توسعه سامانه اسكن ليزري سه بعدي آشكارساز جريان فلورسنت در تحقيقات هيدروليك محيط زيست

Three Dimensional Laser Scanning System for Illumination of Fluorescent flow for the Environmental Hydraulic investigations

مدل سازي و مشاهده پديده هاي هيدروليكي در آزمايشگاه را شايد بتوان اولين گام در راستاي شناخت رفتار پيچيده فرايندهاي طبيعي در مكانيك سيالات دانست. در اين زمينه از زمانهاي دور، روش هاي مختلفي براي اندازه گيري پارامترهاي جريان توسعه داده شده است. اين روش ها عمدتا مبتني برآشكارسازي و ثبت متغيرهاي جريان در شرايط مختلف حركت مي باشند. با توسعه تكنولوژيهاي ديجيتال در سالهاي اخير، روش هاي متنوعي براي ثبت ميدان غلظت و سرعت جريان بدون ايجاد تداخل در محيط توسعه داده شده است. اين روش ها بيشتر مبتني بر شبيه سازي جريان در محفظه هاي شيشه اي و شفاف، آشكارسازي جريان با كمك ليزر، فلورسنت و ذرات ريز و ثبت تصاوير با دقت و فركانس بالا جهت پردازش ثانويه است. در اين تحقيق، قابليتهاي سامانه اسكن ليزري سه بعدي كه جهت آشكار سازي جريان آغشته به فلورسنت در مطالعات هيدروليك محيط زيست براي اولين بار در ايران و در دانشگاه صنعتي نوشيرواني بابل توسعه داده شده است، معرفي مي گردد. در اين سامانه با تابانيدن نور ليزر به جت حاوي فلورسنت و تحريك آن، نسبت به آشكارسازي جريان و ثبت تصاوير آزمايش در طول موج خاصي از نور نارنجي اقدام مي گردد. علاوه بر تصوير سازي دو بعدي، اين سيستم به نحو خاصي طراحي شده است كه امكان ثبت و تصويرسازي سه بعدي آزمايشات را نيز داشته باشد. به كمك اين سامانه همچنين با تصويربرداري سريع با نرخ HZ 100 از آزمايشات، تحليل فركانسي جريانهاي آشفته، توسعه طيف انرژي آشفتگي و محاسبه پروفيل شدت و قدرت آشفتگي امكان پذير مي باشد

Introduction The experimental modeling and laboratory observation is probably the first step in the recognition of the flow complicated behavior in fluid mechanics. Since long time ago, various methods have developed for the measurement of the flow parameters. These methods are based on the illumination and inscription of flow variables in different conditions. Facilities and equipment were temperature and conductivity probes for scalar quantities and Hot-wire anemometers and Acoustic Doppler Velocimeters for velocity measurement as a vector variable in each point. Such equipment will cause disturbances in the flow as they are intrusive into the body of the ambient water. The measurements are point-based and data sampling needs too many probes for each test. Therefore, these probes do not appropriate for data sampling in many applications of the experimental fluid mechanics, especially in small scales. With recent progress in digital technologies, there are various methods have newly developed for the inspection of concentration and velocity field that are non-intrusive. These methods are more based on flow simulation in the transparent chamber, flow illumination with laser and fluorescent or small particles and filming the flow with high accuracy for later visual processing. Methodology In this paper, the capabilities of the three-dimensional laser scanning system are exhibited which is developed for first time in Iran at Babol Noshirvani University of Technology (BNUT). It includes a water tank, pomp, the three-dimensional laser scanning system, high-speed camera, and data processing apparatus that all located in the darkroom. The optical system consists of two fast scanning mirrors that drive the beam from an argon-ion laser through the flow in a programmed pattern. The system is controlled by a computer for overall timing control, and image capture. Having added an infinitesimal quantity of a fluorescent dye (Rhodamine 6G, Sigma-Aldrich, St. Louis, Missouri), the discharged effluent would be fluoresced under the impression of the laser. So due to the function of laser beam, the jet of fluorescent illuminated and recorded in the wavelength of orange light. The orange filter is used to filter out all the scattered lights of the green laser to increase the contrast, and accordingly quality of the images. The apparatus is set in a glass-made tank with length, width, and height, respectively. A charge-coupled device (CCD) camera with the resolution of x pixels was successively capturing the reflected light in the separated illustrations, at approximately 100 frames per second. Each captured illustration had to be modified for laser attenuation and sensor response at each pixel by using clear and dyed water. Having used image processing techniques in a software coded in C#, subsequently, the stream of images for unsteady flow and also time-averaged results were obtained. So the images are processed by a specially written computer program NITLIF, which is a new version of TFLOOK that previously was developed by Tian and Roberts (2003) at Georgia Institute of Technology. This software through lengthy computational procedures that explained by Tian and Roberts (2003) computes concentration pixel-by-pixel after a complicated calibration process. The images then turn into a real scale of position, time and concentration for every single frame in Tecplot. The program eventually can time-averaged the frames and placed them next to each other to form a two dimensional or three-dimensional configuration of flow dynamic. The accuracy of the dilution measurements is computed . It should be noticed that this system had been originally incepted by Tian and Roberts (2003) and the one that developed here is the new version of it that upgraded for temporal analysis and space-time evolution of the concentration field. Results and discussion The system is designed in a way that can record and visualize the three-dimensional configuration of the flow. Due to fast recording of the experiments with our high-speed camera with the frequency of 100 Hz, this apparatus is appropriately able to physically analyze the turbulence of the flow, turbulence energy spectrum and intensity and strength profile of the flow. The turbulence is a fluid motion that characterized by chaotic changes in flow variables. Getting 100 Hz data from each point in this system makes us be able for the frequency analysis of flow turbulent properties. Conclusion As a demonstration, the results of our observation for an inclined dense jet are exhibited. Th temporally-averaged intensity along vertical cut and energy spectrum are plotted at jet maximum height together with the instantaneous and time-averaged 2D and 3D configurations of the flow. Turbulence kinetic energy spectrums are well fitted with the power law of Kolmogorov theory for the inertial subrange. The time-averaged intensity distribution shows that for the location of maximum high, eddies are always present in centerline which shows the dominance of jet-like behavior in this point.