47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition
5 - 8 January 2009, Orlando, Florida
AIAA 2009-887
Numerical Investigation of Segmented Actuation Slots
for Active Separation Control of a High-Lift
Configuration
Tobias Höll∗, Bert Günther† and Frank Thiele‡
Berlin Institute of Technology, Department of Fluid Mechanics and Engineering Acoustics,
D-10623 Berlin, Germany
The control of the flow over the flap of a three-element high-lift configuration is investigated numerically by solving the unsteady Reynolds-averaged Navier-Stokes equations
(URANS). At a Reynolds number of Re = 750 000 the flow is perturbed by periodic blowing/suction through slots near the flap leading edge. The main focus is on comparing the
manner with wich the flow field reacts differently if this actuation is segmented in the
spanwise direction. Previous results show that the mean aerodynamic lift can be enhanced
by excitation with continuous slots in the spanwise direction. On this basis an assessment
of the additional gain in lift provided by segmented excitation slots is conducted. Furthermore, the influence of a phase shift of the actuation on the flow field is investigated.
Nomenclature
c, ck
cL
Cµ
f, F+
H
Re
St
ua , uexc
u∞
α, δf , δs
∆t
Φ
clean chord length, flap length (ck = 0.254c)
lift coefficient
2
a
momentum coefficient Cµ = Hc uu∞
k
frequency of periodic excitation, non-dimensional excitation frequency F + = f uc∞
slot width (H = 0.00186ck )
Reynolds number based on chord length
Strouhal number
amplitude velocity of excitation in the slot, excitation velocity
inflow velocity
angle of attack on the main airfoil, flap deflection angle, slat deflection angle
time step size
sweep angle
I.
Introduction
The sophisticated high-lift devices of a modern commercial airplane consist of a slat and single or multiple flaps. These elements have to generate a tremendous amount of lift during take-off and landing in
order to reduce take-off speeds and runway lengths. Because of the enormous cost, complexity and weight
of these devices, aerodynamic research and development aims at simplification of these whilst maintaining
their effectivity. Conventional methods are currently incapable of achieving further significant improvement.
∗ Research Associate, email: tobias.hoell@cfd.tu-berlin.de, Institut für Strömungsmechanik und Technische Akustik - ISTA,
Müller-Breslau-Str. 8, Berlin, 10623, Germany, AIAA student member.
† Research Associate, email: bert.guenther@cfd.tu-berlin.de, Institut für Strömungsmechanik und Technische Akustik - ISTA,
Müller-Breslau-Str. 8, Berlin, 10623, Germany, AIAA student member.
‡ Professor, email: frank.thiele@cfd.tu-berlin.de, Institut für Strömungsmechanik und Technische Akustik - ISTA, MüllerBreslau-Str. 8, Berlin, 10623, Germany, AIAA member.
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Institute
of Aeronautics
and Astronautics
Copyright © 2009 by Tobias Höll, Bert Günther, Frank Thiele.American
Published by
the American
Institute of Aeronautics
and Astronautics, Inc., with permission.
However, one promising means to reduce flow separation is offered by active flow control methods. Both
numerical and experimental investigations have shown that the effectiveness of high-lift configurations can
be significantly improved by delaying flow separation on the flap.1–3
This paper describes a numerical investigation of the control of the flow over the flap of a three-element
high-lift configuration by means of periodic excitation.4 An unsteady wall jet emanating from the single
slotted flap shoulder close to the leading edge is used to excite the flow and thus provoke either delay of
separation or full reattachment.5 The test model consists of a swept wing with an extended slat and a single
slotted Fowler flap.
The investigations are mainly focused on a comparison between the use of continuous and segmented excitation slots in spanwise direction.6 It is investigated whether longitudinal vortices can be induced by segmented
excitation slots, which are expected to give rise to a further delay of flow separation. It will be analyzed
whether an enhancement in the effectivity of active flow control can be achieved using segmented instead
of continuous excitation slots on the flap. Furthermore, the segmented actuation itself is analyzed in detail.
The question whether a phase shifting of the actuation between both segments or an equiphase actuation
provides a larger gain in lift is clarified. Moreover, the gain in lift when only one actuated segment of both
consecutive slits is used is evaluated in contrast to both actuated segments and the continuous actuation.
The motivation of this investigation is twofold: On the one hand, both in wind tunnel experiments and on
real airplanes continuous slots cannot be realized because of technical reasons, therefore segmented actuation
slots are applied. On the other hand, due to the possibility to react to the local flow conditions and a larger
variety of parameter value combinations, the use of segmented actuation slots provides the possibility to
further improve the effectiveness of active flow control.
II.
Active Flow Control
Active flow control methods are characterised by arrangements with an excitation mechanism that inserts
external energy into the flow (figure 1).
In the last decades, a large number of experimental and numerical studies have shown the general effectiveness of active flow control for single airfoils. In most investigations, leading edge suction is applied
to delay transition;7 nonetheless, jet flaps are also employed for lift increase and manoeuvrability. Surface
suction/blowing can be used to rapidly change the lift and drag on rotary wing aircraft.8 However, most
control techniques considered in the past demonstrated low or negative effectiveness.
In further investigations, oscillatory suction and blowing was found to be much more efficient with respect
to lift than steady blowing.9, 10 The process becomes very efficient if the excitation frequencies correspond
to the most unstable frequencies of the free shear layer, generating arrays of spanwise vortices that are
convected downstream and continue to mix across the shear layer.11 Suction and blowing can be applied
tangentially to the airfoil surface,12 perpendicular13 or with cyclic vortical oscillation. In order to create an
effective and efficient control method, previous studies have been primarily focused on the parameters of the
excitation apparatus itself.
Apart from the investigation of the active flow control apparatus itself, further peripheral points have to
be considered. For example, can the energy demand be satisfied? Is the energy balance positive at all? Is
the benefit higher than the effort? Additionally, if the actuators must not fail during take-off and landing
because of the required gain in lift, then they have to be redundant with a probability of failure of less than
1 · 10−9 per flight hour. Furthermore, the robustness of the actuators against rain, hail, ice and dust has to
be insured.
To sum up, a great deal of issues must be investigated before active flow control is ready for application on
a real aircraft. Overviews of the active flow control technique are given by Wygnanski and Gad-el-Hak.14, 15
III.
Simulation
General Description
The numerical test model represents the practically-relevant SCCH (Swept Constant Chord Half model)
high-lift configuration that has already been used for several experimental studies targeting passive flow and
noise control concepts.16–18 The three-dimensional wing has a sweep angle of Φ = 30◦ and a constant chord
length in the spanwise direction. The numerical investigation is focused on a wing with infinite span in
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Figure 1. High-Lift configuration with periodic wall jet on the upper surface of the flap
order to reduce the computational costs. The simulation of this infinite wing comprises only one part of the
three-dimensional effects generated by the sweep.
c
SCCH
δS = 26.5°
δ F = 37°
Figure 2. Sketch of the original SCCH high-lift-configuration.
The typical three-component setup consists of a main airfoil equipped with deployed slat and flap with
0.158 c and ck = 0.254 c relative chord lengths respectively (figure 2). All profiles have blunt trailing edges.
The separation position of the flow on the upper flap surface reaches its most upstream position at a flap
deflection angle of 37◦ . The angle of attack is fixed at α = 6◦ for the whole configuration, which is situated
in the typical range of approach for civil aircraft. In addition, the flow over the flap is detached whereas the
flow over the slat and the main wing are still fully attached. With these settings, the area of separated flow
above the flap is maximised and better suited to the application of active flow control.
For the current numerical investigations the freestream velocity corresponds to a Reynolds number of
Re = 750 000 based on the chord of the clean configuration (with retracted high-lift devices). This Reynolds
number is chosen according to the wind tunnel experiments. Therefore, a later comparison between simulation and experiment is possible. Earlier simulations by Günther et al.6 have been conducted at a Reynolds
number of Re = 1 · 106 . As a consequence, the effect of a change in Reynolds number can also be assessed
with regard to lift.
Numerical Method
The numerical method applied is based on a three-dimensional incompressible finite-volume scheme for
solution of the Reynolds-averaged Navier-Stokes equations. The three-dimensional method is fully implicit
and of second order accuracy in space and time. Based on the SIMPLE pressure correction algorithm, a
co-located storage arrangement for all quantities is applied. Convective fluxes are approximated by a TVD
scheme. In previous investigations of unsteady turbulent flows, the LLR k-ω turbulence model by Rung &
Thiele19 exhibited the best performance.
Computational Mesh
Figures 3(a) and 3(b) show the mesh around the slat, the main airfoil and the flap. The dimensions of
the computational domain are 15 chords forward, above and below the configuration and 25 chords behind.
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Figure 3(b) shows the two-dimensional mesh around the entire configuration. The two-dimensional computational c-type mesh consists of 90,000 cells in total. The non-dimensional wall distance of the first cell
centre remains below Y + = 1 on the entire surface.
The three-dimensional grid is based on an expansion of the two-dimensional mesh into the third direction.
For consideration of an infinite swept wing, 40 layers of the two-dimensional mesh are combined to resolve
a three-dimensional wing section using around 4,000,000 cells in total. These 40 grid layers in the spanwise
direction are necessary to resolve the actuation segments sufficiently, because the length of one segment is
0.2 c relative chord lengths. Each segment consists of 20 layers in the spanwise direction and 6 layers in the
streamwise direction (figure 3(a)). Both segments are connected to each other without considering a strip
between them.
The infinite spanwise domain is simulated by means of periodic boundary conditions.
Segment I
Segment II
(a) surface mesh of the computed wing segment
(b) 2-dimensional mesh in the region of the configuration
Figure 3. Computational mesh
Time Step Size
From initial numerical investigations of the configuration without excitation, characteristics of the unsteady
behaviour are already known. A separate study of the influence of the time step size indicated that a typical
time step of ∆t = 2.1 × 10−3 c/u∞ is sufficient to obtain results independent of the temporal resolution. All
computations presented here are based on this time step size, which allows a resolution of around 200 time
steps per oscillation cycle for a non-dimensional oscillating frequency of F + = 0.6.
Boundary Conditions
All flow quantities including the velocity components and turbulent properties are prescribed at the wind
tunnel entry. The level of turbulence at the inflow is set to T u = u1∞ ( 32 k)1/2 = 0.1% and the turbulent
viscosity to µt /µ = 0.1. At the outflow a convective boundary condition is used that allows unsteady flow
structures to be transported outside the domain. The complete airfoil and flap surface is modeled as a nonslip boundary condition. As the resolution is very fine, a low-Re formulation is applied. The wind tunnel
walls are neglected in the far field.
Excitation Mechanism
To model the excitation apparatus, a periodic suction/blowing type boundary condition is used. The perturbation to the flow field is introduced through the inlet velocity on two small wall sections, arranged in
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the spanwise direction, representing the excitation slots:
n−1
c
+
uexc (τ, n) = ua · sin
·π
· sin 2π ·
·F ·τ
nmax − 1
ck
|
{z
}
|
{z
}
spatial velocity distribution time-dependency
r
with
ua
F+
τ
= u∞
= fper ·
c
Cµ
H
ck
u∞
u∞
c
1 ≤ n ≤ nmax
= t·
where ua is the amplitude velocity of the perturbation oscillation, F + is the non-dimensional perturbation
frequency, τ is the dimensionless time given in convective units of the whole configuration, H is the slot width
(H = 0.00186 ck ), Cµ is the non-dimensional steady momentum blowing coefficient and n is the actual grid
layer of each actuation slot (1 ≤ n ≤ nmax , nmax = 20 grid layers per actuation slot). Both intensity and
excitation frequency can be chosen differently for each slot. Current research activities moreover focus on two
further issues: A variable streamwise excitation direction (instead of perpendicular to the wall) and different
excitation modes (e.g. pulsed blowing/suction). Furthermore (in contrast to previous investigations6, 11 ),
now the excitation velocity is not only characterized by a time-dependency, but also by dependency of the
location on the segment. However, for the cases with a continuous slot a simple block profile is used for
the excitation velocity. Using the sin-function for the segmented slots results in a velocity distribution on
each slot which is zero at the segment boundaries and has its maximum in the segment center (figure 4).
The present use of the sin-function for the numerical realization of the spatial velocity distribution on a
slot exhibits inapplicabilities in some details. The realization by means of a tanh-function would appear
to be more appropriate regarding smoothness to the zero velocity at the slot boundaries. Therefore this is
currently under investigation and will be presented in the future.
The oscillating jet is emitted perpendicular to the wall segment of the excitation slot, and is located at 6%
chord behind the flap leading edge.
Figure 4 shows an example of the segmented actuation, where at two different spanwise locations a snapshot
of the flow conditions is depicted to illustrate the distributed excitation in the spanwise direction on the flap.
Figure 4. Antiphase excitation on the flap with segmented actuation slots
IV.
Results
In the following, the methods described are applied in order to analyze the difference between active flow
control on a high-lift configuration with a continuous slit and two slit-segments in the spanwise direction. At
first a basic description of the flow physics for the unexcited case is given. Secondly, the results of the excited
cases with one continuous slit are presented, followed by results of the cases with segmented actuation. For
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the segmented actuation the main focus is on investigating the influence of a phase shift in the actuation on
both segments on the flow of the flap. Furthermore, the difference between a high excitation intensity and
a low excitation intensity is evaluated.
Unexcited Flow
As the set-up is completely three-dimensional, the investigations of the unexcited flow are aiming at understanding the separation process on the flap and determining the main drivers that trigger flow separation
(see figure 12(a)). Factors such as the sweep angle of 30 degrees generate a strong cross flow on the main
wing and especially on the flap. The flow field of the SCCH -configuration without excitation is characterised
by massive separation above the upper surface of the flap. The mean separation point is located at 6% chord
length behind the flap leading edge, and downstream a large recirculation region occurs. This is characterized by near-wall flow in the upstream direction with a dominant component in the spanwise direction. This
component is also produced by the local pressure gradient in the spanwise direction (sweep) and is strongly
developed in the slow detached flow. The unsteady behaviour of separated flow is mainly governed by large
vortices shed from the flap trailing edge that interact with the vortices generated in the shear layer between
the recirculation region and the flow passing through the slot between main airfoil and flap nose (see arrows
in figure 12(a)). These vortices show a twisted character as a result of the sweep (figure 5(a)). More details
are given by Günther et al.6
All these findings indicate that the chosen setup is predestined for the application of active flow control techniques, because of the massive flow separation on the upper surface of the flap. In previous investigations
by Günther et al.6 different setups have been tested, but this one proved to be most appropriate. Thus, this
angle of attack (6◦ ) and this flap deflection angle (37◦ ) are chosen for the flow control cases.
(b) Excited flow (F + = 0.6, Cµ = 50 · 10−5 )
(a) Unexcited flow
Figure 5. Illuminated streamlines showing the 3D character of the flow above the flap of the infinite wing
Excited Flow
Following the investigation of the unperturbed flow, the flow control mechanisms are applied. All flow control
computations use the baseline case solutions as initial flow conditions. The simulations are performed with
two different intensities (Cµ = 50 · 10−6 and Cµ = 300 · 10−6 ) at a constant excitation frequency of F + = 0.6.
The excitation mode is sinusodial blowing and suction.
Excitation with a Continuous Slot
Figure 6(a) presents three-dimensional results of the excitation in the simulation with different frequencies
compared to two-dimensional results. These results have been obtained by Günther et al.6 in earlier
investigations at a Reynolds number of Re = 1 · 106 and with 16 grid layers in the spanwise direction. The
diagram shows the difference of the lift coefficient relating to the unexcited case depending on the excitation
frequency. The lift coefficient can be enhanced by 17% compared to the baseline simulation in 2D and up
to 12% in 3D. According to Günther et al.,6 the reason for the smaller enhancement for the infinite swept
wing compared to the two-dimensional case seems to be the three-dimensionality of the flow induced by the
sweep. By using the optimum parameter values, the large-scale vortex shedding from the flap trailing edge
can be nearly eliminated (figure 5(b)).
Furthermore, Günther et al.6 present results of a variation of the excitation intensity at a constant nondimensional frequency of F + = 0.6 (see figure 6(b)). This diagram shows, that the optimum actuation
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20%
20%
16%
16%
12%
12%
∆cL [%]
∆cL [%]
intensity for the 3D case can be found at Cµ = 50 · 10−5 . The largest overall value regarding the gain in
lift can be found at Cµ = 300 · 10−5 . The excitation values for the cases with segmented actuation slots are
chosen on this basis.
Current simulations with the wing segment consisting of 40 grid layers and at the lower Reynolds number
of Re = 750 000 showed that for the 3D case the enhancement in lift is on a level of around 18%. This
is approximately the same value like the gain in lift for the 2D cases at a Reynolds number of 1 · 106 (see
figure 6).
8%
8%
+
(2D) Fopt = 0.6
-5
4%
4%
(2D) Cµ = 114×10
opt
+
(3D) Fopt = 0.6
-5
(3D) Cµ = 50×10
0%
opt
0
0.5
1.0
1.5
0%
2.0
0
100
+
(a) Increase in the mean lift coefficient versus the
non-dimensional excitation frequency
200
300
Cµ
F
400
500
-5
[ × 10 ]
(b) Increase in the mean lift coefficient versus the
non-dimensional excitation intensity
Figure 6. Numerical results for excited flow of the infinite wing with one continuous slit
Excitation with Segmented Slots
The next approach is to use segmented actuation slots. As shown in figure 7, sinusodial blowing and suction is
applied as well, but with the possibility of using different values for intensity and frequency for each actuation
slot. Furthermore, a phase-shift between both sinusodial excitation modes can be applied. Figure 7 shows
as an example two snapshots taken from the slice in the center of the actuation segments (light blue). They
illustrate both the contours of the velocity perpendicular to the surface and velocity vectors, showing a
situation were one segment is blowing out and the other one is sucking in (top) and suction on both slots
(bottom). Both examples are shown for different amplitude velocities (intensities) for each slot.
Figure 7. segmented actuation at 6% chord of the flap, v-velocity contour and vectors
These examples emphasize that a large variety of combinations of parameter values is possible. Using this
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range of parameters, it is investigated whether it is possible to generate longitudinal vortices which might
be able to weaken effects that are detrimental to the lift. Furthermore, it will be assessed if an enhanced
level of flow mixing can be achieved, resulting in a gain in lift compared to the excitation with a continuous
slit. This approach is promising because the turbulent energy increases due to the excitation by segmented
slots. Unlike in a two-dimensional case the turbulent energy dissipates not only in a 2D-plane but in all
three spatial dimensions. Therefore, the lift decreases in the three-dimensional cases compared to the twodimensional ones. The excitation of longitudinal vortices is however a promising means to introduce more
turbulent energy into the area of recirculation.
First of all, an overview of the investigated cases is given (illustrated in figure 8 and listed in table 1).
Mainly, four different situations regarding phase shift between both actuation segments are compared. An
equiphase case, i.e. no phase shift, and an antiphase case, i.e. a phase shift of π. Furthermore a case
with a phase shift of π/2 and a case where only one segment is actuated have been considered. These
phase shifts seem to be particularly interesting, because minima, maxima and zero-crossings of the actuation
coincide with each other (see figure 8). The different cases are run at intensities of Cµ = 50 · 10−5 and
Cµ = 300 · 10−5 and a dimensionless excitation frequency of F + = 0.6. These values are chosen based on
earlier investigations by Günther et al.,6 which showed that at Cµ = 50 · 10−5 the ideal ratio between gain
in lift and energy consumption is obtained. At an intensity of Cµ = 300 · 10−5 the maximum gain in lift is
achieved, disregarding the energy consumption.
Table 1. Overview of the different phase shifts
case
phase shift
Cµ · 10−5
F+
a)
0
300
0.6
b)
π
50, 300
0.6
c)
π
2
50, 300
0.6
d)
one actuated segment
50, 300
0.6
Excitation
Seg.1
Segments
Cases
t
H
Seg. 2
c)
Seg. 2
t
t
t
t
C, G
D, I
Seg.1
Seg. 1
Seg.2
Seg. 1
Seg.1
b)
t
Seg.2
Seg. 2
Seg.1
Seg. 1
Seg.2
a)
d)
t
B, F
Seg. 1
Seg. 2
Seg.2
blowing
t
suction
Figure 8. Time evolution for the different cases with segmented actuation
In the following, the resulting gain in lift for all cases is presented. Table 2 gives an overview of the
total gain in lift for all cases compared to the unexcited case. It is seen that for the higher excitation
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intensity also the gain in lift is higher, as was shown by earlier investigations. For a momentum coefficient
of Cµ = 300 · 10−5 the continuous excitation for the 3D grid with 40 grid layers in the spanwise direction
reaches a gain in lift of around 19% compared to the unexcited case. With the use of segmented actuation
slots the gain in lift can be enhanced by up to 23% for the case with no phase shift and 24% for the antiphase
excitation (phase shift of π). Interestingly, actuating only one of both consecutive slots yields 18% gain in
lift, which is almost the same level as the continuous excitation.
For a lower intensity of Cµ = 50 · 10−5 however, the benefit of the segmented actuation decreases. The
antiphased excitation reaches the same level in lift as the continuous exctiation (16% compared to 18%),
but actuating only one segment (half of the span) only provides a gain in lift of 9%. All these values show
that segmented actuation slots feature a different gradient of the lift increase at a rising excitation intensity.
Therefore, more investigations with a variation of excitation intensities are required to depict this fact in
detail. Due to the fact that no continuous slot over the whole span can be used on either wind tunnel
models or real aircraft in the future, the behaviour of the segmented actuation at different intensities, nondimensional frequencies and at different phase shifts must be investigated.
Figure 9 depicts a decomposition of the changes in lift into the components slat, main wing and flap for all
excitation cases. The right hand side of the figure shows how the lift coefficient without excitation is divided
into parts for all components. Both for the unexcited flow and the excited cases, the greatest portion of the
lift is produced by the main wing (around 80% of the total lift). For the excited flow, the major part of the
gain in lift also is produced by the main wing, namely between 6% and 16%. Although the flow control slots
are positioned on the flap, the lift is also increased on the slat and the main wing. This is due to the fact
that flow separation on the flap is suppressed and therefore the circulation around the whole wing increases.
Furthermore, the departure angle of the main airfoil is increased, which also leads to an increase in lift of
the main wing.11 In comparison with the continuous actuation, the segmented actuation slots provide a
higher gain in lift directly on the flap (≥ 3%). The continuous excitation provides around 2.5%. The same
applies to the component of the gain in lift of the slat. Furthermore, exciting the flow with an intensity of
Cµ = 300 · 10−5 results in a gain in lift ≥ 16% on the main wing. However, this does not apply to the lower
momentum coefficient.
Table 2. Overview of the total gain in lift for all cases, compared to unexcited case
case
excitation
∆cL (total)
A
B
C
D
−5
Cµ = 50 · 10 , continuous
Cµ = 50 · 10−5 , one segment
Cµ = 50 · 10−5 , antiphased
Cµ = 50 · 10−5 , phase shift π/2
+18%
+9%
+16%
+14%
E
F
G
H
I
Cµ = 300 · 10−5 , continuous
Cµ = 300 · 10−5 , one segment
Cµ = 300 · 10−5 , antiphased
Cµ = 300 · 10−5 , equiphased
Cµ = 300 · 10−5 , phase shift π/2
+19%
+18%
+24%
+23%
+24%
In order to evaluate these findings more precisely, a frequency analysis of the lift coefficient has been
conducted for both intensities. Figures 10 and 11 show the dominant frequencies for the unexcited case, the
continuous excitation, the antiphase excitation and for the cases where only one actuation segment is used.
The Strouhal number is formed with the flap chord of ck = 0.254c.
For the unexcited case, the fundamental component can be found at a Strouhal number of St = 0.3, produced
by the vortex shedding on the upper surface of the flap. These large coherent structures are detrimental
to the lift, therefore the excitation mechanism is used to improve this flow condition. For the case with
the continuous actuation slot, the first harmonic lies at St = 0.6, consistent with the non-dimensional
excitation frequency of F + = 0.6. This can be found for both the Cµ = 50 · 10−5 and the Cµ = 300 · 10−5
cases (figures 10(b) and 11(b)). For the segmented actuation slots however, just as in the gain in lift, a
difference between both intensities is yielded. For the higher excitation intensity, according to the frequency
analysis, the formation of large coherent structures is suppressed. Unlike with the continuous excitation,
the first harmonic at St = 0.6 is suppressed and only the second harmonic at St = 1.2 can be observed,
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-5
-5
Cµ=50×10
24%
Cµ=300×10
3.8%
22%
110%
3.8%
change in mean lift coefficient ∆cL
20%
100%
12.0%
18%
16%
90%
2.5%
3.1%
2.5%
80%
3.2%
70%
14%
2.5%
16.3%
16.3%
12%
60%
16.4%
10%
13.9%
12.6%
8%
1.9%
80.5%
50%
12.6%
40%
10.7%
9.4%
6%
30%
5.6%
4%
20%
2%
3.8%
2.5%
2.5%
1.9%
C
D
1.3%
0%
A
B
2.5%
2.5%
E
F
3.1%
mean lift coefficient cL without excitation
3.8%
10%
3.8%
7.5%
0%
G
H
I
excitation case
Figure 9. Change in lift coefficient for each component of the high-lift configuration
but with a factor of 2 reduction in signal strength (see figure 11(c)). For the lower momentum coefficient of
Cµ = 50 · 10−5 however, a dominant frequency at St = 0.17 is yielded (see figure 10(c)). On the other hand,
the first harmonic at St = 0.6 is also suppressed. Hence, the frequency analysis provides one possibility to
identify why the gain in lift for an antiphase excitation is smaller at this lower intensity. For the antiphase
case at the higher intensity, large coherent structures seem to be eliminated, because the first harmonic is
suppressed. At the lower momentum coefficient of Cµ = 50 · 10−5 , the first harmonic is also suppressed
(figure 10(c)). However, a dominant frequency at St = 0.17 occurs. This dominant frequency indicates that
a very long-waved structure is introduced into the flow field, possibly an interaction between the wake of the
slat and the structures generated by the actuation. This phenomenon is eliminated at higher intensities. This
has to be investigated in detail with a long-term average and an associated study of the vortex structures
in the wake of the flap. These findings also re-emphazise that so far this is not a comprehensive study but
rather the beginning of an extensive assessment of the use of segmented actuation slots.
However, to get more information about the vortex structures which develop both at the continuous and the
segemented actuation, different snapshots of the λ2 -criterion are compared in the following.
For selected cases, isosurfaces of the λ2 -criterion at a value of -200 are shown in figure 12. It is evident
that for the unexcited case (figure 12(a)) large scale vortex shedding occurs on the upper surface of the flap.
This also reflects the dominant frequency at St = 0.3 and the relatively low lift coefficient. The continuous
excitation then provides a considerable improvement of the flow above the flap. A displacement of the
flow separation in the downstream direction combined with improved mixture of the flow can be observed
(figure 12(b)). However, coherent structures still occur (dominant flap based frequency at St = 0.6 with
a large amplitude), which prohibit ideal lift performance. The largest gain in lift at Cµ = 300 · 10−5 is
provided by the antiphased excited case (together with the case at a phase shift of π/2), which is depicted
in figure 12(c). It can be noted from the isosurfaces that this excitation form provides a further increased
in the flow mixing above the flap, further displaced flow separation in the downstream direction and a
large number of small three-dimensional vortex structures in contrast to a small number of large coherent
structures. These highly three-dimensional structures offer more interaction among each other. Therefore
the dissipation rate in enhanced. This already was indicated by the findings of Günther et al.11 In general,
there is a link between the actuation of small three-dimensional structures and the gain in lift.
The actuation type where only one segment is used provides a gain in lift of 18% at an intensity of Cµ =
300 · 10−5 and 9% at Cµ = 50 · 10−5 . Hence, it achieves less gain in lift than those actuation types with
both segments actuated, although with less energy consumption. As depicted in figure 12(d), the large scale
vortex shedding on the upper surface of the flap is also suppressed. However, the reattachment of the flow on
the flap is inferior compared to the antiphase case. Furthermore, the level of mixture of the flow is worse and
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0,03
0,03
continouus excitation
0,025
Amplitude of lift coefficient
Amplitude of lift coefficient
unexcited
0,02
0,015
St = 0.3
0,01
0,005
0
0
0,5
1
2
1,5
2,5
0,025
0,015
0,01
0,005
0
0
3
St = 0.6
0,02
St = 1.2
1
0,5
Strouhal number
(a) Unexcited case
St = 0.17
one segment actuated
Amplitude of lift coefficient
Amplitude of lift coefficient
3
0,03
antiphase excitation
0,02
0,015
0,01
0,005
0
0
2,5
(b) Continuous excitation
0,03
0,025
2
1,5
Strouhal number
St = 1.2
0,5
1
2
1,5
2,5
0,025
0,02
0,015
0,005
0
0
3
St = 0.6
0,01
1
0,5
Strouhal number
2
1,5
2,5
3
Strouhal number
(c) Antiphase excitation
(d) One segment actuated
Figure 10. Spectra of the lift coefficient for Cµ = 50 · 10−5
0,04
unexcited
Amplitude of lift coefficient
Amplitude of lift coefficient
0,04
0,035
0,03
0,025
0,02
0,015
St = 0.3
0,01
0,005
0
0
0,5
1
2
1,5
2,5
0,035
0,02
0,015
St = 1.2
0,01
0,005
0,5
1
Strouhal number
2,5
3
(b) Continuous excitation
0,04
0,04
antiphase excitation
0,035
Amplitude of lift coefficient
Amplitude of lift coefficient
2
1,5
Strouhal number
(a) Unexcited case
0,03
0,025
0,02
0,015
0,01
St = 1.2
0,005
0
0
continouus excitation
0,025
0
0
3
St = 0.6
0,03
0,5
1
2
1,5
2,5
0,03
0,025
0,02
St = 0.6
0,015
0,01
0,005
0
0
3
Strouhal number
one segment actuated
0,035
0,5
1
1,5
2
2,5
Strouhal number
(c) Antiphase excitation
(d) One segment actuated
Figure 11. Spectra of the lift coefficient for Cµ = 300 · 10−5
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3
relatively large structures form, which propagate to a large extent in the y-direction (see figure 12(d)). The
continuously excited case at Cµ = 50 · 10−5 (figure 12(e)) looks similar to the case with continuous excitation
at the high momentum coefficient. It also provides a large gain in lift, namely 18% in total. Unlike the results
at the higher excitation intensity, the antiphase case at Cµ = 50 · 10−5 does not achieve an improvement of
the gain in lift. This is also reflected in the isosurfaces of the λ2 -criterion. Figure 12(f) shows large vortex
structures similar to the ”one-segment” case at Cµ = 300 · 10−5 (figure 12(d)).
(a) Unexcited flow
(b) Continuous excitation at Cµ = 300 · 10−5
(c) Antiphase excitation at Cµ = 300 · 10−5
(d) One segment actuated at Cµ = 300 · 10−5
(e) Continuous excitation at Cµ = 50 · 10−5
(f) Antiphase excitation at Cµ = 50 · 10−5
Figure 12. Snapshots of the isosurfaces of the λ2 -criterion at a value of -200
V.
Conclusion
A numerical investigation of active flow control by means of periodic suction and blowing has been
conducted. Both continuous and segmented actuation slots have been used on an industrially-relevant threeelement high-lift configuration. Preliminary work by Günther et al.6 showed that continuous actuation
already provides a significant gain in lift. Based on these results, the use of segmented actuation slots was
evaluated at two different excitation intensities and a non-dimensional excitation frequency that proved ideal
in previous investigations. Furthermore different phase shifts of the actuation on both consecutive slots were
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applied.
As a major result of the investigation, the use of segmented actuation slots instead of one continuous slot
provides a further gain in lift at an excitation intensity of Cµ = 300 · 10−5 . Chosing different phase shifts
of the actuation only has a minor influence. Mostly the improved mixing of the flow and the excitation of
highly three-dimensional structures on the flap are responsible for the larger gain in lift compared to the
continuous excitation. According to a frequency analysis, coherent structures are suppressed at the antiphase
excitation. Isosurfaces of the λ2 -criterion support these results.
At the lower excitation intensity of Cµ = 50 · 10−5 however, the segmented actuation does not provide an
improvement regarding lift. The antiphase excitation only reaches approximately the same lift coefficient as
the continuous excitation. Additionally, the frequency analysis shows a dominant frequency at a very low
Strouhal number of St = 0.17 which indicates that long-waved structures are formed. These give rise to a
lower gain in lift. Furthermore, snapshots of isosurfaces of the λ2 -criterion show that relatively large vortex
structures form on the upper side of the flap, which are obviously unfavourable for the gain in lift.
To sum up, it can be stated that more simulations at a large variety of excitation intensities (and frequencies)
are needed to evaluate the use of segmented actuation slots more precisely. It has to be investigated how the
gain in lift correlates with increasing excitation intensity. Furthermore, the vortex structures on the upper
side of the flap must be analyzed in more detail, for example with a feature based analysis.11 Nevertheless,
this investigation provides interesting and promising inital results of the segmented actuation for active
separation control and is the start of a comprehensive study of the subject.
Acknowledgements
The research project is funded by the Deutsche Forschungsgemeinschaft (German Research Foundation)
as part of the Collaborative Research Centre 557 Complex turbulent shear flows at TU Berlin. The authors are
also grateful to Tino Weinkauf, Zuse Institute Berlin, who provided the illuminated streamline visualization
of the flow above the flap.
References
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