Strong secondary flow results in substantial aerodynamic loss for highly-loaded turbine. Accurate prediction of the complicated flow structures proposes challenges to the widely used Reynolds Average Navier-Stokes (RANS) approach. This work employs the hybrid RANS/Large Eddy Simulation (LES) method to study the unsteady flows for a highly-loaded turbine blade, with both flat endwall and optimized contoured endwall. Evolution of the unsteady flows in the endwall region is analyzed, with emphasis on the loss generation mechanism. Results show that for the flat endwall, the horseshoe vortex system is highly unsteady and is a significant source of unsteadiness in both the passage and the wake region. It contributes to the unsteady passage vortex, also the earlier breakdown of the trailing edge shedding vortex. The Probability Density Function (PDF) histogram of velocity in the wake region is bimodal, implying the perturbations from two mechanisms. For the contoured endwall, the unsteady evolution of the horseshoe vortex is blocked, which results in significantly reduced unsteadiness, also the merge between the horseshoe vortex with the passage vortex is prevented. Effect of the flow unsteadiness on the loss generation is assessed based on the entropy generation rates contributed by the time-averaged flow and the fluctuations. The effect of unsteadiness is two-fold: unsteady perturbations trigger the vortex breakdown into small-scale structures and thus weakened wake velocity deficit and loss generation; however, in both the passage and the wake region the entropy generation by the fluctuations are remarkable. For the contoured endwall, due to the reduced flow unsteadiness, loss generation contributed by the fluctuations are much smaller compared to the flat endwall. The results highlight the importance of including the loss generation by the fluctuations, also a possible mechanism to reduce the secondary loss by attenuating the flow unsteadiness with the contoured endwall.

强二次流导致高负载涡轮机的大量空气动力损失。对复杂流动结构的准确预测对广泛使用的雷诺平均纳维-斯托克斯 (RANS) 方法提出了挑战。这项工作采用混合 RANS/大涡模拟 (LES) 方法来研究具有平坦端壁和优化轮廓端壁的高负载涡轮叶片的非定常流动。分析了端壁区域非定常流动的演变，重点是损失产生机制。结果表明，对于平端壁，马蹄涡系统高度不稳定，是通道和尾流区域不稳定的重要来源。它有助于不稳定的通道涡流，也有助于后缘脱落涡流的较早崩溃。尾流区域速度的概率密度函数 (PDF) 直方图是双峰的，这意味着来自两种机制的扰动。对于异形端壁，阻断了马蹄涡的非定常演化，从而显着降低了不稳定性，也防止了马蹄涡与通道涡的融合。流动不稳定对损失产生的影响是根据时间平均流动和波动贡献的熵产生率来评估的。不稳定的影响有两个：不稳定的扰动触发涡旋分解成小尺度结构，从而减弱尾流速度赤字和损失产生；然而，在通道和尾流区域，波动产生的熵是显着的。对于轮廓端壁，由于流动不稳定性降低，与平坦端壁相比，波动造成的损失产生要小得多。结果强调了包括波动产生的损失的重要性，这也是通过用轮廓端壁衰减流动不稳定性来减少二次损失的可能机制。