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16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 1 -
Particle Image Velocimetry at a Generic Space Launcher Model at Mach 5.9
Marcus Casper1,*, Sören Stephan1, Peter Scholz1, Rolf Radespiel1
1: Institute of Fluid Mechanics, Technische Universität Braunschweig, Braunschweig, Germany
* correspondent author: [email protected]
Abstract This contribution discusses particle image velocimetry measurements at a generic space launcher model with the focus on liquid tracer particles at a Mach number of M=5.9. The test facility, the Hypersonic Ludwig Tube Braunschweig, is a blow down type wind tunnel which allows unit Reynolds numbers between Re=3·10
6 m
-1 and Re=20·10
6 m
-1 at a Mach number of M=5.9. Due to the conditions inside the storage tube, a
pressure of up to 30bar and a temperature of up to 623K, the material of the tracer particles have to be chosen carefully. The presented work includes a qualification of the atomizer ATM210 with two different oils, Emery 3004 and Plantfluid. The results show that the quality of the tracer particles at the outlet of the atomizer complies with the requirements. During the wind tunnel measurements the effects of the aerosol facility operating parameters, wind tunnel parameters, and laser energy to the tracer particles were investigated. Both oils were tested and the oil Plantfluid was found to be more resistant to higher temperatures. Additionally, the number of particle images increased at higher laser energy. However, the resulting velocity vector fields show that the quantity of tracer particles inside the boundary layer, the wake, and the recirculation area behind the generic space launcher model was insufficient to measure turbulent stresses. A mean velocity vector field, based on 160 double images, could be computed. Based on numerical flow simulations and particle path calculations the problems with the tracer particles were analyzed. Measurements inside the recirculation area, using local seeding, was also tested.
1. Introduction
Particle image velocimetry is a promising and established non-intrusive measurement technique.
Due to the fact that the velocity is measured indirectly by tracer particles these particles must
comply with important requirements. Among the optical properties of the tracer particles, one
important requirement is the relaxation time which could be influenced by the size as well as the
density of the tracer particle. There are basically two kinds of tracer particles, solid [4, 6, 8, 10, 11]
or liquid [7], which can be used in hypersonic test facilities. Among other things the choice of a
solid or liquid material depends on the conditions inside the wind tunnel. Hypersonic test facilities
can have temperatures higher than 1500K [4] and, thus, solid materials like titanium dioxide or
alumina oxide are used [5]. These materials are available as powders. The resulting tracer particles
are usually agglomerated which increases the tracer particle size [8]. If needed, the agglomerate can
be broken down to primary particle size and, finally, the nano particles can be prevented to
reagglomerate [10]. Based on solid nano tracer particles the measured velocities are in good
agreement with theory [5, 10]. However, in hypersonic test facilities solid tracer particles used to
seed the wind tunnel flow can cause problems. The tracer particles pollute and harm moving parts
of the wind tunnel, such as valves and pumps. These problems can be solved using liquid, or rather
oil, based tracer particles. Additionally, oil based tracer particles like Emery 3004 have a reduced
mass compared to solid tracer particles of same size. Thus, the relaxation time is reduced and this
will enhance the resulting velocity vector field. However, a new technical problem appears with the
storage tank of a blow down facility. For example hypersonic Mach 6 Ludwieg tubes operate with
stagnation pressures of around 30 bars and total temperatures between 400K and 800K. At these
conditions an oil-air mixture could result in an explosion and, thus, the aerosol has to be handled
with care [1] .
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 2 -
2. Experimental Setup
Wind tunnel
The measurements were conducted in the Hypersonic Ludwig Tube Braunschweig (HLB) which is
a blow down type wind tunnel as seen in figure 1. The high-pressure section consists of 17m long
storage tube including a 14m non-heated part and a 3m heated part. Inside this section a pressure of
up to 30bar and a temperature of up to 623K are possible. The low-pressure section consists of a
laval-nozzle, a test section, and a 6m³ vacuum tank. The high- and low-pressure sections are
separated by a fast acting pneumatic valve. Inside the test section unit Reynolds numbers between
Re=3·106m
-1 and Re=20·10
6 m
-1 at a Mach number of M=5.9 are possible. The measurement time
per wind tunnel run is approximately 80ms. Detailed information about the test facility are
described in [2].
The particle image velocimetry (PIV) measurements presented in this work were conducted with a
pressure of 15.7bar and a temperature of 418K inside the storage tube, if not noted otherwise. The
resulting unit Reynolds number is Re=16·106m
-1.
Figure 1 The Hypersonic Ludwig Tube Braunschweig (HLB) and the aerosol facility
Partcile image velocimetry setup
The PIV setup is presented in figure 2a. The laser light source was a Nd:YAG double pulse laser
system with an energy of 150mJ per pulse. The laser light sheet with a thickness of less than 1mm
was focused by one pair of lenses consisting of a plano-concave lens with a focal length of -50mm
and a plano-convex lens with a focal length of 75mm. A cylindrical lens with a focal length of -
12.5mm forms the light sheet. To observe the effect of an increased laser energy this cylindrical
lens was exchanged by a lens with a focal length -25mm and then -50mm. The camera Imager Pro
X 11M with a Tamron SP AF 180mm F/3.5 Di LD[IF] Macro 1:1 recorded the PIV images. The
camera aperture was set to 3.5 to maximize the brightness of the particle images. The complete
setup was mounted on a scaffold which had no direct contact to the wind tunnel and was placed on
small rubber mats. This prevented misalignments of the light sheet due to vibrations caused by the
wind tunnel. The atomizer ATM 210 (Topas) was used to generate oil based tracer particles. It
allows pressures up to 10bar at the outlet. The particle size distribution of DEHS based aerosols has
a peak at 200nm using a pressure difference between inlet and outlet of approximately 5bar. Two
oils, the standard oil Emery 3004 (Cognis Corporation) and the oil Plantfluid (Bechem), were tested
during the measurements. The aerosol for every run was pumped (figure 1, Aerosol LP) into a
aerosol tank with a volume of 10litres. To inject the aerosol into the storage tube the aerosol tank
was pressurized (figure 1, In) and connected to the storage tube (figure 1, Aerosol HP). Finally, the
aerosol tank was depressurized (figure 1, Out).
Aerosol
LP
Aerosol
HP
ATM 210 In
Out
Storage tube
Heater Laval-nozzle
Test section
Fast acting valve Diffusor
Vacuum
tank
Aerosol tank
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 3 -
Figure 2 The experimental setup (a) and the generic space launcher model with calibration grid (b)
Generic space launcher wind tunnel model
The 144mm long front part of the generic space launcher model consists of a spherical nose with a
radius of 10mm and a cone with an angle of 36°. The rear part is a 328.6mm long cylinder with a
diameter of 108mm. The wind tunnel model is made of Plexiglas and painted in dull black.
Transition is fixed with a ring of corundum grain (400µm) glued on the surface of the conical part
98mm behind the nose. A tail sting with a diameter of 43mm connected the wind tunnel model with
the diffuser downstream of the test section.
Data eavluation
To calculate the velocity vectors of the PIV measurements the software Davis 7.2 was used. The
recorded raw images were preprocessed applying a shift correction and a filter (subtract sliding
minimum). Areas with laser light reflections were masked. Finally, the velocity vectors were
calculated.
The tracer particle images of the PIV measurements were counted by a program based on Matlab
R2008a. The program analyzed an area of 44.53 x 44.53mm² above the wind tunnel model. Before
the counting was initialized, every individual image was prepared to account for the differences in
background brightness, and variations in the brightness and size of the tracer particle images. First,
the non-uniform background brightness in the image was reduced by using two filters, a wiener 2
lowpass-filter to remove noise and a medfilt 2 median-filter to remove the tracer particle images.
The resulting background image was used to remove unfocused bright areas from the non-filtered
image. The new image with homogeneous background had some noise and, thus, it was also filtered
with the wiener 2 lowpass-filter. This prepared image was converted into a binary black white
image. The level to differ between black and white was a mean value of all local minima in this
image with an additional offset. This level ensures that no noise was interpreted as tracer particle
image. The local minima should not exceed this level and the violations were counted. The
maximum violations were 5.3% of the registered tracer particle images.
The tracer particles paths were also simulated in the present work. For this purpose, numerical flow
simulations based on the Reynolds-averaged Navier-Stokes equations were obtained from RWTH
Aachen [3]. These computations used the same flow geometry and the same Reynolds and Mach
number. The simulation data was then input to Tecplot 360 2010 that allows to compute tracer
particle paths for given particle data.
Camera
Laser
Light sheet optics
Test section
U∞
a) b)
Corundum
Calibration grid
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 4 -
3. Results The manufacturer of the aerosol generator ATM210, Topas, reports a particle size distribution with
a peak at 200nm atomizing the oil DEHS with a pressure difference of approximately 5bar between
the inlet and the outlet. As different oils were used in the present work the resulting particle size
distribution was analyzed here. Both oils, Emery 3004 and Plantfluid, were atomized using a
pressure of 5 bar at the inlet. Two oil temperatures, 18°C and 50°C, were investigated. This varied
the viscosity of the oil which could influence the particle size distribution. Note that the oil
Plantfluid has an increased viscosity compared to the oil Emery 3004 and, thus, the oil Plantfluid
was heated to 50°C during the PIV measurements. The particle size distribution was measured by a
Fast Mobility Particle Sizer (FMPS, TSI Inc.) which detects particles sizes between 5.6 nm and
560nm. It should be noticed that particle sizes larger than 560nm are detected as smaller particles.
The results are presented in figure 3.
Figure 3 Particle size distribution absolute (a) and normalized (b) of the oils Emery 3004 and Plantfluid
Figure 3a shows that the peak of the particle size distribution is at 200nm independent of the oil and
the oil temperature but it seems that the number of particles changes. Thus, the particle size
distribution is normalized by the number of particles detected between 50nm and 560nm (Emery
3004: 9.97·105 particles at 18°C and 12.52·10
5 particles at 50°C; Plantfluid: 11.54·10
5 particles at
18°C and 12.82·105 at 50°C). The normalized particle distribution in figure 3b indicates that more
smaller particles and less larger particles are produced with the oil Plantfluid compared to the oil
Emery 3004. The influence of the temperature seems to be small. Obviously, with respect to the
particle size distribution there exists no reason to exclude any of the oils from the PIV
measurements. More than 85% of the detected particles are smaller than 300nm.
Before a wind tunnel run was initialized a certain volume filled with tracer particles was injected
into the storage tube. Figure 4a shows the theoretical saturation of oil inside the storage tube based
on a fixed amount of oil per run. The amount of oil within the storage tube increases with every
aerosol injection until the aerosol losses due to a wind tunnel run are equal to the amount of injected
oil. Figure 4b presents the detected tracer particle images of the observed start-up phases of seeding
the storage tube. The figure includes our standard injection procedure using two injections of
aerosol which was produced by the atomizer during a runtime of 9 minutes. Additionally, the figure
includes three start-up procedures with a different first injection. The storage tube pressure was
initially set to 6bar and the aerosol was directly pumped into the storage tube for 20 minutes.
a) b)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 5 -
However, the resulting first runs in figure 4b are identical. The observed start-up procedure
indicates that only a partial amount of the injected tracer particles are visible. After three wind
tunnel runs the number of detected tracer particles is approximately constant. This differs from the
theoretical saturation. Possibly a part of the tracer particles evaporated or were lost by wall contact
during the time between two runs of about 20min. An aerosol inlet closer to the fast acting valve
could increase the number of tracer particles but this causes other problems [1].
Figure 4 Theoretical saturation of oil (a) and (b) detected tracer particle images inside the wind tunnel using the oil Emery 3004
During the measurements the injection procedure of the aerosol was optimized. The atomizer
always filled up the aerosol tank first. Then, the aerosol tank was pressurized to a higher level than
the storage tube, in order to transfer the particles into the storage tube. Figure 5a presents the
detected tracer particle images based on a pressure of 25bar and 18bar within the aerosol tank. The
figure clearly shows that a lower pressure results in more detected tracer particles. This indicates
that tracer particles collided with the pipe walls at higher aerosol tank pressures causing particle
losses.
Figure 5 Influence of (a) the pressure inside the aerosol tank using the oil Emery 3004 and (b) the oil
a) b)
a) b)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 6 -
In a next step the oil was changed from Emery 3004 to Plantfluid. Plantfluid was first tested at the
same conditions inside the wind tunnel and the aerosol facility. The detected number of Plantfluid
and Emery 3004 based tracer particle images is identical as seen in figure 5b. This result
corresponds to the measured particle size distributions in figure 3.
Figure 6 Influence of (a) the laser energy using the oil Emery 3004 and (b) the temperature of the storage tube
The influence of the laser energy was analysed by decreasing the length of the light sheet. This was
realized with two different cylindrical lenses. The standard cylindrical lens had a focal length of -
12.5mm to illuminate the complete rear part of the generic space launcher model. To increase the
laser energy by a factor of 4 a cylindrical lens with a focal length of -50mm was used. The result is
presented in figure 6a. It shows that more tracer particles are detected with an increased laser
energy. Thus, there were small tracer particles which did not scattered enough light to be detected.
However, the increased laser energy also caused a problem. The reflections on the surface of the
wind tunnel model were increased. This impaired measurements inside the boundary layer. Several
approaches exist for reducing the reflections of the present set up and these should be explored in
future work.
The influence of the temperature in the heated part of the storage tube is displayed in figure 6b. The
number of Emery 3004 based tracer particles decreases at an increased temperature of 450K. The
number of Plantfluid based tracer particle images at 450K is similar to the value of Emery based
tracer particle images at 418K. The results of figure 5b and figure 3 indicate that there are no losses
of Plantfluid tracer particles due to the increased temperature. This allows higher temperatures
inside the storage tube which is useful to avoid condensation in the test section.
Qualitative data of raw images of Emery 3004 (a) and Plantfluid (b) based tracer particles are
presented in figure 7, to supplement the quantitative results of figure 6b. The raw images are
recorded at a temperature of 450K inside the storage tube. The raw images confirm the count result
of the detected tracer particle images. Figure 7b also shows the low seeded area behind the generic
space launcher model. The number of small tracer particles is too low, even though the tracer
particles are more resistant to higher temperatures. Thus, it should be clarified if the procedure to
transfer the aerosol into the storage tube influences particle size distribution.
a) b)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 7 -
Figure 7 Raw images with (a) Emery 3004 and (b) Plantfluid based tracer particles at a temperature of 450K inside the storage tube The mean velocity vector field in figure 8a is based on 160 double images which were measured
with the oil Emery 3004. The resolution is 1.02 x 1.02 mm² and every third velocity vector is shown.
The coloured contour represents the velocity. The mean velocity vectors in the complete upper area
1
1
2
2
1
1
b)
a)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 8 -
are available whereas the mean velocity vectors in the wake are partly unknown. The basic problem
is that the single velocity vector fields have more blank spaces in the boundary layer and the wake
due to less tracer particles as seen in figure 7b.
Figure 8 Mean velocity vector field (a) and number of velocity vectors (b). Tracer particles: Emery 3004
Figure 8b visualizes the problem. The figure presents the number of velocity vectors normalized by
the number of double images. In the lower boundary layer every mean velocity vector is based on
less than 5% of the possible velocity vectors. This value increases to 50% 5mm above the surface
and the isoline of this value in the wake is horizontal.
Figure 9 Raw image with Emery 3004 (main flow) and TiO2 (recirculation area) based tracer particles
1
1
a) b)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 9 -
Based on the results obtained so far we have to conclude that oil based seeding of the wind tunnel
free stream does not allow for measuring the wake region. One solution could be local seeding but
the flow should not be influenced by any active device needed to transport the tracer particles into
this area. In one approach the tracer particles are placed in the area before the wind tunnel run starts.
This approach is tested by two wind tunnel runs with some non-prepared and hence agglomerated
titanium dioxide powder. The test section was opened and the powder was positioned behind the
generic space launcher model on the tail sting. Then, the test section was closed, evacuated, and one
wind tunnel run was done. One example is presented in figure 9. The wake of the generic space
launcher model is filled with titanium dioxide particles. Such a method of local seeding will
possibly allow to measure turbulent stresses in this area. Additionally, only a small quantity of
powder is needed which should not harm the wind tunnel.
These first results demonstrate that local seeding of the recirculation area is feasible. However, PIV
data evaluation parameters must be adjusted to obtain resolution of both the outer flow field and
details of the wake flow. We note that local seeding of the wake does not resolve the lack of tracer
particles in the forebody boundary layer and, hence, within parts of the wake shear layer.
In the following analysis we therefore assess the inhomogeneous tracer particle distribution
observed along the model forebody. This is performed by using a numerical simulation of the
generic space launcher model at the same Reynolds- and Mach number [3]. The numerical
simulation allows to analyze various sources of particle image inhomogeneities as observed in the
measurements. Here we assume that the tracer particles are homogeneously mixed inside the
storage tube. In ideal conditions the tracer particles react to changes of the flow immediately and,
thus, the tracer particles will be distributed as the flow density. In a hypersonic Mach 6 blow down
facility one observes three major density variations: 1.) drop of the density about the nozzle, 2.)
density increase across shock waves, and 3.) density drop across boundary layers. To visualize the
ideal tracer particle distribution around the generic space launcher model figure 10a shows the
density close to the surface at the beginning and the end of the cylindrical part. The presented
density distributions in z-direction may be regarded as an upper bound of possible seeding based on
homogeneously distributed tracer particles inside the storage tube.
Figure10 Density ratio (a) and comparison of the density ratio with the number of velocity vectors (b) at x=-10mm
An indicator for the observed tracer particle distribution can be derived from the counted velocity
vectors as presented in figure 8b. With respect to all double images and the used PIV data
evaluation method a large number of tracer particles at one position will more often result in a
a) b)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 10 -
velocity vector compared to a position with less tracer particles. In order to make the flow density
and the velocity vector count comparable, both are normalized with their uniform values outside the
boundary layer edge. The result is presented in figure 10b. Comparing the ideal and the observed
gradients show that the trends, an increase of the ratio from the surface to the outer flow, are similar.
The difference between both lines could be a result of the relaxation time of the tracer particles. At
the end of the conical part and the begin of the cylindrical part non-ideal tracer particles are shifted
away from the wall due to the relaxation time. This could reduce the number of the tracer particles
close to the surface at the trailing edge.
Figure 11 Streamlines and particles paths at (a) the nose and (b) the end of the conical part
The paths of ideal tracer particles are the streamlines as presented in figure 11. Figure 11a shows a
streamline which starts in front of the generic space launcher model and figure 11b shows a
streamline which starts at the end of the conical part. Based on the assumption that a continuum
flow exists tracer particles with given mass and locally calculated drag coefficient CD=var. can be
simulated and compared with streamlines. The simulated tracer particles start at the same position
as the streamlines. Figure 11a presents the simulated tracer particles with a diameter of 200nm and
1000nm at the nose. The deviations from the streamlines are insignificant independent of the tracer
particle diameter. The influence of the drag coefficient is checked for the tracer particles with the
diameter of 200nm. The drag coefficient is set to the fixed values CD=[1, 10, 100, 1000]. The
deviation between the particle path with fixed and locally calculated drag coefficient decreases with
a higher drag coefficient. This indicates that the locally calculated drag coefficient, which changes
due to the changes of the tracer particle velocity, has a mean value in the order of 1000.
Figure 11b shows the begin of the cylindrical part. Again the sensitivity of the tracer particle path
due to the drag coefficient is displayed. The drag coefficient depends on the flow regime and must
be calculated with respect to the Reynolds and Mach number as well as the Knudsen number [9].
The authors [9] recommend the formula
( )[ ] ( )CKnξkkCD
687.0Re15.01
Re
24+= (1)
because it can be used in rarefaction regimes over broad ranges of Reynolds number (Re≤200),
Mach number (M≤1) and Knudsen number (continuum to free molecular). The formula differs to
the formula which is used by Tecplot 360 2010 to calculate the drag coefficient. Based on a tracer
a) b)
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 11 -
particle path with locally calculated drag coefficient the velocity difference between the tracer
particle and surrounding flow can be estimated. These estimates were used at the end of the conical
part and the beginning of the cylindrical part for evaluations of equation (1) which resulted in drag
coefficient around CD=3. Thus, the particle paths based on CD=1 and CD=10 in figure 11b are
possibly more realistic representations of real tracer particle behaviors. We note that the tracer
particle with a diameter of 200nm and a drag coefficient of CD=1 results in displacement of 0.6mm
versus the streamline at the trailing edge of the generic space launcher model. This value increases
to 1.6mm based on a tracer particle with a diameter of 1000nm and a drag coefficient of CD=1. Thus,
it seems that both the flow density close to the model surface and relaxation time of tracer particles
contribute to the observed decrease of tracer particles close to the wall.
4. Conclusions and future work
The present work shows that it is
possible to use oil based tracer
particles in a blow down type
wind tunnel with heated storage
tube. An alternative oil,
Plantfluid, is presented which is
more resistant to higher
temperatures. The number of
detected tracer particles can be
increased by using low pressure
differences to transfer the tracer
particles from the aerosol tank to
the storage tube and by using
high levels of laser energy for
PIV measurements.
The results also show that there
are still problems to seed the
flow with tracer particles independent of the used oil. In hypersonic flows the seeding density
decreases inside the boundary layer compared to the outer flow. This problem can only be solved by
additional tracer particles injected inside the storage tube or close to the surface of the generic space
launcher model. The tracer particle path simulation shows that oil based tracer particles with a
diameter of 200nm have only small deviations from the streamlines, as the relaxation time is short.
The atomizer ATM210 is well suited to produce the needed tracer particles but the effect to the
tracer particles during the procedure to inject the particles into the storage tube of the wind tunnel is
still unknown. Thus, the particle size distribution in the storage tube should be measured as well as
to check the possibility of droplet growth. One approach to simplify the aerosol transport into the
storage tube is to integrate the atomizer nozzle into the storage tube. Figure 12 displays that this can
be accomplished with only a few modifications of flange and support tubing.
The problem, that there are no tracer particles inside the recirculation area, must be solved with
local seeding. The tests show that injected tracer particles remain in the recirculation area even
though the tracer particles were injected prior to a run. If titanium dioxide particles will be used the
particles should be prepared [10].
compressed air
oil reservoir and pressure reducer
tracer
particles
ATM210
nozzle
level
sensor
Figure 12 Sketch of an integrated atomizer nozzle
16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012
- 12 -
Acknowledgment
The work was funded by the German Research Foundation (Deutsche Forschungsgemeinschaft,
DFG) within the framework Sonderforschungsbereich Transregio 40 (Technological foundations
for the design of thermally and mechanically highly loaded components of future space
transportation systems). The authors want to thank I. Kirsch and E. Uhde (Fraunhofer Wilhelm-
Klauditz-Institut, WKI) for providing the equipment for the measurements of the particle size
distribution as well as V. Statnikov for providing the numerical Simulation.
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