Pressure measurements

The “pressure” concept is very relevant in physics and aerodynamics, since it lays the basis for the generation of fluid forces and loads on objects. “Pressure” is a derived quantity defined as force per unit area. “Force” and “area” are also derived from three fundamental quantities: “length,” “mass,” and “time”; therefore, the first one is usually written as a combination of the last three ones. Typically, the concept of pressure is wrongly confused with the one of “force,” mainly due to the historical ways of measuring pressures by applying forces on known areas. While pressure stresses can be measured in solids by exploiting the stiffness characteristics of materials and structures, in fluids (i.e., gas and liquids) the measuring procedure is rather different. When considering fluids indeed, the movement of their constituting molecules cannot be neglected. An interesting relation can be derived, that states how the statistical kinetic motion of the molecules constituting the fluid medium can be linked to its resultant pressure. Understanding the connection between the statistical characteristics of the constituting elements of matter and the macro characteristics of the medium is a critical step to avoid many misunderstandings. Together with the fluid pressure, other properties such as temperature and density can be linked to the kinetic motion of the fluid molecules, due to the connection with 110the random motion of the particles moving in the medium. In the following paragraphs, the concept of pressure will be explained with respect to many different applications, first introducing it at the molecular level and then studying it in several pressure transducers. Different typologies of sensors can be chosen to measure unsteady/steady or high/low pressures (example in Figure 5.1). The following paragraphs will be dedicated to the presentation of the most known configurations in experimental aerodynamics, with specific interest to their implementation and characteristics.