شيب شكن قائم با تبديل همگرا و با جريان زير بحراني در بالا دست

چكيده انگليسي :

Vertical drop with a contraction transition and subcritical flow Upstream Mohammad Hemen Jannati h jannaty@cv iut ac ir 2009 03 07 Department of Civil Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiMohammad Reza Chamani mchamani@cc iut ac irMohammad Karim Beirami beirami@cc iut ac irAbstractA vertical drop structure or free over fall is a common feature in both natural and artificial channels Natural drops are formed by river s bed erosion while artificial drops are built in irrigation system toreduce the channel slope to designed slope Due to more energy dissipation drops are also applied instepped spillways More energy dissipation is obtained through mixing of the falling jet in the pooldownstream of the drop This will help to reduce the size of the energy dissipater that is generallyprovided at the toe of drops and stepped spillways Vertical drop structures and stepped spillways usually have the same upstream and downstream widths This condition leads to the formation of negative pressure under the nape of falling jet and to thecavitation of the brink depth of drop In this work contraction transitions are used at drops to eliminatethe negative pressure and to increase the energy loss Based on some previous investigators assumptions and some new assumptions five analyticalmodels are presented to predict end depth ratio relative energy loss relative rolling depth and relativedrop length Experiments on physical models of six different contracting transitions and two drops ofdifferent heights are conducted in the Hydraulics Lab Several runs of physical models have beenundertaken to measure required parameters The ratio of the critical depth to the drop height was in therange of 0 07 to 0 41 Time averaged point velocities at upstream and downstream sections threetransverse sections were measured with a prandtl tube A total of 58 tests were run for the two drops provided a range of discharge from 7 to 40 L s For the proposed model of the end depth ratio the continuity equation based on the Ferro s work 1999 was used The model of the end depth ratio Model I is based on simulation of the flow over asharp crested weir and predicts the flow discharge and Froude number over a free over fall To evaluate the energy loss White s Gill s and free jet models have been modified Themodified models of White Gill and free jet model were called Models II III and IV respectively InModel V the relative drop length was investigated using the relationship of Models II to IV Model II predicts the energy loss and the relative pool depth relatively well In this model thepresence of the pool was neglected in calculating the velocity of the falling jet The experimentalvalues of energy losses are mostly higher than the predicted ones A good agreement was observedbetween the theoretical and the experimental results for the relative pool depth Model III didn t estimate the energy losses well The experimental values of energy losses areconsiderably higher than the Model III s predicted values The experimental results of the relative pooldepth were very close to the results of the Model II Comparing with experimental results Model IV was more accurate in predicting the flowcharacteristics as compared with the other models As the ratio of the width of the brink drop to thewidth of channel increases the accuracy of the model predictions also increases The results of ModelV were in good agreement with the experimental results This research indicated that the energy lossesof the vertical drops with the contracted transitions are always higher than those of drops without thetransitions Key wordsHydraulic structures Vertical drop structures Contraction transitions Subcriticalflow Energy Losses

هيمن جنتي، محمد

شيب شكن قائم با تبديل همگرا و با جريان زير بحراني در بالا دست

سازه هيدروليكي , افت انرژي