Study On The Improvement Of The Bend Flow Pattern With Pyramid

Bend flow is a common flow pattern in nature and hydraulic structures.After entering the bend,the flow makes curve movement under the action of centrifugal force and gravity,the flow structurereadjusts,and the flow pattern is gradually deteriorating.The main adverse flow patterns include local scour of concave bank,superelevation of water surfaceand sedimentation of convex bank,etc.In order to improve the flow pattern of the bend,many engineering measures have been put forward to improve the flow pattern of the bend,including various types of groins,inclined sills,guide walls,guide wings,etc.The pyramid,the research object of this paper,is similar to groin and guide wing in some aspects,but they are fundamentally different.For example,the upstream face of the pyramid is parallel to the direction of the water flow,which has less effect on the flow resistance of the flow passage;the pyramid makes the water flow produce a transverse velocity from the concave bank to the convex bank,which can balance the flow between the two banks.The pyramid has been used in some practical projects,but there are a few researches on it.In order to provide reference for the engineering application,this paper studies the influence of the pyramid on the water surface line,flow velocity distribution,circulation flow and turbulence intensity of the bend by combining the numerical simulation and model test.The following research results are obtained:?1?The improvement effect of the pyramid on the water surface line of the bend is studied,and the influences of the bottom width and the position of the pyramid are considered.Comparing the two working conditions of setting the pyramid or not using the model test data,it is found that the superelevation of water surface at the downstream of the bend obviously decreases after setting thepyramid.Compared with that without pyramid,the water surface fluctuation increases,but the water surface fluctuation decreases with the increase of the distance.In general,setting pyramid can significantly improve the downstream water level of the bend.The fluctuation of the downstream water surface is greatly affected by the bottom width of the pyramid,and the larger the bottom width of the pyramid is,the larger the ratio of advancement and the fluctuation of the downstream water surface are.?2?The improvement effect of the pyramid on the velocity distribution in bend is studied,and the influences of the bottom width of the pyramid,the flow rate and the position of the pyramid are considered.With the numerical simulation method,the plane velocity distribution,the longitudinal velocity distribution and the transverse velocity distribution of the bend are analyzed.The results show that the setting of the pyramid in the bend has a significant improvement effect on the velocity distribution of the bend:when the pyramid is not set,the high-velocity area of the bend transfers from the convex bank to the concave bank at the top of the bend,and the high-velocity area maintains on the left bank at the downstream straight section,resulting in the severe erosion damage of the concave bank.After the pyramid is set,the high-velocity area of the bend returns to the axis.The distribution of the plane flow field in the bend is much sensitive to the bottom width of the pyramid,and if the bottom width is too large or too small,the improvement effect will be poor.?3?The improvement effect of the pyramid on the circulation flow in the bend is studied,and the influences of the bottom width of the pyramid and flow rate are considered.Setting the pyramid produces a large transverse velocity downstream from concave bank to convex bank,which destroys the original structure of the circulation flow in the bend and makes the downstream erosion and deposition of the bend return to equilibrium from"concave erosion and convex deposition".?4?The influence of the pyramid on the turbulence intensity of the bend is studied,and the influences of the bottom width of the pyramid and the flow rate are considered.The transverse and longitudinal turbulence intensity distributions of the bend are obtained by numerical simulation The results show that the influence of setting the pyramid on the turbulence intensity distribution of the bend mainly occurs near the downstream of the pyramid on the concave bank,and the area with larger turbulence intensity downstream of the pyramid is"saddle-shaped",which is mainly composed of the backflow area near the downstream of the pyramid and the hydraulic jump area downstream of the jet flow.?5?By analyzing the results of the numerical simulation control group,the empirical formulas of the flow velocity V₀ on the side of the pyramid,the bottom width of the bend b and the flow rate Q are obtained,and the velocity distribution formula of the section of the pyramid is fitted according to the velocity formula of the pyramid side.