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

EXPERIMENTAL STUDIES OF BOUNDARY LAYER DYNAMICS VIA ACTIVE
MANIPULATION OF LARGE-SCALE STRUCTURES
Mitchell Lozier
Dept. of Aerospace and Mech. Engineering
University of Notre Dame
Notre Dame, IN, 46556, USA
mlozier@nd.edu

Flint O. Thomas
Dept. of Aerospace and Mech. Engineering
University of Notre Dame
Notre Dame, IN, 46556, USA
fthomas@nd.edu

Stanislav Gordeyev
Dept. of Aerospace and Mech. Engineering
University of Notre Dame
Notre Dame, IN, 46556, USA
sgordeye@nd.edu

ABSTRACTThe dynamic response of a zero pressure gradient turbulent
boundary layer (TBL) to a an active flow control actuator was
experimentally studied using particle imaging velocimetry(PIV). In previous experiments using a single hot-wire; it was
shown that the synthetic large-scale structure (LSS) introducedby this plasma-based actuator in the outer region of TBL had astrong modulating effect on the near-wall turbulence. Results
showed that close to the actuator, an actuation frequency
comparable to the burst/sweep frequency of the near-wallstructure created the strongest modulation effect while farther
downstream, an actuation frequency related to the streamwise
wavelength of the naturally occurring LSS resulted in thestrongest modulation effect. In the study reported here, an
improved plasma-based active flow control device was placed in
the same region of the TBL to introduce a periodic synthetic LSS
in order to study the response of the TBL to these large-scaledisturbances. Planar PIV was used to measure the time-resolved
two-dimensional velocity downstream of the actuator at a single
streamwise location. Using PIV, the modulating effect on thenear-wall turbulence is described in more detail. The results are
discussed and compared with previous hot-wire measurements
and numerical simulations.

In general, the prevailing views fall into two broad groups:(1) those that focus on the influence of outer layer LSS on thenear wall turbulence generation mechanism; the so-called” topdown” mechanism and (2) those that view the near wallmechanism as largely autonomous, and the outer LSS results as
a consequence of the near-wall turbulence; the “bottom-up”
mechanism. Several different models have sought to couple thetwo regions, with one of the oldest and most notable being the
Attached Eddy Model [5], which suggests self-similar eddies
with a typical topology of the structure. Mathis et al [4] alsomodelled these interactions using a correlation-based statisticalmodel where the near-wall effects are predicted from thestatistics of the large-scale structures. Both models have provenincredibly useful in understanding the structure of turbulent
flows but are limited in that they are statistical in nature and failto capture the underlying dynamic interaction of the structures.
Resolvent analysis is a recent method that overcomes thislimitation by looking at spatial-temporal interactions in wallbounded turbulent flows, thereby providing insight into the
dynamics of the structures [6]. Resolvent analysis has provensuccessful in identifying a key wavenumber-frequency “kernel”
or “skeleton” of turbulent pipe flow and answering fundamental
questions about the structure of wall-bounded turbulent flows,
see e.g. [7].
A vast majority of studies and models regarding therelationship between the near-wall and outer layer large scale
structures deal with natural un-manipulated TBLs and apply
various conditional averaging techniques are applied to study
their interactions [4]. We take the view that to clarify the
dynamics of the large-scale structure, one needs to analyse theflow’s response to an external large-scale perturbation. Such a
dynamic systems approach in which the boundary layer isperturbed via an actuator is particularly well suited to gaining
insights regarding the underlying flow physics which is essential
for the design of novel active flow control strategies. It
artificially introduces a well-defined perturbation with a given
frequency and/or spatial scale that allows quantification of thenonlinear TBL response. This approach allows, for instance, thestudy of triadic interactions between various scales of motion. It
also provides a well-defined phase reference by which toperform a phase-locked analysis. In a similar manner, periodic
perturbations can also be experimentally introduced into aturbulent boundary layer through a dynamic (temporally
oscillating) roughness, which provides a reference phase to

INTRODUCTION
It is now widely recognized that vortical large-scale
structures (LSS) play a key role in governing the dynamics ofwall bounded turbulent flows. The effect of these LSS in
turbulent boundary layers (TBL) on technologically relevant
flow properties (e.g., friction drag, noise generation, aero-optical
distortions, flow separation etc.) have been extensively
documented [1-3]. This influence of the LSS on the TBL
dynamics was shown to increase with Reynolds number [3]. The
dynamics of the LSS have also been shown to be correlated with
the near wall small scale turbulence [3,4]. These findings suggest
the potential effectiveness of flow control strategies focused onaltering the LSS dynamics in to achieve desired technologicalgoals (e.g., reduced drag, noise reduction and separation
control). Such strategies could lead to enormous performancegains and cost savings. However, to date, this potential remains
largely unrealized due, in large part, to an incomplete
understanding regarding the genesis, subsequent evolution andinteraction of LSS in the inner and outer regions of the TBL.

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

Figure 1. (a) picture of plasma-based ALSSA device and (b) schematic of experimental PIV set-up.
electrodes were installed on both sides of the actuator plate. The
alternating current dielectric barrier discharge (AC-DBD)
plasma that formed on top of the actuator was produced using ahigh voltage AC source. This source provided a 40kV peak-topeak sinusoidal waveform excitation to the electrodes at afrequency of 4 kHz. The streamwise length of the actuator waschosen to be 𝐿 = 32 𝑚𝑚 (< 1 𝛿) to minimize the downstream
wake without creating arcing. The spanwise length of theactuator plate was 𝑊 = 25 𝑐𝑚 (8 𝛿) wide. Airfoil-shapedvertical supports were used to position the actuator plate at a
fixed wall-normal distance of ℎ/𝛿 = 0.3 (ℎ = 200) . To
introduce periodic forcing, the sinusoidal waveform wasmodulated by a square wave at the forcing frequency, 𝑓 =80 𝐻𝑧, (𝑓 𝛿/𝑈 = 0.4) with a fifty percent duty cycle.
A schematic of the experimental set-up is shown in Figure1(b). To quantify the effect of the synthetic large-scale structureon the near-wall turbulence a time-resolved planar PIV system
was used. To perform PIV measurements, the flow was seededwith DEHS particles (diameter <1μm) and illuminated with acontinuous 532 nm laser sheet ( < 1𝑚𝑚 spanwise thickness)directed through the top of the test section. The laser sheet was
focused onto a small streamwise region (∆𝑥 = 6𝑚𝑚, ∆𝑥/𝛿 =0.2) to produce enough intensity from the continuous laser
source to perform PIV. Images were captured with a Phantom
v2512 high-speed camera using a 5𝜇𝑠 exposure at a resolutionof 384 × 280 pixel. The measurements presented here weretaken at the streamwise locations of 𝑥/𝛿 = 5. The frame rate of
the camera was 20 kHz (100 U∞/), and spatial resolution of thecalculated vector fields are x+ =y+ = 16. The PIV acquisitionand plasma forcing were triggered simultaneously so that thecollected data were phase-locked to the plasma actuation cycle.
This PIV configuration resulted in measurements that were time
resolved, two-dimensional velocity fields which are also spatial
resolved in the wall-normal direction.

isolate synthetic large-scale structures and small-scale flow
structures [8-10]. Instead of introducing the perturbation very
close to the wall, Ranade et al. [11] performed an experimental
study where the perturbation was introduced in the outer region
as a forced shear layer and the turbulence inside the boundary
layer was found to be both amplified and modulated by theexternal forcing. In summary, the introduction of periodic
perturbations of the scales of interest has proven an effective
way to characterize the flow phenomena of interest.

APPROACH AND EXPERIMENTAL SET-UP
In our previous work [12,13], we introduced large-scale
spanwise vorticity into the outer portion of the TBL using anovel active flow control device, called an Active Large-Scale
Structure Actuator (ALSSA), see Figure 1(a). The ALSSA wasconfigured to produce a spanwise uniform plasma-induced
pulsed jet that serves to introduce coherent spanwise vorticity
into the TBL at a specified frequency and wall-normal location.
To isolate the interactions of interest, the experimental studies
were performed at 𝑅𝑒 = 1770,, which is low enough that
there was no strong naturally occurring large-scale structure in
the outer portion of the turbulent boundary layer. This approachallowed the controlled introduction of a synthetic periodic largescale structure into the TBL. The fixed forcing frequency
provides a well-defined reference frequency by which to phaselock the TBL response and allows clear separation between thelarge-scale structure and the resulting changes to the TBL
dynamics. The advantage of using the plasma-based ALSSA
method of actuation is that it introduces the periodic structure ata user-selected location away from the wall without directly
affecting the near-wall turbulence.
The experiments were performed in a low-turbulence,subsonic, in-draft wind tunnel located at the University of Notre
Dame. To create a canonical turbulent boundary layer, a 2meter-long by 0.6-meter-wide boundary layer development plate
was installed in the centre height of the tunnel test section. Main
turbulent boundary layer parameters are summarized in Table 1.

RESULTS
Profiles of the streamwise component of mean velocity andturbulence intensity are shown in Figure 2. In Figure 2(a) the
canonical mean velocity profile measured using PIV matches the
DNS results done with similar turbulent Reynolds number [14].
There is a slight deficit in the mean velocity around the actuatorlocation and extending down towards the wall when the plasma
forcing was added. This effect is mostly due to the presence of
the actuator plate which creates a wake within the TBL [12]. In
Figure 2(b) the canonical turbulence intensity profile is slightly
attenuated in the near-wall and log-region due to spatial

Table 1. Turbulent boundary layer parameters

33.2
mm

U
7.95
m/s

u
0.304
m/s

Cf

Re

Re

0.00391,770

690

The rectangular actuator plate was made of a 2 mm thick(0.06𝛿) sheet of Ultem dielectric polymer. To create plasma, two212th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
averaging caused by the PIV windows. This effect is consistent
with the spatial attenuation predicted for hot-wires in [3] asindicated by the ’ × ’ in Figure 2(b). When the plasma forcing is
added there is a deficit in turbulence intensity below the actuator.
This reduction in the turbulent intensity in the log-region was
found to be mostly due to the presence of the actuator plate [13].
The largest reduction in the turbulence intensity was observed
near the location of the actuator, indicated by the dashed line in
Figure 2(b). The reduction of the turbulent intensity wasobserved to extent all the way to the buffer region (y+~20), as
well as in the region above the actuator location. All these results
are consistent with the results of the hot-wire experiments
documented previously [12,13].

the plate described above [12,13]. In Figure 3(b) the measured
canonical Reynolds stress (−𝑢𝑣 ) is consistently lower than the
DNS data throughout the entire boundary layer region due to a
combination of the spatial attenuation effects described earlier.
When the plasma forcing is added there is a significant decreasein the Reynolds stress around the actuator location that extends
both above and below the actuator, like the actuator response forthe wall-normal turbulence intensity. In Figure 3(c) the
measured canonical turbulence production −𝑢𝑣

matches

u2rms /u2U+

the DNS results well. When the plasma forcing is added there isa consistent decrease in turbulence production that extends
downwards towards the wall. This largest decrease in turbulencein turbulence production occurs around the centre of the logregion and is nearly equal to the canonical production above theactuator location.

Figure 2. (a) mean streamwise velocity and (b) streamwise
turbulence intensity profiles. Vertical dashed line indicates
actuator location. Dotted line is DNS data [14]. Black ‘ × ’
represents expected attenuation of turbulence intensity due tospatial averaging. 𝑥 = 5𝛿, ℎ = 200, 𝑓 = 80𝐻𝑧.
In addition to the streamwise velocity components, wallnormal components of velocity have been obtained using PIV.
The wall-normal turbulence intensity, Reynolds stress, andturbulence production are presented in Figure 3. In Figure 3(a)the measured canonical turbulence intensity is less than the DNS
results due to spatial attenuation in a manner consistent with thestreamwise results presented above. When the plasma forcing is
added there is a decrease in the turbulence intensity at theactuator location that extends both above and below the actuator.
It is expected that for both the streamwise and wall-normal
velocity components there will be a reduction in fluctuations dueto the presence of the actuator plate with some increase inturbulent fluctuations in the region of the plasma forcing above

Figure 3. (a) wall-normal turbulence intensity (b) Reynold stress(−𝑢𝑣)and (c) mean turbulence production −𝑢𝑣

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profiles.

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
Vertical dashed line indicates actuator location. Dotted line is
DNS data [14]. 𝑥 = 5𝛿, ℎ = 200, 𝑓 = 80𝐻𝑧.
The mean velocity, turbulence intensity, and Reynolds stressresults show different behaviour than those seen in other forced
turbulent boundary layers, specifically bottom-up actuation
schemes [16]. It was shown that in wall actuated TBL there are
generally increases in the turbulence intensity and Reynolds
stress in the log and outer regions of the TBL. In this experiment
the actuator plate creates a decrease in these quantities while theplasma forcing creates a slight increase above the actuator
resulting in an overall decrease in turbulence intensity andReynold stress.
Because the PIV measurements were phase-locked to theplasma forcing, phase-locked analysis of the PIV measurements
was performed using the framework described in [11,12,13],
where a triple phase-locked Reynolds decomposition of thestreamwise velocity is used,𝑢(𝑥, 𝑦, 𝑡) = 𝑈(𝑥, 𝑦) + 𝑢(𝑥, 𝑦, 𝜑) + 𝑢′(𝑥, 𝑦, 𝜑, 𝑛).
Here u is the instantaneous velocity, U is the time mean
component of velocity, 𝑢 is a phase dependent or modal velocity
component, 𝑢′ is a fluctuating turbulent component, φ is thephase, and n is the number of realizations. To study thefluctuating component, it is convenient to remove the timeaveraged term from the phase-locked fluctuating component andinvestigate the residual phase-changing term only,Δ𝑢(𝑦, 𝜙) = 𝑢(𝑦, 𝜙) − 〈𝑢(𝑦, 𝜙)〉 ,where,𝑢(𝑥, 𝑦, 𝜑) = (〈[𝑢 (𝑥, 𝑦, 𝜑, 𝑛)] 〉 )Figure 4. Phase-locked variations in (a) streamwise and (b)wall-normal modal velocity. Actuator location indicated byhorizontal dashed line. 𝑥 = 5𝛿, ℎ = 200, 𝑓 = 80𝐻𝑧.

The same decomposition was used for the wall-normal, vcomponent.
The streamwise and wall-normal components of the modal
velocity are shown in Figure 4. There are strong modal
fluctuations around the actuator location that extend all the way
to the near-wall region, as well as to the region above theactuator, for both streamwise and wall-normal components. The
strongest variations are around the location of the plasma forcing(𝑦 = 300 [13,15]) and the geometric centre of the log-region(𝑦 = 100) where naturally occurring LSS would be present inhigher Reynolds number boundary layers. The positivefluctuations in wall-normal modal velocity appear to slightly
lead the positive fluctuations in streamwise modal velocity inphase. The modal velocity near the wall lags the velocity in the
log-region due to the slower convective speed near the wall.
These results are consistent with the hot-wire results presented
in [12,13]. In addition, the extracted modal velocity agrees very
well with spatial linear input-output analysis [15]. The modal or
large-scale response of the TBL to these synthetic outer-layer
LSS is very similar to the large-scale response of the TBL to the
dynamic roughness observed in [16].

The streamwise and wall-normal components of the residual
turbulence are presented in Figure 5. In Figure 5(a) the positivefluctuation in residual turbulence above the actuator location isa signature of the convecting synthetic LSS as described inprevious experiments [12,13]. The region of positive fluctuation
below the actuator is referred to as the region of modulated nearwall turbulence. This region of modulated turbulence occurs
directly below the convecting LSS, and the size, shape andamplitude of the modulated turbulence obtained using PIV isconsistent with the results obtained with a hot-wire [12,13]. The
region of modulated turbulence extends from the wall throughthe log-region and has an inclination consistent with canonicalnear-wall structures [3,13]. The region of modulated streamwise
turbulence is slightly leading the positive fluctuations instreamwise modal velocity in phase. This region is also nearly inphase with the wall-normal modal velocity near the wall. In
Figure 5(b) there are no significant regions of modulated wallnormal turbulence below the plate, but there are somefluctuations above the plate which are slightly out of phase withthe streamwise residual turbulence in a manner consistent withthe components of the modal velocity.

412th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
the region of the synthetic LSS (𝑦 = 300). Near the centre of
the log-region positive changes in Reynolds stress areapproximately in phase with the positive fluctuations in thestreamwise modal velocity. These positive Reynolds stressfluctuations also slightly trail the region of modulated turbulence
as seen in Figure 5(a). Around the actuator location and near thewall the fluctuations in Reynolds stress are slightly behind thefluctuations at 𝑦 = 100 in phase. This phase delay is likely due
to a wall-normal spreading of the Reynolds stress away from thecentre of the log-region.
These distinct phase regions highlight the modulating effectof the synthetic LSS. The synthetic LSS cause large-scale
velocity fluctuations in the region between the plate and the wall.
These fluctuations are correlated with increases in residual
turbulence and Reynolds stress. These distinct regions show thatthe fluctuations in turbulence parameters within the log-region
are not induced directly by the LSS but rather are being
modulated by LSS presence. The strong changes to turbulence
parameters within the log-region, where the natural coherent
LSS exist, reinforces previous observations that the forced TBL
behaves similarly to a higher Reynolds number canonicalboundary layer [12,13] when exciting the scales close to thenaturally occurring LSS.

Figure 5. Phase-locked variations in (a) streamwise and (b) wallnormal components of residual turbulence. Actuator location
indicated by horizontal dashed line. 𝑥 = 5𝛿, ℎ = 200, 𝑓 =80𝐻𝑧.

Figure 6. Phase-locked variations in Reynolds stress (−𝑢𝑣)Actuator location indicated by horizontal dashed line. 𝑥 =5𝛿, ℎ = 200, 𝑓 = 80𝐻𝑧.

Figure 7. Phase-locked variations in (a) spanwise vorticity and(b) fluctuating velocity quadrants. Actuator location indicated byhorizontal dashed line. 𝑥 = 5𝛿, ℎ = 200, 𝑓 = 80𝐻𝑧.

The phase-locked decomposition, described above, can also
be used to study the modal changes in the Reynolds stress(−𝑢𝑣). The results are shown in Figure 6. There are multiple
regions where the modal variations in the Reynolds stress havedistinct phases. The strongest variations in Reynolds stress arearound the geometric centre of the log-region (𝑦 = 100) and

Finally, the phase-locked variations in spanwise vorticity
and fluctuating velocity quadrant analysis are presented in
Figure 7. From Figure 7(a) the vortical signature of the LSS
above the actuator plate can be seen as well as the induced
vorticity in the near-wall and log-region. The fluctuations in5

12th International Symposium on Turbulence and Shear Flow Phenomena (TSFP12)
Osaka, Japan (Online), July 19-22, 2022
spanwise vorticity match the phase of the Reynolds stress andturbulence production fluctuations. It should be kept in mind thatlater phase corresponds to the earlier spatial location, if a frozen
flow assumption is used,
𝑥=𝑥−[5] de Silva, C. M., Hutchins, N. & Marusic, I.,
2016,“Uniform momentum zones in turbulent boundary layers,”
J. Fluid Mech., 786, pp. 309-331.
[6] McKeon, B. J. and Sharma, A. 2010 “A critical layer
framework for turbulent pipe flow,” J. Fluid Mech., 659, pp.
336-382.
[7] Sharma, A.S., and McKeon, B.J., 2013. On coherent
structure in wall turbulence J. Fluid Mech. 728 pp. 196–238.
[8] Jacobi, I., and McKeon, B. J., 2011, “Dynamic roughness
perturbation of a turbulent boundary layer,” J. Fluid Mech., 688,
pp. 258–296.
[9] Jacobi, I., and McKeon, B. J., 2013, “Phase relationships
between large and small scales in the turbulent boundary layer,”Exp. Fluids, 54(3), p. 1481.
[10] McKeon, B. J., Jacobi, I., and Duvvuri, S., 2018,“Dynamic roughness for manipulation and control of turbulent
boundary layers: an overview,” AIAA J., 56(6), pp. 2178–2193.
[11] Ranade, P., Duvvuri, S., McKeon, B., Gordeyev, S.,
Christensen, K., and Jumper. E.J., 2019,” Turbulence Amplitude
Amplification in an Externally Forced, Subsonic Turbulent
Boundary Layer,” AIAA J., 57(9), 3838-3850[12] Lozier, M.E., Thomas, F.O., and Gordeyev, S., 2020,”
Streamwise Evolution of Turbulent Boundary Layer Response
to Active Control Actuator,” AIAA Paper 2020-0097.
[13] Lozier, M.E., Thomas, F.O., and Gordeyev, S., 2021,”
Turbulent Boundary Layer Response to Active Control Plasma
Actuator”, AIAA Paper 2021-1455.
[14] Jiminez, J., et. al. 2010, “Turbulent Boundary layers andchannels at moderate Reynolds number,” JFM 657, 335-360.
[15] Liu, C., et. al. 2022* “Spatial input-output analysis oflarge-scale structures in actuated turbulent boundary layers,”
AIAA J.
[16] Huynh, D., McKeon, B., 2020, “Characterization of theSpatio-Temporal Response of a Turbulent Boundary Layer toDynamic Roughness,” Flow, Turbulence and Combustion 104,
293-311.
[17] Nagano, Y., and Tagawa, M., 1995, “Coherent motions
and heat transfer in a wall turbulent shear flow,” J. Fluid Mech.,Vol. 305, pp. 127–157.
[18] Midya, S., Thomas, F. O. and Gordeyev, S. V., “On theSpectral Decomposition of Skewness in Canonical and ActuatedTurbulent Boundary Layers,” Proceedings of the 12th
International Symposium on Turbulence and Shear Flow
Phenomena (TSFP12), Osaka, Japan, 2022.

𝑈.

Here x is the streamwise pseudo-spatial coordinate, 𝑥isthe measurement location, and 𝑈 is an appropriate convective
velocity for the actuator location, see [12] for details. Thus, from
Figure 7(b) the quadrant analysis shows that the fluctuations in
the velocity in physical space follow the 𝑄4 → 𝑄1 → 𝑄2 → 𝑄3
pattern. This sequence corresponds to the dynamics of the largescale structures observed in canonical turbulent boundary layers
and plays the most important role in the dynamics and transportof near-wall turbulence [17]. Both the vorticity and quadrant
analysis results are consistent with the results of the spatial
input-output analysis [15]. It does demonstrate that the synthetic
LSS introduced by the plasma actuator has a very similar
dynamic effect on the turbulent boundary layer as the naturallyoccurring LSS in the higher Reynold number boundary layers.

CONCLUSIONSPIV measurements of the top-down plasma actuated TBL
were made at a single streamwise location and compared toprevious experimental works [12,13,16] and numericalsimulations [14,15]. In terms of statistical quantities, the plasmabased top-down actuation method resulted in an opposite effectto bottom-up wall actuated methods [16] where all the twodimensional Reynolds stresses decreased in the log and outerregions instead of increasing due to the presence of an actuatorplate. Phase-locked analysis of the modal velocity and residual
turbulence showed good agreement with previous work [12,13]
as well as numerical simulations [15]. Additionally, analysis of
the variations phase-locked variations in Reynolds stress showedthat the LSS have a predominantly modulating effect within the
log-region and on the near-wall turbulence. Specifically, the LSS
create variations localized within the log-region which drive themodulation of the near-wall turbulence. These observations
along with the canonical patterns from the quadrant analysis
show that the TBL is responding to the presence of LSS in
manner consistent with higher Reynolds number canonicalboundary layers. These results are consistent with the spectral
decomposition of skewness measurements presented in Midya et
al [18] which show significant differences between the canonicaland actuated TBLs in the logarithmic region.

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