst
pressure in excess of 30 MPa, and was hydraulically pressure tested to 8 MPa.
.
FIG. 1 – 75mm Shock Tube Arrangement
The gas gun was operated by charging the 2.5m long driver section with compressed air up-to
maximum pressures of 3.8 MPa, when a commercially available bursting disc, situated at the end of
the driver section, failed discharging the air into the barrel of the gas gun. To achieve lower
pressures of approximately 1 MPa the bursting discs were replaced with a pneumatically actuated
50mm ball valve.
In order to ensure a high degree of safety the projectile was fired through a trap consisting of a
length of 20 cm diameter perforated steel pipe. This prevented the projectile from escaping into the
room housing the gun, if the projectile should recoil from the test vessel. The perforations allowed
for the use of high speed photography, providing a limited visualization of the projectile impact,
and also provided some relief from the muzzle blast. The instrumented test vessel including the
perforated section of the gas gun can be seen in Fig. 2.
FIG. 2 – Instrumented Test Vessel and Gas Gun
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
Projectiles and Velocity Measurement
The original test specification required projectiles weighing approximately 4.5 kg to provide
continuity with the Concorde accident. The weight was, however, reduced to 2.5 kg as even at the
lowest velocities the heavier projectiles caused failure of the tank. The projectiles were machined
from cast nylon-6 rod for which a detailed list of material properties was available. Following
machining the projectiles were 50cm long with a diameter of 74mm.
The impact velocities of the projectiles were recorded using an optical system based upon a
split laser light beam which was directed through two holes in the gun barrel near to the muzzle.
The light beam was detected by two photodiodes for which the outputs were monitored by an
electronic counter. The velocity of the projectile was then determined from the time taken for both
light beams to be broken. For some of the test cases the velocity was also determined from high
speed video recordings of the tests However, there were discrepancies between the two
measurement techniques. The reasons for the discrepancies were not established.
Test Vessel
The test vessel comprised of a rectangular steel tank of internal dimensions 2 x 1 x 1 m. The
vessel was constructed from 8mm mild steel and was designed to have rigid sides with flexible
front and back panels in order to simplify the later numerical modelling. The flexible front and
back panels were secured with bolts enabling interchanging of the panels, allowing a number of
materials / thicknesses to be tested. The vessel was stood vertically a short distance from the gas
gun (see Fig 2.) with one of its square faces secured to the ground, to enable the impact response of
the flexible front and back surfaces to be determined. The close proximity of the test vessel to the
gas gun ensured that a planar impact occurred. The vibrations arising during the impact were
measured with Kistler piezoelectric accelerometers. The pressures within the fluid were measured
using 0-250 bar Kistler piezo-resistive pressure transducers.
P3
P2
Acc B
Face C (rear)
Face A (front)
P1
Impact Point
Acc A
y
z
x
(0, 0, 0)
Face B
Impact Point
597, 560, 0 (Face A)
Accelerometer A
380, 560, 0 (Face A)
Accelerometer B
560, 1440, 0 (Face A)
Pressure Transducer 1
0, 1970, 500 (Face B)
Pressure Transducer 2 200, 1593, 900 (suspended inside tank)
Pressure Transducer 3 910,563,900 (suspended inside tank)
FIG. 3 – Test Vessel Instrumentation
In early tests all of the transducers were attached to the tank walls, however, in later tests two
of the pressure transducers were suspended inside the tank in an attempt to de-couple the extreme
vibrations. The Final test vessel instrumentation can be seen in Fig. 3.
The test data was obtained using a 4000 series micro-link system configured to a logging rate
of 200 kHz. The data collection commenced upon the breaking of the first light beam used in the
velocity measurement.
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
3 Numerical Modelling
Fluid Structure Interaction
The numerical models were developed using LS DYNA explicit dynamics software [8]. This
particular software package was selected due to the use of explicit formulation which is particularly
suitable for problems with short duration such as impact or explosion. The programme also
enables the modelling of fluid structure interactions using Arbitrary Lagrangian Eulerian ALE
formulation with a fluid structure coupling algorithm.
Typically structural finite element analysis uses Lagrangian formulation. In pure Lagrangian
finite element analyses the computational mesh moves with the material, automatically following
the material deformation. This method is only efficient and accurate for small to moderate
deformations. In the case of large deformation problems the elements become highly distorted,
reducing both accuracy and the stable time step size of the solution. In the case of the fuel tank
impact problem if both the fluid and the structure are modelled using a Lagrangian formulation the
fluid elements in the impact region become severely distorted, reducing the stable time step size,
significantly increasing the CPU time.
An alternative method is employed within LS-DYNA in which a single or multi material
Eulerian formulation can be used to eliminate mesh distortion in the fluid. In an Eulerian
formulation the finite element mesh is fixed in space and the material is able to flow through the
fixed mesh hence eliminating the problems associated with mesh distortion. However, as the
material is transported from one element to its neighbour nonlinear transport terms are introduced
into the problem. The transport of mass, momentum, internal energy and state variables must be
computed using advection algorithms such as the donor cell algorithm, a first order scheme or the
Van Leer algorithm a second order advection algorithm.
Within the LS DYNA programme the ALE formulation is implemented using an approach
referred to as an Operator Split. Firstly a Lagrangian step is performed in which the finite element
mesh deforms with the material. In the second phase, the advection step, the solution is mapped
from the distorted Lagrangian mesh back onto the original mesh for a single or multi material
Eulerian formulation. The solution can also be mapped onto an arbitrarily distorted mesh for an
ALE formulation.
In the case of the fuel tank impact the fluid inside the tank can be modelled using an Eulerian
formulation for which the computational mesh is fixed. The tank structure can be modelled using a
Lagrangian formulation as the mesh deformation is not too severe. Euler Lagrange coupling is then
employed to enable the modelling of the interaction between the Lagrangian structure mesh and an
Eulerian fluid mesh.
The fluid structure coupling algorithm implemented within the LS DYNA programme allows
the fluid material to flow around the structure but prevents penetration of the fluid material into the
structure mesh. The flow of the fluid is prohibited from flowing through the structure using a
penalty coupling algorithm. In this algorithm penalty forces are applied to both the fluid and the
structure. As soon as a fluid particle penetrates into a Lagrangian structure a force of recall is
applied to both fluid particle and structural node in order to return the fluid particle to the surface of
the structure hence preventing penetration. The penalty forces are proportional to the penetration
depth and penalty stiffness, behaving like a spring system.
Application to Fuel Tank
The previously described fluid structure coupling algorithm implemented within the LS DYNA
programme was used to model the Fuel Tank Experiment. Figure 4 below shows a cut –away
view of the finite element mesh for the case of the full tank. The fluid inside the tank is modelled
using solid elements with an Eulerian element formulation. The material is modelled using a null
material model with a Gruneisen Equation of State with the properties of water. Surrounding the
fluid material is a region of air. This region acts as vacant cells into which the fluid material can
flow during the impact. This region was also modelled using an Eulerian formulation, null material
model with a linear Polynomial Equation of State and was assigned the properties of Air. In order
to allow the water to flow into the air region during the impact the two meshes must share common
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
nodes at their interface. Next the tank structure mesh is superimposed upon the Eulerian fluid
mesh. The Tank was modelled using Lagrangian shell elements and was assigned the material
properties of steel. The projectile was modelled with rigid solid elements again with a Lagrangian
formulation and was assigned the material properties of nylon.
Eulerian
Fluid Mesh
Eulerian
Air Mesh
Lagrangian
Structure Mesh
superimposed
within fluid
FIG. 4 – Tank Finite Element Model – Cut Away View
The nodes on the base of the tank were fixed in all degrees of freedom as in the experiment.
The projectile was modelled initially touching the tank surface at the point of impact. The contact
between the projectile and the tank was modelled using a surface to surface contact interface. The
fluid structure interaction algorithm was employed, coupling the inside surface of the tank to the
fluid particles inside the tank, preventing the fluid from penetrating through the tank walls.
The analysis was run for 0.16 seconds with a time step size of 5 micro-seconds which
corresponded to the data capture time and the 200 kHz sampling rate of the data logger used in the
experiment
4 Results
Figure 5 below shows the experimental and numerical acceleration time history at the point of
impact for the partially full tank impacted at 14m/s
3.0E+04
2.5E+04
2.0E+04
Acceleration
1.5E+04
1.0E+04
5.0E+03
0.0E+00
-5.0E+03
-1.0E+04
-1.5E+04
-2.0E+04
Experim
-2.5E+04
-3.0E+04
0.00E+00
4.00E-02
8.00E-02
Tim
1.20E-01
Numerica
1.60E-01
Time
FIG. 5 – Acceleration Time History at the Point of Impact (ACC A) for Partially Full Tank 14m/s impact
From Fig 5. it can be seen that the numerical model accurately predicts the peak acceleration at
the point of impact. Good agreement can be seen between the experimental and numerical results;
however, the numerical results exhibit more noise than the experimental results. The addition of
damping to the numerical model may decrease the oscillations seen later in the solution.
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
Figure 6 below shows the experimental and numerical acceleration time history at the top of
the impacted panel (Acc B) for the partially full tank impacted at 14m/s.
1.40E+04
1.20E+04
8.00E+03
Acceleration
Acceleration
1.00E+04
6.00E+03
4.00E+03
2.00E+03
0.00E+00
-2.00E+03
-4.00E+03
-6.00E+03
-8.00E+03
-1.00E+04
0.00E+00
Numeric
Experime
4.00E-02
8.00E-02
Time
1.20E-01
1.60E-01
Time
FIG. 6 – Acceleration Time History at the top of the impacted panel (ACC B) for Partially Full
Tank -14m/s impact
Good agreement is again seen between the experimental and numerical results for the top of the
impacted panel. As expected the peak accelerations are much less than at the point of impact.
Again the numerical results display a more oscillatory nature particularly towards the end of the
solution. Numerical damping would improve the overall decay of the curve. The peak acceleration
measured in the experiment has been cut off above 5000 m/s2 due to the range of the accelerometer
selected. However, the overall shape of the remainder of the curve shows good agreement with the
numerical model.
Figure 7 below shows the experimental and numerical acceleration time history at the point of
impact for the partially full tank impacted at the higher speed of 21 m/s
FIG. 7 – Acceleration Time History at the Point of Impact (ACC A) for Partially Full Tank
21m/s impact
Again good agreement can be seen between the peak experimental and numerical acceleration
results for this higher impact speed. Again the numerical results would benefit from some
damping.
Figure 8 shows the experimental and numerical results for the second accelerometer at the top
of the impacted panel. As with the lower speed test the peak experimental acceleration is cut off at
±5000 m/s2. The actual peak acceleration was therefore not captured during the experiment. As
with the previous results the numerical results appear to be much more oscillatory than the
experiment results.
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
In comparison to the earlier case in which the projectile speed was lower there is not a great
difference in the peak acceleration, however, the numerical results are slightly more oscillatory for
the higher speed case than for the lower speed impact.
1.4E+04
1.2E+04
1.2E+04
1.0E+04
1.0E+04
Acceleration
1.4E+04
Acceleration
8.0E+03
6.0E+03
4.0E+03
2.0E+03
0.0E+00
8.0E+03
6.0E+03
4.0E+03
2.0E+03
0.0E+00
-2.0E+03
-2.0E+03
-4.0E+03
-4.0E+03
-6.0E+03
-6.0E+03
-8.0E+03
-1.0E+04
0.0E+00
4.0E-02
8.0E-02
1.2E-01
Numerica
-8.0E+03
Experim
1.6E-01
-1.0E+04
0.0E+00
4.0E-02
8.0E-02
1.2E-01
1.6E-01
Time
FIG. 8 – Acceleration Time
Time History at the top of the impacted panel (ACC
B) for Partially Full
Tank 21m/s impact
Figure 9 shows the acceleration time history at the point of impact (Acc A) for the final case in
which the full tank is impacted at 21m/s. It can be seen that the measured peak acceleration has
been cut off at ±20000 m/s2. The peak has been lost due to the incorrect selection of the
accelerometer range. Despite the loss of peak it can be seen that the predicted numerical peak was
lower than the measured peak acceleration. In this case the experimental results display more noise
than the numerical results.
Figure 10 shows the results for the top of the impacted panel from the final case in which the
tank was completely full and impacted at a speed of 20m/s. Again from the experimental results it
can be seen that the incorrect accelerometer range was selected for the test as the peak acceleration
has been lost. However, the remainder of the curve shows good agreement with the numerical
results. In this case the numerical results are slightly noisier than the experimental results.
FIG. 9 – Acceleration Time History Acc A Full Tank
6.0E+03
Acceleration
5.0E+03
Acceleratio
4.0E+03
3.0E+03
2.0E+03
1.0E+03
0.0E+00
-1.0E+03
-2.0E+03
-3.0E+03
-4.0E+03
Experime
-6.0E+03
0.0E+00
Time (s)
Numeric
-5.0E+03
4.0E-02
8.0E-02
FIG. 10 – Acceleration Time History Acc B – Full Tank
1.2E-01
Time (s)
1.6E-01
Résumé pour le 1ercolloque du GDR interactions fluide-structure – 26-27 sept. 2005
5. Conclusions
In conclusion the projectile impact of a fluid filled tank has been investigated experimentally
and numerically. The design of the large scale experiment was described and results were
presented for three cases in which a partially full tank was impacted at two projectile speeds, 14m/s
and 21m/s, and a full tank was impacted at 20m/s. A numerical modelling procedure was described
in which full fluid structure interaction using a penalty coupling algorithm was employed to couple
the fluid particles to the structural nodes. The numerical method was used to model the fuel tank
experiment.
Upon comparison of the experimental and numerical results it was shown that the peak
acceleration and overall shape of the acceleration time history has been predicted fairly accurately
by the numerical model. In some cases the experimental peak was not captured due to incorrect
selection of the accelerometer range during the experiment. In such cases the remainder of the
curves within the accelerometer range show good agreement with the numerical model.
In most cases the numerical results were more oscillatory than the experimental results. It is
thought that either using material damping or damping within the fluid structure coupling algorithm
will improve the numerical response.
Future work will include modelling the pressure time history and comparing the results with
the experiment.
ACKNOWLEDGMENTS
The authors would like to thank Health and Safety Laboratory Buxton for the use of the
experimental results.
References
[1] UK Aircraft Accident Investigation Board, 1985, “Report on the Accident to Boeing 737-236,
G-BGJL at Manchester International Airport on 22 August 1985”, Aircraft Accident Report form
the British Department of Transport No 8/88 (EW/C929).
[2] National Transportation Safety Board, 1995, “Uncontained Engine Failure/Fire ValuJet Airlines
Flight 597, McDonnell Douglas DC-9-32, Atlanta Georgia, Aircraft Accident Report NTSB
ATL95MA106.
[3] Moussa, N. A., Whade, M. D., Groznam D. E., et al., 1997, “The potential for Fuel Tank Fire
and Hydrodynamic Ram from Uncontained Aircraft Engine Debris” DOT/FAA/AR-96/9, US
Department of Transportation Federal Aviation Authority.
[4] Anderson, C. E., Sharron, T. R., Walker, J. D., et al, 1999, “Simulation and analysis of a 23mm
HEI Projectile Hydrodynamic Ram Experiment” International Journal of Impact Engineering, 36,
pp. 981-997.
[5] Knight, N. F., Jaunky, N., Lawson, R. E., et al, 2000, “Penetration Simulation for Uncontained
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[6] Ambur, D. R., Jaunky, N., Lawson, R. E., et al, 2001, “Numerical Simulations for High Energy
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