Collaborative Research: Coordinated Experiments and Simulations of Near-Surface Turbulent Flow over Barchan Dunes: Informing Models of Dune Migration and Interaction

建议编号：1603211/1604155/1603254拟议研究的重点是探索沙丘在大范围、沙漠环境或海底环境中的迁移。提出的协作方法包括实验、理论和模拟，以了解湍流下沙丘的迁移过程。这项工作的发现可能对风沙沙丘的迁移具有重要意义，这一问题对于预测和缓解发生在美国西部和世界其他地方的沙漠化至关重要。了解和预测水下沙丘的形态动力学对于管理水道和量化营养物质和污染物的传输至关重要。同样，了解风沙沙丘的这些过程对于确定荒漠化过程的特征和为数值天气预报提供信息也很重要。在大沙丘间距时，可以使用基于平均床面剪应力的泥沙输送模型来准确地预测沙丘运动学。然而，绿巨人通常出现在田野中。由于沙丘的迁移率与大小有关，不均匀的沙丘区域将导致不同的沙床间距，较小(较快)的沙丘接近较大(较慢)的沙丘。因此，当两个沙丘很接近时，上游的沙丘将产生非定常的湍流尾迹，这将塑造下游沙丘的形态，由于空间非均匀湍流，引入了显著的形态复杂性。目前最先进的模型并没有捕捉到这样的影响。(这些方法模拟床形演变，但不解析上覆流动；相反，它们使用湍流的空间均匀表示作为流动输入)。根据初步研究，以及现有沙丘模拟方法的不足，提出了一个假设，即阐明相互作用的沙丘地貌动力学的“缺失环节”是沙丘间空间产生的湍流的非定常性质。建议开展一项合作研究工作，利用创新的测量方案和科学计算，辅之以自然界的观测。这一结果将改变人们对固定床沙丘与沙丘相互作用的认识，并将为进一步的地形动力学模型奠定基础。对于目标沙丘配置，平面和体积PIV测量将在折射率匹配的环境中进行(允许前所未有的光学访问流动)。这些实验将验证大涡模拟，这将使更大的空间体积和更广泛的沙丘场参数被探索，后者由现场观测提供信息。最终的任务将包括将已确定的湍流非均质性纳入现有的地形动力学模型中，以检验该假说。拟议的工作不仅将通过其对工程湍流的影响在流体动力学方面产生影响，而且还将对包括边界层气象学、地貌学和沉积学在内的其他学科产生影响。

PI: Christensen, Kenneth / Best, James / Anderson, WilliamProposal Number: 1603211 / 1604155 / 1603254The focus of the proposed research is to explore the migration of sand dunes in large scale, desert environments or ocean floor environments. The proposed collaborative approach involves experiments, theory and simulations to understand the process of dune migration under turbulent flow. The findings of this work can be important for Aeolian dune migration, a problem that can be critical to predicting and mitigating desertification as it occurs in the Western US and in other places in the world.Understanding and predicting the morphodynamics of subaqueous barchan dunes is critical for management of waterways and quantifying transport of nutrients and pollutants. Similarly, understanding these processes for aeolian barchan dunes is important for characterizing desertification processes and for informing numerical weather prediction. At large dune spacings, barchan kinematics can be accurately predicted using sediment transport models predicated on mean bed shear stress. However, barchans typically occur in fields. Since dune migration rate is size dependent, a heterogeneous dune field will result in variable bedform spacing, with smaller (faster) dunes approaching larger (slower) dunes. Thus, when two dunes are in close proximity, the upstream one will produce an unsteady, turbulent wake that will sculpt the morphology of the downstream one, introducing significant morphological complexity owing to spatially-heterogeneous turbulence. Such effects are not captured by current state-of-the-art models. (These approaches simulate the bedform evolution but do not resolve the overlying flow; they instead use a spatially homogeneous representation of turbulence as the flow input). Informed by preliminary research, and compelled by deficiencies in existing dune modeling approaches, it is hypothesized that the "missing link" to elucidating the morphodynamics of interacting barchan dunes is the unsteady nature of the turbulence generated within the inter-dune space. It is proposed to pursue a collaborative research effort that leverages an innovative measurement protocol and scientific computing, complemented by observations in nature. The results will transform the understanding of dune-dune flow field interactions for fixed-bed dunes as a function of proximity and volumetric ratio and will form the basis for advancing morphodynamic models. Planar and volumetric PIV measurements will be conducted in a refractive-index-matched environment (allowing unprecedented optical access to the flow) for targeted dune configurations. These experiments will validate large-eddy simulations that will enable larger spatial volumes and a much broader suite of dune field parameters to be explored, with the latter informed by field observations. The culminating task will involve incorporating the turbulence heterogeneity identified into existing morphodynamic models to test the hypothesis. The proposed work will have an impact not only in fluid dynamics through its impact on engineering turbulent flows, but also to other disciplines including boundary-layer meteorology, geomorphology and sedimentology.