پديد آورندگان :
خياط رستمي، بابك شركت آب منطقه اي اردبيل , حسن زاده، يوسف دانشگاه تبريز - دانشكده فني مهندسي عمران , خياط رستمي، سيامك دانشگاه محقق اردبيلي
كليدواژه :
برگشت آب , پرش هيدروليكي , FLOW- 3D , پل هاي تاريخي , هندسه دهانه
چكيده فارسي :
قرار گرفتن پل در آبراهه، مشخصات جريان را تغيير مي دهد. اغلب اين تغييرات از جزئيات هندسي پل ناشي مي شوند. در معماري پل هاي تاريخي، قوس هاي نيمدايره و جناغي به عنوان هندسه دهانه به كار رفته است. پل هاي تاريخي ميراث ارزشمندي هستند كه شناخت آنها از جنبه تداوم فرهنگي اهميت دارد. در اين تحقيق، تاثير سه هندسه دهانه مستطيلي، نيمدايره و جناغي بر پديده هاي برگشت آب و پرش هيدروليكي تحت شرايط جريان سطح آزاد و نيمه مستغرق با نرم افزار Flow-3D مدلسازي عددي گرديد و با نتايج آزمايشگاهي دانشگاه بيرمنگام صحت سنجي شد. تعيين موقعيت سطح آزاد با روش VOF و مدلسازي آشفتگي با روش K-ɛ دو معادله اي انجام پذيرفت. يافته ها نشان داد كه هندسه نيمدايره از نظر ميزان ايجاد خيزاب و تخريب ناشي از پرش هيدروليكي در بستر، بر هندسه جناغي برتري دارد. گذشته از عوامل سازه اي و روشهاي اجراء، يافته هاي تحقيق حاضر را مي توان يكي از علل هيدروليكي تحول هندسه دهانه از فرم جناغي به نيمدايره طي دوره صفويه تا قاجاريه دانست.
چكيده لاتين :
Establishing a bridge in a waterway changes flow characteristics and emerges phenomena like three dimensional flow, turbulent flow, variation of the shear stress, variation of the channel conveyance etc. Most of these changes derive from geometric details of bridge, of which the opening geometry is one of the most important factors. Circular and pointed (nested) arches have been used as opening geometry in historical bridges for centuries. Historical bridges are valuable heritage and protection of them is important because of cultural continuity and knowing native knowledge. In this research, the effect of three bridge opening geometry on backwater and hydraulic jump phenomena was numerically studied by Flow-3D software (version 10). Flow-3D is a famous commercial CFD package with explicit and implicit solvers and five turbulence models. It solves Reynolds-Averaged Navier Stokes (RANS) equations via Volume of Fluid (VOF) method as a Finite Difference formulation. In this research bridge models are located in the middle of an 18-meter-long compound channel. This compound channel resembles a waterway in a symmetric flood plain. Three solid geometries of two opening bridges with a simple rectangular pier, designed using AutoCAD, included semicircular, rectangular and pointed arch cross sections. Thereafter, they were converted into the STL format. After checking the errors of these solid geometries by pyADMesh tool, they were imported in Flow-3D. Unit system was set to SI and single water phase at 20ºC was set to fluid. Boundary conditions of the numerical models were discharge as inflow, water depth as outflow, wall as bed and channel sides and symmetry as free surface. Sensitivity analysis was conducted on two uniform cell sizes (1 and 2 cm) in longitudinal axis. Therewith non-uniform cell sizes were selected to discretize numerical domain into three mesh blocks. The characteristics of numerical domain were 400 cells at upstream, 100 cells at opening, 450 cells at downstream in X axis and 30 cells at flood plain1, 40 cells at waterway, 30 cells at flood plain2 in Y axis, and 26 cells at free surface, 30 and 40 cells at submerged levels of low flows and high flows in Z axis, respectively. Also sensitivity analysis was conducted on bed roughness which is an important parameter of free flows. Five low flows (sub-soffit) discharges (21, 24, 27, 30, and 35 lit/sec) and four high flows (super-soffit) discharges (40, 45, 50, and 60 lit/sec) were studied. The performance of Flow-3D numerical models was tested using experimental data obtained from the test series which were conducted at the Hydraulic Laboratory, Birmingham University, on two opening semicircular bridge model in compound channel. In those experiments with low flows, the flow didn’t touch the bridge opening soffit, so the discharges were extrapolated by means of a MATLAB code. Free surface was modeled by VOF and turbulence was modeled by two equations K-ɛ methods. The results indicate that Circular opening geometry produces less afflux and upstream flooding and less possibility of downstream bed destruction, so it has advantages on pointed arch geometry. Out of the structural reasons, all of these results may be considered as the hydraulic reason of evolution of the pointed arch to the semicircular geometry from the Safavid to the Qajar era.
