基于RANS/LES混合模型的90°方形弯管内流分析

Analysis of internal flow in 90° square section curved duct based on hybrid RANS/LES models

为了了解90°方形弯管内流速度、压力分布、流动结构等流场参数带来的影响,分别对2种RANS/LES混合模型DDES和SAS在离心力作用下的弯管流场做数值模拟,并将流动横截面速度分布和外弧面流向压力系数分布与试验结果进行对比分析.为此,进一步比较了2种模型的瞬时流动结构和湍动能分布.结果表明:DDES和SAS的速度分布曲线及外弧面压力系数变化趋势相近,但是DDES在近壁区速度曲线和外弧面壁面压力系数分布上更接近试验结果;根据壁面小涡的尺度分布,DDES模拟结果在贴壁和近壁处有多种尺度的小涡,而SAS结果仅在贴壁面处存在若干无规则小涡;DDES能模拟出理论上四涡交界处的低湍动能区,而SAS未能得到此现象.

When Reynold-averaged Navier-Stokes(RANS)method was applied to simulate flow fields under the action of centrifugal force,it failed to sufficient accuracy.Thus,hybrid RANS/LES(large eddy simulation)method was proposed to improve the accuracy,where RANS was for modelling flow near a wall and LES was for simulating flow behaviour far away from the wall.Here,two kinds of RANS/LES models,such as DDES and SAS,were selected to simulate the flow field in a 90°square section curved duct where the action of centrifugal force was dominant to fully understand effects of the force on flow field information such as flow velocity profile,pressure distribution and flow structure in the curved duct.The predicted circumferential velocity profiles in cross-sections and the longitudinal distributions of pressure coefficient on the concave surface were compared with the experimental results.Furthermore,the instantaneous flow structure and turbulent kinetic energy distributions were contrasted between two models.It is showed that the velocity profiles and pressure coefficient curves predicted with DDES is similar to those with SAS,however,the results near the wall with the former are even closer to the measurement than that with the latter.In terms of the scale distribution of small eddies near the wall,DDES can obtain multiscale small eddies close to the wall,while SAS can capture irre-gular small eddies adherent to the wall only.For the turbulent kinetic energy field,DDES can represent the low turbulent kinetic energy region at the intersection of four vortices predicted theoretically,but SAS fails to obtain this phenomenon.