12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

SEPARATION CONTROL WITH AIR-JET VORTEX GENERATORS OF
SHOCK/BOUNDARY-LAYER INTERACTIONS ON FLEXIBLE PANELS

Anne-Marie Schreyer
Deepak Prem Ramaswamy
Institute of Aerodynamics and Chair of Fluid Mech.Institute of Aerodynamics and Chair of Fluid Mech.
RWTH Aachen University
RWTH Aachen University
Wüllnerstr. 5a, 52062 Aachen, Germany
Wüllnerstr. 5a, 52062 Aachen, Germany
a.schreyer@aia.rwth-aachen.ded.ramaswamy@aia.rwth-aachen.de

Christopher Julian Schauerte
Institute of Aerodynamics and Chair of Fluid Mechanics
RWTH Aachen University
Wüllnerstr. 5a, 52062 Aachen, Germanyc.schauerte@aia.rwth-aachen.de

ABSTRACT

stood, although progress has been made (see e.g. Daub et al.
(2016); Spottswood et al. (2019); Hoy & Bermejo-Moreno(2021)). Also, the response of this coupled system to separation control has not been studied in detail so far.
The objectives of this research are to a) elucidate the fundamental mechanisms of interactions among shock waves, turbulent boundary layers, and the compliant panels over whichthese STBLI develop, and b) assess the effectiveness of separation control with AJVGs on STBLIs over flexible panels.
An experimental approach, including flow visualizations
with a schlieren setup, velocity measurements with particleimage velocimetry (PIV), and digital image correlation (DIC)
to study the panel deformation and flutter, is used to analyze
this complex fluid-structure interaction (FSI). Here, we present
an analysis of the effect of panel aspect ratio and AJVG control
on the flow topology, turbulence evolution in the interaction
region, and panel-flutter behavior.

Shock/turbulent boundary-layer interactions (STBLI)
with separation are ubiquitous in aerospace-transportation applications and detrimentally affect the aerodynamic performance and system integrity. We therefore aim to understandthe effect of separation control with air-jet vortex generators(AJVGs) on STBLI over both rigid and compliant walls, withthe ultimate goal to reduce structural vibrations induced by the
low-frequency unsteadiness of the shock system. An experimental study of a 24◦ compression-ramp interaction at Mach
2.52 over rigid and flexible panels of varying aspect ratio, withand without the influence of an array of AJVGs was carriedout. The flow topology, turbulence behavior in the interaction
region, and panel-flutter behavior are presented and discussed.

INTRODUCTION
Shock/turbulent-boundary-layer interactions (STBLIs)
occur in a variety of external and internal flows in supersonicflight and propulsion. For example, shock waves are generated
at compression surfaces in the inlets of air-breathing engines.
For strong shocks, the flow separates, which detrimentally affects the engine performance. Related effects include loss of
total pressure, inlet instability, locally high thermal loading,and structural fatigue. To the latter, low-frequency motionsof the shock/separation-bubble system (see e.g. Clemens &Narayanaswamy (2014)) are particularly relevant. If a resonant frequency of the structure is approached, induced structural vibrations may compromise the structural integrity.
It is therefore of great interest to decrease shock-induced
separation and control the shock-oscillation frequency. Small
sub-boundary-layer vortex generators, in particular air-jet vortex generators (AJVGs), have shown promising effectivenesson rigid surfaces (see e.g. Ramaswamy & Schreyer (2021)).
As many surfaces and structures in aerospace systems are
not rigid, the strong and unsteady aerodynamic loads imposed
by STBLIs result in coupled dynamics with compliant surfaces. STBLIs over rigid walls have been extensively studied
for several decades (Gaitonde (2015)), but the effects of wall
flexibility on the phenomenon have not yet been fully under-

EXPERIMENTAL SETUP AND CONDITIONSThe experimental investigations for this study were conducted in the trisonic wind-tunnel facility at RWTH Aachen
University. The intermittently-operating vacuum-storage facility achieves stable run times of 3 − 4 s. Air as the working
fluid is supplied to a settling balloon. To avoid condensation
effects, the air is dried and the relative humidity is kept below 6%. The test section has a cross-section of 0.4 × 0.4 m2 ;optical access is provided by two circular windows on eitherside of the test section and one on the top wall. The ambientconditions in the laboratory set the wind tunnel stagnation conditions; the selected free-stream Mach number of M∞ = 2.52
thus determines the Reynolds number (Re∞ = 9.6 · 106 1/m).
To create a STBLI, a 24◦ -compression ramp is installed
on a flat-plate model (see Fig. 1). The fully turbulent incoming boundary layer has a thickness of δ0 = 10.4 mm at
x = 4.5δ0 mm upstream of the ramp corner.
A cavity covered by the respective flexible panel is implemented into the flat plate upstream of the ramp corner. The
steel panels (AISI 304) have a thickness of h = 0.15 mm anda flutter parameter of λ = 509. Panel widths of 40, 80, and1

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

Figure 2. Schlieren visualizations for the rigid panel andAR1 flexible panel without and with control (averaged from
20 instantaneous images; depth of focus: 40 mm).

Figure 1. Schematic of the wind-tunnel model and PIV setup(top). Detail view of the AJVG array (bottom).

is used for correlations with the oscillating panel. The panel
was illuminated continuously with off-the-shelf tungsten lightsources and recorded with two Photron SA.5 high-speed cameras setup in stereoscopic configuration. The cameras looked
down onto the model from the side at an angle of approximately 30◦ . Images were recorded with a resolution of 1024 x
640 pixels (9.8 px/mm) at an exposure time of approx. 100 µs.
The schlieren and DIC systems are synchronized using an
ILA high-speed synchronizer, recording at 9.3 kHz.

120 mm at a length of 80 mm create panel aspect ratios of 0.5,
1.0, and 1.5 (cases AR0.5, AR1, and AR1.5, respectively). The
upstream and downstream edges of the panels are fixed, bothlateral edges are free. The pressure in the cavity is equal to
the free-stream static pressure. The panel length was selected
such that the compression-ramp-induced shock wave is locatedapproximately along the spanwise flexible-panel center line.
For the control cases, a one-row array of AJVGs (jetorifice diameter d jet = 1 mm, jet spacing D = 8d jet , injection
angles φ = 45◦ and θ = 90◦ ; see Fig. 1) is installed at 120 mm(=11.5δ0 ) upstream of the ramp corner. Each jet is injected at
a pressure of 1 bar and with a mass-flow rate of 0.0041 kg/s.
Two-component PIV measurements were conducted in
the streamwise/wall-normal center plane of the model (equivalent to the location of an AJVG orifice for the controlcases). We used a 527 nm Litron NANO-L pulsed PIV laser
with a maximum pulse energy of 200 mJ to illuminate the
DEHS seeding particles. Images were recorded with four
FlowSense EO 11M cameras (C1 – C4 in Fig. 1), equippedwith 4008 px×2672 px CCD sensors and Tamron SP AF
180mm f/3.5 objectives. Two cameras each observed thesame field of view (FOV): the first FOV covered the entire
flexible-plate region (C3, C4; −8.8δ0 ≤ x ≤ 1.9δ0 ), the second FOV covered the STBLI (C1, C2; −5.8δ0 ≤ x ≤ 4.1δ0 ).
The FOV was split to obtain a sufficiently high spatial resolution (40 px/mm). The respective cameras observing the same
FOV were triggered alternately to increase the overall acquisition rate to 10 Hz. PIV measurements are available for the
rigid surface, the AR0.5, and AR1.5 flexible panels.
Focusing-schlieren visualizations were carried out withthe system described by Schauerte & Schreyer (2018). Images are taken at the model-center plane using a PCO Dimax
HS4 high-speed camera at a resolution of 1402 x 502 px andwith an exposure time of 2 µs.
For the DIC measurements of the flexible-panel motion,a speckle pattern was applied to the matte-black painted panel
surface. A reference image at the rest position of the panel

RESULTSIn this article, we present the first analysis of this data set,
including a discussion of the flow topology, the evolution of
the turbulent velocity field upstream of the ramp corner, andthe flexible panel flutter frequency and deflection modes. Boththe effects of panel aspect ratio and of separation control with
AJVGs are analyzed.

Flow topology
The overall flow topology does not fundamentally differ
for STBLIs on rigid and compliant surfaces. A large-scale
shock-induced separation forms in the vicinity of the ramp corner (see the flow visualizations in Figs. 2 and 3). In the uncontrolled case (oil-flow visualization for baseline case on a rigid
surface; left image in Fig. 3), the separation line is straight and
2D. In the AJVG-controlled case, the overall separation length
Ls decreases, and the separation line becomes wavy and spanwise periodic.
The presence of a flexible panel leads to an increase in
separation length, with and without control (see Fig. 4; length
between extrapolated mean-shock position on the wall andramp corner in PIV). This increase in separation length is mostprobably related to 3D effects due to the panel deformation
and/or the dynamics of the flow. The shock front in all flexiblepanel cases, for example, is 3D even in the uncontrolled case(see double shock foot in focusing schlieren visualization in
Fig. 2).

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12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
so with increasing panel aspect ratio – thus indicating the upstream shift of the shock with increasing separation length.
Under the influence of control, the urms maximum in theseparation bubble (weakly) increases further initially, then decreases again slightly, before remaining constant for x/δ0 ≥−2. The initial increase is higher for the rigid panel (20% at
x/δ0 = −3 vs. 10% for both flexible panels). For the overall urms after this initial increase, no simple trend regarding
the influence of a flexible panel emerges from this preliminary analysis: for the rigid panel, the urms level returns to thesame value as for the uncontrolled case. For the AR1.5 flexible panel, the maximum marginally decreases by 2%, whereas
for the AR0.5 flexible panel, a 15% reduction in turbulence
intensity is observed for the controlled case. This value is thelowest of all presented cases.
Also for vrms , the overall increase in turbulence intensity
across the shock/boundary-layer interaction is larger for theflexible panel cases (factor of 1.8 vs. 1.62 for the rigid surface). Here, an effect of panel aspect ratio can be observed:
the overall increase is slower for the low-aspect-ratio panel(AR0.5) than for the AR1.5 panel, i.e. the same overall increase is reached over a longer downstream distance.
Under the influence of AJVG control, the continuous increase in vrms after the initial increase over the shock is slower
for the AR0.5 compliant wall, so that at x/δ0 = 0 an overall
decrease in vrms (compared with the uncontrolled case) is observed; the intensity maximum and profile coincide with therigid-panel case for x/δ0 ≥ −2. For the AR1.5 panel, a higher
vrms maximum than in the uncontrolled, rigid and AR0.5 panel
cases is reached.
This behavior indicates complex interactions between
panel flutter, the separation bubble, the shear layer, and thevortex structures induced by the AJVG control. These interactions are also affected by the panel aspect ratio. To get the full
picture, we will analyze the turbulent structures and dynamic
mechanisms in more detail in the future. The turbulence behavior probably varies with the spanwise location inbetween
the jet orifices, just as for rigid surfaces (see Ramaswamy &Schreyer (2021)). PIV data across the model span would be
helpful to study these aspects.

Figure 3. Surface oil-flow visualizations of the STBLI region
without and with AJVG control (rigid panel).

Figure 4.

Separation lengths from PIV.

AJVGs successfully reduce the separation length for both
the rigid surface and the flexible-panel cases (see Fig. 4). For
the compliant wall, the reduction is slightly smaller (7.14%,
from 4.48δ to 4.16δ for the AR0.5 case and 4.38%, from 4.8δ
to 4.59δ for AR1.5) than for the rigid surface (10.42%, from
4.03δ to 3.61δ ), and the control effectiveness decreases withincreasing panel aspect ratio. For the compliant wall, AJVGs
seem to alleviate 3D effects, as e.g. visible in the shock-footvisualization (Fig. 3 (right)).

Velocity field and turbulencePanel flutter

Figures 5 and 6 show the development of the turbulence
intensity of the streamwise (urms ) and wall-normal (vrms ) velocity components, respectively, for the rigid surface and the
40 mm and 120 mm flexible panels without (top) and with control (bottom) upstream of the ramp corner. Three influences
affect the boundary-layer turbulence: the interaction with thecompression ramp, the AJVG control, and the panel flutter due
to flexibility of the surface.
For all cases, the turbulence intensity of both components
initially increases over the shock. After this initial increase, theurms remain on a constant level, while the vrms increase further
due to streamline-curvature effects. The respective intensity
maxima are located within the separation region; towards theboundary layer edge, the turbulence levels decrease to the freestream/measurement-noise level. Due to the displacement effect of the separation bubble, the maxima shift away from thesurface as the ramp corner is approached.
The increase in urms is stronger for the flexible panels (afactor of 2.9 versus 2.6 for the rigid panel) due to the effect ofpanel flutter on the boundary-layer turbulence.
Downstream of the separation location, a secondary maximum farther away from the surface can be observed where theprofile traverses the shock. This maximum is located farther
away from the surface for the compliant wall, and increasingly

The induced dominant oscillation frequency of the panel,extracted from DIC measurements at the center location of
the respective panels, is indicated in Fig. 7. The frequency ismostly constant along the panel in the streamwise direction
and therefore only shown for the one location.
The frequency increases with the aspect ratio of the flexible panel. Under the influence of AJVG control, the frequency
decreases. These trends remain, even when normalized withthe respective separation lengths (StL ).
The decrease is weakest for the square panel (AR1),
where the AJVG control seems to barely affect the induced
panel-flutter frequency.
Overall panel displacements of ±2 mm = ±0.19δ0 are
reached during a flutter cycle. The maximum displacements
are shown exemplarily for the AR1.5 panel in Fig. 8.
In the uncontrolled baseline state, the deflection isstronger in the −z-direction for all tested panel aspect ratios(see Fig. 8 (left) for AR1.5), as one might expect since thepressure fluctuations exerted by the shock impacting the panel
from above.
Under the influence of AJVG control, however, the overall deflections become stronger in the +z-direction for AR0.5(not shown) and AR1.5 (Fig. 8 (right)), and almost equally3

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

Figure 5. Boundary-layer profiles of the root-mean square of the fluctuating component of the streamwise velocity upstream of theramp corner (x/δ0 = 0). Top: no control; bottom: AJVG control.

220

1.8

200

1.6

180

1.4
Flex. panel - no control
Flex. panel - AJVG control
Flex. panel - no control
Flex. panel - AJVG control

160
140

Figure 7.

AR0.5

AR1

AR1.5

1.2

StL=(f Ls)/U

f [Hz]

Figure 6. Boundary-layer profiles of the root-mean square of the fluctuating component of the wall-normal velocity upstream of theramp corner (x/δ0 = 0). Top: no control; bottom: AJVG control.

1

Figure 8. Maximum overall panel deflection for the AR1.5
case. Left: baseline case; right: with AJVG control.

Dominant flutter frequency of the flexible panels.
orthogonal decomposition (see e.g. Berkooz et al. (1993))
of the DIC measurement data. Based on this decomposition,
we intend to clarify how the AJVG control affects the paneldisplacement mode shapes and the energy content associatedwith the respective modes, as well as the influence of panel
aspect ratio on this behavior.
The energy content in the respective eigenmodes, as wellstrong in both directions (≈ ±1 mm) for the square panel(AR1; not shown).
These overall displacements are obtained from superimposing deflection modes.
We analyzed the individual mode shapes associated withthe panel displacement during flutter on the basis of a proper-4

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022
low energy modes: φ7 – φ10 are symmetrical in relation to the
diagonal of the panel.
Applying AJVG control to these cases only weakly affects the contributing panel-deflection modes. Modes φ1 andφ2 are still of the same shape as for the uncontrolled case andtheir energy content amounts to the same total. However, theenergy distribution between the modes is modified, as has been
expected from the overall deflection shown in Fig. 8.
For cases AR1 and AR1.5, the first mode is slightly lessdominant (78% and 75% instead of 93% and 83%, respectively), and more energy is shifted to the second mode φ2 (predominantly) and higher-order modes. This shift in energy isparticularly strong for AR1, which agrees with our previously
discussed observations of the behavior of the overall deflection, which is almost equally strong in both directions for thesquare panel (AR1; not shown) under the influence of control. For AR0.5, the opposite happens and the energy content
φ1 is weakly increased (compare the top and bottom lines inFig. 10).
On the basis of the observed dominant panel-deflection
modes, we may expect additional spanwise variations in turbulence intensity associated with the panel flutter, in particularfor higher aspect ratios. These variations will be superimposedon the spanwise variations associated with AJVG control arrays discussed in the previous section.

Figure 9. Eigenspectra λn and cummulative relative energy∑n for the first n POD modes. Left: baseline case; right: with
AJVG control.

CONCLUSIONS
Motivated by the ubiquity of shock/turbulent boundarylayer interactions (STBLI) with separation in aerospace applications, and the detrimental effects of these STBLI on aerodynamic performance and system integrity, we studied the effectof separation control with air-jet vortex generators (AJVGs)
on STBLI over both rigid and compliant walls. An experimental study of a 24◦ compression-ramp interaction at Mach 2.52
over rigid and flexible panels of varying aspect ratio, with andwithout the influence of an array of AJVGs was carried out.
This article presents the flow topology, development of
turbulence in the interaction region, and panel-flutter behavior.
Both the topology and flutter behavior are qualitatively
similar for all cases; neither the panel aspect ratio nor the separation control fundamentally change the overall flow and theshapes of the dominant deflection modes of the panel.
However, important details of the flow vary with panel
aspect ratio, in particular when interacting with AJVG control.
The separation length and the turbulence amplification
across the STBLI increase for compliant surfaces. The turbulence behavior probably also varies more strongly in the spanwise direction, since the effect of panel flutter is added to the
influence of spanwise location in relation to the AJVG orifices.
One strongly dominant panel-deflection mode was identified for all cases; the mode corresponds to a panel deflectionin the shape of a spanwise valley. The associated panel-flutter
frequency increases with the aspect ratio of the flexible panel.
AJVG arrays to control shock-induced separation are almost equally effective on compliant walls as on rigid surfaces –and even more necessary, since larger separation zones are induced due to the interaction between the STBLI and the panel
flutter. The reduction in separation length with AJVG control decreases with increasing panel aspect ratio. Also the reduction in turbulence intensity under the influence of control
varies with aspect ratio (stronger decrease for smaller aspectratio), as does the energy distribution between the deflection
modes.
The overall behavior for square panels weakly deviates

as the cumulative energy content after a number of n modes ispresented in Fig. 9. The eigenmodes are sorted by decreasing
fractional energy.
The eigenspectra for all cases show one strongly dominant mode. Without control, this first mode φ1 contains atleast 80% of the kinetic energy of the flutter motion, for theAR1 panel even 93%. It can be assumed that this mode corresponds to the extracted dominant panel-oscillation frequency
shown in Fig. 7.
Without AJVG control, the largest part of the energy is
associated with a panel deflection in the shape of a spanwise
valley. For all tested panel aspect ratios, both the first andsecond modes follow this shape (exemplarily shown for theAR0.5 case in Fig. 10), and further higher modes at lower energy also contribute; for the AR0.5 panel, the fourth and fifthmodes have the same shape as modes φ1 and φ2 , respectively(not shown), and even the seventh mode φ7 corresponds to asimilar type of deflection, although with two peaks and one
valley along the streamwise extent of the panel. This shape isto be expected, since the panel flutter is excited by a spanwiseoriented shock-wave front moving up- and downstream acrossthe panel’s streamwise centerline, and the panel is fixed at bothlateral edges.
Two additional mode shapes have weakly significant energy content for the uncontrolled AR0.5 case: φ3 , with up- anddownwards bent lateral free edges, and φ6 , which is symmetric about the panel center point (see Fig.10). Both of theseshapes are also among the six most energetic modes for thehigher-aspect-ratio panels (not shown).
For AR1 and AR1.5, the larger spanwise spanwise extend
enables additional modes including multiple streamwise valleys: φ6 (AR1, see Fig. 11) and φ5 (AR1.5, not shown) with acenter peak and downwards bent lateral edges, and modes φ7
and φ8 with a streamwise peak and valley in the center and bent
lateral edges for AR1.5 (φ8 shown in Fig.12). These modes,
however, have very low energy content (about 0.1%).
The square AR1 panel has an additional interesting set of512th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan, July 19–22, 2022

Figure 10.

POD modes with the largest energy content for the AR0.5 panel. First row: baseline case; second row: with AJVG control.

Figure 11. Additional higher-order POD modes for the AR1 panel, which differ from the AR0.5 modes. Five images on the left:baseline case. Image on the right: with AJVG control.

Figure 12. Additional higher-order POD modes for the AR1.5 panel, which differ from the AR0.5 and AR1 modes. Two images on
the left: baseline case. Image on the right: with AJVG control.

Annual Review of Fluid Mechanics 25, 539–575.
Clemens, N. T. & Narayanaswamy, V. 2014 Low-frequency
unsteadiness of shock wave/turbulent boundary layer interactions. Annual Review of Fluid Mechanics 46, 469–492.
Daub, D., Willems, S. & Gülhan, A. 2016 Experiments on
the interaction of a fast-moving shock with an elastic panel.
AIAA Journal 54 (2), 670–678.
Gaitonde, D. V. 2015 Progress in shock wave/boundary layerinteractions. Progress in Aerospace Sciences 72, 80–99.
Hoy, J. F. & Bermejo-Moreno, I. 2021 Numerical study of
STBLI on flexible panels with wall-modeled LES. In AIAA
Scitech Forum.
Ramaswamy, D. P. & Schreyer, A.-M. 2021 Control of shockinduced separation of a turbulent boundary layer using airjet vortex generators. AIAA Journal 59 (3), 927–939.
Schauerte, C. & Schreyer, A.-M. 2018 Design of a high-speed
focusing schlieren system for complex three-dimensionalflows. In Proceedings of the 5th International Conference
on Experimental Fluid Mechanics, pp. 232–237.
Spottswood, S. M., Beberniss, T. J., Eason, T. G., Perez, R. A.,
Donbar, J. M., Ehrhardt, D. A. & Riley, Z. B. 2019 Exploring the response of a thin, flexible panel to shock-turbulent
boundary-layer interactions. Journal of Sound and Vibration 443, 74–89.

from rectangular panels: the AJVG control seems to barely
affect the flutter frequency and additional deflection modes(symmetric in relation to the panel diagonal) are superimposed. Without control, the first deflection mode for the AR1
panel has an even higher energy content than for the rectangular panels; this difference is mitigated by the AJVG control.
A more detailed analysis of the turbulent structures anddynamic mechanisms will be carried out to explain these observed differences. Overall, the gained knowledge and understanding will allow for the development of effective control setups to mitigate threats to the structural integrity of aerospace
applications that are imposed by shock-induced separation.

Acknowledgements
This research was funded by the German Research Foundation (DFG) in the framework of the Emmy Noether grant“Separation Control with Air Jet Vortex Generator Arrays inTransonic and Supersonic Flow” (grant 326485414).

REFERENCES
Berkooz, G., Holmes, P. & Lumley, J. L. 1993 The proper orthogonal decomposition in the analysis of turbulent flows.

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