Visualization of Flow Field : Application of PLIF Technique

National Key Laboratory of Science and Technology on Tunable Laser, Harbin Institute of Technology, Harbin 150080, China Institute of Opt-Electronics, Harbin Institute of Technology, Harbin 150080, China Science and Technology on Advanced Composites in Special Environment Laboratory, Harbin Institute of Technology, Harbin 150080, China College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China Science and Technology on Space Physics Laboratory, China Academy of Launch Vehicle Technology, Beijing 100078, China


Introduction
The flow around a circular cylinder is a classic problem of hydrodynamics, knowledge of which is essential for basic understanding and technical applications [1].In many vehicles such as cars and aircrafts, there are many cylindrical structures on the surface.In the movement of these vehicles, the interaction between the air and the cylinder is extremely complex, which involves the nonlinear process of flow separation, vortex shedding, lift oscillation, and so on.Therefore, it is more important to understand the law of the flow around the blunt body and its wake flow field.The research has a high theoretical value and the practical significance of the safety of industrial equipment.
In the method of flow field structure measurement, noncontact measurement, for the characteristics of not disturbing the structure of the flow field, has high application value.In the noncontact measurement methods, smokewire technique [2,3], particle image velocimetry (PIV) [4,5], and planar laser-induced fluorescence (PLIF) are the most popular methods to realize flow field visualization.
The smoke-wire technique is often used to visualize the flow in the wind tunnel, and the airflow can be observed by evaporating a smoke-generating liquid from a fine wire by electric heating.Smoke-wire technique is mainly applied to lowspeed flow field measurement, about ten meters per second.PIV technology can measure the two-dimensional speed field in the flow field instantaneously, but particle followability is the main problem.The PLIF technique, with impact less of the flow field structure, high temporal and spatial resolution, and other characteristics, has unparalleled advantages.
PLIF imaging is one of the most promising of these techniques because it is species-and quantum state-specific and is therefore sensitive to species composition, temperature, number density, and velocity [6].In the application of PLIF technology, NASA used NO as a tracer to carry out a large number of experiments [7,8], but NO as a toxic gas, and chemically active, the experimental conditions are very demanding.
This experiment used acetone as the tracer, as acetone has good signal intensity, low toxicity, high vapor pressure, accessible absorption (225-320 nm), and fluorescence (350-550 nm) [9][10][11].Since the peak of the acetone fluorescence is near 440 nm, conventional lenses can capture the fluorescence signal.This experiment was carried out at normal pressure and temperature, and the speed of the flow field was low; in the vortex, the temperature and pressure would not change greatly.When acetone was released into the flow field as a tracer, its fluorescence intensity could approximately represent the spatial distribution of acetone density, which represented the vortex structure.A series of flow field images were taken by a high-speed camera to obtain wake vortex structure development.The main problems addressed in this paper are the delivery of acetone vapor and the exploration of applying PLIF to aerodynamics.

Experimental Details
The experiments were conducted in a one-meter wind tunnel in Nanjing University of Aeronautics and Astronautics.The wind tunnel is a low turbulence, low noise, and low-speed return wind tunnel system, with an open test section of 1.7 m (length) × 1.5 m (width) × 1.0 m (height), and the test section is open.The speed of the wind tunnel is from 3 m/s to 35 m/s, with the turbulence intensity of ε ≤ 0 07%, airflow angle of pitching direction |Δα| ≤ 0 5 °, and airflow angle of yaw direction |Δβ| ≤ 0 5 °.
Acetone was excited by 266 nm laser as a tracer; it was released into the flow field to label the gas stream.The laser beam emitting about 3-4 mJ/pulse at frequency rate of 100 Hz or 500 Hz was formed into a collimated sheet using a combination of cylindrical lenses.Two cylindrical lenses, with focal lengths of −22 mm and 240 mm, formed a cylindrical telescope, spreading the laser beam into a collimated, 70 mm (width) laser sheet.Another cylindrical lens with 100 mm focal length focused the sheet to about 100 μm waist.Acetone vapor was excited by the laser sheet.The fluorescence from part of the illuminated sheet was recorded with an 8-bit intensified CCD camera.The minimum gate width of ICCD was 1 ns.In consideration of the acetone fluorescence lifetime is 8-11 ns, the gate width was set to 20 ns in this experiment, the camera has a maximum frame rate of 500 Hz in full format, and the CCD array was 1280 × 1024 pixels.
The camera was equipped with a Nikkor AF-S 300mm f/4D IF-ED, whose aperture was 77 mm.A Schott glass filter (WG305) was installed in front of the lens to block 266 nm scattered light.The camera was interfaced to a PC computer to control the camera and to record the images.The ICCD camera imaged a 112.5 × 90 mm 2 area of the flow field, giving a spatial resolution of about 88 μm per pixel.The PLIF system timing is controlled by DG645 (Stanford Research Systems).
Figure 1 shows a schematic representation of the experimental set-up.Liquid acetone was stored in a 1.0-liter preheated tank.A certain amount of gaseous acetone and nitrogen as dilution gas flew into the mixing tank, respectively.The molar fraction of acetone in the mixed gas was fixed at 18%.The exit of the acetone tank was connected to a stainless flatbed, controlled by a gas flow meter in order to guarantee a certain flow rate.The mixed gas was injected from a slot and was diluted into the wind tunnel flow field.
In the experiments, the circular cylinder's diameter was 2 mm.The distance between the slot and the cylinder was 5.9 cm.Laser sheet paralleled to the flat, and the distance between the flat and the laser sheet was from 1 mm to 4.5 mm.The Reynolds number was from 70 to 800.The jet direction of the slot was at an angle of 45 °and 90 °to the surface of the flat.It should be noticed that all measurements presented here were performed with N 2 as carrier gas.In the experiments, we investigated the effect of acetone injection angle and the distance between the laser sheet and the flat on the PLIF image.A series of flow field images were taken by a high-speed camera to obtain wake vortex structure development.Depending on a special kind of vortex structure, the vortex's speed and generation frequency are calculated.

Experimental Results
This section presents PLIF visualization results of a circular cylinder wake flow field at various experimental conditions.

PILF Images of Wake Flow Field Depend on Different Jet
Directions. Figure 3 shows wake flow field images that depend on jet directions.In Figure 3(a), the jet direction was 90 °to the surface of the flat.The acetone had an initial injection speed.After jetted from the slot, the acetone had not been able to mix well into the wake field where the sheet was located.This is the main cause of the vague image.In Figure 3(b), the jet direction was 45 °to the surface of the flat.Even though the initial injection speed was the same, the injection direction was more conducive to acetone mixing into the sheet flat.Figure 4 shows the effect of the gas injection direction on the flow of the mixed gas.In Figure 4(a), the jet direction was 90 °.The angle between the tracer speed and the plane was 9.02 °.And when the gas was ejected from the slot, there was no initial speed in the direction of the flow field.
In Figure 4(b), the jet direction was 45 °.The angle between the tracer speed and the plane was 5.76 °.The initial speed in the flow direction (the horizontal green arrow) was about 0.08 m/s, faster than Figure 4(a); the initial speed vertical to the flow direction (the vertical green arrow) was 0.08 m/s, slower than Figure 4(a).This means that when the gas is jetted at a direction of 45 °, the tracer has a better   3 Journal of Spectroscopy followability, and the tracer can be better adhering to the surface of the plate, reflecting the surface flow field structure.In the later part of this paper, the direction of mixed gas injection was 45 °to the surface of the flat.

PLIF Images of Flow Field Transition from Laminar to
Turbulent Flow.In Figure 5, the circular cylinder shows the position of the cylinder's bottom side, because the cylinder was not vertical to the flat in the camera's perspective, the cylinder's position did not coincide with the center of the fluorescent signal.The flow was left-to-right and vertical to the ground.Between the slot and the cylinder, the flow was mostly smooth and laminar.No irregular flow structures were observed upstream of the cylinder.The flow labeled with acetone did not show a visible change in the width before and after passing by the cylinder.The wake flow behind the cylinder showed some visible streaks that did not show up in the upstream flow.These streaks are evidence of perturbations induced by the cylinder and may be associated with streamwise vorticity.The streaks within the roughness wake soon become unstable, oscillating with periodic structures and then breaking down into much more irregular structures.In the diagram, this region of the flow is labeled "transition to turbulence." 4

Journal of Spectroscopy
We can get a clear wake field structure image by the PLIF visualization technology, which provides both highquality wake vortex structure picture and visualization of the formation.Some basic features will be discussed in the next section.
3.4.Wake Vortex Structure Development.Figure 6 is a series of continuous of the wake vortex, which showed the formation and development of wake vortexes.The acetone carried by the air showed the changes of the flow field and vortex generation and shedding.Because the vortex generation and shedding was quasiperiodic, the distance between each adjacent vortex was approximately equal.As we mentioned in the front of this paper that each pixel represents 119 μm actually, the distance between each adjacent vortex can be reversed by the pictures.As the frequency of taking those pictures was known, the frequency of each vortex generation can be calculated.
The average speed of the wake vortex is estimated by measuring the speed of the special structure in the wake field, like the black gap shown in Figure 6. Figure 7 shows the speed and average speed of 10 samples.The average speed was 3.80 m/s, and the RSD (relative standard deviation) was 4.7%.In two adjacent cycles, this gap would appear twice in a particular area.But there are some slight deviations from the two appearances that need to be corrected by the average speed.Figure 8 shows the period and average period time of 20 samples.The average period was 5.07 ms.The RSD was 8.0%.The frequency of the vortex generation was about 197 Hz.The distance of every adjacent vortex was approximately 19.27 mm.
In this experiment, the maximum frequency of the laser is 500 Hz.The maximum stable wind speed produced by the one-meter wind tunnel is about 21 m/s.In order to be able to calculate the speed of the vortex, one vortex need to be captured by two adjacent photos.In this experiment, the maximum wind speed that satisfies the above postulate is about 18 m/s.If this standard is used, if measuring the wake flow field in the supersonic wind tunnel, the frequency of the laser is at least 10 kHz.If the flow speed reaches 10 Mach, the laser frequency should be more than 100 kHz.The abovementioned laser frequency is the lowest frequency required for the corresponding experimental conditions.If the experiment need to be more accurate, a higher frequency laser is required.

Conclusion
The flow field around a circular cylinder has been investigated experimentally in a fully developed one-meter wind tunnel.PLIF is the basic measurement technique to diagnose the wake flow field of the cylinder.Acetone was used as the tracer to label the flow field, the 266 nm laser beam was shaped into a specific size sheet beam by the beam shaping system, which excited the tracer, and the fluorescence signal was captured by a high acquisition frequency ICCD camera and recorded as PLIF image.
In this experiment, some basic problems on applying the PLIF technology to the flow field visualization are discussed: firstly, acetone as the tracer, with its good signal intensity, low toxicity, high vapor pressure, accessible absorption (225-320 nm), and fluorescence (350-550 nm), is very suitable for the experimental study of flow field visualization.Secondly, in the acetone vapor injection to the flow field, the injection speed, angle, and other related factors need to be considered, in order to achieve the best experimental results.Thirdly, this paper gives the correspondence between the speed of the flow field and the frequency of the laser, which could be used as a rough reference standard for future wind tunnel visualization experiments.
In further research, the main direction is to improve the laser repetition frequency and camera resolution, in order to achieve higher temporal and spatial resolution.The period of the wake flow in the sample.Reynolds number = 800, the diameter = 2 mm, the sheet position = 3 mm, and the distance between the cylinder and the slot was 5.9 cm.The jet direction was at an angle of 45 °to the surface of the flat.

Figure 2 :
Figure 2: Spanwise scan of laser sheet.The diameter = 2 mm, sheet positions were 1, 2.5, 4, and 4.5 mm from the flat, Reynolds number = 70, and frame rate = 100 Hz.The jet direction was at an angle of 90 °to the surface of the flat.

Figure 3 :
Figure 3: Wake flow fields depend on different jet directions.(a) The jet direction was at an angle of 45 °to the surface of the flat and (b) was 90 °.The diameter = 2 mm, Reynolds number = 100, and sheet position = 2 mm.

Figure 4 :
Figure 4: Effect of gas injection direction on tracer flow direction.(a) The jet direction was at an angle of 45 °to the surface of the flat, and (b) was 90 °.Flow speed (v 1 ) = 0.7 m/s, and mixed gas outlet speed (v 2 ) = 0.11 m/s.

Figure 5 :Figure 6 :
Figure 5: The typical 100 Hz acetone PLIF image.The diameter = 2 mm; the distance between the cylinder and the slot was 5.9 cm; the distance between flat and laser sheet was 3 mm; Reynolds number = 800; and frame rate = 500 Hz.

Figure 7 :Figure 8 :
Figure7: The speed of the wake flow in the sample.Reynolds number = 800, the diameter = 2 mm, the sheet position = 3 mm, and the distance between the cylinder and the slot was 5.9 cm.The jet direction was at an angle of 45 °to the surface of the flat.