The hydrodynamics of microbial landscapes

水在表面上的流动方式是环境中最难建模和准确预测的现象之一。理解河床上的鹅卵石和砾石之间的复杂流动尤其具有挑战性。在这种环境中，另一个被忽视的复杂层面是附着在表面的微生物（如藻类，统称为生物膜）对流动过程的影响。鉴于生物膜存在于所有自然环境和许多工程环境中，如废水系统，这代表了一个重大的知识差距。但是为什么我们需要了解流动-生物膜的相互作用呢？首先，河流生态学家认识到河床是多种物种的重要栖息地。河床上方的水流进入地下的方式在很大程度上决定了为这个栖息地提供多少氧气和营养物质。其次，渔业管理者早就明白，鲑鱼卵在河床中孵化的可能性将取决于它们产卵的砾石沉积物中是否有持续的含氧水供应。第三，了解生物膜如何影响废水系统内的水流输送或导致饮用水变色是水管理人员的一个重要考虑因素。因此，有很多非常重要的环境和工程背景需要对水如何运动进行详细的预测，然而，没有办法准确地测量或建模，考虑到生物膜可能对这些过程的影响。本提案的总体目标是开发生物膜强迫的微尺度水力响应的定量数值表示。这将通过使用开创性的新实验和数值方法来实现，以应对这一挑战。第一项任务是准确测量床层和床层本身的生物膜和孔隙空间内的流量。这一重大问题将通过在一系列包含完整生物膜培养物的小通道中使用实验室PIV（粒子成像测速）技术来克服。该技术的工作原理是在流动中植入微小的反射颗粒，并提供高强度的激光照明，然后一台摄像机记录下它们如何在生物膜周围的流动中以及在实验通道的孔空间内运动。使用一个特殊的处理器，这些数字图像可以转换成数字数据，准确地记录水流如何流经河床，然后进入河床。这样的测量在以前是不可能的。该项目的第二阶段是利用这个独特的数据集所带来的新认识来开发和测试一个3-D数值模型，该模型可用于进一步了解和探索生物膜在更广泛的环境和工程背景下对床上和床内流动过程的影响。这将使用一种经过特殊修改的计算流体动力学（CFD）模型来实现，该模型将被开发出来，以便它可以解释生活在更稳定的通道表面上的生物膜的动态特性（即它们随着水流移动的事实）。该项目将使用的测量和建模方法的进步代表了真正的突破，将解开从最具挑战性的环境之一中获取有用数据的固有问题。同时，可以广泛使用的数值模型的发展将确保这种新的理解可以被应用和适应，以满足各种现实世界的环境挑战，以及与废水工业等领域相关。

The way in which water flows across a surface is one of the most complex phenomena to model and predict accurately in the environment. Understanding complex flows moving into and from pebbles and gravels on river beds is especially challenging An additional level of complexity that has been largely overlooked in this environment is the effect that microorganisms such as algae, (collectively known as biofilms), attached to surfaces have on the flow processes. Given that biofilms occur in all natural environments and many engineered contexts such as wastewater systems this represents a significant knowledge gap. But why do we need to understand flow-biofilm interactions? Firstly, stream ecologists recognise that the bed of the river is an important habitat for a diverse range of species. The way flow from above the bed makes its way into the subsurface largely dictates how much oxygen and nutrients are supplied to this habitat. Secondly, fisheries managers have long understood that the probability of salmon eggs hatching in river beds will be dependent on a continuous supply of oxygenated water to the gravelly sediments in which they are laid. Thirdly, knowledge of how biofilms can affect the conveyance of flow within wastewater systems or lead to discolouration of potable waters is an important consideration for water managers. There are thus a broad range of highly important environmental and engineered contexts that require detailed predictions of how water moves, yet there is no way of measuring or modelling this accurately which takes into account the effect that biofilms may have to influence these processes. The overall aim of this proposal is to develop a quantitative numerical representation of micro-scale hydraulic response to biofilm forcing. This will be achieved by using pioneering new experimental and numerical approaches to meet this challenge. The first task is to accurately measure flow both right at the bed and within the biofilms and pore spaces of the bed themselves. This significant problem will be overcome by using laboratory PIV (particle imaging velocimetry) techniques in a range of small channels containing intact biofilm cultures. The technique works by seeding the flow with tiny reflective particles, and providing high intensity illumination from a laser, a camera then records how they move within the flow around the biofilms and within the pore spaces of the experimental channel. Using a special processor, these digital images can be turned into numerical data that accurately records how flow moves across and then into the river bed. Such measurements have never been possible before. The second phase of the project is to use the new understanding made possible by this unique dataset to develop and test a 3-D numerical model that can be used to further understand and explore the influence of biofilms on the flow processes at and within the bed over a much broader range of environmental and engineered contexts. This will be achieved using a specially modified computational fluid dynamics (CFD) model which will be developed so that it can account for the dynamic nature of the biofilms (i.e. the fact that they move with the flow) that live on the more stable channel surface.The advances in measurement and modelling approach that will be used in this project represent real breakthroughs that will unlock the inherent problem of gaining useful data from one of the most challenging of environments. Meanwhile, the development of a numerical model that can be widely used will ensure that this new understanding can be applied and adapted to meet a variety of real world environmental challenges as well as being of relevance to areas such as the wastewater industry.