بررسي الگوي جريان ثانويه در كانال روباز با شكل‌هاي مختلف زبري در كف با نرم‌افزار Flow-3D

تخمين سرعت متوسط عمقي و شدت آشفتگي براي شناسايي وجود جريان­هاي ثانويه، شكل و موقعيت آن از اهميت زيادي در علم هيدروليك و مهندسي رودخانه برخوردار است. جريان­هاي ثانويه تابع عواملي همچون شكل زبري بستر كانال، شيب كانال و تنش برشي مي­باشد. مطالعه حاضر به بررسي اثر شكل زبري بر الگوي جريان ثانويه با مدل­سازي عددي در نرم­افزارFLOW 3D با مدل آشفتگي RNG پرداخته است. در نتايج بدست آمده از بررسي­هاي سرعت متوسط، ميانگين خطاي حاصل براي حدفاصل زبري­هاي مثلثي 94/9% و براي روي تاج زبري 71/3% بود و در مورد سرعت برشي، ميانگين خطاي حاصل براي زبري مثلثي در سه مقطع در راستاي كانال 37/6% حاصل گرديد كه نشان از توافق خوب مدل عددي با مدل آزمايشگاهي مرجع است. شرايط جريان در كانال براي سه نوع شكل زبري (مستطيلي، ذوزنقه­اي با زاويه داخلي 80 درجه و ذوزنقه­اي با زاويه داخلي 55 درجه) كه بيشترين كاربرد را در سازه­هاي هيدروليكي دارا مي­باشند طراحي و مورد بررسي قرار گرفت. نتايج بررسي شدت آشفتگي، جريان­هاي ثانويه عرضي و انرژي جنبشي آشفتگي نشان از تاثير بيشتر زبري ذوزنقه­اي با زاويه داخلي 55 درجه نسبت به دو شكل ديگر زبري را نشان داد. ميزان اختلاف شدت آشفتگي در زبري ذوزنقه­اي با زاويه داخلي 55 درجه نسبت به زبري مثلثي 54/4% بود. محل تشكيل مركز هسته سلول­ها در عمق تقريبي2/0 متر شروع شده بود. انرژي جنبشي آشفتگي با هسته مركزي در بالاي سطح زبري با مقدار تقريبي 0002/0 براي تمام زبري­ها شروع شده، اما محل قرار گيري آن با توجه به هندسه زبري متفاوت مي­باشد.

It is important in Hydraulic and river engineering to estimate the mean velocity and turbulence intensity to identify the presence of secondary currents, its shape and position. The flow of channels consists of three conponents of velocity, one component in the direction of flow and two components in the transverse direction of the channel. Due to the heterogeneity of the velocity fluctuations, a series of vortex vortices in the channel section to be formed which is called secondary currents cells. The secondary currents are dependent on factors such as bed roughness, channel slope and shear stress. The present study investigates the effect of bed roughness form on the pattern of secondary currents with numerical modeling in Flow-3D software by using RNG turbulent model. This research has been carried out according to the data of the Negara laboratory model carried out at the Hydraulic Laboratory of Singapore National University. In the results obtained from the mean velocity profile, the mean error for triangular roughness trough was 9.94% and for roughness crest was 3.71%. in the case of shear velocity, the error for triangular roughness was obtained at three cross sections x=4, x=5 and x=6 respectively 6.58%, 6.86% and 5.67% which demonstrates the good fit of the numerical model results with the reference laboratory model. The flow conditions in the channel were designed and studied for three types (rectangular roughness, trapezoidal roughness with an internal angle of 80 degrees and trapezoidal roughness with an internal angle of 55 degrees) of roughness that are most used in hydraulic structures. The results of the study on the turbulence intensity, secondary currents and turbulence kinetic energy showed the effect of trapezoidal roughness with an internal angle of 55 degrees relative to the other two forms of roughness. The difference in the turbulence intensity in trapezoidal roughness with an internal angle of 55 degrees relative to a triangular roughness at a height of 0.1 meter from the channel bed was obtained at about 4.54%, which is affected by a sharp roughness. The location of the center of the contour nucleus begins in the depth of about 0.2 meters, due to channel size B/H=1.5 of less than 5, the channel is narrow and justifies the presence of vortices in the middle. On the other hand, the tendency of cells to the left and right walls of the canal is also affected by roughness geometry with also different from the deformation of the location of these cells. In trapezoidal roughness with an internal angle of 55 degrees, there is a tendency to both walls. The turbulence kinetic energy contour with the center of the nucleus begins at the roughness crest with an approximate value of 0.0002 for all roughness, but its location is different in relation to the roughness geometry. In fact, the turbulence kinetic energy is the effect of oscillating velocity components that the existence of external rotations is the effect of the redistribution of energy by velocity tensor, which is responsible for the formation of secondary current cells