Experimental environmental hydraulics: entrainment and mixing in turbulent flows

日益严格的环境污染法规推动了环境水力学的研究。它关注的是污水的处置和意外泄漏到大气和河流、湖泊和海洋等地表水水体，以及预测污水随后在当地或区域范围内的扩散(长达100公里)。湍流混合，因此，稀释可最大限度地减少或减轻有害影响。湍流的显著特性之一是它能够以极快的速度混合和稀释矢量(流出速度)或标量(污染物)。预测或分析污染物在湍流(出水和/或接收水体)中的扩散会引起工程问题，因此需要了解湍流混合过程。长期的研究目标是更深入地了解湍流输运和混合过程，短期目标是调查以下四个主题。1)利用三维粒子跟踪测速仪(PTV)实验研究了环境湍流对射流动力学的影响及其卷吸机理。最近的工作表明，与普遍的假设相反，环境中的湍流会降低稀释率，这将扩展到研究不断变化的夹带机制。2)将利用PTV和湍流的统计分析对河流汇合处的抑制混合机制进行实验研究，并将使用地貌和地理经验来解释真实环境的结果。3)各向同性均匀湍流中的拉格朗日流动统计量将利用随机喷流阵列产生的近似各向同性均匀湍流中的三维PTV进行实验确定。这一结果将改进环境流的湍流弥散模型。4)考虑泥沙输运的自由水面槽流的SPH-ALE模拟将采用无网格拉格朗日质点方法SPH，该方法适用于边界可变形、界面移动和大变形的问题。SPH的ALE自适应具有更高的计算效率，并改进了边界条件的处理，这对于考虑泥沙输送的自由水面河流流动是很重要的。

The increasingly stringent environmental laws on pollution motivate the study of environmental hydraulics. Its concern is with the disposal and accidental spills of effluents to the atmosphere and to surface water bodies like rivers, lakes and oceans, and the prediction of the effluents' subsequent dispersion at the local or regional scale (up to 100 km). Turbulent mixing and, hence, dilution minimizes or mitigates the detrimental effects. One of the remarkable properties of turbulence is its capacity to mix and dilute either a vector (effluent velocity) or a scalar (pollutant) extremely rapidly. Prediction or analysis of pollutant dispersion in turbulent flows (effluent and/or receiving water body) gives rise to engineering problems for which an understanding of turbulent mixing processes is required. The long term research objective is a deeper understanding of turbulent transport and mixing processes, while the short term goals are to investigate the following four topics. 1) The effect of ambient turbulence on jet dynamics and the entrainment mechanisms will be studied using 3-D particle tracking velocimetry (PTV) experiments. Recent work indicating that turbulence in the ambient reduces dilution rates, contrary to the prevailing assumption, will be extended to examine changing entrainment mechanisms. 2) The suppressed mixing mechanisms at river confluences will be studied experimentally using PTV and statistical analysis of the turbulence and will use geomorphological and geographical experience to interpret the results for real environments. 3) Lagrangian flow statistics in isotropic homogeneous turbulence will be determined experimentally using 3-D PTV in an approximately isotropic homogeneous turbulent flow created using a random jet array. The results will improve turbulent dispersion models of environmental flows. 4) SPH-ALE modeling of free surface channel flows with sediment transport will use the mesh-free Lagrangian particle method, SPH, which is suited to problems with deformable boundaries, moving interfaces and large deformations. The ALE adaptation of SPH is more computationally efficient and improves the treatment of the boundary conditions which is important for free surface river flows with sediment transport.