download : CVC99_PitchingFoil.hwp

Animation of a Pitching Airfoil from PIV Data

 

    Young-Ho LEE1), Kyu-Sik KIM2), Jang-Woon CHOI3), Mi-Young KIM4)

     

    1) School of Mechanical Eng., Korea Maritime University, Professor
            (1 Dongsam-dong Youngdo-ku Pusan, Korea, lyh@hanara.kmaritime.ac.kr)
    2) Image Information Technology Co., President (infor@iitpiv.com)
    3) Image Information Technology Co., Junior Researcher (cjw@iitpiv.com)
    4) Graduate School, Korea Maritime University, Doctoral Student(JYH28@chollian.dacom.co.kr)
     

      Animation procedure from the PIV experimentation is specially emphasized in details to give whole-field and intuitive understanding of complex flow fields encountered in fluid engineering. As an application, a pitching airfoil(NACA0018) immersed in free surface water circulating tunnel is adopted(Re=1.1x104). Obtained images are processed in time sequence by PIV exclusive routines where an efficient and reliable cross correlation algorithm is included for vector identification. All animation jobs are implemented completely on single personal computer environment. Compressed digital images are obtained initially by Motion-JPEG board and various AVI files are finally obtained through graphic processes. As result, dynamic stall phenomena characterizing the pitching airfoil at high angle of attacks are well expressed from instant vector distribution, vortex shedding pattern and streaklines.

Keywords : Animation, PIV, Visualization, Pitching Airfoil, Dynamic Stall, Streaklines 

 

1. Introduction

     Recently, more sophisticated electronic equipments together with versatile illumination choices in PIV(Particle Image Velocimetry) are easily available and fluid engineering researchers are further encouraged to enjoy fantastic visualization world by overcoming hardware difficulties, such as lower resolution of input devices, limited image memory, slow CPU speed in host computer. Nowadays, advanced PIV system produces completely instant 3-D whole flow vector maps by support of the delicate laser and optics arrangement and corresponding identification algorithm. But time-dependent analyses of PIV data are yet confined in limited cases despite its intrinsic capability of continuous acquisition of flow images and subsequent time-sequential process of the recorded images by appropriate PIV identification. As an another attempt in PIV application, animation by post-processing of the time-sequence PIV data is believed to be fully capable of natural understanding of any complex flow. Kobayashi et al.(1993, [1]) represented first complete animation pictures from PIV experiment where a flow of 2-D circular cylinder was adopted. The present study is aimed to represent full animation procedures from the 2-D PIV data which are produced from the dedicated processing routines under PC environment such as Windows98/NT O/S, its software families and built-in image grabber. PIV measurement was carried out for a pitching airfoil which phenomena are characterized by dynamic stall and its related aperiodicity in up and down movement(Raffel et al., 1993[2], ; Oshima et al., 1995[3]) and by unsteady aerodynamics(Fuchiwaki et al., 1998[4]) A model airfoil(NACA0018) is used and its pitching speed is automatically adjusted by a step motor controller in the range of 0-45 angle of attacks. Corresponding Reynolds number(based on chord length and inflow velocity) is 1.1x104 in a circulating free surface water tunnel. As results, animation in full colors containing the temporal information of velocity distribution, vortex shedding and whole-field streaklines is well represented to understand dynamic stall phenomena.

 

2. PIV Processing

(1) Experiment

 

Fig.1 Arrangement of PIV System

 

 

     Figure1 is the general arrangement of the PIV system used in the present animation work. 300mW Argon-Ion laser is sufficient for the good quality image acquisition in case of low representative flow speed(0.11 m/sec). 2-D plane illumination is given by direct approach through cylindrical lens. A high-accuracy CCD camera(SONY,  XC-77RR, B&W, 768x493 pixels) is used as an input device. Flow images from the CCD camera are recorded on a Laser Disk Recorder(SONY, LVR3000AN). To obtain visualization pictures, a circulating free surface water tunnel(measurement size is 1.5m(L) x 0.3m(W) x 0.4m(H) is used. An acryl made support with model airfoil and a reduction gear system(Fig.2) to give accurate pulse signal revolution from step motor controller is immersed into water tunnel. Model airfoil is symmetric and its chord length is 100mm and span width is 17mm. Pitching angle range(0-45。) and its speed is accurately adjusted by step motor controller. The rotating speed is 18.4。/sec in the present case. Tracer particle is PolyVinyl Chloride sphere and its average diameter is 110μm.

     Figure 3 represents a schematic analysed domain and some information helpful to understand flow configuration. The size of the analysed flow region is 232mm(H) x 128mm(V) and its corresponding pixel size is 528 pixels(H) x 291 pixels(V). The height of flow passage between the bottom plate and upper plate is 152mm. Average inflow velocity is 0.11m/sec and its turbulent intensity is unknown.  

 

Fig.2 Airfoil Model with Pitching Mechanism

Fig.3 Analysed Domain and Flow Configuration

 

 

(2) Pre-Processing & Identification

   In the PC based PIV system, digital pixel images are usually generated through a non-compressed image grabber. As an alternative, compressed AVI files in Motion-JPEG format are here created onto free memory space secured in a hard disk through the capture board(miro Computer Products, miroVIDEO DC30plus, 24bits, RGB:256 colors) slit into the host computer. Its spatial resolution is 640 x 480 pixels and 8-bit grey-levels in each color are a little sacrificed according to the compression ratio. 44MB file memory is needed to capture 144 frames at the minimum compression ratio of 2.5 to animate 4.8 seconds pitching cycle satisfying the sampling rate of real-time, 30Hz. Each frame image from Laser Disk Recorder is interlaced into AVI files and at the pre-processing level, it is accurately decomposed into two field images(odd & even, here, 60Hz time resolution) for the cross-correlation identification. Odd and even field separation from the interlaced JPEG file is done after file transformation from the AVI(RGB, 24 bits) to BMP format(Y/C, 8 bits).

   The present study adopts optimized cross correlation PIV and it gives higher measurement accuracy( Lee et al., 1998[5]). The size of correlation domain is 41 x 41 pixels and its searching radius is 12 pixels for the grid formation of 50 x 35. The processing time for the identification per frame on PC(Pentium-II, 233MHz) requires 66 seconds. Outlier elimination process is followed and the divergence-free criterion at every grid is evaluated for the automatic removal. Afterwards, manual operation is added if necessary. Grid reallocation(55 x 40) is given by an appropriate method such as bicubic interpolation.

 

3. Animation

(1) Animation Procedure

    Figure4 and Fig.5 show, in sequence, the graphic procedure of the animation from the PIV database. The procedure to obtain the separate 144 raw files for the pre-processing is first required. The highly sophistigated PIV software, CACTUS'97 is easily adopted to decompose the single but large AVI file into 144-frame sequences for further frame-by-frame process. The obtained file is BMP format in 24 bits RGB colors. But, in usual PIV process, color information is not so available that the RGB information should be transformed into the Y/C signal(Y:luminance, C:chroma). The X.vel 144 binary files at the final stage of Fig.4 are used for averaging five sequential files at all grids. Next, various quantities for flow animation are prepared by calculating the turbulent intensity, turbulent kinetic energy, three Reynolds stress components, vorticity and time-dependent displacement coordinates of all artificial markers for streaklines, trajectories and streamlines.

 

 

Fig.4 Graphic Procedure of Animation(A)

 

 

Fig.5 Graphic Procedure of Animation(B)

 

 

And then, these quantitative informations are changed efficiently into full graphic sources. The visual language, Delphi3.0, which is greatly superior to other languages especially in compile speed, is absolutely contributed to the animation. 144MB X.bmp files are again transformed into non-compressed AVI files. But this file size should be reduced for easy handling. Adobe Premiere is again used for the purpose. The specification of the obtained single AVI file is 382 x 211 pixels in 24 bits resolution and its file size ranges from 2.6MB to 3.3MB in this animation work. Under Windows95/98 operating system, the built-in Active Movie displays automatically these animation pictures on PC monitor by mouse clicks only.        

 

(2) Examples

  As examples of PIV animation work, velocity vectors, vorticity distribution and streaklines are selected to show the overall behavior of dynamic stall phenomena of a pitching airfoil. Figure6 is an instant original image and Figure7 is the case of velocity vector distribution. Figure8 is a vortex shedding pattern and Figure9 shows instantaneous streaklines at 45。 angle of attack. Animation movies are prepared and they are accessed in Internet site, http://www.iitpiv.com/iiTbank/board.cgi?path=AnalExam.

 

Fig.7 Velocity Vectors

 

Fig.8 Velocity Vectors

 

Fig.8 Vorticity Distribution

Fig.9 Streaklines

 

4. Conclusion    

     To understand macroscopic flow phenomena in quantitative description, an animation procedure from the PIV data is newly suggested. Visualized flow is obtained, as an example, from a pitching airfoil model immersed into a circulating water tunnel. For identification, the cross correlation PIV to estimate the peak coefficient pixel by direct calculation is adopted. All animation jobs were completely implemented on a personal computer. Detailed animation procedures were introduced and instant pictures of the spatial distribution of velocity vector, vorticity and streaklines were represented to show macroscopic behavior of dynamic stall phenomena around a pitching airfoil with high angle of attacks.

 

References

1. Kobayashi T., Tsuda N., 1993, "Karman vortex street behind a circular cylinder" in Fantasy of Flow(VSJ ed.), Ohmsha & IOS Press, p.130.

2. Raffel M., Kompenhans J., Wernert P., 1995, "Investigation of the unsteady flow velocity field above an airfoil pitching under deep dynamic stall conditions", Exp. in Fluids, Vol.19, pp.103-111.

3. Oshima H., Ramaprian B.R., 1997, "Velocity measurements over a pitching airfoil", AIAA J. Vol.35 No. 1, pp.119-126.

4. Fuchiwaki M., Tanaka K., Tanaka H., 1998, "Unsteady fluid forces on an airfoil undergoing pitching motion", Proc. of 4th KSME-JSME Fluids Eng. Conf., pp.337-340.

5. Lee Y.H, Choi J.W., Seo M.S., Saga T., 1998, "Analysis of uncertainties from PIV input devices", Proc. of VSJ-SPIE98 Yokohama, pp.232-233.