The mean and instantaneous flow separation of two different three-dimensional asymmetric diffusers is analysed using the data of large-eddy simulations. The geometry of both diffusers under investigation is based on the experimental configuration of Cherry et al. (Int J Heat Fluid Flow 29(3):803–811, 2008). The two diffusers feature similar area ratios of \(\mathrm{AR}=4.8\) and \(\mathrm{AR}=4.5\) while exhibiting differing asymmetric expansion ratios of \(\mathrm{AER}=4.5\) or \(\mathrm{AER}=2.0\), respectively. The Reynolds number based on the averaged inlet velocity and height of the inlet duct is approximately \({\textit{Re}}=10{,}000\). The time-averaged flow in both diffusers in terms of streamwise velocity profiles or the size and location of the mean backflow region are validated using experimental data. In general good agreement of simulated results with the experimental data is found. Further quantification of the flow separation behaviour and unsteadiness using the backflow coefficient reveals the volume portion in which the instantaneous reversal flow evolves. This new approach investigates the cumulative fractional volume occupied by the instantaneous backflow throughout the simulation, a power density spectra analysis of their time series reveals the periodicity of the growth and reduction phases of the flow separation within the diffusers. The dominating turbulent events responsible for the formation of the energy-containing motions including ejection and sweep are examined using the quadrant analysis at various locations. Finally, isourfaces of the Q-criterion visualise the instantaneous flow and the origin and fate of coherent structures in both diffusers.

使用大涡模拟的数据分析了两个不同的三维非对称扩散器的平均和瞬时流动分离。所研究的两个扩散器的几何形状均基于 Cherry 等人的实验配置。(Int J Heat Fluid Flow 29(3):803–811, 2008)。这两个扩散器具有\(\mathrm{AR}=4.8\)和\(\mathrm{AR}=4.5\) 的相似面积比，同时表现出不同的不对称膨胀比\(\mathrm{AER}=4.5\)或\(\mathrm{AER}=2.0\)，分别。基于平均入口速度和入口管道高度的雷诺数约为\({\textit{Re}}=10{,}000\). 使用实验数据验证了两个扩散器中在流向速度剖面或平均回流区域的大小和位置方面的时间平均流量。总的来说，模拟结果与实验数据吻合良好。使用回流系数进一步量化流动分离行为和不稳定性揭示了瞬时反向流动演变的体积部分。这种新方法研究了整个模拟过程中瞬时回流占据的累积分数体积，对其时间序列的功率密度谱分析揭示了扩散器内流动分离的增长和减少阶段的周期性。使用象限分析在不同位置检查负责形成包含能量的运动（包括喷射和扫掠）的主要湍流事件。最后，Q 标准的等面可视化两个扩散器中的瞬时流动和相干结构的起源和命运。