Is Fine-Scale Turbulence Universal?

大多数实际感兴趣的流都是湍流的，因此对这些流的理解对于工程和基本原因都很重要。不幸的是，湍流仍然是“经典物理学中最重要的未解决问题”。它包括范围广泛的三维运动，从大而慢到小而快。最小（和最快速）的运动耗散了流动的动能，并决定了物体的阻力，污染物的分散和化学混合。不幸的是，直到最近，由于这些运动非常微小，使得它们在具有实际重要性的流动中无法进行实验和计算。因此，湍流的预测是建立在这些“精细尺度”的不确定理论和模型的基础上的，这些尺度被认为对所有流动都是相同的，即普遍的。没人知道这是真是假。要回答这个问题，需要对一系列流体进行测量，使用能够解决它们在空间中随时间演变的完整结构的技术。申请人开发的新技术首次使这些运动的全面测量成为可能。计算方法的类似进步为与此类测量进行有意义的比较提供了机会。在伦敦帝国理工学院、剑桥大学和南安普顿大学之间的这项合作倡议中，我们的目标是开发和采用一系列先进的激光诊断技术来测量一系列“典型”湍流剪切流的时间分辨、精细尺度的三维特征，以测试普遍性假设。还将测量直接数值模拟可获得的雷诺数，特别是为此目的进行交叉比较和验证。实验技术将基于电影扫描和层析粒子图像测速（PIV）技术。无论普适性假设是否成立，都将获得必要的信息来建立基于物理的精细尺度模型，这些模型可以解释多尺度相互作用。这些数据以及3D PIV软件将在网上提供给英国和世界各地的研究人员。

Most flows of practical interest are turbulent and hence the understanding of these flows is important for both engineering and fundamental reasons. Unfortunately, turbulence remains ``the most important unsolved problem of classical physics''. It consists of a wide range of three-dimensional motions, from large and slow to small and fast. The smallest (and most rapid) motions dissipate the kinetic energy of the flow and determine drag on bodies, dispersion of pollutants and chemical mixing. Unfortunately the very smallness of these motions has, until recently, made them inaccessible to both experiments and computations in flows of practical importance. Predictions of turbulent flows have thus been based on uncertain theories and models of these ``fine scales which are assumed to be the same for all flows i.e. universal. No-one knows if this is true or not. Answering this question requires measurements of a range of flows using techniques capable of resolving their full structure in space as it evolves in time. New techniques developed by the applicants have, for the first time, made the full measurement of these motions possible. Similar advances in computational methods has provided the opportunity for meaningful comparisons with such measurements.In this collaborative initiative between Imperial College London, the University of Cambridge and the University of Southampton, we aim to develop and employ a series of advanced laser diagnostic techniques to measure the time-resolved, three-dimensional features of the fine-scales in a series of `canonical' turbulent shear flows in order to test the hypothesis of universality. Measurements will also be taken at Reynolds numbers accessible to Direct Numerical Simulations especially carried out with this purpose for cross-comparison and validation. The experimental techniques will be based on cinematographic scanning and tomographic Particle Image Velocimetry (PIV) techniques. Regardless of whether the universality hypothesis holds or not, the necessary information to formulate physics based fine-scale models that can account for the multi-scale interactions will be obtained. The data as well as the 3D PIV software will be made available online for researchers in the UK and around the world.