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.

超高速設備

有許多空氣動力學實驗設備可以在遠遠高出以前各章介紹的各類風洞所能達到的速度下進行試驗與研究。在這些設備里，高的速度通常是以犧牲其他某個試驗參數來達到的，比如馬赫數，壓力或運轉時間等。

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 The enthalpy no -longer increases linearly with temperature and the gas law p = pRT is no longer valid.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. 

一般地說，這些設備之所以問世，是因為空氣動力學家認為，高速飛行中出現的氣動力問題不可能完全通過高超聲速風洞試驗來解決；因為在高超風洞中，風洞的運行溫度只能達到避免發(fā)生液化作用的程度。持這種懷疑態(tài)度有幾個理由。為了在風洞氣流速度等于飛行器在某一高度下的飛行速度的同時，使風洞試驗段的靜溫和靜壓與該高度上大氣的數值相同，風洞的總溫和總壓就必須非常高。要在試驗段中復現靜溫、靜壓和速度的重要性是根據這樣一個事實提出來的，即只有這樣做模型附近（激波后及附面層內）的溫度和壓力才能與飛行器飛行時所處的條件相當。因為在高溫下空氣的特性與低溫下的完全不同，所以，人們認為使模型附近具有適當的溫度與壓力是很重要的。在高溫下，焓不再隨溫度線性增加，氣體定律p=pRT也不再成立。人們已經認識到，在高溫下，焓不僅取決于溫度，而且取決于壓力。

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.

高溫氣體中發(fā)生的這種使氣體定律p=pRT不能成立，并對焓產生很大影響的過程稱為離解。離解是用來描述氧和氮的雙原子分子（O2和N2)分解為氧原子和氮原子（O和N)這樣一個過程的術語。在某一溫度下（這要取決于壓力)，空氣的內能會變得很高，致使雙原子分子中的兩個原子之間的結合鍵松弛，并互相分離。在振動自由度被完全激發(fā)之后，就會發(fā)生這種現象。要把雙原子分子離解為原子需要大量能量。這就是在高溫下焓值隨溫度不正常變化的原因。發(fā)生離解作用時，因為存在的分子數增加了，所以，氣體定律不再成立，每一個離解的氧分子變成兩個氧原子。