بررسي آزمايشگاهي خصوصيات پرش هيدروليكي در حوضچه هاي آرامش همگرا

Experimental Investigation of Hydraulic Jumps Characteristics in Stilling Basins with Convergent Walls

همگرايي ديواره‌هاي حوضچه آرامش يكي از راهكارهاي بهبود پرش هيدروليكي از نظر افزايش عمق پاياب و استهلاك انرژي در پايين-دست سرريزهاي سدهاي بزرگ مي‌باشد. در تحقيق حاضر تشكيل پرش هيدروليكي در حوضچه‌هاي آرامش همگرا به‌ازاي سه هندسه متفاوت و اعداد فرود اوليه 3.17 , 4.46 مورد مطالعه آزمايشگاهي قرار گرفت. عمق‌هاي مزدوج و سرعت پرش در محور مركزي حوضچه‌هاي آرامش همگرا اندازه‌گيري شد. علاوه بر آن، بيشينه ارتفاع امواج ضربه‌اي توليد شده در مقاطع همگرا اندازه‌گيري شد. مقادير اندازه‌گيري شده عمق‌هاي مزدوج نسبي و استهلاك انرژي پرش با نتايج حاصل از روابط تحليلي ارائه شده توسط ساير محققين مورد مقايسه قرار گرفت. ميانگين خطاي نسبي محاسبه پارامترهاي مذكور به‌ترتيب 9.75% , 17.15%. بدست آمد. نتايج نشان داد كه به‌ازاي عدد فرود يكسان، افزايش زاويه همگرايي باعث كاهش عمق‌هاي مزدوج نسبي و طول پرش و افزايش استهلاك انرژي مي‌شود. اما از سوي ديگر افزايش زاويه همگرايي با تشديد امواج ضربه‌اي و در نتيجه اختلال در ايجاد پرش پايدار همراه بود. به‌ازاي زاويه همگرايي يكسان، اثر انحناي ديواره حوضچه نيز بررسي شد و با توجه به تأثير ناچيز آن بر بهبود خصوصيات هيدروليكي پرش، از نظر اقتصادي مقرون به صرفه نمي‌باشد. نتايج تحقيق حاضر براي مهندسين طراح مي‌تواند بسيار مفيد باشد.

Walls convergence of stilling basins is one of the ways to improve hydraulic jump to increase tailwater depth and energy dissipation at downstream of spillways of high head dams. On the other hand, occurrence of hydraulic jump in converged sections is accompanied with formation of shock waves. Technically, production and development of the mentioned waves are undesirable due to amplification of mixture of water and air and resulting, disturbance outbreak on occurrence of stable hydraulic jump. Many studies have been conducted on the characteristics of hydraulic jump over gradually expanding cross sections, but comparatively few have been carried out on basins with convergent wall. In this research, occurrence of hydraulic jump in stilling basin with convergent wall was studied using experimental model for three different geometry and initial Froude number equal to 3.17 and 4.46. Experiments were conducted in a flume with a length of 6m, width of 1m and depth of 0.7m. Angels of convergence (7.7º and 19.5º) and type of stilling basins walls (straight and curved) were intended as geometric variables. In all experiments, widths of upstream and downstream channels were considered 80 and 40 cm, respectively (contraction ratio=0.5). The flow discharges were measured by an ultrasonic flow-meter having the accuracy of 0.02 lit/s. Values of instantaneous velocity were measured in 10 vertical sections in centerline of the convergent stilling basins using an electromagnetic 2-D velocity meter having the accuracy of 0.5 cm/s. Maximum height of produced shock waves in the contraction sections and conjugate depths of hydraulic jump were measured by a point gauge having the accuracy of 0.1 mm. The measured values of conjugate depths ratio and energy dissipation were compared with the obtained results of analytical equations presented by Sturm (1985) and Montes and Chanson (1998). The average relative errors of calculation of the mentioned parameters were respectively achieved 9.75% and 17.15%. It should be mentioned that the equations tended to underestimate the conjugate depths ratio and energy dissipation values. The velocity and turbulence intensity profiles were demonstrated and analyzed based on the mean values of instantaneous velocity and minor fluctuation of instantaneous velocity. The effects of convergence angle and curvature of basin wall were investigated on changes trend of the profiles. The results showed that changes of the convergence angle has a considerable impact on the conjugate depths ratio, energy dissipation and length of hydraulic jump. As for a constant Froude number, increasing of the convergence angle to approximately 12º was averagely accompanied with decrease of the conjugate depths ratio and hydraulic jump length to the 34.4% and 35.5%, respectively and increment of the energy dissipation to the 33.2%. It should be mentioned that increasing of the convergence angle caused intensification of the shock waves. Moreover, effects of curvature of basin wall were investigated for an equal convergence angle. As regards it had insignificant impact on improvement of hydraulic jump characteristics and difficulty of its implement, so it is not economical. The obtained results of the present research can be very useful for designer engineers.