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Pressure The pressure of a fluid is another important consideration for computing aerodynamic forces. When a fluid moves over or through an object, it exerts small pushes on the surface of the object. These pushes are defined as the pressure exerted on the object. Pressure is measured as force per unit area, because the pushes occur over the entire surface of the object. The units for pressure include mass times a length divided by time squared (the units for a force) divided by the area of the surface (length squared). exert 施加力，作用力，发挥，尽力 In metric units, pressure is measured in Newtons per square meter (N/m2) or Pascals (Pa). A Newton, a force unit, is 1 kilogram times 1 meter divided by seconds squared. A Pascal is 1 Newton per meter squared. In the English system, pressure is most commonly measured in pounds per square inch (psi). In either system, pressure is sometimes measured in atmospheres, a measure of how the pressure differs from the atmospheric pressure in a typical room on the earths surface. Atmospheric pressure has been defined experimentally as 14.7 psi or 101 kPa. A kilopascal (kPa) is 1000 pascals. Sometimes pressure is given in inches of mercury (inHg) or millimeters of mercury (mmHg) because old-style manometers and barometers (instruments to measure pressure) used to measure the length of a thin column of mercury to compute the pressures. metric 度量 pounds per square inch 磅每平方英寸 atmospheres 大气 atmospheric 大气压的，常压的 experimentally 实验上，用实验方法 mercury 汞，水银 manometers （液体）压力计 barometers 气压表（计） column 柱，柱状物 deceive 欺骗，蒙蔽，伪装，掩饰 govern 管理，支配，调整，控制，操纵运输 Pressure can be a powerful quantity. It can also be deceiving. A small pressure spread over a very large area can add up to be a very large force. Air pressures decrease as the altitude increases; pressures also decrease when the speed of the fluid increases. When the temperature of a fluid increases, so does the pressure. The rules that govern these changes are part of the study of aerodynamics, and the pressures on an airplane contribute directly to its flight capabilities! Density Density is a measure of how much mass (the amount of molecules) is included in a given volume. Another way to think about it is that it can be considered the measure of how tightly the molecules are packed in a volume or object. The units for density are mass per volume, or kg/m3, for example, When we talk about the density of a fluid, we often compute the density using a unit volume, say 1m by 1m by 1m(1m3). We do this for two reasons: first, we dont have to know exactly how large our volume is (could be tough in the earths atmosphere, or in a large ocean!), and second, it allows us to compare densities at different conditions a hot day versus a cold day, or high altitudes versus lower altitudes. tough 坚韧的，强韧的，坚强的，结实的 The units for density are mass per volume, and the most common measurements are kg/m3, slugs per gallon, or slugs per cubic foot (slugs/ft). What is a slug? The US uses terms for mass and weight interchangeably, but science requires precise definitions of properties and their units. A slug is equal to 32.174 pounds mass. Scientists and engineers have defined the terms pounds force (1bf) and pounds mass (1bm) to help differentiate between a mass and a force like weight. slug 斯（拉格）质量单位 32.2磅 A fluid with a lot of molecules tightly packed together has a high density; one that has fewer molecules would have a lower density. Water, for example, has a much higher density than air. A 10 gallon fish tank with water in it has much more mass in it than a 10 gallon tank with air in it. Since it has more mass, it will weigh more (more on that in a later section). In addition, the density is used to define whether a fluid is incompressible or compressible. If the density of the fluid is fixed (constant), the fluid is incompressible; neither the mass or the volume can change. Water (an incompressible liquid) poured from an 8 ounce cylindrical glass will still have 8 ounces in a large round bowl. the amount of mass and the volume stay the same. If the density can change, the fluid is compressible. Gases are compressible fluids; they will expand to fill a new volume. The mass doesnt change, but the volume increases, so the density of the gas decreases in the new volume. tank 水箱，大容器 compressible 可压缩的，可浓缩的 All of the properties are linked together. If the pressure or the temperature of a fluid changes, its density will usually change, too. The density of air on a hot day is lower than the density of air on a cold day. At high altitudes, where the pressure is lower, the density is also lower. This is why an aerodynamicist must pay attention to all of the properties of the fluid when trying to define flow conditions. Viscosity This is one of the most difficult properties on this list to define. It is a measure of how much a fluid will resist flowing. If you spill water on an inclined board, it will run quickly down the board. However, if you Spill honey on the same board, it will travel down the board much more slowly. Honey has a much higher viscosity than water. It is said that honey is a more viscous fluid than water. viscosity 粘性，粘度 resist 阻碍，反抗，抵抗 spill 流出，溢出 honey 蜂蜜 viscous 粘性的 The units for viscosity come from the mathematical definition of the property. When a fluid flows over a surface , it exerts a force (measured in Newtons, for example) on it. The viscosity is measured by dividing this force by the speed of the fluid flowing (meters per second, m/s) and the thickness of the fluid layer (meters). So the units for the viscosity are Newton-seconds per square meter (N s/m). However, the convention used by scientists and engineers is to define the property using units of mass. Since a Newton is equal to a kilogram per second squared (kg m/s), the more commonly used units are kilogram per meter second (kg/m s) for the metric system, and pounds mass per foot second(1bm/ft s) in the English system. mathematical 数学的，数学上的，精确的 convention 条约，协约，集会，会议 Force 力 Weight and Gravity 重量和重力 Velocity 速度 Acceleration 加速性 Different ways air moves 空气运动的不 同方式 Speed of sound 声速 Mach Number 马赫数 Friction 摩擦力 Boundary Layer 附面层，边界层 Laminar Boundary Layer 层流边界层 Turbulent Boundary Layer 紊流边界层 Transition 过渡，转变 Flow Separation 气流分离 Buoyancy 浮力，浮性，弹性，恢复力 Streamlines and Flow Patterns 流线和气 流特性曲线 Shocks 冲击震荡 Perfect Gas law 理想气体定律 Bernoullis theorem 伯努力定理
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