Delayed detached eddy simulation (DDES) has been proved to be suitable for the numerical simulation of massively separated flow. Whereas, there are still some drawbacks in the treatment of gray area, which is the transition zone between Reynolds-Averaged Navier–Stokes (RANS) and large eddy simulation (LES). In this paper, a modified DDES with shear layer adapted (SLA) subgrid length scale was employed, which takes advantage of the peculiarities of flow and grid topology in the initial shear layer, it can rapidly destabilize the separated shear layer and accelerate RANS to LES transition. To evaluate the performance of modified DDES versus conventional DDES, two typical separated flows are considered, they are the flow over backward-facing step with fixed geometry-induced separation and wall-mounted hump with non-fixed pressure-induced separation. The fifth-order Adaptive Dissipative Compact Scheme (ADCS) is also formulated to reduce numerical dissipation in grey area. The results show that the gray area can be slightly alleviated by ADCS, but it cannot be effectively mitigated with conventional DDES model. The visualizations of instantaneous flow reveal that the modified DDES is capable of unlocking the Kelvin–Helmholtz instability rapidly and accelerating the transition to resolved turbulence in the initial shear layer, which is strongly delayed by conventional DDES. The time-averaged pressure and skin friction coefficients show the mitigation of delayed transition as well. The distributions of mean velocity and Reynolds stress of modified DDES exhibit a rapid development in the initial shear layer; thus, more turbulent structures can be distinguished and the accuracy of results can be enhanced.

事实证明，延迟分离涡模拟（DDES）适用于大分离流的数值模拟。然而，在灰色区域的处理上仍然存在一些缺点，该区域是雷诺平均纳维斯托克斯（RANS）和大涡模拟（LES）之间的过渡区域。本文采用了一种具有剪切层适应（SLA）子网格长度尺度的改进DDES，该方法利用了初始剪切层中流动和网格拓扑的特殊性，可以快速使分离的剪切层不稳定并加速RANS到LES过渡。为了评估改进的DDES与常规DDES的性能，考虑了两个典型的分离流，它们是具有固定的几何形状引起的分离的向后步骤上的流动和具有非固定的压力引起的分离的壁挂式驼峰。还制定了五阶自适应耗散紧凑方案（ADCS），以减少灰色区域的数值耗散。结果表明，ADCS可以稍微缓解灰色区域，但常规DDES模型无法有效缓解。瞬时流动的可视化结果显示，改进的DDES能够迅速释放开尔文-亥姆霍兹不稳定性，并加速初始剪切层到已解决湍流的过渡，而传统DDES则大大延迟了该过程。时间平均压力和皮肤摩擦系数也显示了延迟过渡的缓解。改性DDES的平均速度和雷诺应力分布在初始剪切层中快速发育; 因此，