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， WilliamProposal Number: 1603211 / 1604155 / 1603254拟研究的重点是探讨沙丘在大尺度、沙漠环境或海底环境下的迁移。采用实验、理论和模拟相结合的方法来理解湍流作用下沙丘迁移的过程。这项工作的发现可能对风沙沙丘迁移很重要，这个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barchan沙丘的形态动力学对于水道管理和营养物和污染物的量化运输至关重要。同样，了解风成barchan沙丘的这些过程对于表征沙漠化过程和为数值天气预报提供信息也很重要。在较大的沙丘间距下，利用基于平均床层剪应力的输沙模型可以准确地预测沙丘的运动学。然而，barchans通常发生在田地里。由于沙丘迁移速率与大小有关，非均质沙丘场将导致不同的床型间距，较小（较快）的沙丘接近较大（较慢）的沙丘。因此，当两个沙丘靠近时，上游沙丘会产生一个不稳定的湍流尾迹，它会塑造下游沙丘的形态，由于空间非均质湍流而引入显著的形态复杂性。目前最先进的模型没有捕捉到这种影响。（这些方法模拟了河床的演变，但不能解决上面的流动问题；相反，它们使用空间上均匀的湍流表示作为流动输入）。根据初步研究，以及现有沙丘模拟方法的不足，我们假设，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非定常性质。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。研究结果将转变对固定床沙丘流场相互作用的理解，将其作为接近度和体积比的函数，并将为推进形态动力学模型奠定基础。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访问流），用于目标沙丘配置。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套件得以探索，后者由实地观测提供信息。最终的任务将包括将湍流异质性识别到现有的形态动力学模型中，以检验假设。这项工作不仅会对流体动力学产生影响，还会对边界层气象学、地貌学和沉积学等其他学科产生影响。

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.