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1、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 u
2、nit 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vIn metric units, pressure is measured in Newtons per s
3、quare 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 a
4、tmospheres, 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. vSometimes pressure is given in inches of mercury (inHg) or millim
5、eters 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. vmetric 度量vpounds per square inch 磅每平方英寸vatmospheres 大气vatmospheric 大气压的，常压的vexperimentally 实验上，用实验方法vmercury 汞，水银vma
6、nometers （液体）压力计vbarometers 气压表（计）vcolumn 柱，柱状物vdeceive 欺骗，蒙蔽，伪装，掩饰vgovern 管理，支配，调整，控制，操纵运 输v 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
7、 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! vDensity vDensity is a me
8、asure 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 t
9、he density of a fluid, we often compute the density using a unit volume, say 1m by 1m by 1m(1m3). vWe 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
10、 different conditions a hot day versus a cold day, or high altitudes versus lower altitudes.vtough 坚韧的，强韧的，坚强的，结实的vThe 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 a
11、nd 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. vslug 斯（拉格）
12、质量单位32.2磅vA 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.
13、 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. vIf the density of the fluid is fixed (constant), the fluid is incompressible; neither the mass or the volume can change. Water
14、 (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 d
15、oesnt change, but the volume increases, so the density of the gas decreases in the new volume. vtank 水箱，大容器vcompressible 可压缩的，可浓缩的v 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
16、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 vThis 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 w
