Toward a wall-stress model for LES over rough boundaries ; contributions to experimental and simulation technology

为了对壁面湍流模拟研究做出贡献，本项目开发了相关的测量技术，提出了一种混合LES-RANS壁面应力建模技术和一种新的模拟高浓度分散颗粒湍流的虚拟域方法。在测量技术方面，我们开发了用于开放水道的立体PIV（粒子图像测速）。首先，我们指出了相机标定的一些简单而新颖的方面。接下来，我们提出了一种新的图像处理技术来直接测量墙壁上的剪切速率，既可以从单个摄像机看到，也可以从两个摄像机视图中重建三维图像。这种“PIV/界面梯度”技术成功地应用于正弦壁的实验图像。这些基本贡献将有助于将（立体）PIV应用于可能接近复杂边界的区域。为了能够对沉积物床附近固液两相流动进行数值模拟，我们扩展了Kajishima的虚拟域技术来处理高颗粒浓度的情况。对“关闭”和“打开”旋转鼓进行的测试，部分填充了玻璃珠，在床角和其他重要参数上取得了很好的一致。在这些测试之后，我们考虑了一个包含1500个球形珠子的双周期“最小”湍流通道。调整参数使屈服粒子雷诺数在15左右，无量纲剪切应力（Shields参数）在0.06 ~ 1.1范围内。在这个范围内，沉积物通量的结果与Meyer-Peter Mueller方程的预测在20%以内一致。为了解决与复杂形状边界上的LES相关的尚未解决的理论问题，PI还根据附近的涡旋平流导出了壁面“斑块”上的压力阻力方程。

To contribute to research on simulation of wall turbulence, the present research project has developed relevant measurement technology, and has proposed a hybrid LES-RANS wall-stress modeling technique and a new fictitious-domain method for simulating turbulent flow with highly concentrated dispersed particles.Concerning measurement technique, we have developed Stereo PIV (Particle Image Velocimetry) as applied to an open water-channel. First, we pointed out some simple but new aspects of camera calibration. Next, we proposed a new image-processing Technique to directly measure the rate of shear at a wall, both as seen by a single camera, and as reconstructed in 3D from two camera views. This "PIV/Interface Gradiometry" technique was successfully applied to experimental images from a sinusoidal wall. Such basic contributions will facilitate application of (stereo) PIV to regions near possibly complex boundaries.To allow numerical simulation of solid-liquid two-phase flow near a sediment bed, we extended Kajishima's fictitious-domain technique to handle cases of high particle concentration. Tests on "closed" and "open" rotating drums, partially filled with glass beads, gave good agreements on bed angles and other important parameters. Following these tests, we considered a biperiodic "minimal" turbulent channel containing 1500 spherical beads. Parameters were adjusted to yield particle Reynolds number around 15, and nondimensional shear stress (Shields parameter) in the range 0.06 1.1. Over this range, results for sediment flux agreed to within 20% with the prediction of the Meyer-Peter Mueller equation.To contribute to the unresolved theoretical issues related to LES over a boundary of complex shape, the PI has also derived an equation for the pressure drag on a "patch" of wall in terms of the advection of vorticity nearby.