自由活塞激波风洞产生的入射激波在行进过程中存在较大衰减，这种现象不仅降低了风洞喷管贮室的焓值、压力的量值和平稳性，而且也制约了风洞有效试验时间。针对自由活塞激波风洞结构特点，试图揭示入射激波衰减的主导原因。在忽略一些偶然性随机性因素后，重点对黏性衰减和反射膨胀波作用两个因素的影响进行了分析和比较。结果表明，在风洞主膜片打开时刻，活塞前脸与主膜片之间的短促距离，加剧了反射膨胀波的影响，在很多情况下，这是导致激波衰减的更为主要的因素。出于降低激波衰减和延长风洞有效试验时间的实际工程需要，提出了变截面活塞压缩器的设计构型。随后的理论研究显示，该构型能够实现活塞充分减速并达到安全速度，被压缩气体（驱动气体）能够形成平稳的压力/温度平台，满足激波管驱动需要。
Attenuation of incident shock waves in free piston shock tunnels is of great important, which not only reduces the enthalpy, pressure and steadiness, but also imposes constraints on valid testing time. In this study, an attempt of investigation of main reasons of attenuation of shock wave was carried out based on the structural features of free piston tunnels. After neglecting some random factors, two important factors of attenuation, viscosity of the boundary layer and the reflected expansion wave, were studied. The results show that the latter factor plays major role in free piston shock tunnels. When the main diaphragm ruptures, the front face of the piston is very close to the main diaphragm, which leads to reflected expansion wave to catch up with the interface or shock wave earlier. A new compression tube with an abrupt area change was proposed, which can delay the interaction between the reflected expansion wave and the interface (or shock wave). In the new compression tube, the steady pressure and temperature of driver gas could be formed in the end of the tube while the free piston decelerated to a safety speed.
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