Advances in Magnetic Resonance Imaging (MRI) of Flow

低速下稳定的流体流动是层流的：流体层以有序的方式平滑地滑过另一层。当流动足够快或发生在一个足够大的容器中时，流动是湍流的：流体的混合是复杂和不可预测的。自列奥纳多·达·芬奇（Leonardo da Vinci）时代以来，湍流流体的流动一直令科学家着迷，但尽管经过了近600年的研究，我们仍然偶尔会对湍流的特征感到惊讶。这对科学家和工程师来说是一种挫折，因为了解湍流在大气中，在河流中，在水坝和涡轮机中，在鱼类和潜艇周围，在化工厂中，在岩石中以及在人体内部都是很重要的。为了提高我们的理解，我们测量了流体的流动。这一建议涉及改进一般流体流动研究的测量方法的发展，特别是湍流流体的流动。我们将使用磁共振成像（MRI）设备来测量湍流，该设备在原理上与医院扫描中使用的设备相同。核磁共振成像是测量流体流动的绝佳工具，因为它可以看到泡沫或浆液等不透明流体的内部，而且它根本不干扰流体流动——不需要插入探针或示踪剂。我们特殊的MRI流量测量方法是独一无二的，有两个原因。测量时间非常短（对于MRI），这使我们能够测量比其他MRI方法更广泛的流速范围。我们数据采集的所有部分的测量时间都是恒定的，这消除了其他MRI方法容易产生的某些伪影，特别是在泡沫等包含液体和气体混合物的流动中。在这个建议中，我们开发了一些硬件在我们的方法中使用，我们扩展了该方法来测量所有方向的湍流特性，我们改进了我们用来选择我们感兴趣的区域的技术，我们研究了一些流体中的湍流，这是其他技术无法处理的。所有这些发展将使我们对看似复杂的湍流现象有新的认识。

Steady fluid flow at low speeds is laminar: layers of fluid slide smoothly past one another in an ordered way. When the flow becomes fast enough or when it happens in a sufficiently large vessel, the flow is turbulent: there is complicated and unpredictable mixing of the fluid. Turbulent fluid flow has fascinated scientists since the time of Leonardo da Vinci, but despite nearly 600 years of study, we are still occasionally surprised by the features of turbulence. This is a frustration for scientists and engineers, because an understanding of turbulent flow is important in the atmosphere, in rivers, through dams and turbines, around fish and submarines, in chemical plants, through rocks and inside the human body. In a quest to improve our understanding, we make measurements of fluid flow. This proposal concerns the development of improved measurements for the study of fluid flows in general, but turbulent fluid flows in particular. We will use magnetic resonance imaging (MRI) equipment, which is in principle identical to those used in hospital scans, to measure turbulent flow. MRI is a fabulous tool for the measurement of flow, because it can look inside opaque fluids like foams or slurries and because it does not disturb the flow at all - no probe or tracer need be inserted. Our particular MRI flow measurement method is unique for two reasons. The measurement time is very short (for MRI), which allows us to measure a much broader range of flow speeds than other MRI methods. The measurement time is constant for all parts of our data acquisition, which eliminates certain artefacts to which other MRI methods are prone, particularly in flows like foams, which contain a mixture of liquid and gas. In this proposal we develop some of the hardware used in our method, we extend the method to measure the character of the turbulent flow in all directions, we refine the techniques that we use to select our region of interest within the flow and we study turbulence in some fluids that no other technique can cope with. All of these developments will afford us new insight into the deceptively complicated phenomenon of turbulence.