Turbulence Driven Hyporheic Exchange Processes with Highly Permeable Channel Beds

湍流驱动的具有高渗透性河床的潜流交换过程本项目的主要目的是研究由偶发性湍流事件和相干流造成的时空异质性对具有渗透性河床的潜流交换的影响。最根本的挑战是评估连贯流动结构对潜流交换过程的影响，以及提供流体和溶质流入和流出潜流区的准确预测的能力。这将是第一个调查瞬时流场的研究，并将推进对水动力学和潜流带运输过程的了解，这对定量分析河流生态系统至关重要。所提出的数值方法，即大涡模拟，对于具有明显周期性成分的流动和/或湍流传输由存在于可渗透层或流动障碍物上的大尺度涡流动力学决定的流动，提供了令人印象深刻的预测精度。这项研究将是第一个明确地模拟构成隐隐带的单个粒子的研究。因此，控制流和输运过程被明确地表示出来，并且可以在微观细节上进行研究。将研究两种不同的情况，(a)流过平坦的砾石型床，其中隐性交换仅由床的质地产生和诱导的大规模湍流引起；(b)流过埋在砾石型床中的淹没和突出的砾石型障碍物，其中隐性交换由准稳定压力梯度和从流动障碍物脱落的旋涡导致的大规模连贯流动结构驱动。定量交换将通过在流体动力学研究中加入示踪剂研究来完成，这样可以直接计算质量和动量交换。通过对平均流场和瞬时流场的详细统计分析，特别是在界面处，例如使用象限分析、概率密度函数等，可以进一步证明相干结构对次隐交换的影响。所提出的研究结果是对湍流驱动的隐性交换的物理机制的全面理解，并将为不同条件下的汇率提供非常准确的量化。

TURBULENCE DRIVEN HYPORHEIC EXCHANGE PROCESSES WITH HIGHLY PERMEABLE CHANNEL BEDSThe main objective of this project is to investigate the effect of spatial and temporal heterogeneity created by episodic turbulent events and coherent flows on hyporheic exchange with permeable channel beds. The fundamental challenge is the evaluation of the effect of coherent flow structures on hyporheic exchange processes and the ability to provide accurate predictions of fluid and solute influxes into and outfluxes from the hyporheic zone. This will be the first study that investigates the instantaneous flow field and will advance knowledge of the hydrodynamics as well as transport processes of hyporheic zones, which are critical to quantitatively analyze stream eco-systems. The numerical method proposed, i.e. Large-Eddy Simulation, provides impressive predictive accuracy in flows which feature pronounced periodic components and/or in which turbulence transport is dictated by the dynamics of large scale eddies as present in flows over permeable beds or around flow obstacles. This study will be the first to model explicitly the individual particles that constitute the hyporheic zone. Consequently, governing flow and transport processes are represented explicitly and can be studied in microscopic detail.Two distinct scenarios will be investigated, (a) flow over a flat gravel-type bed where hyporheic exchange is solely induced by large-scale turbulence created and induced by the texture of the bed and (b) flow over submerged and emergent boulder-type obstruction buried in a gravel-type bed, where hyporheic exchange is driven by both quasi-steady pressure gradients and large-scale coherent flow structures as a consequence of vortex shedding from the flow obstruction. Quantification of the exchange will be accomplished by adding a tracer study to the hydrodynamic investigations, with which mass and momentum exchange can be calculated directly. Further evidence of the effect of coherent structures on hyporheic exchange will be provided by a detailed statistical analysis of the mean and instantaneous flow field especially at the interface for instance with quadrant analysis, probability density functions, etc. The outcome of the proposed research is a comprehensive understanding of the physical mechanisms of turbulence driven hyporheic exchange and will provide very accurate quantification of exchange rates for different conditions.