PSFVIP-8: The 8th Pacific Symposium on Flow Visualization and Image Processing, Moscow, Russia, August 21st-25th, 2011
SYNCHRONIZATION OF PARTICLE IMAGE VELOCIMETRY AND BACKGROUND ORIENTED
SCHLIEREN MEASUREMENT TECHNIQUES
P. Bencs1, Sz. Szabó
1, R. Bordás
2, K. Zähringer
2 and D. Thévenin
2
1 Department of Fluid and Heat Engineering, University of Miskolc, Hungary
2 Laboratory of Fluid Dynamics and Technical Flows, University of Magdeburg, Germany
ABSTRACT
Flow around cylinders has been investigated
experimentally for a long time and has a very broad
literature. The main objective of the present experimental
investigation is to determine the temperature- and velocity
field around a heated cylinder for the case of low
Reynolds number and forced convection. The present
experiments were also carried out in a wind tunnel using
PIV and BOS systems. Detailed information about
measurement systems and first results are presented in this
study.
INTRODUCTION
Bluff bodies placed in a flow, such as electrical
transmission lines, cartridge heaters, pipes of heat
exchangers, factory chimneys and so on, often have a
different temperature compared to that of the
surroundings. The structure of the flow developing around
bluff bodies has been investigated for a long time [1, 2].
The Kármán vortex street was and is examined by
numerous researchers, both experimentally and
numerically. Nevertheless, the question arises as to how
this vortex street is modified by a heated cylindrical bluff
body. What is the influence of heating on the frequency of
the detaching vortices, the structure of the vortices and the
location of the detachment? Many of these questions have
already been answered by the help of numerical
simulations and of measured velocity distribution using
Particle Image Velocimetry (PIV) and the vortex
distributions obtained from this [3, 4]. A further question
is the heat loss caused by the vortex structure and the
forced convection. To tackle this question, the
Background Oriented Schlieren (BOS) method is applied
here. At the same time, first steps have been taken towards
determining temperature and vortex distributions
simultaneously, which are introduced in this paper. Main
objective and novelty of this work is the solution for the
mentioned measurement problem with a single camera.
The objective of this work was to carry out non-
intrusive measurements of both temperature and flow
fields, by means of BOS and PIV respectively, using the
experience from previous research [5-8]. The flow was
investigated behind a heated cylinder, mounted in a
Göttingen-type (closed-loop) wind tunnel, with suitable
conditions. Future intention is to validate existing
numerical calculations.
BACKGROUND Z-type Schlieren system was used to determine the
temperature field in the first phase of this study [9] (see
Figs. 1 and 2). Advantages of this system: real-time
temperature distribution visualization, low cost of
measuring equipment and software. Disadvantages:
determination of simultaneous velocity and temperature
distributions requires a separate system and a complicated
procedure; measurement area is limited by mirror size.
PIV-BOS technique with one camera has been
developed to overcome these disadvantages.
Fig. 1. Principle of Z-type Schlieren measurement
technique
Fig. 2. First Schlieren pictures (300
oC and velocity range:
0-0.3 m/s)
EXPERIMENTAL SETUP
The experimental setup (Fig. 3) is mounted in a
closed-loop wind tunnel. The cross section of the test area
had the dimensions of 500x600 mm.
Mean velocity was set to v=0.3 m/s, since this was the
minimum stable velocity of the wind tunnel in this
configuration. This led to a wind tunnel Reynolds number
of Re=11,000, calculated from the mean flow velocity in
the test section, the hydraulic diameter of the wind tunnel
and the viscosity of air at ambient temperature. Two
transparent windows were mounted on both sides of the
measurement section, with a hole in the middle, used to
mount the heated cylinder perpendicular to the main flow
direction (see Fig. 3). The cylinder with a diameter of
Flow direction
PSFVIP-8: The 8th Pacific Symposium on Flow Visualization and Image Processing, Moscow, Russia, August 21st-25th, 2011
d=10 mm was electrically heated by an adjustable
transformer. The mean temperature of the cylinder was
measured by a thermocouple and the power of the
transformer was set to the required value. The cylinder
Reynolds number was Recyl=200, calculated with the
mean flow velocity, the diameter of the cylinder and the
viscosity of air at ambient temperature.
Fig. 3. Schematics of the experimental setup
PIV/BOS SYSTEM
The system used for the present measurement was a
regular 2D-PIV system, consisting of the components
listed in Table 1.
Table 1. Description of the PIV/BOS system
Component /
Manufacturer
Remarks
Double frame CCD
camera /
Dantec Dynamics
Flow Sense 2M/E with
8 bit resolution, recording
frequency: 15 Hz
Lens /
Nikon
Manual Focus Nikkor
180 mm; f-number: 11, focus set
to ~4 m
Double pulse
Nd-YAG laser /
Litron
Power: 2x300 mJ at
532 nm, max. frequency:
fr= 15 Hz
High-energy
mirrors /
CVI Melles Griot
for a wavelength of
532 nm
Laser sheet-optics /
LaVision f = -10
Timer box /
Self-produced
TTL logical electronic unit to
trigger laser and LEDs
PC with a frame
grabber card and
PIV software /
Dantec Dynamics
For image data acquisition and
for processing of acquired data
The applied software for the acquisition and evaluation
of data was commercial PIV software package (Dynamics
Studio 3.0 from Dantec Dynamics), used for both PIV and
BOS measurements. The PIV measurements are only
briefly discussed here, since there are numerous
publications describing the principles of PIV (e.g., [4]).
The same camera was used for both PIV and BOS
measurements. The camera was calibrated with the help of
a calibration plate to set the pix/mm factor and to eliminate
possible distortion. Camera optics was focused on the
calibration plate and the f-number (the focal length of the
lens divided by the “effective” aperture diameter) was set
to 11.
Timer Box and synchronization
PIV and BOS pictures were recorded successively
(timing scheme is shown in Fig. 4). The measurement area
was lit by the laser (at PIV recordings). The background
(during BOS measurements) was lit by LEDs (shown in
Fig. 6). This timer box (with timer electronics) was
developed for the present PIV/BOS measurements. A
block diagram of the timer box is shown in Fig. 5. Main
task of the timer box was to trigger the laser and LEDs
during the alternating PIV/BOS recordings so that the
timing scheme in Fig. 4 was assured. The essence was,
that one single camera could record successive PIV and
BOS images with relatively small time intervals. For PIV
evaluation double frame images were recorded, while in
case of BOS recordings, only the second frame was lit and
used for the correlation. The reference image for BOS was
taken prior to the measuring sequence.
Fig. 4. Timing diagram
The timing diagram of the synchronization method
assuring that the temperature and velocity information
were synchronized as shown in Fig. 4, with the time-
intervals:
1,2 1A s , 1,500B= s , 66,667C = s .
TTLDelay
Generator
HUB
Laser Q-Switch 2
Converter
LEDs
Flashlamp 2
Laser Q-Switch 1
Flashlamp 1
AC 230 V
Q1
(in)
Q2
(in)
Q1
(out)
Q2
(out)
+ -
-+
Fig. 5. Schematics of the timer box setup
PSFVIP-8: The 8th Pacific Symposium on Flow Visualization and Image Processing, Moscow, Russia, August 21st-25th, 2011
Both PIV and BOS images were made in the same
recording. The measurement area was lit by the laser (at
PIV recordings). The background (for BOS
measurements) was lit by LEDs (Fig. 6). LEDs were
placed between the wind tunnel and the background plane
(Fig. 3).
LEDs
Background
Fig. 6. Experimental LEDs setup
The time lag between two succeeding frame pairs was
specified by the recording frequency of the applied
camera:
1/ 1/ 15 66,666 .C fr Hz s (1)
Therefore, the time difference between two PIV and two
BOS recordings was:
2 .P B= = C = 136,333 s (2)
This means a recording frequency of 7.33P Bfr = fr = Hz
for separate PIV and BOS image sequences. According to
previous research [7], the vortex shedding frequency for
the present case is vfr 4.85 Hz , considering both
branches of the vortex street. Thus, the recording
frequency is about 3 times larger than that of the vortex
shedding, when considering a single branch. Therefore,
even the present camera with a recording frequency of
15 Hz is suitable to capture each vortex. Thus, an
interpolation of the velocity field and its derivatives was
possible for time instances between two recordings. Of
course, the accuracy of interpolation is expected to
increase with higher recording frequencies.
From the timing scheme (Fig. 4) it can be seen that the
PIV (velocity) and BOS (temperature) distributions are
not recorded simultaneously but successively: PIV1,
BOS1; PIV2, BOS2; … PIVi, BOSi,; …. Time instances
belonging to the recordings are P,1 ,
B,1 ; P,2 ,
B,2 ; …
P,i , B,i ; …, respectively. Therefore, during the
evaluation, each deflection vector pair of two consecutive
BOS images was linearly interpolated according to the
time instance of the enclosed PIV image. The temperature
P,iT belonging to the velocity P,iv of a given time
instance P,i can be interpolated using the relation:
, , 1
, , 1 , , 1
, , 1
.P i B i
P i B i B i B i
B i B i
T = T T T
(3)
PIV Measurements
For PIV measurements the background was not
illuminated and the TTL electronics turned on the laser
light. Oil droplets of 3 µm in diameter were added to the
flow as tracer particles and the measurement plane was lit
through the light sheet optics by a doubled Nd:YAG
double pulse. The velocity field was calculated from the
scaled images using cross-correlation with a 64x64 pixel
interrogation area, using 75% overlap. The resulting
vector maps were then exported to ASCII files for later
visualization using Mathcad® v14 and Matlab® R2009a.
BOS Measurements
For BOS measurements a background with white noise
dots was printed and placed 519 mm behind the plane of
focus. The background was illuminated homogeneously
with LEDs (every second double frame), such that the
same f-number could be applied as in case of the PIV
measurements. The Schlieren recordings were carried out
in double frame mode (where only the second frame was
used). The time lag between two double frames,
B=1,500 µs (see Fig. 4), was important for the calculation
of the deflection from the exported correlation
information. The cross-correlation was carried out with an
interrogation area of 32x32 pixels and an overlap of 75%.
The results were also exported into an ASCII file for later
post processing and visualization in Mathcad® and
Matlab®. The displacement vectors resulting from PIV
analysis must be translated into density gradient vectors in
order to move the BOS analysis towards completion. By
assuming the flow is strictly two-dimensional, the density
gradient along any given light ray passing through the
Schlieren object can then be assumed constant [10]. Given
these assumptions, the relation between image
displacement and density gradient can be simply written
using two algebraic equations. Eq. (4). defines the
relationship between angular deflection of a light ray
and image displacement d as
/ ,Ddh z (4)
where h is the physical dimension of a pixel in the
background plane (i.e., a conversion between
displacement in pixel units to a length unit) and Dz is the
distance between background plane and Schlieren object.
Eq. (5). defines the relation between density gradient
and angular deflection as
,K W (5)
where W is the width of the Schlieren object. The
variable K is the Gladstone-Dale constant, which is
found using the relation between density and the
index of refraction n as shown in Eq. (6).
1 .n K (6)
Finally, the temperature field was calculated using the
ideal gas law and presented as a contour plot.
RESULTS
Raw PIV (tracers with laser lighting) and BOS
(background with LED lighting) recordings are presented
in Fig. 7.
PSFVIP-8: The 8th Pacific Symposium on Flow Visualization and Image Processing, Moscow, Russia, August 21st-25th, 2011
Fig. 7. PIV and BOS raw pictures (300
oC)
The vortex shedding can be clearly recognized in the
PIV image (Fig. 7, left). Even this image shows the
connection between the vortex shedding and the
temperature field. The dark regions represent the change
in physical condition of the oil fog used for the
visualization. These dark regions appear due to higher
temperatures and mark at the same time the vortices. The
diffraction, caused by the air density change near the
heated cylinder, can slightly be seen slightly in the BOS
picture near the heated cylinder (white circle in Fig. 7).
The periodicity of vortices and temperature are shown
in Figs. 8 and 9. The origin 0, 0x = y = is defined by
the intersection of the ,x y plane and the axis of the
cylinder, which is perpendicular to this plane. Figure 8
depicts the positive (yellow) and the negative vortices
(magenta) and the vorticity (amplitude). The vorticity
peaks decrease progressively downstream of the cylinder.
Fig. 8. Vorticity field
In Fig. 9 the temperature field is presented. Directly
behind the cylinder two peaks representing high
temperatures are followed by two rapidly decreasing but
explicit wakes.
These temperature regions follow the path of the
vortices and indicate that the heat is transported in
packages from the cylinder. It can also be noticed that the
temperature equalization increases downstream of the
cylinder, i.e., the distance grows between the parallel
wakes.
Figure 10 shows both measured and interpolated
contour plots of vorticity and temperature in two
successive time instances. For comparison, the
interpolation was carried out for vorticity (right) as well.
Although it is a vector field, and the interpolation was
carried out separately for both vector components, the
result is satisfactory. The vortex street is not decomposed;
moreover it suits both the preceding and the next
following vorticity fields.
Fig. 9. Temperature field
Regarding the temperature and vorticity fields,
presented in Figs. 10 and 11, following statements can be
made:
The experimental setup is suitable for simultaneous
velocity and temperature measurements for the present
case, even with a relatively slow camera.
Velocity and temperature fields can be determined
using a single camera and the developed timer box.
Examining the vortices, we find that the lower branch
of the vortex street is more regular. A possible reason for
this is the rising heat packages collapsing with the upper
branch. This can also be seen in the temperature field,
where the upper branch is much less ordered.
The temperature field diverges more than the vortex
street. This is probably also caused by the previously
mentioned phenomenon of heat diffusion.
The high peaks behind the cylinder in the temperature
field can be explained by the closeness of the heated
cylinder. However, the high temperature differences - i.e.,
the large density gradients - might require an additional
BOS background image. It is an interesting question
whether a relation could be found between the resolution
of the BOS background and the expected density
gradients. This might improve the accuracy of the
temperature measurement (results of the cross
correlation).
Comparing the two image sequences, it is clear that the
distribution of the temperature peaks is similar to that of
the vortices, but not identical. The reason for this is
probably an optical problem: PIV visualizes an image at a
well defined plane, illuminated by the laser sheet, whereas
BOS recordings represent light refraction in the whole
focal depth. Furthermore, light rays arriving to the camera
chip are not parallel to each other, thus in particular at the
boundary region of the recorded BOS image the light ray
crosses vortices in different phases and incident rays of
light are not parallel to the heated cylinder (see Fig. 12).
PSFVIP-8: The 8th Pacific Symposium on Flow Visualization and Image Processing, Moscow, Russia, August 21st-25th, 2011
BOS (temperature) ms
x 1000 y 1000, T,
0
x 1000 y 1000, T,
65
.2
x 1000 y 1000, T,
13
3.3
19
8.5
measured
interpolated
measured
y [
mm
] y
[m
m]
y [
mm
] y
[m
m]
interpolated
Fig. 10. Temperature field behind the cylinder
ms
PIV (vorticity)
0
65
.2
x y, ω, ( )
13
3.3
x y, ω, ( )
19
8.5
x y, ω, ( )
measured
measured
y [
mm
] y [
mm
] y [
mm
]
y [
mm
]
interpolated
interpolated
Fig. 11. Vorticity field behind the cylinder
PSFVIP-8: The 8th Pacific Symposium on Flow Visualization and Image Processing, Moscow, Russia, August 21st-25th, 2011
Fig. 12. Geometrical properties of the optics
CONCLUSIONS
Measurement results presented in this work confirm
that the BOS system is suitable to visualize and quantify
the temperature field of the vortex street behind a heated
cylinder in a wind tunnel. The developed Mathcad® and
Matlab® codes were successfully applied to the
calculation of the temperature field from the measured
deflection, resulting from density variations in the flow.
Thanks to the employed timer box, temperature and
velocity measurements could be reasonably synchronized.
However, considerable improvements - especially
concerning timing method and optics (a convex lens to
generate field of view parallel to the cylinder) - are still
required in the existing system to make more reliable and
comparable measurements. In order to analyze images in a
further step, the recording quality and frequency must be
increased to get more reliable images (a high speed
camera to decrease time delay between two recordings). It
should also be checked whether it is necessary to change
the resolution of the BOS background according to the
expected density changes in the flow.
ACKNOWLEDGEMENTS
The authors are grateful to NKTH-OTKA (68207) and
to the Hungarian-German Intergovernmental S&T
cooperation programs P-MÖB/386 for the financial
support of this research.
The described work was carried out as part of the
TÁMOP-4.2.1.B-10/2/KONV-2010-0001 project in the
framework of the New Hungarian Development Plan. The
realization of this project is supported by the European
Union, co-financed by the European Social Fund.
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field of view