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

PI：Christensen、Kenneth/Best、James/Anderson、William 提案编号：1603211 / 1604155 / 1603254 提案研究的重点是探索沙丘在大尺度、沙漠环境或海底环境中的迁移。所提出的协作方法涉及实验、理论和模拟，以了解湍流下沙丘迁移的过程。这项工作的发现对于风沙沙丘迁移非常重要，这一问题对于预测和减轻美国西部和世界其他地方发生的荒漠化至关重要。了解和预测水下新月形沙丘的形态动力学对于水道管理和量化营养物和污染物的运输至关重要。同样，了解风成新月形沙丘的这些过程对于描述荒漠化过程和为数值天气预报提供信息也很重要。在大沙丘间距下，可以使用基于平均床剪切应力的沉积物传输模型来准确预测新月形运动学。然而，新月形植物通常出现在田间。由于沙丘迁移速率取决于大小，因此异质沙丘场将导致可变的床形间距，较小（较快）的沙丘接近较大（较慢）的沙丘。因此，当两个沙丘非常接近时，上游沙丘将产生不稳定的湍流尾流，从而塑造下游沙丘的形态，由于空间不均匀的湍流而引入显着的形态复杂性。当前最先进的模型无法捕捉到此类效应。 （这些方法模拟床形演化，但不解析上覆流；而是使用湍流的空间均匀表示作为流输入）。根据初步研究，并由于现有沙丘建模方法的缺陷，推测阐明相互作用的新月形沙丘形态动力学的“缺失环节”是沙丘间空间内产生的湍流的不稳定性质。建议开展合作研究工作，利用创新的测量协议和科学计算，并辅以自然界的观察。研究结果将改变对固定床沙丘沙丘与沙丘流场相互作用作为邻近度和体积比函数的理解，并将构成推进形态动力学模型的基础。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进入流动）针对目标沙丘配置。这些实验将验证大涡流模拟，从而能够探索更大的空间体积和更广泛的沙丘场参数，后者由现场观测提供信息。最终的任务将涉及将识别的湍流异质性纳入现有的形态动力学模型中以检验假设。 拟议的工作不仅将通过其对工程湍流的影响而对流体动力学产生影响，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.