Hypervelocity Facilities

There are a number of experimental aerodynamic facilities that allow testing and research to be done at velocities considerably above those achieved in tunnels of the types described in the preceding chapters. Usually in these facilities the high velocities are achieved at the expense of some other test parameter,such as Mach number,pressure,and/or run time.

超高速设备

有许多空气动力学实验设备可以在远远高出以前各章介绍的各类风洞所能达到的速度下进行试验与研究。在这些设备里，高的速度通常是以牺牲其他某个试验参数来达到的，比如马赫数，压力或运转时间等。

Generally speaking,these facilities have come into being because the aerodynamicist does not believe that the aerodynamic problems of high-speed flight are completely answered by tests in hypersonic wind tunnels, where the tunnel operating temperature is only high enough to avoid liquefaction. There are several reasons for this skepticism. In order for the static temperatures and pressures in the test section of a wind tunnel to equal values at some altitude in the atmosphere at the same time that the velocity in the wind tunnel equals the flight velocity of an aircraft at that altitude, the total temperatures and pressures in the wind tunnel must be very high. The importance of duplicating static temperature, static pressure,and velocity in the test section arises from the fact that only in this way will the temperatures and pressures in the vicinity of a model (behind shock waves and in boundary layers) correspond to conditions for the vehicle in flight.‘Having the proper temperatures and pressures in the vicinity of the model is considered important because at bigh temperatures,the characteristics of air are completely different from those at low temperatures. The enthalpy no -longer increases linearly with temperature and the gas law p = pRT is no longer valid. It is noted that enthalpy at high temperatures is dependent on pressure as well as temperature. 

一般地说，这些设备之所以问世，是因为空气动力学家认为，高速飞行中出现的气动力问题不可能完全通过高超声速风洞试验来解决；因为在高超风洞中，风洞的运行温度只能达到避免发生液化作用的程度。持这种怀疑态度有几个理由。为了在风洞气流速度等于飞行器在某一高度下的飞行速度的同时，使风洞试验段的静温和静压与该高度上大气的数值相同，风洞的总温和总压就必须非常高。要在试验段中复现静温、静压和速度的重要性是根据这样一个事实提出来的，即只有这样做模型附近（激波后及附面层内）的温度和压力才能与飞行器飞行时所处的条件相当。因为在高温下空气的特性与低温下的完全不同，所以，人们认为使模型附近具有适当的温度与压力是很重要的。在高温下，焓不再随温度线性增加，气体定律p=pRT也不再成立。人们已经认识到，在高温下，焓不仅取决于温度，而且取决于压力。

The process taking place in air at high temperatures which causes the gas law p = pRT to be invalid and which has a major effect on the enthalpy is called dissociation. Dissociation is the term given to the process in which diatomic molecules of oxygen and nitrogen (O2 and N2) are broken up into atomic oxygen and nitrogen(( and N). At some temperature,which is dependent on pressure,the internal energy level of the air becomes high enough that the two

atoms of the diatomic molecules begin to loose their bonds and fly apart.'This occurs after the vibrational degree of freedom is fully excited.Considerable energy is required for breaking the diatomic molecules into atoms. This explains the unusual variation of enthalpy with temperature at high temperature levels. The gas law is not valid when dissociation is present because the number of molecules present is increased. Each molecule of O2 that is dissociated becomes two molecules of O.

高温气体中发生的这种使气体定律p=pRT不能成立，并对焓产生很大影响的过程称为离解。离解是用来描述氧和氮的双原子分子（O2和N2)分解为氧原子和氮原子（O和N)这样一个过程的术语。在某一温度下（这要取决于压力)，空气的内能会变得很高，致使双原子分子中的两个原子之间的结合键松弛，并互相分离。在振动自由度被完全激发之后，就会发生这种现象。要把双原子分子离解为原子需要大量能量。这就是在高温下焓值随温度不正常变化的原因。发生离解作用时，因为存在的分子数增加了，所以，气体定律不再成立，每一个离解的氧分子变成两个氧原子。