پديد آورندگان :
احساني، مجيد دانشگاه صنعتي اصفهان - دانشكده مهندسي آب و سازه هاي هيدروليكي , بهنام طلب، احسان دانشگاه حكيم سبزواري - دانشكده فني و مهندسي - گروه مهندسي عمران , صالحي نيشابوري، علي اكبر دانشگاه تربيت مدرس تهران - دانشكده مهندسي عمران و محيط زيست
كليدواژه :
سرريز نيلوفري , مدلسازي عددي , كاويتاسيون , شعاع زانويي , Flow3D
چكيده فارسي :
سرريز نيلوفري شامل سه بخش اصلي تاج دايره اي، شفت قائم همراه با يك زانويي معروف به گلوگاه و مجراي تقريباً افقي است. در بخش زانويي سرريزهاي نيلوفري انتظار ميرود بدليل وجود جريان با سرعت بالا كه منجر به جداشدگي و افت فشار ناشي از زبري و انحناء ميشود، كاويتاسيون اتفاق بيفتد. اين موضوع باعث شده است اين سازهها يكي از مستعدترين سرريزها براي وقوع كاويتاسيون باشند. در پژوهش حاضر امكانپذيري وقوع كاويتاسيون در زانويي سرريز نيلوفري با مدلسازي عددي با استفاده از نرمافزار Flow3D مورد بررسي قرار گرفت. مقايسه نتايج مدل عددي با نتايج مدل فيزيكي كه در مركز تحقيقات آب در ايران ساخته شده نشان داد كه تطابق مطلوبي بين مدل عددي و مدل فيزيكي وجود دارد. اثر شعاع زانويي بر نتايج فشار در بدنه سرريز نيلوفري در دبيهاي مختلف و احتمال وقوع كاويتاسيون با استفاده از شاخص كاويتاسيون در بخش ديگري از اين تحقيق مورد بررسي و تحليل قرار گرفت. نتايج اين تحقيق نشان داد تغيير در شعاع زانويي با توجه به تأثير آن بر انحناءِ خطوط جريان، اثر قابل توجهي بر بهبود يا افزايش خطر كاويتاسيون خواهد گذاشت. نكته قابل توجه اين است كه تغيير در شعاع زانويي در هر يك از حالتهاي رژيم جريان آزاد و رژيم جريان مستغرق داراي رفتارهاي متفاوتي است. در رژيم جريان آزاد، افزايش شعاع زانويي خطر كاويتاسيون را تشديد ولي در رژيم جريان مستغرق يا پر بودن مقطع زانويي، افزايش شعاع زانويي خطر كاويتاسيون را كاهش ميدهد.
چكيده لاتين :
Morning glory spillway is one of the most susceptible cases for the occurrence of the cavitation phenomena. At the entrance of the spillway, the junction location of the funnel entrance, the vertical shaft and within the elbow, cavitation is expected. In the present study, the flow characteristics included the inlet and outlet discharge and the velocity and pressure close to the elbow walls were calculated for Alborz dam spillway by using of Flow3D software and the results were compared with the experimental data. The experimental data has been presented by water research institude. The VOF method was used to simulate the flow surface and the RNG turbulence model was used for flow turbulence simulation. The comparison of the numerical model results and the experimental data showed the good agreement between them. Also, the occurrence probability of the cavitation in various parts of the morning glory spillway was investigated using of the cavitation index. The results showed, the maximum pressure head in different discharges and for different elbow radius, have been located in median of piezometer in the bottem of elbow. Therefore, the cavitation index for these areas is more than the other points. The points of the entry to the elbow and the end points are the most probable points for the probability of the occurrence of the cavitation due to geometric changes that cause the change of direction to flow. As a result, it is suggested that, if possible, the curvature changes in the entrance area from the vertical shaft to the elbow and also from the elbow to the horizontal tunnel gradually. In different discharge and different radius of the elbow, the cavitation index for bottem of the elbow will increase as a result of dynamic impacts on the bottem of the elbow, but the cavitation index, toward to the end of the curvature, will be lower due to the contraction end of the cross section. The results showed that radius change of the elbow due to considerable effect on the flow field, has the significant effect on the occurrence probability of the cavitation. The radius change of the elbow has the different results for the submerged flow and free surface regimes. In the free flow regime, by increasing the radius of the elbow, the occurrence probability of the cavitation will increase, and in the submerged flow regime, by increasing the radius of the elbow, the occurrence probability of the cavitation will decrease. It was also determined that in the free flow regime, the momentum is the dominant mechanism and in the submerged flow regime, the energy parameter is the dominant mechanism. Also, increasing the radius of the elbow will improve the performance of the elbow roof in terms of the cavitation phenomenon. Consequently, it is suggested, as usually morning glory spillway is designed for the free flow regime, this spillway to be designed with a smaller radius. Also, by increasing of the discharge in these spillways, the cavitation index increases and the occurrence probability of the cavitation decreases.
