3D-3C PIV method by using W-shaped light sheet and color PIV

10TH INTERNATIONAL SYMPOSIUM ON PARTICLE IMAGE VELOCIMETRY – PIV13 Delft, The Netherlands, July 2–4, 2013

3D-3C PIV method by using W-shaped light sheet and color PIV

1

Department of Mechanical Engineering, University of Yamanashi, Kofu, Japan sfunatani@yamanashi.ac.jp

ABSTRACT

This paper describes the principle of the 3D-3C PIV system which is based on expanded visualized area by deforming the shape of a laser light sheet into a “W” shape. When the shape of the light sheet is W-shape, it becomes possible to visualize parts of a field at various x, y and z positions simultaneously. Since velocity distribution with depth information can be obtained by using the W-shaped light sheet, 3D velocity distribution can be evaluated by interpolating the velocity distributions on each W-plane. The proposed 3D PIV technique is applied to the measurement of the velocity distribution in a vertical buoyant jet. This technique is well suited for measuring the velocity field of an airflow. The color PIV method using the digital SLR camera is also discussed for to maintain the good spatial resolution of the 3D PIV method.

1. Introduction

The instantaneous measurement of 3D-3C velocity fields is essential for studying various fluid flows. For this purpose, a tomographic particle image velocimetry (PIV) method was developed, which was successfully applied to an actual flow field [1]. However, the method often involves the observation of ghost vectors derived by the principle of measurement algorithm, and also requires several high-resolution cameras for obtaining original images of the tomographic PIV. In traditional PIV, the laser light sheet used for visualization is flat and can only be used to visualize a single 2D plane. Although the visualized area has been improved to 3D by the application of a pair of scanning light sheets [2-3], the scanning velocity is limited by the frame rate of the camera, and the 3D volume cannot be instantaneously visualized. To solve these problems, we propose a new method for the instantaneous visualization of the entire 3D field by deforming the light sheet. Using a W-shaped light sheet, we were able to simultaneously visualize different parts of a field in the x, y, and z directions (Figure1). The velocity distributions could therefore be obtained where the W-shaped light sheet intersected the flow field, and the 3D velocity distribution could be evaluated by interpolating the velocity distributions on each W-plane. However, the proposed PIV method reduces the number of velocity vectors in the x-y plane and significantly deforms the shape of the correlation area. It is therefore necessary to increase the number of pixels of the camera to maintain the good spatial resolution of the traditional PIV velocity distribution. Nevertheless, there have been increases in the number of pixels of digital single-lens reflex (SLR) cameras in recent times, and their emerging large-scale commercial production has been accompanied by reduced costs. Besides, the number of pixels can be easily increased by lining up multiple cameras.

Figure 1 Schematic view of 3D-3C PIV using W-shaped light sheet. The steps for the reliable application of the proposed 3D-3C PIV are as follows:

(1) Developing a method for generating a W-shaped light sheet of scanning laser beams using a rotating mirror. (2) Establishing the color PIV method for the digital SLR camera.

(3) Evaluating the uncertainty resulting from the deformation of the light sheet. (4) Applying the W-shaped scanning to an actual flow field.

Steps 1 and 2 are discussed in this paper.

2. Measurement methods

2.1 Developing a method for generating a W-shaped light sheet of scanning laser beams using a rotating mirror The experimental apparatus used for the 3D velocimetry are shown in Figure 2. The volume of the test section was 400 × 250 × 280 mm3. The test section was visualized using a 445 nm diode laser (1500 mW). The laser light sheet was generated by scanning a laser beam using a rotating mirror. A color camera (Nikon D5100, 14 bit, 4928×3264 pixels) was used to record the visualized image. The F-number of the camera was set to 16 to minimize the defocusing due to the deformation of the light sheet.

The time chart of the scanning method is shown in Figure 3. The W-shaped light sheet illustrated in Figure1 was generated using two pairs of rotating mirrors.

Figure 2 Experimental apparatus.

Time / ms Mirror 1 Mirror 2 Laser Trigger 0 10 20

Figure 3 Time chart of the 3D PIV system using W-shaped light sheet.

2.2 Color PIV method

Figure 4 shows the flowchart of the three-color PIV method. The three illumination colors were generated by diode lasers of wavelengths 671 nm (red), 532 nm (green), and 445 nm (blue). The visualized images were obtained by a color charge-coupled device (CCD) camera with color (red, green, and blue) pixel image grids. The power of the red laser was set to 400 mW and those of the others were adjusted to equalize the intensities of the colors of the visualized images. The three laser beams were combined into a single RGB laser beam by two dichroic mirrors before conversion into the light sheet. Each illumination had time intervals t1 and t2, and the velocity range could be changed by adjusting

t1 and t2. The visualized images, which had the three colors of the tracer particles, were decomposed into the three

channels (red, green, and blue). The PIV analysis was conducted on each channel. Laser

Test section

Mirror

W-shaped Light sheet

3. Results

Figure 5 shows the 3D-2C velocity distributions in the vertical jet. The figure shows the instantaneous structure of the vertical plume that was generated by a nozzle and interacted with the stagnant surrounding air. We observed that the 3D PIV method was suitable for measuring the velocity field of an air flow.

Figures 6(a)–(c) are examples of the visualized color images obtained by the proposed PIV method. In this study, only two kinds of laser beams (red, 671 nm; and green, 532 nm) were used for illumination. The time interval was set to 5 ms. The visualized images show that the displacement of the tracer particles of each color was successfully observed. Figure 6(d) shows an example of the velocity distribution evaluated by the color PIV method. An upward flow generated by natural convection was observed, although many of the velocity vectors had a magnitude of zero. It is considered that the green (532 nm) particles had an effect on the red image because the transmission ratio of the red image at 532 nm was about 1%. The PIV system should be improved to eliminate this effect. The average velocity of the natural convection of a wall with a grooved pattern was 0.043 m/s, which was almost the same as that of a hot wire anemometer. May we also note that the color PIV method was successfully used to investigation the heat transfer and fluid flow characteristics of a natural convection.

Figure 4 Flowchart of color PIV.

Figure 5 3D-2C Velocity distribution of vertical jet.

x / mm z / mm y / mm -30 0 -15 15 0 30 = 10 mm/s 0 30

Red laser Blue laser

Green laser Time Laser power t1 t2 Velocity distribution PIV RGB RG Wavelength combiner Visualized image

4. Conclusions

The proposed 3C-3D PIV is suitable for measuring the velocity field of an airflow. The method was successfully used to investigate the heat transfer and fluid flow characteristics of a natural convection. The details of the other steps of the method will be discussed in a future paper.

Figure 6 Visualized color image and velocity distribution.

Acknowledgements

This research was supported by the Grant-in-Aid for Young Scientists (B) No.24760131 of the Japan Society for the Promotion of Science (JSPS).

REFERENCES

[1] Elsinga G E, Scarano F, Wieneke B, van Oudheusden B W, “Tomographic particle image velocimetry” Exp. Fluids 41 (2006) pp.933– 947

[2] Fujisawa N, Funatani S, “Simultaneous measurement of temperature and velocity in a turbulent thermal convection by the extended range scanning liquid crystal visualization technique” Exp. Fluids 29 (2000) pp.S158–165

[3] Fujisawa N, Funatani S, Katoh N, “Scanning liquid-crystal thermometry and stereo velocimetry for simultaneous three-dimensional measurement of temperature and velocity field in a turbulent Rayleigh-Bérnard convection” Exp. Fluids 38 (2005) pp.291–303

(b) Original image (d) Velocity distribution x / mm 0 10 -10 0 10 y / mm = 0.02 m/s (a) Red image