Research On Relationship Between Form Drag And The Dead Zone Of Flow Around A Square Pier

The flow resistance of the pier has always been the focus of fluid mechanics,which is of great significance.Square as a important section of pier,getting more extensive application.Based on the physical model test and numerical simulation,the flow characteristics around the square pier,the mechanism of the form drag of the square pier and its relationship with the dead zone were analyzed.The research can enrich the existing theory of flow resistance,and provide some reference for the future design and construction of hydraulic structures.First of all,we studied the flow around a square pier by the physical model test based on Particle Image Velocimetry(PIV)and pressure sensors.The change law of the free surface shape around the square pier was studied by the image recognition technique.We obtained the change laws of the maximum backwater value before the pier and the maximum drop value of the pier side with the Reynolds number,and then we studied the characteristic of water level along x axis at different Re.The three-dimensional mean-flow structure near the square pier was explored by coupling the mean flow field of the vertical faces of middle axle(y/D=0)at differentRe and the horizontal planes at differentRe and water depth(z/h),the influence of Re and z/h on the flow characteristics around a square pier was simultaneously acquired.The influence of Re on the vorticity field around the square pier was obtained by discussing the time-averaged vorticity distributions of the intermediate depth of water(z/h=0.5)planes and the vertical faces of middle axle at different Re.Combining with theoretical analysis,we found that there is a good quadratic function relationship between the dimensionless time-average area of the dead zone,the dimensionless time-average volume of the dead zone,the dimensionless time-average from drag,the time-average coefficient of non-dimensional new resistance formula and Re by discussing the relationship between the form drag and the dead zone of flow around the square pier under a two-dimensional plane(z/h=0.5)and a three-dimensional condition.In the larger current condition,there is a good linear relationship between the dimensionless time-average from drag and the dimensionless time-average size of the dead zone.Secondly,the flow around the square pier was studied by using the validated three-dimensional fine numerical model based on large eddy model.The VOF method was used to track the free liquid surface around the square pier,and the morphological characteristics of the free surface under different Re were acquired.The transient flow field near the square pier and the vertex shedding frequency were analyzed in detail,and we found that the Stroud number St?0.139 at Re=l5000,the same as the previous results.By analyzing the time-averaged flow field at the horizontal planes of the bedside(z/D=0.01)and the transverse faces of middle axle(x/D=0)near the square pier,the hydrodynamic characteristics near the square pier were further supplemented.Finally,the relationship between the flow resistance of the square pier and the volume of the dead zone was discussed based on the numerical model,and we found that there is a good quadratic function relationship between the dimensionless time-average volume of the dead zone,the dimensionless time-average from drag,the time-average coefficient of non-dimensional new resistance formula and Re.there is a good linear relationship between the dimensionless time-average from drag and the dimensionless time-average volume of the dead zone.This is similar to the physical model test results.The differences between the two research methods were subsequently analyzed.