1. 1
Analysis of Arch-Fairing Flow Control System in Boundary
Layer Research Tunnel
Thomas J. Setzera
Utah State University, Logan, UT 84322
and
David M. Schatzmanb
and Jacob S. Wilsonb
U.S. Army Aviation Development Directorate—AFDD, Moffett Field, CA 94035
For most rotorcraft, a major contribution to total vehicle drag comes from the rotor hub.
Significant research into reducing drag on rotorcraft has been conducted, and numerous
experiments have focused on the hub in particular. The purpose of this project is to investigate
the effectiveness of reducing separated flow and unsteadiness behind a finite circular cylinder,
which represents a simplified flow field of the rotor shaft and hub. This experiment expands
on prior research conducted on finite circular cylinders in turbulent flow conditions. The
current experimental effort is concentrated on the development of a passive semicircular arch-
fairing mounted downstream of the cylinder. To better control the wake flow, the arch-fairing
has an airfoil-shaped cross section. Several geometries and locations of the arch-fairing are
tested to determine the best design. Measurements of cylinder drag loads, surface pressure,
and velocity profiles, are conducted to assess the effectiveness of the concept. Oil surface flow
visualization is utilized to compare the flow patterns with and without the arch-fairing and
evaluate the impact on the flow field.
Nomenclature
CD = total drag coefficient
CD,p = pressure drag coefficient
Cp = pressure coefficient
D = measured drag force on cylinder
d = diameter of cylinder
H = height of cylinder
ht = height of wind tunnel test section
i = iteration index for numerical analysis
N = total number of iterations for numerical analysis
Pxx = common logarithm of energy content
p = total measured pressure
p∞ = free stream static pressure
RA,cyl = aspect ratio of cylinder
RT,M = ratio of model area to tunnel area
Re = Reynolds number
S = cross-sectional area of cylinder
St = cross-sectional area of wind tunnel
u = velocity
U∞ = free stream velocity
wt = width of wind tunnel test section
x = streamwise coordinate from leading edge of cylinder or test section entrance
y = vertical coordinate from wind tunnel floor
α = angle of attack
θ = cylinder angle measured from leading edge
a
Undergraduate Intern, Aeromechanics Branch, NASA Ames Research Center.
b
Research Engineer, Aeroflightdynamics Directorate, Ames Research Center/M.S. 215-1.

2. 2
µ = dynamic viscosity
ρ = density
I. Introduction
NE major contribution to drag on rotorcraft comes from the rotor hub, accounting for between 30-40% of the
total drag. The wake produced by separated flow behind the hub can also cause issues with tail performance if it
is not controlled. The arch-fairing is a relatively new concept for passive flow control (PFC); this device is intended
to reattach separated flow behind the rotor hub and reduce hub drag without requiring additional energy input. It has
an airfoil-shaped cross section so as to better control wake flow, providing a streamlined shape for the wake to flow
over.
Since the arch-fairing is a design that has not yet been tested, this experiment seeks to determine the effectiveness
of the arch-fairing as a PFC device. Several measurements are taken, including velocity profiles, drag force on the
cylinder, and surface pressure, to quantify this experiment’s findings. Additionally, oil surface flow visualization are
utilized to observe the flow patterns with the arch-fairing and compare them to flow patterns without. If this experiment
shows the arch-fairing to be an effective PFC device, it could help set the stage for future research in drag reduction
and flow control of rotorcraft.
II. Background
There have been many experiments conducted on finite circular cylinders in flow, particularly in turbulent flow
conditions. When air flows over such a cylinder, the flow separates and causes several types of vortices to form. These
include a horseshoe vortex at the cylinder’s base, an arch vortex immediately behind the cylinder, tip vortices on the
top of the cylinder, and trailing vortices well behind the cylinder1
. Together, these vortices form a wake region behind
the cylinder. Controlling this wake region has been a significant challenge in engineering for some time, and there has
been much research conducted on separated flow patterns and how to control them.
Placing objects downstream from the cylinder can affect the flow patterns to some degree. Depending on where
the object is placed, separation or reattachment may occur sooner or later. This is especially true for bluff bodies
asymmetrically offset from the center of the cylinder2
. Therefore, placement of the arch-fairing relative to the circular
cylinder is a crucial aspect of this experiment.
III. Experimental Setup
This experiment is conducted in the boundary layer research tunnel, owned and operated by the U.S. Army
Aviation Development Directorate—AFDD. The tunnel is located in a small lab behind the 7-by-10-foot wind tunnel
at NASA Ames Research Center. In order to avoid wall effects, the ratio of the model cross-sectional area to the test
section area should be kept between 5% and 10%.
The cylinder has a height of 6 inches and a diameter of 4 inches, resulting in an aspect ratio of 1.5 so as to eliminate
streamwise vortices near the bottom of the cylinder3
. It also has a hollow interior to allow for the placement of
instrumentation, as well as a cap to prevent air from flowing into the top of the cylinder and potentially skewing the
measurements taken during the experiment. Small holes on the interior of the top and sides of the cylinder allow for
the placement of pressure taps, while the bottom cap is attached to a load cell to record drag force measurements.
The baseline for this experiment will be the cylinder tested without an arch-fairing behind it, tested at a free stream
velocity of U∞ = 75 ft/s. When the arch-fairing is positioned behind the cylinder, the characteristics of the wake and
separated flow are expected to vary from the baseline. Altering the configuration of the arch-fairing, such as using
elliptical arch pieces, changing the height, or repositioning the entire assembly, is expected to further change the flow
patterns behind the cylinder.
A. Description of Arch-Fairing
The arch-fairing consists of several different components: a curved arch piece, extension pieces of varying lengths,
and flanged baseplates to secure the arch-fairing to the wind tunnel floor. Each component was manufactured using
3D printing techniques from Nylon 12 GF. The pieces are joined together by rectangular connectors 1/8 inch long.
Each arch piece has a semi-minor axis, or height, of 1.5 inches, and the span of each piece varies as a function of the
height. The baseplates of the arch-fairing are secured to the test section floor with 2-mil thick Mylar tape. A Clark-Y
airfoil is used as the cross section for the arch-fairing, chosen for its streamlined shape and performance characteristics
at low Reynolds numbers. The airfoil has a 2-inch chord, a maximum thickness of 11.7% of the chord, and no
adjustment factor. Figure 1 shows one of the arch-fairing configurations used in this experiment, as well as the
direction of flow in the wind tunnel.
O

3. 3
Fig. 1 Example arch-fairing assembly with 2-inch-tall extension pieces, circular arch piece at zero angle of
attack, and flanged baseplates.
The extension pieces for the arch-fairing come with three different heights: 0.5 inch, 1 inch, and 2 inches. The
extension pieces and the baseplates also have a cross section shape of a Clark-Y airfoil. Different arch-fairing heights
can be attained by using different combinations of the extension pieces. The seams between the pieces are taped over
during testing to ensure that they stay together. Additionally, the span of the arch-fairing can vary if the circular arch
piece is exchanged for elliptical arch pieces of varying aspect ratios; doing so moves the ends of the arch-fairing
outside the wake region, changing the flow patterns behind the cylinder. The angle of attack of the arch pieces can
also vary, and doing this may also alter the flow patterns significantly. The multiple height, span, and angle of attack
configurations of the arch-fairing are tested at different distances behind the cylinder to determine the effect of
streamwise position on the finite cylinder wake.
B. Boundary Layer Wind Tunnel
This wind tunnel is typically used for testing turbulent boundary layer flow over a range of pressure gradients. It
is an open-circuit, blow down wind tunnel, with a fan-drive in front, followed by a diffuser and settling chamber with
a honeycomb section and six sets of screens. The entrance to the test section measures three feet wide by one foot tall.
The height of the test section ceiling can be adjusted at different points, up to a maximum of two feet above the floor
of the wind tunnel. For this experiment, the ceiling height will be adjusted such that a zero pressure gradient (ZPG) in
a turbulent boundary layer is maintained in the test section. Freestream turbulence levels in the center of the ZPG test
section are less than 0.1%. Baseline measurements showed that the cylinder flow transitioned between U∞ = 55-65
ft/s; this experiment evaluates the arch-fairing’s performance at U∞ = 75 ft/s.
The wind tunnel has a rectangular opening, and the ceiling height of the test section remains relatively constant to
achieve a ZPG flow. The cross-sectional areas of the wind tunnel and cylinder, as well as the area ratio, are given by
Eqs. (1).
𝑆" = ℎ" 𝑤"
𝑆 = 𝐻𝑑
𝑅),+,- =
𝐻
𝑑
𝑅.,/ =
𝑆
𝑆"
×100%
(1)
The ratio of the model area to the wind tunnel area is determined to be 5.56%, which is within the specified range
to avoid wall effects from the floor, ceiling, and side walls. For this experiment, the cross-sectional area of the cylinder
will be used as the reference area of the model. Therefore, the measurements taken in the wind tunnel should provide
an accurate estimate of the cylinder’s performance.
Direction of Airflow

4. 4
C. Measurements in Wind Tunnel
In order to assess the effectiveness of the arch-fairing, several different measurements will be taken in the wind
tunnel. These include velocity profiles, turbulence, drag force, surface pressure, and the flow field over the cylinder
and arch-fairing. All quantitative measurement data collected from the wind tunnel are transferred to a Pacific
Instruments PI660 data acquisition (DAQ) system, which then transmits the information to a virtual instrument (VI)
computer program developed in LabVIEW. The experiment will consider these measurements for the baseline case
and the cases with the arch-fairing in place. Figure 2 shows the complete experimental apparatus inside the wind
tunnel, including one configuration of the arch-fairing, the circular cylinder, and some of the instruments used in this
experiment.
Fig. 2 Complete experimental setup inside boundary layer wind tunnel.
1. Velocity Profiles
Velocity measurements are taken at several different locations in the wind tunnel using a pitot-static probe. These
locations are typically some distance behind the cylinder, showing what the trailing velocity profile looks like for the
arch-fairing case in comparison to the baseline. Velocity measurements are also taken at other locations in the
downstream direction, starting with the “front” end of the cylinder, in order to determine the separation and
reattachment points.
It is necessary to take velocity measurements at different heights in order to get a complete velocity profile at each
measurement location4
. The dynamic pressure is measured at each location with the pitot-static probe. The flow in the
wind tunnel is assumed to be incompressible, so a variation of the Bernoulli equation can be used to determine the
velocity, given by Eq. (2) below5
:
𝑢 =
2(𝑝8 − 𝑝)
𝜌
(2)
Additionally, the wind tunnel conditions such as freestream dynamic pressure, atmospheric pressure and test
section total temperature are measured and monitored through the data system. Another important parameter that will
be considered is the Reynolds number, which is determined from the free stream velocity, object geometry, and
thermophysical air properties. These properties include the density and dynamic viscosity of air, which are calculated
from the measured tunnel conditions during each run. The Reynolds number for the cylinder is then calculated as
follows:
Re> =
𝜌𝑈8 𝑑
𝜇
(3)
The Reynolds number of the cylinder is about 150,000 for each case tested. Knowing how the arch-fairing affects
the turbulent wake region behind the cylinder is also important to determine its effectiveness. To measure the change
in turbulence levels, a Kulite probe will be used in the same fashion as the Pitot tube to measure unsteady dynamic
pressure fluctuations. The Kulite is positioned at a specific streamwise position behind the experimental apparatus and
takes frequency and energy measurements for the baseline and with the arch-fairing mounted at different positions.
Arch-Fairing
Pressure
Taps
Cylinder
Pitot Tube

5. 5
2. Surface Pressure
Surface pressure measurements along the cylinder surface and the centerline of the tunnel are taken using a series
of pressure transducers as part of a Scanivalve system. The cylinder surface, from the leading edge back, has taps
located every 15 degrees from 0-345 degrees, at y/H ≈ 0.5. The pressure coefficients at each point along the cylinder
and centerline are calculated as
𝐶B =
𝑝 − 𝑝8
1
2
𝜌𝑈8
C
(4)
The cylinder pressure drag coefficient based on the cylinder surface pressure coefficients can then be determined
by
𝐶D,B = 𝐶B cos 𝜃 𝑑𝜃
I
J
(5)
However, since the pressure readings are taken at discrete locations, Eq. (5) must be rewritten to solve for the
pressure drag coefficient numerically. The simplest way to accomplish this is to use the trapezoidal rule7
, which should
provide a reasonably accurate initial estimate. The expression for the pressure drag coefficient can then be written as
𝐶D,B ≈
𝜃LMN − 𝜃L
2
𝐶B LMN
cos 𝜃LMN + 𝐶B L
cos 𝜃L
P
LQN
(6)
Pressure drag is typically the largest component of the total drag of a cylinder, while the rest of the drag comes
primarily from the profile of the cylinder6
.
3. Drag Force
A load cell below the cylinder is used to measure the drag force. This load cell is mounted on a bracket, which is
attached to the bottom of the baseplate, and the bottom of the cylinder has an extended structure that attaches to the
top of the load cell. After the drag force is measured, the reference area of the model is used in the calculation of the
drag coefficient, given by
𝐶D =
𝐷
1
2
𝜌𝑈8
C 𝑆
(7)
After the total drag and pressure drag coefficients are calculated, it is necessary to see how much the coefficients
change from the baseline when different arch-fairing configurations are used. This is done by simply subtracting the
drag coefficients with the arch-fairing from those of the baseline:
Δ𝐶D = 𝐶D TUVWVXY − 𝐶D ZU[]VX
Δ𝐶D,B = 𝐶D,B TUVWVXY
− 𝐶D,B ZU[]VX
(8)
D. Oil Surface Visualization
To analyze the flow field on the cylinder and arch-fairing, the entire experimental apparatus is mounted on glossy
black sheets, which are then painted with a special mixture that glows when a light is shined on it. The parts are also
painted with this mixture, showing the flow fields on the cylinder and arch-fairing surfaces. A camera then records
images of the flow fields after each test so that the flow patterns with the arch-fairing can be compared to the baseline
flow patterns. The light casts shadows from the test apparatus, so multiple views of each flow field are photographed
to record all flow patterns and how the arch-fairing affects them. Detailed analysis of the flow fields includes
examining the vortices, streamlines, and reattachment points.
IV. Results
During the analysis of the centerline pressure coefficients, the arch-fairing is tested at three different spans, five
different heights, three different angles of attack at a span of 6 inches, and several distances behind the cylinder. The
data obtained from the arch-fairing tests are then compared with the baseline data to assess the arch-fairing’s
performance and effectiveness at its intended task.

6. 6
A. Velocity and Turbulence Profiles
The arch-fairing has a significant impact on the velocity profile at different locations and configurations. The first
major velocity test conducted examines the arch-fairing’s performance at different distances behind the cylinder,
detailed in Fig. 3. For this test, the Pitot probe is positioned at a distance of x/d = 3, and the arch-fairing consists of a
6-inch span piece at 0 degrees angle of attack and 1-inch-tall extension pieces, for a total height of 2.5 inches
(y/H = 0.42). For all cases, the Pitot probe is positioned along the centerline of the wind tunnel. Compared to the
baseline, the arch-fairing seems to slow down the flow at each streamwise location. When the arch-fairing is placed
at x/d = 2.5, there is a shallow velocity gradient up to y/H ≈ 0.4, which then becomes very steep. This could mean that
the flow accelerates somewhat at this location.
The second major velocity test involves examining the arch-fairing at different angles of attack. This test is done
by using three different 6-inch-span pieces; no other spans are considered at this time. The Pitot probe is moved
forward to a distance of x/d = 2.5, and the arch-fairing is again tested at a height of 2.5 inches. Also, two different
streamwise distances behind the leading edge of the cylinder are considered. Figure 4b shows that the flow is faster
than the baseline below y/H ≈ 0.15 when x/d = 2.0, but only for the 0-degree and 4-degree pieces. The 8-degree piece
has a shape similar to that of the baseline, albeit at reduced magnitudes. On the other hand, Fig. 4a indicates that at
x/d = 1.5, the arch-fairing angle does not affect the velocity profile very much above a certain point. It is also possible
that reverse flow occurs near the floor of the wind tunnel behind the arch-fairing, which may affect the flow patterns
in this area.
Fig. 3 Impact of arch-fairing streamwise position on velocity profile.
a) b)
Fig. 4 Impact of arch-fairing angle of attack on velocity profile at distances behind cylinder of a) x/d = 1.5 and
b) x/d = 2.0.
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
1.2
u/U
∞
y/H
Baseline
AF @ x/d = 1.5
AF @ x/d = 2.0
AF @ x/d = 2.5
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
1.2
u/U
∞
y/H
Baseline
AF, 0 deg, x/d = 1.5
AF, 4 deg, x/d = 1.5
AF, 8 deg, x/d = 1.5
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
1.2
u/U
∞
y/H
Baseline
AF, 0 deg, x/d = 2.0
AF, 4 deg, x/d = 2.0
AF, 8 deg, x/d = 2.0

7. 7
The third major velocity test examines the effect of different arch-fairing heights on the velocity profile. The Pitot
probe remains at the position of x/d = 2.5 for this test, and all tests are conducted at zero angle of attack. Different
heights are obtained by using different extension pieces or removing the extension pieces altogether. Examining the
impact of the arch-fairing’s height at different spans also affects the shape of the velocity profile. Each arch-fairing
configuration is tested at a streamwise distance of x/d = 2.0.
As with the angle of attack test, the height test shows that there may be areas of reverse flow behind the arch-
fairing. Both graphs show that at H = 1.5 inches and 2.5 inches, there are regions with very steep velocity gradients in
the y-direction. These steep gradients occur between 0.22 ≤ y/H ≤ 0.42. Additionally, for H = 3.5 inches, a steep
velocity gradient occurs at y/H ≈ 0.6, while the slope matches that of the baseline everywhere else.
After all velocity profile sweeps have been completed, the Kulite probe takes turbulence data for the baseline and
different streamwise positions of the arch-fairing. Three different vertical positions of the Kulite are considered, since
turbulence, like velocity, varies with height. Figure 6 shows the results of the Kulite survey, comparing the arch-
fairing results with those of the baseline.
Almost every case in each of the velocity tests shows that the arch-fairing slows down the flow above a height of
y/H = 0.25. The Kulite survey also shows that there is a broadband reduction in turbulence levels with the arch-fairing
present, especially with the fairing mounted sufficiently aft of the cylinder. Figure 6 seems to show that each Kulite
case follows the -5/3 turbulence slope well. The next major survey of the arch-fairing involves examining the
centerline and cylinder pressure distributions.
a) b)
Fig. 5 Impact of arch-fairing height on velocity profile at arch spans of a) 4 inches and b) 6 inches.
Fig. 6 Impact of arch-fairing height on turbulence at vertical position of y/H = 0.25.
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
1.2
u/U
∞
y/H
Baseline
AF, 0 deg, x/d = 2.0, h = 1.5"
AF, 0 deg, x/d = 2.0, h = 2.5"
AF, 0 deg, x/d = 2.0, h = 3.5"
0 0.2 0.4 0.6 0.8 1 1.2
0
0.2
0.4
0.6
0.8
1
1.2
u/U
∞
y/H
Baseline
AF, 0 deg, x/d = 2.0, h = 1.5"
AF, 0 deg, x/d = 2.0, h = 2.5"
AF, 0 deg, x/d = 2.0, h = 3.5"
10
1
10
2
10
3
10
-7
10
-6
10
-5
10
-4
10
-3
Pxx
Freq , Hz
Baseline, y/H = 0.25
AF @ x/D = 1.5, y/H = 0.25
AF @ x/D = 2.0, y/H = 0.25
AF @ x/D = 2.5, y/H = 0.25
-5/3 Slope

8. 8
B. Pressure Distributions
As the configuration of the arch-fairing is changed, the way that it impacts the centerline and cylinder pressure
distributions also changes. The first test in examining the pressure variation is to vary the airfoil angle of attack for
the 6-inch span piece. Each configuration is tested at an arch-fairing height of 2.5 inches. These pressure distributions
are collected at a streamwise distance of x/d = 2.0, and the centerline results can be seen in Fig. 7. Figure 8 shows the
cylinder surface pressure distributions for each of the arch-fairing span configurations at the same height and location.
Along the centerline, the 0-degree arch piece seems to provide the greatest reduction in the pressure coefficients
behind the cylinder. The cylinder angle range of 0° ≤ θ ≤ 180° represents the left side of the cylinder, while 180° < θ
≤ 360° represents the right side of the cylinder. The cylinder pressure distribution on the right side does not change
very much from the baseline with the angle of the arch-fairing. However, the left side changes so that it almost exactly
matches the right side; the magnitude of the pressure coefficients is reduced.
Fig. 7 Effect of arch-fairing angle at x/d = 2.0 on centerline pressure distribution.
Fig. 8 Effect of arch-fairing angle at x/d = 2.0 on cylinder pressure distribution.
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
25 35 45 55 65 75 85
CenterlinePressureCoefficient
x/d
Baseline
0 deg
4 deg
8 deg
-2
-1.5
-1
-0.5
0
0.5
1
0 60 120 180 240 300 360
CylinderPressureCoefficient
Cylinder Angle, degrees
Baseline
0 deg
4 deg
8 deg

9. 9
The second test in investigating the pressure distribution is to vary the height of the arch-fairing while using one
arch piece. For this test, the 6-inch span piece at zero angle of attack is used, and the height is changed by using
different extension pieces. As with the previous pressure survey, these pressure distributions consider the case where
each arch-fairing configuration is located at x/d = 2.0. Results for this case are found in Figs. 9 and 10, again showing
the centerline and cylinder pressure distributions, respectively.
When considering the pressure distribution on the cylinder surface, changing the arch-fairing height has almost
the same impact as changing the angle of the arch piece. Along the centerline, a height of 1.5 inches pushes the
reattachment point back, while the other heights move it forward slightly. From both the cylinder and centerline plots,
it is evident that a height of 2.5 inches provides the greatest reduction in the pressure coefficients.
Both sets of graphs give a good indication of how each arch-fairing configuration alters the centerline and cylinder
pressure distributions. From additional pressure distribution analysis, it is determined that placing the arch-fairing at
x/d = 2 generally means that it will perform well at its intended task. Knowing the pressure distributions helps with
finding out how they affect the drag of the cylinder, especially when the arch-fairing is added in.
Fig. 9 Effect of arch-fairing height configuration at x/d = 2.0 on centerline pressure distribution.
Fig. 10 Effect of arch-fairing height configuration at x/d = 2.0 on cylinder pressure distribution.
-1.2
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
25 35 45 55 65 75 85
CenterlinePressureCoefficient
x/d
Baseline
H = 1.5"
H = 2.5"
H = 3.5"
-2
-1.5
-1
-0.5
0
0.5
1
0 60 120 180 240 300 360
CylinderPressureCoefficient
Cylinder Angle, degrees
Baseline
H = 1.5"
H = 2.5"
H = 3.5"

10. 10
C. Drag Coefficients
Once the drag forces for the baseline and all arch-fairing cases are found, the total drag coefficients are calculated
using Eq. (7). The pressure drag coefficients for each configuration are then found by applying Eq. (6) to the cylinder
pressure distributions. When applying Eq. (8), a negative sign indicates a reduction from the baseline, while a positive
sign indicates an increase from the baseline. The table below shows the total drag and pressure drag coefficients of
the 6-inch-span arch-fairing compared with the baseline.
At a streamwise position of x/d = 2.0, the arch-fairing at a height of 2.5 inches and a 4-degree angle of attack
provides the largest reduction of the total drag coefficient. However, it also results in a moderate increase of the
pressure drag coefficient. Examining the results of the other configurations presented in Table 1, it is evident that a
decrease in the total drag coefficient correlates with an increase in the pressure drag coefficient.
Table 1: Impact of arch-fairing at x/d = 2.0 on total drag and pressure drag coefficients.
Configuration CD ΔCD CD,p ΔCD,p
Baseline 0.489 0 0.417 0
2.5”, 0 deg 0.473 -0.015 0.423 0.006
2.5”, 4 deg 0.466 -0.023 0.428 0.011
2.5”, 8 deg 0.480 -0.009 0.435 0.018
1.5”, 0 deg 0.484 -0.005 0.451 0.034
3.5”, 0 deg 0.479 -0.010 0.420 0.003
D. Arch-Fairing Flow Patterns
After all other data for this experiment have been collected, the flow patterns over the cylinder and arch-fairing
are examined using oil visualization. The baseline is analyzed first, followed by a case with a specific arch-fairing
configuration. First, a horseshoe vortex is observed, starting from the leading edge of the cylinder and moving back.
An arch vortex is indicated by two oil swirls behind the trailing edge of the cylinder. The expanding region behind the
arch vortex indicates the turbulent wake region. These features can be found in Fig. 11a below, along with the free
stream and other features. Once the arch-fairing is added in, the flow patterns change. At the leading edge of the arch-
fairing, the horseshoe vortex turns toward the direction of the free stream. The arch vortex behind the cylinder is
reduced, as is the turbulent wake region. This is consistent with earlier analysis that shows that the arch-fairing reduces
the magnitude of turbulence. Figure 11b shows the flow patterns with the arch-fairing in place.
a)
b)
Fig. 11 Oil visualization of cylinder flow patterns for a) baseline and b) arch-fairing.

11. 11
V. Conclusion
From initial testing in the boundary layer wind tunnel, the arch-fairing seems to perform reasonably well as a PFC
device. Most importantly, it reduces turbulence behind the cylinder by reattaching the downstream flow sooner. The
results of the oil visualization test also show that the arch-fairing helps in eliminating vortices on or near the cylinder.
As a very new design, the arch-fairing will need to be analyzed further, but this experiment shows that it seems
promising. Future studies could include active flow control incorporated into the arch-fairing design.
One possible application of the arch-fairing design is in the reduction of rotor hub drag on helicopters and other
rotorcraft. As mentioned previously, the hub causes a significant amount of drag due to its blunt shape, so the arch-
fairing could help in reducing drag in this region and improving helicopter performance. It would also mean a higher
payload capacity, which would be helpful in many civil, commercial, and military applications.
Acknowledgments
Thomas J. Setzer would like to thank NASA and the Utah Space Grant Consortium for all their hard work in
making this internship possible, as well as Bill Warmbrodt for his mentorship and putting together excellent activities
for the interns. I would also like to thank everyone at the U.S. Army Aviation Development Directorate—AFDD for
all their help and guidance on this project, particularly David Schatzman and Jacob Wilson for their mentorship.
References
1
Pattenden, R. J., Turnock, S. R., and Zhang, X., “Measurements of the Flow Over a Low-Aspect-Ratio Cylinder Mounted On
a Ground Plane,” Experiments in Fluids, Vol. 39, No. 1, 12 May 2005, pp. 10-21.
2
Yucel, S. B., Cetiner, O., and Unal, M. F., “Interaction of Circular Cylinder Wake with a Short Asymmetrically Located
Downstream Plate,” Experiments in Fluids, Vol. 49, No. 1, 13 Mar. 2010, pp. 241-255.
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