SYNCHRONIZATION OF PARTICLE IMAGE...

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


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


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.


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).


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


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.

REFERENCES

[1] Adrian, R.J.: Particle-Imaging Techniques for

Experimental Fluid Mechanics. Annual Reviews in

Fluid Mechanics, 23(1), pp. 261-304, (1991).

[2] Williamson, C.H.K.: Vortex dynamics in the cylinder

wake. Annual Review of Fluid Mechanics, 28(1), pp.

477-539, (1996).

[3] Venkatakrishnan, L. and Meier, G.E.A.: Density

measurements using the Background Oriented

Schlieren technique. Experiments in Fluids, 37(2), pp.

237-247, (2004).

[4] Wang, A.B. and Trávniček, Z.: On the linear heat

transfer correlation of a heated circular cylinder in

laminar crossflow using a new representative

temperature concept. International Journal of Heat

and Mass Transfer, 44(24), pp. 4635-4647, (2001).

[5] Baranyi, L., Szabó, S., Bolló, B., and Bordás, R.:

Analysis of Flow Around a Heated Circular Cylinder.

Proceedings, 7th JSME-KSME Thermal and Fluids

Engineering Conference, Sapporo, Japan, (No. 08-

201.), A 115. pp. 1-4, (2008).

[6] Bencs, P., Bordás, R., Zähringer, K., Szabó, S., and

Thévenin, D.: Towards the Application of a Schlieren

Measurement Technique in a Wind-Tunnel.

Proceedings, MicroCAD International Computer

Science Conference, Miskolc, Hungary, pp. 13-19,

(2009).

[7] Baranyi, L., Szabó, S., Bolló, B., and Bordás, R.:

Analysis of Flow Around a Heated Circular Cylinder.

Journal of Mechanical Science and Technology, 23,

pp. 1829-1834, (2009).

[8] Bencs, P., Szabó, Sz., Bordás, R., Thévenin, D.,

Zähringer, K. and Wunderlich, B.: Investigation of the

Velocity (PIV) and Temperature Field (BOS) of a

Heated Cylinder in a Low Re-number Flow.

Proceedings, ISFV14 - 14th International Symposium

on Flow Visualization, EXCO, Daegu, Korea, pp.

234/1-234/8, (2010).

[9] Bencs, P., Szabó, Sz.: Application of Z-type Schlieren

technique for Flow Visualization around Heated

Cylinder. Proceedings, ISFV14 - 14th International

Symposium on Flow Visualization, EXCO, Daegu,

Korea pp. 240/1-240/7, (2010).

[10] Richard, H. and Raffel, M.: Principle and

applications of the background oriented Schlieren

(BOS) method. Measurement Science and

Technology, 12, pp. 1576-1585, (2001).

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