شبيه سازي سه‌ بعدي حفره‌ي آب شستگي اطراف تكيه گاه پل با ديواره‌ي قايم با نرم افزار FLOW-3D

3-D Simulation of Scour Hole at Vertical-Wall Bridge Abutment

آب شستگي اطراف تكيه گاه پل ها يكي از عوامل اصلي شكست پل هاست و شكست پل ها خود ممكن است منجر به تلفات مالي و حتي جاني شود. به همين دليل پيش بيني عمق آب شستگي قبل از طراحي پل براي پيشگيري از شكست آن امري ضروري است. در اين نوشتار، با استفاده از نرم افزار FLOW -3D، كه يك نرم افزار قوي در مدل سازي جريان و انتقال رسوب است، آب شستگي اطراف تكيه گاه پل با ديواره‌ي قايم شبيه سازي شده است. از اين نرم افزار براي حل معادلات سه بعدي نوير- استوكس به روش حجم محدود استفاده شده است. مدل آشفتگي RNG براي مدل‌سازي ميدان جريان در اطراف تكيه گاه، محلي كه گردابه هاي نعل اسبي تشكيل شده و جريان آشفته غالب است، به كار برده شده است. مدل مورد استفاده، مقدار عمق ميانگين جريان و مقدار بيشينه‌ي عمق آب شستگي و محل وقوع آن را با دقت خوبي در مقايسه با نتايج آزمايشگاهي پيش بيني مي ‌كند؛ به طوري كه بيشينه‌ي عمق آب شستگي پيش بيني‌شده در شبيه‌سازي فقط 7% با مقدار مدل آزمايشگاهي تفاوت دارد.

Scour at bridge abutments is one of the most prevalent causes of bridge failure, which may result in traffic disturbance and even loss of life. Hence, predicting the depth of a scour hole before bridge design and construction to prevent failure is essential. In this paper, using a CFD model, scour at a vertical-wall bridge abutment was simulated. The model has been used to solve a three dimensional, transient, Navier-Stokes equation. A nonlinear RNG turbulence model was used for modeling of the flow field near the abutment, where horseshoe vortices are formed and turbulent flow is more dominant. The sediment scour model uses two concentration fields: suspended sediment and packed sediment. The suspended sediment advects and drifts with the fluid, due to the influence of the local pressure gradient. The packed sediment represents sediment that is bounded by neighboring sediment particles, and eroded when it becomes suspended sediment. Suspended sediment can become packed sediment if fluid conditions are such that the sediment drifts towards the packed bed more quickly than it is eroded away. According to the results, the horse shoe vortices developed upstream of the abutment, and wake vortices developed downstream of it, and negative velocities have been observed in laboratory results. Mesh sensitivity analysis has been done to find the best mesh for predicting flow and scour depth. The results showed that the larger mesh predicts scour depth better. However, if the mesh becomes very large, because of large errors in the developed flow depth, the scour depth may be predicted wrongly. The maximum scour depth in each three mesh analysis has been overestimated, but, due to the complexity of scour phenomena and the large number of parameter effects, its overestimation is acceptable. It was found that there is good quantitative and qualitative agreement between the experimental results and results of this study.