3.1 Description of Fluid Motion and the Theory of Relative Motion

Fields represent the distributions of physical quantities in the space, and the physical quantity can be scalar or vector. As what is represented in its name, flow fields describe the distribution of physical quantities that are related to fluid in space. Typically, flow fields can be classified into 2 categories: flow fields with scalar quantities (quantities with size but not direction) and flow fields with vector quantities (quantities with size and direction). As abbreviated, they can also be called scalar fields of flow and vector fields of flow.

It is really important to realize how flow fields work!!! The reason is that flow fields can quantify and visualize the flow of fluids, and it will be easier to explain physical phenomena (like the generation of lift) with the help of visualized flow fields.

Apart from the classification of flow fields, a critical concept in a flow field can be introduced. It is called pathlines. Pathlines are fictional lines in a flow field. It describes the motion of a fluid microelement. Combined with colors, pathline plots can visualize the motion of fluid in an intuitive way.

Play the video below to see visualization of flow fields around a tilted board based on Ansys Fluent

But here comes a question: how can we simplify the analysis of the flow fields if the object in the fluid is moving? The solution is simple. According to the principle of relative motion, the motion of the objects in the fluid can be equalized as the motion of fluid in an area with static objects. This is called the theory of relative motion of fluid.