航空发动机专业英语之流体的物理特性12.ppt

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 earth’s 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 aerody