مطالعه عددي پارامترهاي موثر بر استهلاك انرژي سراشيب هاي بلوكي

Numerical Study on Energy Dissipation of Block Ramps

يكي از كارآمدترين انواع مستهلك كننده هاي انرژي در كانال هاي باز، سراشيب هاي بلوكي است، كه آنها را مي توان نوع خاصي از شيب شكن هاي همراه با بلوك اغتشاشي، ولي با ساختار طبيعي سنگدانه اي و غير سيماني، در نظر گرفت. در آزمايشگاه ها براي بررسي رفتار جريان روي اين سازه ها، مدل هاي فيزيكي مختلفي ساخته شده اند، كه با چشم پوشي از هزينه و زمان بر بودن آزمايش ها، مشكلات ناشي از تغييرات مقياس را نيز به همراه دارند. امروزه با استفاده از كدهاي ديناميك سيالات محاسباتي (CFD)، رفتار هيدروليكي سيال بررسي مي شود. در اين پژوهش به شبيه سازي جريان روي يكي از انواع سراشيب هاي بلوكي پرداخته شده و سازوكار استهلاك انرژي روي آن بررسي مي شود. براي اين منظور از نرم افزار FLOW-3D، مدل سطح آزاد VOF و مدل آشفتگي شبيه سازي گردابه هاي بزرگ (LES) استفاده شده است. نتايج حاكي از استهلاك انرژي بيشتر براي آشفتگي بيشتر جريان و افزايش آشفتگي با تشكيل جريان برگشتي است. در نتيجه، استهلاك انرژي به ترتيب در جريان غير مستغرق و سپس در حالت هاي تداخل دنباله ها و زبري منفرد از جريان مستغرق بيشتر است. براي افزايش بيشتر استهلاك انرژي بايد يا تا حد امكان از مساحت مقطع عرضي موثر كاست و يا با ثابت نگه داشتن مقطع عرضي موثر و حجم دبي عبوري و استغراق، عوامل زبري را كاهش داد.

One of the most efficient energy dissipatoion structures in open channel flow are block ramps; which have been considered and used most often recently because of the simple performance. This kind of energy dissipators can be assumed as a particular type of baffle block chutes, but with natural base materials like sands and gravels and without cement. In laboratories, several physical models have been built and examined regarding their efficiencies to dissipate energy of flow. These experimental studies are not only too expensive and time consuming, but also some of the hydraulic conditions like higher velocities could not be obtained in controlled conditions of laboratories. Moreover, there are always problems with changing scales in laboratory studies. Hence, using computational fluid dynamics (CFD) codes are to some extent more favorable to engineers with which fluid hydraulic behavior is more widely examined. This paper at first, reviews the classifications of these structures regarding their fabrication and roughness from various point of views, and then deals with the formation of some types of flow regimes with different volume flow rates on them. In continuation, to study the three dimensional flow field around block ramps, Flow-3D software is utilized and hence, the mechanism of energy dissipation on various flow regimes on these structures has been surveyed. In this research, incompressible fluid assumption is made and Large Eddy Simulation (LES) turbulence model is used. Since it was a shallow water condition, Volume of Fluid (VOF) method is used to calculate the free surface elevation. The numerical results showed the more energy dissipation with more turbulence and more turbulence with forming the recirculating flow and extending the distance of the reattachment point from the boulders. Consequently, energy dissipation is maximum for lower volume flow rate and higher slope of the ramp. Since in these two conditions, the flow involves with roughness of the ramp more and more; hence it gets easier for turbulent flow to happen. In other words, energy dissipation is also maximum for the non-submerged boulders. Flows for the cases in which boulders are submerged have different mechanism of energy dissipation. In these types of flows, energy dissipation is maximum for the wake-interface flow condition and isolated roughness flow condition, respevtively. Therefore, in order to enhance the energy dissipation in flow over block ramps, it is advised to reduce the effective cross section or reduce the submergence of scale roughness while maintaing the same effective cross section and the volume of fluid flowing over the structure. Changing the boulder ordering from rows to staggered would be another way to increase the energy dissipation.