![]() However, the molecules are further apart and much more mobile in fluids. The difference between solids and fluids is that the molecules are mobile in a fluid and can be easily deformed by external forces. Solids are primarily rigid and have shapes that are difficult to change under the action of external forces. In a solid, the molecules are tightly packed in a lattice and have no mobility other than for tiny vibrations around their fixed positions. Unlike solids, fluids are substances with relatively mobile molecules, as illustrated in the figure below. Fluids can be liquids (e.g., water) or gases (e.g., air). Understand what streamlines are in a flow and how to calculate their locations.įluids are substances with mass and volume but no predefined shape.Know how to calculate the viscosity of air using Sutherland’s law.Use the equation of state to relate gas properties, i.e., pressure, density, and temperature.Become familiar with the parameter used to describe the behavior of a fluid, including pressure, density, temperature, viscosity, flow velocity, and the speed of sound.Understand the concept of a continuum model for describing a fluid.The relationships between pressure, temperature, and density can be established using thermodynamic principles formally embodied in an equation of state. For example, increasing the pressure of air, such as by compressing it, is accompanied by an increase in density and temperature. Changing the value of one property inevitably means that the values of other properties may also vary. They apply to bulk matter or a finite group of molecules rather than to each molecule, which has net physical dimensions much greater than the mean free path between the molecules this approach is known as a continuum assumption.įurthermore, it must be recognized that these fluid properties will not be independent of one another, i.e., they will have interdependencies. They are further referred to as macroscopic properties. These also point properties in that their values can change from point to point in the fluid they may also vary with respect to time at a given point. Pertinent properties of fluids include pressure, density, temperature, viscosity, flow velocity, and the speed of sound. For example, concepts of mass, weight, energy, work, power, etc., are all essential physical properties relevant to how we describe the world and problem-solving. In all branches of science and engineering, properties are defined to help describe how things behave around us in the physical world. One must learn about fluid dynamics and aerodynamics before any further understanding of the characteristics of flight vehicles is possible. To understand the action of aerodynamic flows on flight vehicles, it is first necessary to become intimately familiar with the fundamental physical properties used to describe the behavior of fluids and the relationships between them. Fluids can be liquids or gases, and air is a gas. ![]() Aeronautical and astronautical engineers must understand the behavior of fluids under a broad range of conditions. 9 Fundamental Properties of Fluids IntroductionĪerodynamics is the underpinning of atmospheric flight, so understanding aerodynamic principles, or, more generally, fluid dynamic principles, is one key to successfully designing all types of flight vehicles.
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