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Unformatted text preview: AA 311 Lecture 2: Basic Fluid Properties Reading:  Chapter 2.1-2.3. Aerodynamics deals primarily with the flow of air, or more generally, any gas. In case we deal with some other gas, we sometimes talk about gas-dynamics . The science dealing with the motion of water, or in general any liquid, is called hydrodynamics . A great deal of aerodynamics is concerned with the phenomena upon which flight depends, hence aerodynamics is considered the science of flight. An object moving through air experiences aerodynamic forces acting on it. These aerodynamic forces are resolved into two mutually perpendicular components: one called lift , in a direction normal to the flight direction, the other called drag in the direction opposed to that of motion. Heavier-than-air atmospheric flight vehicles need airflow around the vehicle to produce the necessary lift that counters their weight. The airflow at the same time also produces a drag force. This drag force has to be countered by thrust, which is produced by the aircraft’s propulsion system. Based on the free-body diagram of Figure 1, the force balance for steady level flight can be written as T = D W = L, where T is the thrust force produced by the engine and propeller, D is the aerodynamic drag force, L is the aerodynamic lift force, and W is the weight of the vehicle. Figure 1: From . Clearly, the smaller the drag force, the less force needs to be produced by the engine to counter it, and hence less fuel is consumed. The question of how to find the optimum force balance that keeps the airplane in the air with the minimum propulsive force is a topic of aircraft performance , which will be covered later in the course. Basic concepts in aerodynamics Fluids (gases or liquids) are made up of tiny molecules. In principle, one could study the forces exerted on an object in a moving fluid by keeping track of every single fluid molecule in the flow, and how the exchange in kinetic energy and momentum impacts the object exposed to the flow. Considering the large number of molecules even in a very small volume element (for example a volume of 10- 12 cc of air contains about 2 . 7 × 10 7 molecules), it is clear that such a molecular description 1 is not a happy prospect. Besides, in general we are not interested in the mechanics of the individual molecules, but rather we would like to develop relationships between the macroscopically observable quan- tities pertaining to the fluid at rest or in motion. Such properties are called macroscopic properties, or bulk properties. We regard the fluid as a continuous distribution of matter: a continuous medium , or continuum . An infinitesimally small element of such a continuous medium is called a fluid element , or fluid particle . Four fundamental quantities of interest in aerodynamics are pressure, density, temperature and velocity, and we will review these fundamental physical quantities next....
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This document was uploaded on 02/05/2012.
- Fall '09