Download SHOCK TUNNEL FREE FLIGHT FORCE MEASUREMENTS USING

SHOCK TUNNEL FREE FLIGHT FORCE MEASUREMENTS USING A COMPLEX MODEL
CONFIGURATION
Klaus Hannemann (1), Jan Martinez Schramm (2), Stuart Laurence (3), Sebastian Karl (4)
(1)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, [email protected]
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, [email protected]
(3)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, present address: Department of Aerospace
Engineering, University of Maryland, College Park, MD 20742, USA, [email protected]
(4)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, [email protected]
(2)
ABSTRACT
The free flight force measurement technique is a very
attractive tool to determine forces and moments in
particular in short duration ground based test facilities.
With test times in the order of a few milliseconds,
conventional force balances cannot be applied here. The
technique has been applied in a number of shock tunnels
utilizing models up to approximately 300 mm in length
and looking at external aerodynamics. In the present
study the technique is applied using a complex 1.5 m
long hypersonic integrated supersonic combustion
ramjet (scramjet) engine consisting of intake, combustor
and thrust nozzle. For this type of engine the design
objective is a combustor with efficient mixing and
combustion within the shortest possible length, but still
robust enough to operate in various operational
conditions. In the framework of the EU co-funded
project LAPCAT II, a M=7.4 scramjet powered small
scale flight experiment (SSFE) configuration was
designed. Since free jet testing of the complete
combustion flow path is a mandatory step within the
design roadmap of future engines, ground based testing
of the SSFE engine was conducted in the High Enthalpy
Shock Tunnel Göttingen (HEG) of the German
Aerospace Center (DLR). This type of facility allows
duplication of flight conditions in excess of M=8. Here
tests were performed simulating Mach 7.4 flight
conditions in approximately 28 km altitude. The
numerically predicted thrust of the engine could be
confirmed in HEG by free flight force measurements
based on displacement measurements utilizing optical
tracking. Combining these experimental results with
computed aerodynamic data of the complete SSFE
showed that for a selected flight condition a positive
aero propulsive balance of the complete configuration
could be achieved.
balance). The design process of the scramjet engine is
reported in [6]. The main focus of the experimental
campaign in HEG was to measure the thrust generated
by the scramjet engine. The free flight model technique
in conjunction with optical tracking was applied. The
thrust increment of the engine was determined by
comparing the measured force in streamwise direction
obtained by fuel on and fuel off experiments. In HEG
the wings of the SSFE configuration were removed (see
Figure 1). Combining nose-to-tail computations of the
SSFE configuration and the HEG model configuration
conducted by DLR and ESA-ESTEC using the DLR
TAU code [7] with the DLR thrust measurements in
HEG, the aero propulsive balance of both configurations
could subsequently be analysed. While these results are
already discussed in [2], the focus of the present article
is on describing the complex model set-up and the
optical tracking procedure. Details regarding the
numerical modelling of the flow past the complete and
the truncated SSFE configuration (see Figure 1) in HEG
free stream conditions are given in [7].
1. INTRODUCTION
One goal of the EU co-funded research project
LAPCAT II [13] was the design of a M=7.4 supersonic
combustion ramjet (scramjet) powered small scale flight
experiment (SSFE) configuration. Within the design
phase of the SSFE the major goal was that the vehicle is
able to generate net thrust (positive aero propulsive
Figure 1. Schematic of the SSFE configuration (top)
and the wind tunnel model configuration with removed
wings used in the HEG test campaign (bottom)
A summary of the force measurement technique
development at HEG and an overview of the free flight
technique utilizing light and stiff models for external
aerodynamics investigations is given in [1].
2. EXPERIMENTAL SETUP
High Enthalpy Shock Tunnel Göttingen, HEG
The HEG of DLR is one of the major European
hypersonic test facilities. This free piston driven shock
tunnel ([3],[4]) was commissioned for use in 1991 and
has been utilized extensively since then in a large
number of national and international space and
hypersonic flight projects. Originally, HEG was
designed for the investigation of the influence of high
temperature effects such as chemical and thermal
relaxation on the aerothermodynamics of entry or reentry space vehicles. Subsequent extensions of the HEG
operating range included the establishment of a number
of low total specific enthalpy conditions and of
complete scramjet engine ground-based testing
capabilities [3].
pulse of gas to a hypersonic convergent - divergent
nozzle at stagnation pressures of up to 200 MPa, and
stagnation enthalpies of up to 23 MJ/kg. Regarding the
test gas, no basic limitations exist. The operating
conditions presented in the present article are related to
the test gas air. Additionally, operating conditions using
nitrogen and carbon dioxide exist. Additional
information about the working principle of the facility,
its capabilities and former use can be found in, e.g., [3],
[4]. The HEG operating condition used in the presented
experimental investigation is HEG Condition XIII. In
Table 1, the nominal reservoir and free stream
conditions are given.
p0
[MPa]
17.7
h0
[MJ/kg]
3.24
T0
[K]
2695
P
T
[Pa]
[K]
2024
264

[g/m3]
27.2
u
[m/s]
2398
M
[-]
7.36
Table 1. Nominal reservoir and free stream for HEG
operating condition XIII
Figure 2. Photographic views of HEG
Wind Tunnel Model Setup
Compared to the utilization of wind tunnel models
which are derived, e.g., from capsule type
configurations (see, e.g., [1]), the configuration and the
objective of the measurements considered here imply
additional requirements. The SSFE engine is
approximately 1.5 m long and in order increase the
stiffness and to reduce the weight as much s possible,
the main body of the wind tunnel model was
manufactured from a solid block of aluminium (see
Figure 4). The manufacturing of this part occupied a
seven degrees of freedom CNC bed-type milling
machine for a period of four month.
Figure 3. Schematic of HEG
The overall length of HEG is 62 m and it weighs 280 t.
Approximately a third of its weight is contributed by an
inert mass (see Figure 2, left picture) which is used to
reduce the tunnel recoil motion. The compression tube
is closed by a hydraulic oil system at the main
diaphragm station. The shock tube is connected to the
nozzle of the tunnel at the downstream closure, which is
also driven by oil hydraulics to close and seal the
tunnel. The compression tube has a length of 33 m and a
diameter of 0.55 m. The shock tube is 17 m long with a
diameter of 0.15 m. The HEG was designed to provide a
Figure 4. SSFE wind tunnel model main body during
manufacturing in the DLR workshop; in order to
enhance the stiffness of the model it was machined from
a solid block of aluminium
An additional requirement is to implement suitable
mechanisms to both release the model allowing freeflight during the test duration, and terminate its motion
following the conclusion of this time period. In Figure 5
the hanging mechanism attached to the central beam of
the HEG test-section roof is shown. The model is
initially suspended in the test section by means of two
kevlar/aramide threads. The longitudinal separation
between the two threads is 590 mm, and a thread
diameter of 0.8 mm was chosen to support the 85 kg
heavy model. Each of the threads is arranged in a Vconfiguration passing through the model body and then
attached on either side to the appropriate clamping
mechanism. Employing V-configurations provides
better alignment stability in the roll and yaw axes than
single attachment points on the model. The use of a
front-and-back two thread configuration, together with
height adjustment at each of the clamping devices,
means that the model height and angle of attack can be
adjusted independently of one another and an
appropriate initial alignment of the model relative to the
support legs as well as to the flow direction can be
easily obtained. In Figure 6 the attachment points of
the front thread to the hanging mechanism as well as the
two points of entrance of this thread into the model
body are shown.
that this will have a negligible effect on the angle of
attack. For example, a 2 ms delay would produce a
worst-case change of 5x10-4 deg.
The model catching mechanism comprises two
cylindrical buffers, each mounted between a pair of legs
(see Figure 7). Each buffer consists of a steel rod with
two O-rings of slightly larger diameter attached to
provide a damped termination to the model motion
without damaging the internal model components. The
legs not only provide supports for the buffers, but also
provide routes by which the fuel line and any required
cables (depending on the particular configuration
chosen – fully free flying or weakly constrained
configuration) can enter into the model without being
exposed to the flow. In order to allow the different
initial angles of attack required by the experimental
campaign, the height of both buffers can be adjusted via
slots in the legs.
Figure 7. Model catching mechanism; cut through the
model showing the leg-mounted buffers used to
terminate the model motion
Figure 5. SSFE model hanging and height adjustment
components
The hydrogen injection system must be fully integrated
into the model. Hydrogen is injected into the combustor
by two semi struts at the front part of the combustor and
one full strut further downstream [7]. The hydrogen
supply system of the wind tunnel model consists of two
individual injection units. Each unit supplies one strut
assembly. The schematic given in Figure 8 shows the
setup exemplary for one hydrogen flow path.
Figure 6. Attachment points of the kevlar threads at the
hanging mechanism (left) and the model (right)
The height adjustment on both supports is realized by
means of a central travelling screw with a guiding pin to
either side to ensure alignment is maintained. To allow
free flight during the test time, some method of
detaching the threads is necessary. For this purpose
razor blades are incorporated in the model close the
entrance points of the threads (right part of Figure 6).
The razor blades are mounted horizontally directly
behind the threads; when the flow arrives, it will push
the threads back against the razors, shearing them off
without leaving any significant excrescences on the
model surface. There may be a delay between the
cutting of the front and rear threads, but it can be shown
Figure 8. Schematic of the hydrogen injection system
The hydrogen is supplied through a valve to a pipe
crossing the wind tunnel wall. The pipe is hooked up to
a thin and flexible capillary pipe. The capillary line is
attached to the wind tunnel model and supplies the fuel
to one litre steel fuel tanks (see Figure 9). Once the
system is filled to the required pressure, the external
valve is closed and the hydrogen is injected into the
plenum via a fast acting solenoid valve a few
milliseconds before the HEG run is initiated. The
capillary pipe connection to the model was designed
such that no impact on the model is generated during the
free flight phase in case the weakly constrained
configuration is applied. This was confirmed by
dedicated test runs in HEG (see Figure 17).
Figure 9. Hydrogen injection system integrated in the
free flight wind tunnel (photo of the system (top) and
schematic (bottom): (1) Pipe, (2) tanks, (3) pipe, (4)
solenoid valve, (5) pressure tap, (6) plenum chamber
below injector blocks.
HEG run, i.e., to combine the force measurements with
combustion flow path pressure measurements, the
model is additionally equipped with 40 pressure
transducers distributed on the intake-, combustor- and
nozzle-section (Figure 10). In the combustor, all sensors
are mounted on the side-walls; this choice was made
based on CFD predictions showing that the effect of
reflected shocks is much reduced in comparison to the
lower wall, meaning that the measured pressure will be
much less sensitive to the exact location of the shocks.
This should, therefore, provide a more meaningful
comparison with CFD results. In the nozzle section, at
several downstream positions pressure gauges are
located on both the nozzle centreline and sidewall; by
this point downstream, the pressure gradients on the
lower wall have become somewhat shallower in the
downstream
direction,
making
lower-wall
measurements more worthwhile here. For realizing the
completely free flying arrangement, on board data
acquisition units are required. Due to constraints
regarding the available space in the model the number
of units and consequently the number of data channels
are limited. In order to measure pressures at all selected
locations simultaneously in one run [12], wire
connections to an external data recorder are needed
resulting in a weakly constrained arrangement.
Figure 11. Schematic of the free flight wind tunnel
model installed in the HEG test section
Figure 10. Pressure sensor locations on the SSFE
model intake (top), combustor (middle) and nozzle
(bottom)
In order to obtain additional information from each
The integration of the free flight model in the HEG test
section is shown schematically in Figure 11. The inlet is
positioned in the core flow (test flow rhombus) of the
hypersonic nozzle of HEG. However, the downstream
part of the model is exposed to non-uniform flow
outside the test rhombus, and, therefore, does not
correspond to the situation experienced in flight
conditions. For that reason differential force
measurements using fuel on and fuel off conditions
which allow eliminating the influence of the nonrepresentative flow past the model outside the test
rhombus were conducted. Consequently, the tests in
HEG provide the thrust increment which is generated
solely by the internal propulsion flow path.
3. OPTICAL TRACKING
The displacement of the model was recorded using a
visualization based tracking technique. A symmetric
trapezoidal shaped tracking object was installed on the
model. This object is positioned on the top surface of
the model just downstream of the cowl leading edge
such that it lies inside the field of view of the HEG
windows (Figure 12). The length of the top face of the
object is 10 mm; the two sides have an angle of 45
degree and the overall height is 14.5 mm. The tracking
object movement was visualized using the HEG
Schlieren setup, consisting of a conventional Z-fold
arrangement as shown in Figure 12. Additional
information about the Schlieren setup and its utilization
can be found in [4] and [11]. The Schlieren setup was
adjusted such that the target was imaged on the camera
chip covering the sensitive area. A Phantom v1210
digital camera was used. Two 1.5 m focal-length
spherical mirrors (H1) collimate the beam from the light
source to pass through the test section and then refocus
it on the opposite side. The focal plane (FP) of the H1mirrors lies in the middle of the model on the tracking
object.
leading edge and ensured that any unstart of the intake
would be detectable. The light source employed was a
Cavilux Smart laser which provided repetitive 10 ns
pulses at 690nm. The Phantom v1210 camera recorded
the images, typically at 25,000 fps with a resolution of
896x512 pixels. A narrow band-pass filter was placed in
the optical path slightly ahead of the knife-edge in order
to remove any extraneous light, e.g., from tunnel selfluminosity. The visualization system also allowed
observing the detachment of the front support threads.
In Figure
13, three images resulting from one
experiment are shown. The initial state of the model
(before the arrival of the flow), the detachment of the
front threads, and the clean configuration during the test
time are visualized. In most of the HEG run the threads
completely detached well before the onset of the steady
test time. The basis of the methodology pursuit to
evaluate the motion of the model is given in [1], [8], [9]
and [10].
Figure 13. Image sequence showing the tracking object
and cowl leading-edge region; (top) before flow arrival;
(middle) during thread detachment; (bottom) during
steady test time
Figure 12. Position of tracking object on the SSFE
model in HEG (top); setup of the Schlieren visualization
system used for optical tracking of the model motion
(bottom)
The focal point is denoted by (R), and lens (L) is used to
focus the image on the camera chip (FP). A vertical
knife edge was placed at the focal point, which allowed
the visualization of the flow structures at the cowl
In order to locate the contour of the model, the recorded
images are treated with a Sobel convolution mask [14]
which determines the first order derivative of the pixel
intensity in vertical and horizontal direction. This filter
includes a discretised Gaussian smoothing. An example
is given in Figure 14. Here an image recorded prior to
the run is shown after treatment with the Sobel-Filter.
The identified model contour is clearly visible. Here the
pixel intensity represents the magnitude of the
computed combined gradients in horizontal and vertical
directions.
acceleration within this time period are denoted by the
bold black lines in Figure 16.
0.25
0.10
0.20
0.08
0.06
u [m/s]
Figure 14. Tracking image after application of a SobelFilter (original image given in Figure 13 (top))
x [mm]
0.15
0.10
0.04
0.05
0.02
250
0
1
2
3
4
5
6
7
8
0.00
0
1
2
3
4
5
6
7
8
t [ms]
t [ms]
30
25
20
15
2
10
5
0
350
300
0.00
a [m/s ]
The next step in the processing procedure is to mark all
pixels with intensity above a user defined threshold. The
result of this process is shown in Figure 15. In the top
plot, all marked pixels are shown in grey. The pixels
marked in red and blue are subsets which the tracking
algorithm identifies as pixels defining the model
contour. This is done by comparing the analytical
description of the tracking object with the coordinates of
the marked pixels.
pixels with a gradient value above
user definded treshold
pixels afer first optimization step
‐5
‐10
1
2
3
4
5
t [ms]
6
7
8
y [Pixel]
200
Figure 16. Displacement (top left), velocity (top right)
and acceleration (bottom) of the tracking object in x
direction
150
100
50
Figure 15. Algorithm to identify pixels describing the
tracking object; all pixels with a gradient value above a
user defined threshold and set of contour pixels after
first optimization step (upper); final set of pixels
describing the contour (lower)
The free flight technique applied to the SSFE model in
HEG posed several challenges on the optical tracking
technique. The model size of the SSFE is larger than the
optical windows available at HEG, which means that
only a part of the model (the tracking object) could be
visualized. The assumption has to be made that the
motion of the tracking object represents the motion of
the complete model. Due to the mass of the model, the
resulting motion is small. In Figure 16, it can be seen
that the distance the model moves during the test time is
only a fraction of a millimetre. The intake crotch region,
the detachment process of the threads and the motion of
the tracking object had to be visualized simultaneously.
The observation of the intake crotch region is important
to monitor the operation mode of the intake, i.e., to
identify in case the intake would show unstart during
the test time. The behaviour of the threads allows
judging the release time of the model, and the target
itself is used to determine the motion of the complete
model, as already discussed.
An example for the measured movement in x-direction
of the tracking object for a fuel-off run is shown in
Figure 16 in the upper plot. The displacement is
smoothed by applying a Savietkzy-Golay-Filter. By
subsequent differentiation with respect to time the
velocity and the acceleration of the tracking object is
obtained (Figure 16). For the run considered here, an
approximately constant acceleration, i.e., force on the
model is obtained between 4.5 and 7 ms. The
interpolated displacement (using a quadratic fitting
function) and the constant velocity increase, i.e.,
For the present measurements, a resolution of
approximately 0.02 mm per pixel was obtained.
Together with the error resulting from the camera
timing, a precision of 10% can be determined when
assuming a constant acceleration and using the
displacements and times encountered in the experiments
in an error propagation method (see e.g., [8]). This
precision would hold if the determination of the motion
would rely only on the information of one pixel. As
discussed, the determination is based on a large number
of pixels defining the contour of the target and the
0
‐50
400 450 500 550 600 650 700 750 800 850 900
x [Pixel]
350
300
250
pixels afer first optimization step
final set of pixels describing the contour
y [Pixel]
200
150
100
50
0
‐50
400 450 500 550 600 650 700 750 800 850 900
x [Pixel]
precision improves by this statistics to roughly 1.5%.
Nevertheless, the motion of the model deviates from the
fitted motion assuming a constant acceleration by
approximately 7%. Therefore, this value is taken here as
a conservative error estimation.
model does not represent this case. Based on TAU
predictions the wind tunnel model configuration
experiences a negative acceleration (net thrust),
however, the non-representative external flow past the
model in HEG generates a higher drag, and, therefore, a
positive acceleration (net drag) even in the case of fuel
injection.
30
Angle of attack ‐2°
Fuel‐On
Fuel‐Off
25
20
2
a [m/s ]
15
10
5
0
‐5
Figure 17. Comparison of acceleration in streamwise
direction using the weakly constrained and free flying
model configuration
As outlined above, several reasons were identified to
consider a weekly constraint free flight model
configuration (re-fuelling of the hydrogen tanks during
run preparation, recording of pressure data from all
gauges installed in the model). The line and cable
connections were designed such that an impact on the
force measurements is minimized. This approach was
validated by comparing the force measurements
obtained for the weekly constraint configuration with a
completely free flying model configuration, i.e., without
any connection between the model and the support
system. The measured acceleration in x‐direction is
compared in Figure 17. No measurable influence of the
line and cable connections used on the weakly
constrained configuration can be detected during the test
time window.
4. RESULTS
The results of the HEG SSFE force measurements and
the comparison with computational predictions are
reported in [2]. Here a summary of the key results is
provided.
The net thrust increment generated by the internal
engine flow path is determined by subtracting the force
measured with fuel injection from the force measured
without fuel injection. The measured acceleration for
the fuel off and fuel on (fuel equivalence ratio of  ≈
1.0) cases at an angle of attack of  = -2o are given in
Figure 18. In both cases the acceleration is positive, i.e.,
during the available test time, the model is moving
downstream. However, the acceleration obtained from
the run with fuel injection is significantly reduced
compared to the run without fuel injection. It should be
emphasized that the force acting on the model in HEG
cannot be directly compared to the numerical prediction
given in [2]. In the computations a uniform free stream
is applied whereas in HEG the external flow past the
‐10
1
2
3
4
5
6
7
8
9
t [ms]
Figure 18. Acceleration in streamwise direction derived
from the measured displacement of the tracking object;
angle of attack  = -2o
The difference between the absolute thrust and absolute
drag are referred to as aero propulsive balance here. If
the thrust exceeds the drag, the aero propulsive balance
is positive, and vice versa (see [2]). The measured net
force increment (thrust solely generated by the internal
combustion flow path for a fuel equivalence ratio of  ≈
1.0) and  = -2o is given in Table 2. Additionally, the
corresponding numerical TAU code prediction is
included. The numerical result under predict the
measured thrust by 7.6%.
 = -2°

Thrust
[N], HEG Thrust [N],
TAU Difference
[%]
0.99
580 ± 42
536
7.58
Table 2. Measured and computed thrust increments of
the internal combustion flow path;  denotes the fuel
equivalence ratio
Figure 19. Aero propulsive balance for the SSFE
configuration based on combining experimental (HEG)
and numerical (TAU) data; fuel equivalence ratio
 ≈ 1.0
The aero propulsive balance determination for the
complete SSFE configuration based on a combination of
measured (HEG) and computed (DLR TAU code) data
is summarized in Figure 19. Combining the measured
thrust of the internal propulsion flow path and the
computed drag of the complete vehicle for fuel-off
condition, a positive aero propulsive balance of 73N is
obtained for  = -2o (see Figure 19). Even when taking
into account the uncertainty of the thrust measurement,
a positive balance of 73N - 42N = 31N is still obtained.
5. SUMMARY AND CONCLUSIONS
The complete scramjet flow path of the LAPCAT II
small scale flight experiment (SSFE) configuration was
tested in the High Enthalpy Shock Tunnel Göttingen
(HEG) of DLR. The thrust increment between fuelled
and unfuelled operation was detected with the free flight
technique (in fully free flight and/or weakly constrained
configuration). This technique was used in HEG for the
first time in combination with such a complex model
configuration. For an angle of attack of  =-2°, and an
equivalence ratio of  ≈ 1.0, the combination of
measured and computed forces demonstrated that a
positive aero propulsive balance could be achieved, i.e.,
the SSFE configuration would generate net thrust. It
should be emphasized that the numerical predictions are
based on the assumption that the boundary layer is fully
turbulent. Since the flow on the external part of the
vehicle and on the intake will be transitional, the above
values represent a conservative estimate.
ACKNOWLEDGMENTS
A part of this work was funded by the ‘Long-Term
Advanced Propulsion Concepts and Technologies II’
project investigating high-speed transport. LAPCAT II,
coordinated by ESA-ESTEC, is supported by the EU
within the 7th Framework Programme Theme7
Transport, Contract no.: ACP7-GA-2008-211485.
Further info on LAPCAT II can be found on
http://www.esa.int/techresources/lapcat_II. The support
of the HEG team in preparing the wind tunnel model
and during the performance of the test campaign is
highly acknowledged.
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