Application of Particle Image Velocimetry Technology in Water Tunnel

JOURNAL OF CHINA ORDNANCEApplication of Particle Image VelocimetryTechnology in Water TunnelHU Hai-bao(胡海豹), SONG Bao-wei(宋保维), LIU Zhan-yi(刘占一)，SUN Chuan-dong(孙传东)College of Marine, Northwesterm Polytechnical University, Xi' an 710072, ChinaAbstraet: A display and analysis systemxr bermetical flow field in water tunnel is developed based on the particle imagevelocimetry(PIV) technology. It consists of some subsystems, such 8s lamp bouse, imaging, image processing and ana-lyzing, control and particles putting. Taking the flow fields around a sphere and a cylinder model as an example underhigher Renolds number 5.8X 105 in water tunnel, we inquired into the application of the system to water tunnel experi-ment by cross correlation algorithm. The measure eror is less than 2% . The test results show that the developed systemcan measure and display the flow field accurately in complex flow field without touching and disturbing. The researchwork indicates that PIV has lots of potential applications in the underwater flow field measurement.Key Words: hydromechanics, PIV, water tunnel,flow field, cross correlationCLC Number: 0357.5; V211.7Documeat Code; AArticle ID: 1673-002X(2007 )04-0308-05mark, University of Edinburgh and Roll Royce Com-Introductionpany, United Kingdom are at the forefront in the re-Particle image velocimetry, called as PIV, cansearch and commercial application. They have devel-measure the flow velocity and direction by trackingoped a variety of flow field test equipment used to dif-tracer particle. It is a multi- disciplinary flow fieldferent fluids'3]. Since 1990s， in China， this testmeasurement technology which combines laser, digi-technology has been concemned by many research in-tal signal processing, image and graphics processing,stitutes, such as Bejing University, Tsinghua Uni-computer, application of modern optical and micro-versity, Tianjin University， Dalian University ofelectronics technologies1-21. It is an important fea-Technology and the China Ship Scientific Researchture of PIV technology that it breaks the limitation ofCenter(3). They carried out researches on image pro-single point measurements in space and can record allcessing, mathematical processing and applications inrelated flow information in a plane without contactingthe flow field test inside engine cylinder and detectionand disturbing, thus, obtain the instantaneous veloci-of marine motion, etc.ty field, the pulse velocity field, vortex field andIn view of those advantages, PIV can ,be appliedReynolds stress distribution of the plane. It is suitableto flow field test directly. It has been generally recog-for the flow field measurement of unsteady complexnized. In order to explore the feasibility of PIV inflow，such as turbulence.water tunnel tests, a flow field display and analysisIn PIV area, TSI company, United States, ILAsystem is developed. It can be used to record in real-company，Germany， Dan Tec Corporation， Den-time and analyze the instantaneous dynamic flow fieldRecived 200701-26中国煤化工Spomsored by National Nature Science Foundation of Chinas (10672136)M出CNMH GBiogrnphies HU Hai bao(1979- -), leturer, dotoral student, buabab@o.wu.... .. Wa15 , poftesor, doctoral adviser,songbaowei@ nwpu. edu. c一308一HU Hai-bao, et al. / Application of Prticle Image Velocimery Technologs in Water Tunnelaround crafts in a water tunnel.from region B, is the best matched region with re-gion A, then the distance Os between regions A and1 Equipment and SchemeA' can be regarded as the displacement of tracer par-1.1 Water Tunnelticle in time interval Ot. The velocity can be obtainedThe developed system can be applied suessfullyfrom the ratio of Os and Ot. Fig. 2 shows the princi-in a high-speed water tunnel, as shown in Fig. 1.ple of PIV[4] .The body of the tunnel is a hermetical and cycledA: matching piccestainless steel pipeline flld with water. Driven by anaxial pump, the water flows along the pipeline cycli-cally and forms a stable uniform flow field at thewotking segment. The flow velocity and pressure canL))be adjusted in accordance with the requirements ofsimilar condition and remained stable. Experimentalmodels in the working segment are connected with{a) Tirue Isome test equipment to measure their hydrodynamicB:seurchingparameters.piece(的z以A'mathing picce(b) Time tAFig.2 Principle of PIV131.2.2 System compositionThe PIV flow field measurement system for thewater tunnel includes following parts.1) Double pulse Nd: YAG laser-allance; 2- rexperimental model; 3- working segment; 4-suge chamber; 5- gas removing tower; 6- filter; 7- wate: punp:The flow field parametersn are analyzed by tracer8- vacuum box; 9- -vecuum pump; 10- pressure tube; 11- Dparticles moving together with the fluid in the PIVpressor; 12- -exial pump; 13- -DC motorsystem, thus, the laser subsystem to iluminate theFig.1 Sketch map of water tonnel experiment equipoentparticles is very important to the image quality. It1.2 Flow Field Display and Analysis Systemshould have good beam quality, stable output power1.2.1 Working principleand be easy to use. Meanwhile, the double pulse pro-PIV technology uses two successive laser to ira-duced by the laser should be high precision and havediate fluid, in which tracer particles are put already,an adjustable time delay to satisfy the demands ofrecord the instantaneous images by CCD camera, prodouble-exposal. Thus, the subsystem uses Nd: YAGcess the images by auto correlation or cross correlationgreen laser(5-7]. Its technical parameters are listed inalgorithm and obtain a two dimensional velocity dis-Table 1. The interval between two laser pulses is ad-tribution on the transverse section of the flow field fi-justable and ranges from 100 ns to 100 ms.nallyf4l. In the image processing, a piece of region2) Double exposal interframe CCD digital cameraA. called as matching piece, is selected in the l mo-中国煤化工ross crrlation al-ment image as study object, a bigger region B, whichgoritHCN M H G are exposed twiceincludes A and is called as searching piece, is selectedshould be recorded in two frames respectively. Thein the 1+ Ot moment image. If region A ', searchedinterval between two frames should range from mi-JOURNAL OF CHINA ORDNANCE, 2007, Vol.3, No.4croseconds to milliseconds to ensure their correlation.vide interface with other software so that the flowThus, a fast continuous double exposal interiramefield calculation results can be directly transmitted toCCD digital camera must be chosen. In Fig.3, theother software for further processing and analysis.interframe technique is ilustrated.5) Particle putting subsystem .It can be seen from the PIV principle that a suc-Table 1 Technical pararmeters of lasercessful dissemination and appropriate concentration ofwavelength532 nmpower (pulse)≥100 mJpulse frequency1- 15 Hzpulse width6-8nsthe tracer particles can guarantee perfect PIV test re-beam diameter6 ombeam divergence 1 mradsultl81. Since the particle putting subsystem is used inspatial pattermenergy stabilitythe hermetical water tunnel, it is difficult to design_power drift<5%biss of pulse<6n5and arrange. After repeated discussion and attempt, ahigh pressure injection method is adopted for putting_ L . minimum iterame intervalNo.T fname No.2 frumpeexposal ofthe particles finally, as shown in Fig. 5. This subsys-cameratem ensures the particles to be injected stably in pres-- pulse itevelsure condition.delay of pulsetpipelinevalvewater tunnclNo.1 lascer pulseNo.2 laser pulsecontrolsignal0=FFig.3 Principle of interframe techniquehigh concentration3) Control subsystemwaterparticles mixed liquidThe control subsystem consists of a multiple syn-chronous pulse generator and corresponding controlFig.5 Sketch of particke potting setsoftware which can set time accurately for laser pulsegeneration and image capture according to different2 System Operationflow measurement requirements. It ensures that theFirstly，particles, which have good scatter andwhole system works harmoniously and captures high-follow abilities, are put into water so that a certainquality image.particle concentration in the whole flow field can beextemal riggcr___reached. Secondly, the double pulse laser device iscontrol signalof camcrastarted, of which laser has good capability of penetra-pricle-putingbility and adjustable interval through frequency dou-im-phasebling crystal, and become a sheet-like light source toilurminate a thin flow layer through an optical systemcontrol signal ofcomposed of cylindrical lens and spherical mirrors.oflaserintcrlaceThe CCD digital camera set in the perpendicular di-rection with light source can capture the image ofmoving particles in flow layer and send it to acquisi-FIg.4 Skelch of signal flow directiontion card. By using auto correlation or cross correla-of synchronous controllertion algorithms, the velocity distribution of the flow4) Image processing and analysis subsystemfield can be obtained.A professional software PivView is used for im-This PIV system has fllwing three operationage processing, which bas the functions of image geo-modes.metric correction, flow field calculation and analysis,中国煤化工image preprocessing and fixed object shielding. It canJHC N MH Gits images to theanalyze and calculate the pair of particle images andacquistion cara I a maximum speed of 30 frames perdisplay or save the final test results. It can also pro-second, at the same time, PIV control software can一310一HU Hai-bao, et al. / Application of Particle Image Velocimery Techmology in Water Tunnelshow the images in a display screen. This mode isthe flow velocity. After preliminary analysis, thesuitable for some real- time monitoring applications,possible factors resulting in the measurement error aresuch as focusing and calibrating.as follows.2) Single exposure imagingIt is mainly used for capturing single- frame se-quential images in the same interval when water tun-halr of sphere“20nel runs in a low speed state.3) Double exposure imaging“-2002040080100./mm，The trigger pulses for two exposures are ad-(a) Vclocity vector(bi Strcam lmejustable. So, this mode ensures that the laser pulsesFig.7 Flow field of sphere sarfaceand exposures are synchronous in time. It is the mainTable 1 Test results of fnow veloeity without models m/soperation mode in the PIV system.setting velocity3 Test Resultsitem64.94To validate the adaptability and precision of thisMt= 100 us5.964.983.98PIV system, a series of tests for a sphere and a cylin-5.933.91der model in the water tunnel are carried out. The5.853.94test results in different flow velocities indicate that0t= 150 u5.843.96the measurement errors are almost the same. Some5.924.933.92representative results are given in this paper. Table 2Ot=200us5.88is the test results for a flat flow field in different flow5.943.92_velocities. The flow velocities as reference are mea-sured by the velocimetry measurement system of the0t= 300pus4.883.97water tunnel itself. The velocity distribution of the4.923.955.87flat flow field is shown in Fig.6. Fig. 7 shows the△t= 400 usflow field around a standard sphere model in flow ve-4.97locityof 6 m/s. Fig.8 and Fig. 9 ilustrate the flowaverage/m's5.89field around a cylinder model.variance/m2.s 20.001 70.00070.0006relative error/ %1.81.21)In the water tunnel, there are more solid pre-cipitation particles with different forms and sizes.They are regarded as valid particles and results in a .few bigger errors at some testing points for their poor-1500 150follow capabilty.(a) Velocity vecter(b) Strcam line2) Recently, in the test, the diameter of theFig.6 Flow field in work section of watertracer particles ranges from 100 pm to 200 μm. Thetunnel without tested modelbetter diameter range is between 5 μm and 20 μm.Table2 and Fig. 6 show the PIV systerm has aBut this kind of particles is more expansive than thathigh precision in flat flow field. The average velocitywe use. The bigger particles result in a worse effecteror in the whole flat flow field, except a fewon following the water flow and errors in velocitypoints, is less than 2% from 4 m/s to 6 m/s and themeas中国煤化工coherence error is less than 0.8%. In addition, theTYHCNMHG8 and 9 that thetest results do not depend upon the double exposurePIV system can display the complex flow field aroundinterval (Ot). Ot can be setting freely according totest models accurately. The system breaks the limita-一311-JOURNAL OF CHINA ORDNANCE, 2007, Vol.3, No.4tions of other measurement mnethods which measure atranges from 100 μm to 220 pm, thus, their followingsingle point only, and can record the dynamic velocitycapabilities are relatively poor. Recently, we are try-field distribution in real- time instantaneously. In ad-ing to develop some tracer particles with smaller di-dition, the development proces of boundary layer onameter than 100 μm.the cylinder surface in the flow direction can be4 Conclusionsshown clearly in Fig.9.This paper shows that PIV system can measurethe flow field in a hermetical water tunnel precisely.-00It can be used in the measurements of the flow field-150-around vatious underwater vehicle models. The study50 100 150indicates that the PIV technology has tremendous ad-x/mmvantages, such as non- contact, non- disturbance and(间) Velocity vetor(b) Stream linehigh precision, in the whole flow field measurement.Fig.B Flow field outstde head ol cylinderReferences[1] TANG Jing-yuan. Laser measurement technology and100 150applications in industry[D]. Xi' an;Xi' an Jiaotong Uni-(旧) Velocity vector(b) Sircam lincversity, 2001 :10 - 34, (in Chinese)2] LIU Bac-jie, WANG Guang- hua, GAO Ge. ApplicationFig.9 Flow field outside middle section of eylinderof on-line PIV system in low speed wind tunel[J]. Ex-These results indicate that the PIV system canperiments and Measurements in Fluid Mechanics, 1998,measure the flow field in the water tunnel rapidly,12(2):55 - 63. (in Chinese)precisely and obtain more information, and does not3] SHANG Yan-feng. Research on PIV calculating methodinterfere with the measured flow field. It can alsoand application of digital image processing in PIV[D].display the instantaneous flow field of non- steadyZhengzhou; North-China Universty of Water Consetvan-cy and Electric Power, 2003:7- 52. (in Chinese)flow, and give the corresponding quantitative param-eters and changes which are difficult to obtain by oth-[4] YANG Xiaolin, YAN Jing. Principles for the measure-ment and image pocssing using PIV[J]. J Xihuaer conventionail technologies.Univ. (Nat. Sei. ), 2005, 24(1):19- 20. (in Chinese)In addition, the test capability of the system is5] BU Yi-kun, XUE Qing-hua. ZHENG Quan, et el. De-insufficient if the measured region closes to the mod-sign and manufacture of optical thin films for 500.8 nme. After analysis, the reasons and solutions are asNd: YAG laser[J]. Acta Photonica Sinica, 2006， 35follows.(1): 79 - 83. {in Chinese)1) The connection rod for the model is poor in [6] FarleyR w, DAOP D. Development of an iteteriyr.rigidity. The model always vibrates rapidly duringsummed multiple wavelength Nd: YAG laser for a ruggedsolidstate sodium Lidar System [J]. Aplied Optics,the test process, then it is not at the same location in1995, 34 (21): 4269 - 4273.different images, thus, the flow field near its surfacecan not be measured accurately. To restrain the vi-[7] YANG Yong-ming, ZH0U Rong, GUO Zhen, et al.End deformation' s experiment and its computer mimic inbration, the connection rod must be strengthened toLD end pumped Nd: YAG crystal[J]. Acta Photonicaimprove its anti- vibration capability.Sinica, 2005, 34(9):1297 ~ 1300. (in Chinese)2) Because the velocity of the flow near the mod-8] DAI Guang-qing, WU Yan-hua, LIU Qi-hong. Applica-el surface is lower and fluctuates greatly, it requires a中国煤化工acers in PIVUJ]. Jstronger following capability of the tracer particles.2000 32(1):1 -3.MYHCNMHGIn the equipment, the diameter of the tracer particles一312-