凹槽长度对后台阶湍流特性影响研究

Study on effect of groove length on hydraulic characteristics of backward facing step flows

为探究上游凹槽长度对后台阶流动影响,采用数值模拟方法对雷诺数6500、9000下后台阶流动进行研究,得到不同凹槽长度下,凹槽内、后台阶流道内流速及湍动能分布和后台阶时均再附距离的变化规律。结果表明:(1)凹槽可以减小后台阶时均再附距离,但当凹槽长度大于等于7倍台阶高度时,时均再附距离受凹槽长度影响较小,并趋于定值,相比无凹槽情况,时均再附距离平均减小12%。(2)凹槽长度可以改变后台阶分离点断处湍动能分布,增大分离点处湍流度。当凹槽长度大于等于7倍台阶高度时,断面湍动能呈现出S型分布,极大值出现在0.3倍流道高度附近;当凹槽长度小于7时,湍动能分首先在0.05倍流道高度附近达到极大值,而后0.6倍流道高度附近达到极小值。(3)通过缩放尺度因子,得到后台阶流动近似再附距离,解释了湍流度对后台阶流场的作用机理。时均再附距离与近似再附距离有着良好的线性关系,湍流度增大促进自由剪切层的下沉,缩短剪切层再附到下游壁面的时间,进而缩短了时均再附距离。

A numerical simulation method is used to study the effect of groove length on backward-facing step flows at the Reynolds numbers of 6500 and 9000, focusing on the distributions of flow velocity and turbulent kinetic energy and the variations in time-average reattachment distance in the conditions of different groove lengths. The results show that(1) this distance is reduced by the groove, but it is less affected by groove length and tends to a constant value at the length no less than 7 times the step height.This constant distance is 12.0% shorter than that of no groove.(2) At the step section, the distribution of turbulent kinetic energy varies with groove length, and turbulence intensity is increased. With the groove length no less than 7 times the step height, turbulent kinetic energy features an S-shaped distribution, and it peaks at a position of roughly 0.3 times the section height above the step;with a shorter groove length,it peaks at roughly 0.1 times the section height first and then drops to the minimum at roughly 0.6 times the section height.(3) Using the scale factor, an approximate reattachment distance of backward steps,well correlated with the time-average reattachment distance, is calculated to explain the mechanism of turbulence affecting the flows. An increase in turbulence promotes the sinking of the free shear layer and shortens the time for the shear layer to reattach to the downstream wall, thus shortening the time-average reattachment distance.