1. AEROTHERMODYNAMIC STUDY
OF A GENERIC FLAP CONFIGURATION WITH GAP IN HEG
Jan Marinez Schramm(1)
, Bodo Reimann(2)
, Andreas Hoffie(2)
, José M. A. Longo(2)
, Klaus Hannemann(1)
(1)
German Aerospace Center, Bunsenstraße 10, 37073 Göttingen, Germany, jan martinez@dlr.de
(2)
German Aerospace Center, DLR, Lilienthalplatz 7, 38108 Braunschweig, bodo reimann@dlr.de
ABSTRACT
In preparation of the flight of the European
eXPErimental Re-entry Testbed (EXPERT) the
German Aerospace Center (DLR) carried out a number
of experimental and numerical investigations for a
generic flap configuration to improve the understanding
regarding flap efficiency and heating.
1. INTRODUCTION
Within the framework of EXPERT the DLR participates
on the open flap payload 5 to improve the understanding
regarding flap efficiency and heating. The major interest
was to investigate strong viscous interaction effects,
transitional flow behaviour and the effect of high
temperature reacting flow. In preparation of the flight
experimental and numerical investigations of these
effects have been carried out.
For this study a simplified geometry with a cylindrical
nose followed by a ramp with embedded open flap has
been used. In contradiction to the real EXPERT capsule
the generic model offers the possibility to open the flap
gap to measure gap heating. Fig. 1 shows a comparison
of the geometry of the EXPERT capsule and the
investigated generic model.
Figure 1: EXPERT capsule and generic model.
2. HIGH ENTHALPY SHOCK TUNNEL
GÖTTINGEN
The experimental part of the study has been carried out
in the High Enthalpy Shock Tunnel Göttingen (HEG) of
the DLR. The HEG is a free piston driven shock tunnel
([1], [2], [3]) and was developed and constructed in the
framework of the European HERMES program over the
period 1989 – 1991. It was commissioned for use in
1991, at that time being the largest facility of its type
worldwide. Since then it was extensively used in a large
number of national and international space and
hypersonic flight projects. The research activities which
were always strongly linked with computational fluid
dynamics (CFD) investigations range from the
calibration process of the facility and the study of basic
aerodynamic configurations, which are well suited to
investigate fundamental aspects of high enthalpy flows
to the investigation of complex re-entry, hypersonic
flight and integrated scramjet configurations. A
schematic of HEG is given in Fig. 2.
Figure 2: Schematic drawing of the High Enthalpy
Shock Tunnel Göttingen (HEG).
In a free piston driven shock tunnel, the conventional
driver of a shock tunnel is replaced by a free piston
driver. This concept was proposed by Stalker [4]. A
schematic and wave diagram of this type of facility is
shown in Fig. 3. Free piston driven shock tunnels
consist of a secondary reservoir, a compression tube,
separated from an adjoining shock tube via the primary

2. diaphragm, and a subsequent nozzle, test section and
dump tank.
Figure 3: Schematic and wave (x-t) diagram of a free
piston driven shock tunnel like the HEG.
The high pressure air stored in the secondary reservoir
is utilized to accelerate a heavy piston down the
compression tube. The driver gas temperature increases
with the driver gas volumetric compression ratio. When
the main diaphragm burst pressure is reached it ruptures
and the wave process as in a conventional reflected
shock tunnel is initiated (see Fig. 3). A shock wave is
moving into the driven section and the head of a centred
expansion wave is moving into the high pressure region.
The numbers used in Fig. 3 denote distinct regions of
the flow. Region 1 contains the test gas at the initial
shock tube filling conditions and region 4 contains the
hot, compressed driver gas after piston compression.
Region 2 contains the shock compressed test gas, while
in region 3, the driver gas processed by the unsteady
expansion wave is contained. The test and driver gas are
separated by a contact surface. After reflection of the
incident shock wave at the right end wall of the shock
tube, the test gas is brought to rest in region 0.
Subsequently, the reflected shock wave penetrates the
contact surface. Reflected shock tunnels are
characterised by a convergent - divergent nozzle which
is attached to the end of the shock tube. A thin
secondary diaphragm is placed at the nozzle entrance in
order to allow evacuation of the nozzle, test section and
vacuum tank before the run. The nozzle entrance
diameter is chosen sufficiently small such that the
incident shock wave is almost completely reflected. The
stagnant slug of test gas, generated by the shock
reflection in region 0, is subsequently expanded through
the hypersonic nozzle. The nozzle flow starting process
is characterised by a wave system which passes through
the nozzle before a steady flow is established (see
Fig.3). The incident shock wave (a) is followed by a
contact surface (b), an upstream facing secondary shock
wave (c) and the upstream head of an unsteady
expansion (d). The trajectory of the piston is chosen in a
way that after main diaphragm rupture, the pressure and
temperature of the driver gas in region 4 is maintained
approximately constant. This is achieved by selecting
the velocity of the piston at diaphragm rupture, and
therefore the subsequent movement of the piston such
that it compensates for the loss of the driver gas flowing
into the shock tube. For that reason, in contrast to the
constant volume driver of conventional shock tunnels,
the free piston driver is a constant pressure driver. Due
to the large forces occurring during the operation of the
free piston driver, the compression tube, shock tube,
nozzle assembly is allowed to move freely in axial
direction. The test section and the vacuum tank remain
stationary. A sliding seal is used at the nozzle / test
section interface.
Figure 4: Binary scaling of EXPERT trajectory and
HEG conditions with corresponding angle of attack.
Fig. 4 shows the planned re-entry trajectory of EXPERT
and the available HEG conditions in terms of the binary
scaling parameter ρL versus flight velocity u.
Additionally the corresponding angle of attack (AoA)
for the selected trajectory is given.
3. TEST MATRIX
For the wind tunnel campaign, HEG conditions III and
IV have been selected to perform the measurements at
various AoA and flap deflection angles (FdA) with open
and closed slit. In Tab. 1 the experimental configuration
is given together with the boundary layer assumption
(BLA) for the CFD rebuilding for the cases presented in
this paper.
condition AoA FdA gap bla
HEG III 0° 20° open laminar
HEG III 0° 30° open laminar
HEG III 0° 20° closed laminar
HEG III 0° 20° closed transitional
NUM PG 0° 20° closed laminar
Table 1: Test Matrix.
0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
0.0
0.5
1.0
1.5
2.0
2.5
3.0
EXPERT, Lref
= 1.55 m
HEG, Lref
= 0.4m
XXII
XIII
XIV
XXXI
III
IV II
I
ρL[kg/m
2
]
u [km/s]
AoA[°]

3. The free stream conditions for the numerical rebuilding
of the HEG experiments are listed in Tab. 2. A flow
condition with a perfect gas (NUM PG) assumption has
been investigated numerically additionally to study the
influence of high temperature effects.
condition III NUM PG
ρ∞ [g/m3
] 3.2 11.9
T∞ [K] 682.0 43.4
u∞ [m/s] 4672 1151
p∞ [Pa] 681.9 148.1
h∞ [MJ/kg] 13.0 0.7
Ma∞ 8.88 8.72
Re∞ [1/m] 4.53 ּ 105
5.07 ּ 106
c(N2) 0.73553 -
c(N) 0.00001 -
c(O2) 0.13500 -
c(O) 0.07850 -
c(NO) 0.05096 -
Table 2: Free stream conditions for the numerical
rebuilding.
4. EXPERIMENTAL SETUP
The idea for the present double ramp model was to
simplify the shape of the EXPERT vehicle, while a
similar flow topology should be created. The cylindrical
nose creates a strong bow shock and induces strong
relaxation processes of the gas within the shock layer.
The same behaviour is expected for the flow around the
nose region of the EXPERT configuration. A three
dimensional view of the model is given in Fig. 5
together with the coordinate system used in the results
section
Figure 5: Three dimensional view of the double ramp
model.
The flap region of the configuration has been designed
in such a way, that the maximum control surface area
possible in HEG has been realized. The flap is 220mm
long and 240mm wide. The EXPERT flap has a length
of 300mm and is 377mm wide. In contrast to the
EXPERT capsule where the flap hinge line is closed, the
model is designed in such a way, that the flap hinge line
is open, such that the flow can pass underneath the flap.
For further testing however, the slit between the body
and the flap can be closed in order to examine the
influence of the slit. The upper surfaces of body (ramp)
and flap have been instrumented with pressure
transducers and thermocouples.
Figure 6: Side view including main dimensions of the
double ramp model(top) and double ramp model
mounted in the HEG test section (bottom).
The nose part of the model is removable and
instrumented with pressure transducers and
thermocouples in axial and radial direction. The flap
and the model body instrumentation are realized by
variable sensor inserts that enable to change the
locations in the flow direction The variable inserts are
part of the surface and can be adjusted, which allows
placing the transducers at the position at flow features
that intend to be resolved. The minimum spacing is
6mm. Pressure and temperature sensors can be moved
independently and 35 inserts (sensors) are installed into
the model, where 20 are on the model body surface and
25 are one the flap surface. The gap between the model
body and the flap is also instrumented. Six
thermocouples are placed in the symmetry plane; three
pressure transducers and three thermocouples are placed
in the span wise direction. Fig. 6 shows the main
dimensions of the model and a photograph of the model
mounted in the test section of the HEG.

4. 5. DLR TAU CODE
The flow solver used in the present study is the DLR
TAU code. This code is a finite volume Euler/Navier-
Stokes solver, which can handle structured,
unstructured, and hybrid meshes and has already been
applied to a variety of configurations. The Reynolds
averaged Navier-Stokes (RANS) equations are
discretized by a finite volume technique using
tetrahedrons and prisms. Prismatic elements are used for
the boundary layer region, while the tetrahedral ones are
used in inviscid flow regions. The AUSMDV second
order upwind scheme with MUSCL reconstruction is
used for the inviscid fluxes. For time discretization,
including local time stepping, a three stage Runge-
Kutta, as well as an implicit, approximately factored
LU-SGS scheme is implemented. For acceleration,
multi-grid and explicit residual smoothing are available.
Furthermore, parallel computing is possible via domain
splitting and Message Passing Interface (MPI)
communication.
The transition from laminar to turbulent flow is
modelled by prescribing the transition location. For the
present model computations with transitional flow, the
transition line has been set at the windward flap hinge
line. This means that only the windward side of the
flaps was modelled as turbulent; all other parts have
been assumed laminar. In the presented cases, the two-
equation k-ω model described by Wilcox [5] was used.
This model has proven in earlier studies to be the only
one reliable for the investigated Mach number range. To
model the thermo-chemical behaviour of the flow,
Gupta's [6] five species chemical non-equilibrium
model was applied.
6. GRID
All of the numerical grids presented here have been
generated by the CENTAUR software.
Figure 7: Initial (left) and adapted grid (right) for the
open gap case and 20° flap deflection angle.
Major criteria for designing the meshes was to obtain a
high resolved surface grid and a high resolution of the
boundary layer by the prism mesh that will enable to
compute the heat flux at the highest possible degree of
accuracy. Although the flow across the present
configuration remains for the most part laminar, a goal
was to keep the y+ value below the value of 0.5. This
implied a short wall distance of the first prismatic layer.
Grid adaptation by cell division offers the possibility to
insert additional points only in region where clustering
is necessary. While the tetrahedra can be divided along
all edges, the prismatic elements are presently refined
only on their triangular faces. Based on the flow
solution points can be added, redistributed or removed.
Fig. 7 shows the symmetry plane of the initial grid and
the grid after several adaptation steps. Fig. 8 shows the
mesh near the hinge line with open and closed gap.
Figure 8: Enlarged view of the grid with open (left) and
closed gap (right).
7. RESULTS
Fig. 9 shows the computed flow topology with
normalized surface pressure distribution and skin-
friction lines for the ramp configuration with a flap
deflection of 20°. On the left hand side of the picture the
gap is open while on the right hand side it is closed.
Figure 9: Normalized pressure distribution and skin-
friction lines for the configuration with open (left) and
closed gap (right).

5. The skin friction lines show no flow separation on the
fore body in front of the hinge line. In Fig. 10 the same
results are shown for the back side of the model. It is
obvious that for the open gap case, where the flow is
allowed to pass underneath the flap, the heating in the
cavity is not more severe than for the close gap case.
Figure 10: Rear view of the model with normalized heat
flux distribution and skin friction lines. The picture
shows the configuration with open gap on the left-hand
side and with closed gap on the right-hand side.
Figs. 11 and 12 show the topology in the symmetry
plane with density gradient and Mach number contours.
The configuration with closed gap shows at the hinge
line a separation bubble with a shock in front.
Figure 11: Density gradient and Mach number contours
around the hinge line with open gap.
The case with closed gap shows only a very small
region of separated flow inside the gap. This numerical
result is confirmed by the HEG experiment.
Figure 12: Density gradient and Mach number contours
around the hinge line with closed gap.
Fig. 13 shows the computed and measured surface
pressure distribution along the symmetry plane of the
model. While for the closed flap the separation bubble
covers a region from x/L= 1 to 1.25, no separation is
resolved by the instrumentation when the gap is closed.
x/L
p/pstag
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
CFD, closed gap
Experiment, closed gap
CFD, open gap
Experiment, open gap
HEG condition III
aoa = 0°
fdf = 20°
Figure 13: Pressure distribution for the configuration
with 20° flap deflection with open and closed gap.
For higher flap deflection angles experimental and
numerical data differ. Fig. 14 shows normalized
pressure distributions along the stagnation line with
open gap for 20° and 30° flap deflection. Compared to
the 20° case the measurement for 30° flap deflection
shows flow separation which is not reproduced by the
numerical computation. The reason for this is not fully
understood and requires further investigation.
x/L
p/pstag
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
CFD, fdf=30°
Experiment, fdf=30°
CFD, fdf=20°
Experiment, fdf=20°
HEG condition III
aoa = 0°
open gap
Figure 14: Normalized pressure distributions for the
configuration with open gap with 20° and 30° flap
deflection angle.
Fig. 15 shows the normalized distribution of the heat
flux along the symmetry plane. The experimental data
are compared with a laminar and a transitional
computation. For the transitional one the wind-ward
side of the flap is modelled turbulent. The comparison
shows that the flow along the flap matches neither the
laminar nor the fully turbulent heating level. The
characteristic shows a more laminar behaviour.

6. x/L
q/qstag
0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
CFD, laminar
CFD, transitional
Experiment
HEG condition III
aoa=0°
fdf=20°
closed gap
Figure 15: Normalized laminar and transitional heat
flux distribution for a configuration with closed gap.
Fig. 16 shows the cross-wise distribution of the
normalized heat flux inside the gap. The convective gap
heating is relative low. Also the effect of the gap flow to
the convective heating of the cavity behind the flap is
negligible.
y/L
q/qstag
-0.55-0.50-0.45-0.40-0.35-0.30-0.25-0.20-0.15-0.10-0.050.000.05
0.000
0.025
0.050
0.075
0.100
0.125
0.150
0.175
0.200
CFD
Experiment
HEG condition III
fdf=20°
aoa=0°
open gap
Figure 16: Normalized heat flux distribution inside the
gap in span-wise direction.
The comparison of the computed pitching moment is
shown in Fig. 17. Here the results for the numerical
rebuilding of the flow around this generic configuration
in terms of total pitching moment is compared for the
HEG condition III and the numerical perfect gas case.
Due dissociation of molecular nitrogen and oxygen in
the high enthalpy condition, the position of the shock
waves and therefore the surface pressure distribution
changes. For the investigated configuration these high
temperature effects cause a drop in total pitching
moment.
Ma/(Re)
1/2
Cm,y
0.000 0.005 0.010 0.015 0.020 0.025
0.0025
0.0026
0.0027
0.0028
0.0029
0.0030
perfect gas
HEG condition III
High-temperature effect
Figure 17: Influence of real gas effects on the total
value of the pitching moment.
8. CONCLUSION
Experiments in HEG have been carried out for a generic
wind tunnel model, which has been developed to create
a similar flow which the EXPERT vehicle will
experience during re-entry. The generic model could be
designed as such that the maximal control surface area
of the steering flap could be realized for HEG, which is
approximately half the area of the flight configuration.
With the model it was possible to measure surface
pressure and surface heat flux with an open and closed
gap between the model and the flap.
The experimental results for HEG condition III for FdA
of 20° at AoA of 0° show that an open gap reduces the
size of the separation bubble in the hinge line
drastically, e.g. it is not measurable in the experiments
any more. This result has been confirmed by the
numerical rebuilding of these two cases.
The surface heat flux measured on the flap at FdA of
20° for the closed gap configuration (with separation)
show, when compared to the numerical results, that the
reattachment on the surface is not purely laminar but
rather in a state between laminar and fully turbulent.
Comparing the experimental results for FdA of 20° and
30° for an open gap, the separation establishes again for
30° FdA. This result has not been reproduced by the
numerical rebuilding, and further investigation is
required to identify the cause.

7. 9. RFERENCES
[1] Hannemann, K., Schnieder, M., Reimann, B.,
Martinez Schramm, J.: The influence and delay
of driver gas contamination in HEG, AIAA
2000-2593, 21st AIAA Aerodynamic
Measurement Technology and Ground Testing
Conference, Denver, CO, 19-22 June, 2000
[2] Hannemann, K., High Enthalpy Flows in the
HEG Shock Tunnel: Experiment and Numerical
Rebuilding, AIAA 2003-0978, 41st AIAA
Aerospace Sciences Meeting and Exhibit, 6-9
Jan, Reno, Nevada, 2003
[3] Hannemann, K., Martinez Schramm, J., Karl, S.:
Recent Extension to the High Enthalpy Shock
Tunnel Göttingen (HEG), Proceedings of the 2nd
International ARA Days “10 Years after ARD”,
October 21-23, Arcachon-France, 2008.
[4] Stalker R. J.: A study of the free piston shock
tunnel. AIAA Journal, 12(5):2160-2165, 1967.
[5] Gupta, R. N., Yos, J. M., Thompson, R. A., Lee,
K.-P.: A Review of Reaction Rates and
Thermodynamic and Transport Properties for n
11-Species Air Model for Chemical and Thermal
Nonequilibrium Calculations to 30000K, RP
1232, NASA,1990.
[6] Wilcox D. C.: Turbulence Modeling, DCW
Industries, 1998.