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American Institute of Aeronautics and Astronautics1
Experimental Studies of Transitional Boundary Layer ShockWave
Interactions
Z.R. Murphree*, J. Jagodzinski,* E.S. Hood, Jr., * N.T.
Clemens,+ and D.S. Dolling†
Center for Aeromechanics ResearchDepartment of Aerospace
Engineering and Engineering Mechanics,
University of Texas at AustinAustin, Texas 78712-1085
Shock wave boundary layer interactions generated by a cylinder
on a flat plate werevisualized with a particular emphasis on
transitional interactions. Surface-streakline(kerosene-lampblack),
schlieren, and low-repetition rate planar laser scattering were
used tovisualize the flowfield. Furthermore, high-speed (10 kHz)
planar laser scattering in astreamwise-spanwise plane (plan view)
was used to visualize the time-varying structure ofthe separated
flow. The shapes and scales of the transitional interactions are
compared tothose of laminar and turbulent interactions. It was
observed that transitional interactionsare highly variable in their
structure, but they appear to be a composite of the other twotypes
with laminar scaling along the plate centerline and turbulent
scaling in the outboardregion. The plan view planar imaging shows
that even the most laminar interactions exhibitseparated flow
regions that are transitional or turbulent. Furthermore, the
fluctuations inthe size of the separated flow for more laminar-like
transitional interactions weresignificantly larger than for
turbulent interactions.
Nomenclatured = cylinder diameterXcyl = distance from plate
leading edge to cylinder leading edgeXsep = distance from plate
leading edge to primary separation line along plate
centerlineXtrans = distance from plate leading edge to transition
along plate centerlineλsep = distance from cylinder leading edge to
primary separation line along plate centerline
I. Introductionhock wave/boundary layer interactions (SWBLI),
often accompanied by separation, are a ubiquitousphenomenon in
high-speed flight. Most of the work done in this field over the
past 50 years has been in fully-
developed turbulent flows because most practical applications
were at transonic and low supersonic speeds ataltitudes where
Reynolds numbers are large and turbulent flow the norm. An
understanding of these turbulentinteractions is very important for
vehicle design because the interactions result in very high
unsteady thermal andacoustic loads that can result in diminished
component performance and material failure. In contrast,
transitionalshock wave/boundary layer interactions, in which the
incoming boundary layer is in a transitional state, or in
whichtransition is induced within the interaction itself, appear to
be even more unsteady and could have greater adverseeffects, and
yet have received little attention. This lack of attention has
stemmed from both the lack of criticalapplications and from the
formidable challenges that the study of such flows poses to both
experiment andcomputation. However, high Mach number air-breathing
propulsion systems are of increasing interest to the U S AirForce,
and the inlets of such systems will have extremely complex
shock/boundary layer interactions withsignificant regions of
transitional flow. These transitional interactions will have a
powerful influence on the localinlet flow properties and on the
uniformity, the quality, and the steadiness of the flow entering
the combustor. It isfair to say that our current understanding of
transitional interactions is extremely poor. The capabilities of
modern
* Graduate Research Assistant, Member+ Professor, Associate
Fellow† Professor, Fellow
S
44th AIAA Aerospace Sciences Meeting and Exhibit9 - 12 January
2006, Reno, Nevada
AIAA 2006-326
Copyright © 2006 by Z.R. Murphree, J. Jagodzinski, E.S. Hood,
Jr., N.T. Clemens, and D.S. Dolling. Published by the American
Institute of Aeronautics and Astronautics, Inc., with
permission.
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American Institute of Aeronautics and Astronautics2
non-intrusive instrumentation, especially particle image
velocimetry (PIV) and planar laser scattering (PLS), nowoffer an
opportunity to initiate studies of transitional interactions and
build the knowledge base.
II. Experimental Program and TechniquesThe experimental program
was carried out in the high-Reynolds-number Mach 5 blow-down wind
tunnel located
at the Wind Tunnel Laboratories at the Pickle Research Campus of
the University of Texas at Austin. The facilityuses a bottle field
of eight tanks that has a combined storage volume of 140 ft3 and
can be pressurized to a maximumof 2550 psia. The stagnation
temperature and pressure were 653º R and 363 psia. Downstream of
the 2-D Mach 5nozzle, the flow enters a 27 in long constant-area
test section which is 7 in high and 6 in wide.
The interactions for this study were generated by a circular
cylinder on a flat plate. The cylinder could betranslated
streamwise along the plate, and the state of the boundary layer at
a given position would dictate the typeof interaction (laminar,
transitional, or turbulent). Three different plates were used, all
with smooth surface finishesand tapered leading and trailing edges.
The first, used for kerosene-lampblack surface flow visualization,
was an 18inch long flat brass plate that was bolted to the tunnel
walls. The second surface was a 10 inch long brass platedesigned to
provide optical access to study the interactions. This plate was
mounted in the tunnel with a supportingstrut, but the strut
presented so much blockage to the flow that the cylinder had to be
very small in order for thetunnel to start. The third surface was
also a 10 inch long plate, but was mounted without a supporting
strut in orderto reduce the blockage so that larger cylinders (up
to a half inch in diameter) could be used to generate
largerinteractions. The third plate also did away with the threaded
holes for attaching the cylinder in order to reduce
flowdisturbances. In this case the cylinder was held in place with
the compressive force of a screw-jack.
Experimental techniques employed to date have included: a)
surface-streakline visualization using a kerosene-lampblack
mixture, used to determine quantitatively the mean separation line,
b) schlieren imaging, and c) PLS. Theschlieren setup consisted of a
high-speed camera, neutral density filters, a razor blade for a
knife-edge and either aHelium-Neon laser (Spectra-Physics Model
102-1 controlled by a Spectra-Physics Laser Exciter Model 212-1) or
aflash-lamp (EG&G Electro Optics model LS-1130-4 flash-lamp,
2-microsecond duration). The choice dependedwhether overall flow
characteristics or local image quality were of primary concern.
Figure 1 shows the arrangement.Images were acquired using a KODAK
EKTAPRO HS Motion Analyzer, Model 4540 mx or a Cohu 4990.
Thecameras were fitted with either a Computar MC Zoom Macro
1:3.5~5.3 lens for the laser schlieren or a Nikon EDAF Micro Nikkor
200 mm 1:4D lens paired with the flash-lamp. The EKTAPRO camera
processor could record fullframes at 30, 60, 125, 250, 500, 750,
1,125, 2,250 and 4,500 frames per second, whereas the Cohu operated
at 30frames per second.
PLS images were taken using twodifferent setups enabling plan
views (i.e.streamwise-spanwise view) of the flow. Theflow was
seeded with a finely atomizedethanol fog, which was then
illuminated withlaser light. The scattering from the condensedfog
was then captured by a high-speed 1kx1kCMOS array (Photron
FASTCAM-ultimaAPX) with a 50 mm 1:1.4 lens. In both setupsthe laser
sheet was brought in parallel to thesurface of the flat plate
through a sidewindow. In the first setup the camera wasimaging the
sheet through the opposite sidewindow and was therefore positioned
at anoblique angle as shown in Fig. 2. Due to theviewing angle,
these images exhibit some geometric distortion and blurring at the
edges. The light was provided bya flashlamp-pumped
frequency-doubled Nd:YAG laser (Spectra-Physics PIV 400) operating
at 10 Hz and around150 millijoules per pulse. In the second setup
the camera was positioned normal to the plate surface. For these
teststhe light was provided by a diode-pumped Nd:YLF laser
(Coherent Evolution 90) operating at 10 kHz and 9millijoules per
pulse. At this high framing rate the maximum resolution of the
camera was 512x256.
Figure 1. Schematic diagram of flash-lamp schlieren setup.
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American Institute of Aeronautics and Astronautics3
III. ResultsIn the first phase of the study, surface streak line
visualizations were obtained for several cylinder locations
relative to the plate leading edge. Three examples, which
highlight three flow regimes, are shown in Fig. 3. At eachof these
stations, and others, results were repeatable. The pattern shown in
Fig 3a is the classic image seen innumerous studies in which the
incoming boundary layer is fully turbulent. The separation line is
symmetric aboutthe plate-cylinder center line, with separation on
center line occurring about two cylinder diameters upstream of
thecylinder leading edge. In Fig 3b, with the cylinder shifted 1 in
further upstream (4 in downstream of the plateleading edge) the
flow structure and scales change significantly. The separation
scale is significantly larger (oncenter line the primary separation
line now is about 5 cylinder diameters upstream of the cylinder
leading edge) andthe flow structure is no longer symmetric about
the cylinder center line. There is some indication of
multipleseparation lines. The significant variation in spanwise
structure is also evident in the plan view PLS images and inthe
conventional side-view schlieren images which will be shown later.
At this stage it is not clear if the incomingboundary layer is
transitional, if transition occurs in the separated shear layer,
elsewhere in the interaction, or somecombination of the latter.
Which ever it is, it is probable that the location and process will
not be symmetric acrossthe span and will result in an asymmetric
flow structure. It is possible that disturbances originating at the
plateleading edge trigger early transition, or waves from the
tunnel floor or sidewalls hasten or delay transition at
somelocations but not at others. In any event, the global behavior
is repeatable, since the essential features of the imageremain the
same from run to run.
CMOScamera
cylinder
flat plate
laser sheet
M∞=5
(a) (b)Figure 2. Schematic diagram of PLS setup. (a) Overall
assembly and (b) inside the test section
(a) (b) (c)Figure 3. Surface streakline visualization
(kerosene-lampblack) of cylinder-induced interactions in a Mach5
flow. (a) “Fully turbulent”: Xcyl = 5 in, (b) “transitional”: Xcyl
= 4 in, and (c) “laminar”: Xcyl = 2.75 in.
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American Institute of Aeronautics and Astronautics4
When the cylinder is shifted 1.25 in further upstream (Xcyl =
2.75 in) the interaction becomes larger still, andnominally
symmetric about the cylinder center line, indicative of a
“laminar-like” interaction. The maximumprimary separation distance
is slightly larger than in Fig. 3b. Again, it is difficult to
characterize the flowfield asbeing laminar or transitional since it
may well be a mix of these two flows. With respect to primary
separationlocation, Young et al. (1968) reported that in laminar
flow at Mach 3 the center line separation distance varied from4.1
diameters to 6.8 diameters as the flow Reynolds number was
increased by a factor of about 20, indicating aReynolds number
dependence.1 Smaller values were observed at Mach 5. At Mach 5.5,
Hung and Clauss (1980)reported values of 9 to 12 diameters for
laminar flow.2 However, as was pointed out by Özcan and Holt,
theyassumed that separation occurred where the surface temperature
or static pressure first begins to rise above theundisturbed value
and this assumption will overpredict the separated flow scale
significantly since the distance fromthe initial rise in pressure
to the separation location can be several cylinder diameters.3 The
Özcan and Holt studynoted that for laminar cases with a height to
diameter ratio greater than about 4 (essentially a semi-infinite
cylinder)the maximum primary separation distance was typically
between about 6.6 and 7.6 diameters upstream of thecylinder. In
Fig. 3c it is likely that the incoming undisturbed boundary layer
is on the verge of transition, or isperhaps laminar, and transition
occurs early in the separated shear layer. Considering that the
location of primaryseparation on center line decreases from about
6-8 diameters to about two diameters as the flow goes from
entirelylaminar to fully turbulent, a large range of scales are
possible depending on where and how transition occurs in
thedisturbed flowfield.
The normalized separation distances, l sep/d, from the surface
flow visualization of the current study agree wellwith the results
of Kaufman et al., as shown in Fig. 4. 4 In the figure, Xsep and
Xtrans are the locations of separationand the end of transition,
respectively, measured from the plate leading edge. The
aforementioned large change inmean interaction scale as it
changesfrom laminar to transitional toturbulent is evident. The
reduction ofl sep/d from the laminar/transitionalvalues to the
fully turbulent valuesoccurred at the same positions relativeto
transition even though the largestlaminar/transitional extent
ofseparation was about 10% higher thanin Kaufman et al.4
Comparisons withthe data of Özcan and Holt3 andYoung et al.1
suggest that thisdifference in l sep/d is reasonable. Inorder to
make the comparison shownin Fig. 4 it was necessary to employ
acorrelation function developed byRamesh and Tannehill5.
Thiscorrelation predicts the Reynoldsnumber for the onset and end
oftransition (between which is the“transition band”). The value for
theend of transition, which was calculatedto be 4.1 in, was used to
normalize thex-axis of Fig. 4.
Planar laser scattering images for similar cases to those
discussed above are consistent with surface streaklineimages and
bring out more details of these complex flows. Figure 5a shows the
case that presumably corresponds toa laminar interaction similar to
Fig. 3c. In this case the cylinder diameter is slightly smaller
(0.375 in compared to0.5 in and is positioned about 0.7 in closer
to the plate leading edge). Recall that the light sheet is parallel
to and 0.1in above the plate surface and the camera views the
scattering from an oblique angle and so some distortion
andblurriness are present at the edges of the images. Light and
dark regions correspond to regions of high and low fogdensity,
respectively. Low velocity regions are rendered as black owing to
droplet evaporation in the relativelywarm fluid. Figure 5a reveals
a separation shock that stands off from the cylinder by about 3.6
diameters. Note thatthe laser sheet is elevated off the plate and
so the origin of the shock is farther upstream. We should emphasize
thatalthough we assume this shock is a separation shock, we see no
evidence of separated flow immediately downstream
Figure 4. Comparison of data from current study with that
ofKaufman et al. 4
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of it. It is possible that separated flow is not observed
because the laser sheet location was too high to see it. Thisissue
will be explored in future work. We observe that the separation
shock is relatively steady and its shape alongthe centerline of the
plate is almost straight. The latter observation is consistent with
many of the surface streaklinepatterns that we have taken in
“laminar” interactions. In Fig. 5b the cylinder is at the same
location relative to theplate leading edge but a “trip strip” (a
strip of tape) has been attached to the plate surface just
downstream of theleading edge. The interaction now has a mixed
structure with the flow upstream and to the left of the
cylinder(looking upstream) exhibiting a laminar-like character
whereas the flow on the right exhibits a turbulent-likecharacter
with separation much closer to the cylinder. On the left hand side,
downstream of the laminar-likeseparation location dark turbulent
streaks/spots are evident in the image. We believe that a turbulent
separationshock may exist on the left side that is similar to that
on the right, but it cannot be seen due to the blur in the image.We
note that this flow is much more unsteady than the previous case as
the “laminar” shock is highly variable in itsshape and the region
of turbulent flow exhibits large variations in its overall
scale.
M∞= 5
plateleading
edge
"laminar"separationshock
2.1"
cylinder
(a)
"laminar"separation shock
"turbulent"separation
shock
M∞ = 5
boundary layertrip strip
(b)
Figure 5. Planar laser fog scattering images of cylinder-induced
interactions. The 0.375 in cylinder is 2.1 infrom the leading edge.
The view shown is a plan view that exhibits some distortion because
it was taken at anoblique angle. (a) untripped boundary layer, (b)
tripped boundary layer.
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American Institute of Aeronautics and Astronautics6
Figure 6 shows the case when the cylinder is moved about 1.9 in
further downstream. The image now becomesessentially symmetric with
respect to the centerline of the plate (consistent with the surface
streakline visualizations).The dark streaks in the image are
associated with increased temperatures associated with what is
presumably theseparated flow. On center line these dark streaks
first appear about 2 cylinder diameters upstream of the
cylinder,consistent with the location of turbulent separation as
inferred from other techniques. Interestingly, there is
noseparation shock that can be identified in the turbulent cases.
It may be that the laser sheet is too close to the wall
toeffectively see the separation shock. Note that the same flow
structure is observed in Fig. 6 whether the trip strip isin place
or not.
When the flow was imaged at a higher rate (10 kHz vs. 10 Hz),
some very interesting phenomena were observed.The problem with
imaging the separation shock again presented itself, so the state
of the boundary layer could notbe inferred from λsep. However, the
location of transition could be found by imaging the boundary
layerdevelopment of the tripped flow with the cylinder removed.
Figure 7 shows representative images of the boundarylayer
development on the plate. The laser sheet is 0.04 in from the plate
surface, and the flow is left to right. ThisPLS technique seems to
be very effective for imaging this development; laminar,
transitional, and turbulent flowsare clearly distinguishable and
the cylinder could be placed accordingly to generate a certain type
of interaction.
M ∞ = 5
t u r b u l e n ts e p a r a t e d f l o w
Figure 6. Planar laser for scattering images for a case where
the cylinder is farther downstream, thusgenerating a turbulent
interaction. Xcyl = 3.95 in.
(a) (b) (c)
Figure 7. Development of boundary layer on flat plate without
interaction. (a) laminar, (b) transitional, and(c) turbulent. Flow
is from left to right.
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American Institute of Aeronautics and Astronautics7
The changes in scale of the separated flow in both the
streamwise and spanwise directions in a “transitional”interaction
are quite substantial. The shape of the interaction is also highly
variable and very sensitive to the state ofthe incoming boundary
layer. Figure 8 shows four non-sequential images of such an
interaction taken at 10 kHz.These particular images were chosen to
illustrate the unsteadiness of the interaction. This particular
interaction isoccurring towards the laminar edge of the transition
band. The bulge a quarter of the way up the frame remainsfairly
constant and looks to be caused by a roughness element upstream
either on the plate surface or leading edge. Itis also interesting
that the transitional interactions do not seem to be time resolved
at 10 kHz, but the fully turbulentinteractions do. This interaction
extends farther upstream for a given spanwise location than in the
more turbulentcases.
When the interaction occurs farther into the transitional band,
the separated flow begins to look more like thecombined
laminar/turbulent interaction, with laminar scaling close to the
centerline and turbulent scaling off-center,as seen in Fig. 9. The
interactions are more swept and become more steady, relatively
speaking.
A fully turbulent interaction is shown in Fig. 10. The scales of
this interaction are smaller in both dimensionsthan those previous,
and both the scales and the shape remain relatively constant from
frame to frame. While theturbulent case is much more unsteady than
a purely laminar interaction is, the data would suggest that
the
Figure 8. Non-sequential images of a more laminar “transitional”
interaction. Flow is from left to right. Cylinderand dark area to
right of white line added in processing.
Figure 9. Non-sequential images of turbulent “transitional”
interaction. Flow is from left to right. Cylinderand dark area to
right of white line added in processing.
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American Institute of Aeronautics and Astronautics8
unsteadiness is at a maximum near the laminar end of transition.
This may seem counter-intuitive, but another flowfeature was
observed that might be offer some insight into this trend.
When the cylinder was placed in tripped flow that appeared
essentially laminar there were intermittent bursts ofturbulence
extending up to 4.5 diameters upstream and occurring as much as 3.5
diameters off of the centerline, asshown in Fig. 8. These bursts
tend to have a consistent general shape, that of a “V” pointing
downstream, and bear astriking resemblance to Emmons turbulent
spots that appear in the transition process6. The light band
immediatelyupstream of these structures is characteristic of a
shock. These spots were found to occur in intermittent groupings
ofthree or four spots, but within each grouping the spots were
temporally very close together, with as little as 200microseconds
separating them. While no definitive conclusions can be drawn yet,
it is possible that the highlyunsteady nature of the transitional
interaction could be closely related to the formation of these
spots.
Figure 9 shows sample side-view schlieren images of a
transitional interaction. Since these images are the resultof
integration of the light beam across the width of the test section
care is needed in their interpretation. The blurredinclined wave in
the upper half of the image and the clearly defined vertical wave
immediately upstream of the
Figure 8. Turbulent spot formation in interaction region. Flow
is from left to right. Cylinder and dark area toright of white line
added in processing. d = 0.375 in, Xcyl = 5.9. Images are not
sequential.
Figure 10. Sequential images of turbulent interaction imaged at
10 kHz. Flow is from left to right.Cylinder and dark area to right
of white line added in processing.
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American Institute of Aeronautics and Astronautics9
cylinder are easily identified as the rippling plate leading
edge shock wave and the steady cylinder bow shock,respectively.
There appear to be several shock waves that coalesce near the
triple point, with the upstream waveappearing weaker. The upstream
wave is likely the laminar separation shock, whereas the downstream
waves arelikely turbulent separation shocks. The plan-view PLS
images and surface streakline images show that the flowstructure is
highly non-uniform in the spanwise direction and so these waves are
probably generated at differentspanwise locations.
IV. ConclusionThe plan view PLS images have proven to be very
effective in visualizing the global flowfield structure of
laminar, transitional and turbulent interactions. The laminar
interactions are characterized by a single, relativelysteady,
separation shock, which exhibits a large radius of curvature. The
turbulent case is more symmetricallydistributed and more
three-dimensional. The transitional interactions exhibit a
“laminar” separation shock near thecylinder centerline but
turbulent separation shocks in the outboard regions. This
dual-shock behavior leads to aseparation shock structure that
exhibits an inflection point. The PLS images demonstrate that the
flow structure ofthe transitional interactions is highly spanwise
non-uniform and therefore care should be taken when
interpretingspatially integrated techniques such as schlieren or
shadowgraphs. A significant piece of the explanation for thehighly
unsteady nature of the transitional interaction may well lie in the
formation of turbulent spots and theirbehavior within the
interaction itself. Future work should focus on these spots both
independently and incombination with the SWBLI. Additionally,
future work will have to address the PLS imaging of the
primaryseparation shock.
AcknowledgmentsThis work was sponsored (in part) by the Air
Force Office of Scientific Research, USAF, under grant/contract
number FA9550-04-1-0112. The views and conclusions contained
herein are those of the authors and should not be
Cylinder
Plateleading-edge
shock
(a)
Leading-edgeshock
laminar separationshock
Cylinderbow shock
Flow
turbulentseparation
shocks
(a) (b)
Figure 9. Spark schlieren photographs of a transitional
interaction. (a) Schematic showing field of view, (b) and(c) are
sample images.
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American Institute of Aeronautics and Astronautics10
interpreted as necessarily representing the official policies or
endorsements, either expressed or implied, of the AirForce Office
of Scientific Research or the U.S. Government.
The authors would like to thank Dr. Bharath Ganipathisubramani
and Pablo Bueno for their help with theexperimental setup and
execution and Edward J. Zihlman and Frank Wise for their technical
expertise. The authorswould also like to thank machinists Travis
Crooks, David Gray and Joe Edgar
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“Experimental Investigation of the Interactions Between Blunt Fin
Shock
Waves and Adjacent Boundary Layers at Mach Numbers 3 and 5,” ARL
68-0214, Dec 1968.2Hung, F.T. and Clauss J.M., “Three-Dimensional
Protuberance Interference Heating in High-Speed Flow,”
AIAA-80-0289,
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Holt M. (1984), “Supersonic Separated Flow past a Cylindrical
Obstacle on a Flat Plate,” AIAA Journal, Vol.
22 No. 5, pp. 611-617.4Kaufman, L. G., II, Korkegi R. H. and
Morton, L. C., “Shock Impingement Caused By Boundary Layer
Separation Ahead of
Blunt Fins,” ARL 72-0118, Aerospace Research Laboratories,
Wright-Patterson Air Force Base, Ohio, Aug 1972.5Ramesh, M. D. and
Tannehill J. C., “Correlations To Predict Transition In
Two-Dimensional Supersonic Flows,” 33rd AIAA
Fluid Dynamics Conference and Exhibit, June 23-26.6Cantwell,
B.J., Coles, D. and Dimotakis, P., “Structure and entrainment in
the plane of symmetry of a turbulent spot,”
Journal of Fluid Mechanics, Vol. 87, 641-672.