Application of Liquid Coating to a High-speed Moving Surface

液体摩擦改进剂（LFM）应用于铁路轨道，将车轮与轨道之间通常较高的摩擦系数降低到适度的水平。适度的摩擦是足够的爬坡和制动，但与卓越的燃油经济性和减少轨道磨损和噪音。改进LFM的应用方式将降低系统维护成本，每年节省数千万美元的燃料，并减少相应的温室气体排放。虽然本文提出的研究是由LFM应用于轨道的具体技术问题驱动的，但它具有根本性的特征，可能对油漆涂层和液体撞击冷却等其他应用领域产生重大影响。LFM的一种应用方法是将非雾化的、自由表面的LFM射流从火车上引导到轨道上。在撞击点附近，射流呈放射状扩散，就像在静止表面上的射流撞击一样，形成薄层。在离撞击点稍远的地方，射流仍停留在表面，粘性效应导致射流向下游对流，形成整体的u形片层结构。碰撞过程的流体力学将采用先进的实验技术（PIV和LIF）进行研究。在边界层理论的基础上，结合自由表面模型，建立了片层的解析模型。LFM液体是非牛顿的，该理论将扩展到非牛顿撞击的情况，并预测喷射飞溅的开始。将LFM应用于轮轨界面的第二种方法是从固定位置将其渗透到轨道上，当火车经过时，它被车轮拾取。如果做得好，每个轮子骑在一个薄润滑层的LFM。我们已经开始通过实验和分析技术的结合来研究这种涂层过程。我们造了一个1/5比例的机车车轮。我们在LFM中加入微量染料，并使用激光诱导荧光来推断润滑层的厚度。我们将增加这些测量与高速成像，以可视化的电影分裂下游的nip。我们目前的理论假设轮轨之间的流动是近似二维的，并由雷诺润滑理论控制。该理论是为牛顿流体发展起来的，我们将修改该理论以考虑真实LFM的假塑性和弹性行为。当车轮在假塑性流体或弹性流体中滚动时，我们将把扩展理论与薄膜厚度的实验测量进行比较。润滑膜中的压力非常高。我们将研究是否有一个响应这种压力而变形的柔性车轮增加了LFM传递。在某些工况下，轮轨之间不形成润滑层。我们将通过实验和分析来探讨这种润滑失效。

Liquid friction modifier (LFM) applied to railroad tracks reduces the normally high coefficient of friction between the wheel and rail to a modest level. The modest friction is sufficient for hill climbing and braking, but is associated with superior fuel economy and reduced track wear and noise. Improving how LFM is applied would reduce the system maintenance cost and result in tens of millions of dollars of annual fuel savings and commensurate greenhouse gas reductions. Although the research proposed here is motivated by the specific technical problem of LFM application to tracks, it is of a fundamental character that could have significant impact in other application areas such as paint coating and liquid impingement cooling. One way that LFM can be applied is by directing a non-atomized, free-surface jet of LFM from the train onto the track. Close to the impingement point the jet spreads radially, like jet impingement on a stationary surface, forming a thin lamella. Somewhat further from the impingement point the jet remains on the surface and viscous effects cause the jet fluid to be convected downstream, forming an overall U-shaped lamella structure. The fluid mechanics of the impingement process will be studied using advanced experimental techniques (PIV and LIF). An analytical model for the lamella will be developed based on boundary layer theory combined with a model for the free surface. LFM liquids are non-Newtonian and the theory will be extended to the case of non-Newtonian impaction and to predict the onset of jet splash. A second way to apply LFM to the wheel-rail interface is to ooze it onto the track from a stationary location, where it is picked up by the wheels as the train passes. When done properly, each wheel rides on a thin lubrication layer of LFM. We have begun to study this coating process through a combination of experimental and analytical techniques. We have built a 1/5 scale version of a locomotive wheel. We add trace amounts of dye to the LFM and use laser-induced fluorescence to infer the lubrication layer thickness. We will augment these measurements with high speed imaging to visualize the film splitting downstream of the nip. Our current theory posits that the flow between the wheel and rail is approximately two-dimensional and is governed by Reynolds lubrication theory. The theory has been developed for Newtonian fluids and we will modify the theory to consider the pseudoplastic and elastic behaviour of real LFM. We will compare the extended theory to experimental measurements of film thickness when the wheel rolls over pools of a pseudoplastic fluid or an elastic fluid. The pressure in a lubricating film is very high. We will study whether having a compliant wheel that deforms in response to this pressure augments LFM transfer. For some operating conditions no lubrication layer forms between the wheel and rail. We will explore this lubrication failure experimentally and analytically.