شبيه‌سازي آبشستگي و ميدان جريان در اطراف دو پايه‌ي كنار يكديگر با استفاده از مدل عددي SSIIM

Numerical simulation of scour and flow field around side by side piers using SSIIM

آب‌شستگي پيرامون پايه‌‌هاي پلِ در معرض جريان امري اجتناب‌ناپذير است. برآورد عمق آب‌شستگي و فهم الگوي جريان اطراف پايه مي‌تواند كمك زيادي به طراحي ايمن پايه كند. در اين مطالعه از مدل عددي SSIIM به عنوان يك مدل ديناميك محاسباتي سيالات (CFD) براي مدل‌سازي همزمان جريان و رسوب در اطراف گروه‌شمع در معرض جريان استفاده شده است. از داده‌ها و نتايج آزمايشگاهي مربوط به دو پايه‌ي كنار يكديگر به عنوان يك حالت خاص گروه‌شمع براي واسنجي و شناخت بهتر اين مدل استفاده شده است. در اين مدل معادلات سه‌بعدي ناويراستوكس براي جريان به همراه مدل‌هاي آشفتگي kε و kω حل شده و از خروجي‌هاي محاسبات جريان براي حل معادلات انتقال رسوب استفاده شده است. با بررسي الگوي آب‌شستگي دو پايه و مقايسه‌ي آن با مقادبر آزمايشگاهي مشخص شد كه مدل عددي SSIIM، محاسبات جريان رو به پايين در جلوي پايه‌ها را با دقت قابل قبولي انجام مي‌دهد اما اثر گردابه‌هاي دنباله‌اي در پشت پايه‌ها و اثر تداخل گردابه‌هاي نعل‌اسبي بين پايه‌ها را بيش از واقعيت در نظر گرفته و مقادير عمق‌هاي آب‌شستگي در اين نواحي بيش از مقادير واقعي است. الگوي آب‌شستگي حاصل از به كارگيري مدل آشفتگي kε دقت بسيار بهتري نسبت به الگوي آب‌شستگي حاصل از به كارگيري مدل kω در پيرامون دو پايه دارد. البته لازم به ذكر است به دليل پيچيدگي محاسبات جريان و رسوب و ساده‌سازي‌هاي انجام شده براي مدل‌سازي، نتايج بدون اشكال نيست.

Scour around pier in the flow is an Inevitable issue. Estimation of scour depth and understanding the flow field around pier would help us to design with safer factor. The most important factor of scour around pier is changing of streamelines that leads to a system so called local scour. It consist of two vortices: horseshoe vortices and wake vortex. The obstacle creates downflow jet in front of pier that collide with bed sediments and carry them to the downstream, making the horseshoe vortice. The wake vortex is caused by splitting the streamelines and formation of low pressure flow field region and absorption of flow in rear of pier and pick up the bsd sdiments in this district. In this study we used the numerical model SSIIM as a CFD model to simulate flow and scour pattern Simultaneously around a group piers. This model can be used in hydraulic and environment engineering and has the ability of sediment transport calculation in bed transient movement with temporal dependent as the most important advantage in compare with the other CFD models. The verification of this model was implemented by data and results reported for side by side piers examinations as one ofe the group categorize. In this model we considered the kε and kω seperately as a turbulence model to solve the eddy viscousity of 3D NavierStokes flow equations and use their outputs as inputs of sediment transition equations, we used PowerLaw scheme as one of the descritization method of FirstOrder upstreame scheme to solve the flow and sediment equations on the grids. The pressure term of NavierStokes equations in cells was calculated by SIMPLE algorithm which is the FirstOrder upstreame scheme too. Also by changing the G⁄D distant ratio on the other simulation runs, we generated the diagrams with comparative situation with experimental diagrams.Results in the last time of simulation showed there is much more value of horizontal and vertical velocity between the piers than the other sides. It was 57% of final maximum scour depth in first hour of calculation. Similarly to velocity, The final scour patterns showed there is more scour depth counters between the piers. In details it was deriven that the scour depth pattern was symmetric in early time of calculation, but with time passing it appeared more in the region of between the piers. Although Numerical results show the SSIIM model have calculated the erosion depth in front of piers with high accuracy resulted from good calculation of downflow, comparisons between model results and data show the scour depth pattern that the model calculated the wake vortices behind the piers and Interference the horseshoe vortex between the piers with overestimate value and there are deeper countors of scour depth than experiment diagram. Also the RMS index of scour depth has been calculated in the grid and it represented the values of 0.0353 for kε model and 0.0899 for kω model. Therefore, the kε turbulence model resulted better scour depth pattern calculated in compare with kω turbulece model.