The operation of free-piston-driven reflected shock tunnels has largely observed the rule of thumb that the nozzle throat diameter should not exceed one third the shock-tube diameter. This is intended to ensure drainage does not prematurely terminate the test time before wave effects. For successful testing at lower Mach numbers, a higher nozzle throat-to-exit area ratio is required, either resulting in smaller test flows, and therefore experimental models, or necessitating a larger nozzle throat. This work experimentally examines the effect of enlarging the nozzle throat past the widely observed one-third limit in the T4 reflected shock tunnel over a range of operating conditions. This is performed within the context of testing the feasibility of designing a new Mach 6 nozzle with a throat-to-shock-tube diameter ratio of 0.421. It is demonstrated that the increased test-gas drainage caused by an enlarged nozzle throat decreases the speed of the reflected shock and the nozzle-supply pressure. There is also a reduction in constant-pressure test time of 2.3%, although this falls well within normal experimental variability. Quasi-one-dimensional simulations confirm these results and also show that the reduced strength of the reflected shock changes the tailoring of the condition. While this results in the downstream propagation of the contact surface, Pitot measurements at nozzle exit have shown that driver-gas contamination is not a concern at the low-enthalpy conditions of interest. These results demonstrate that it is possible to operate free-piston-driven reflected shock tunnels at throat diameters exceeding the one-third rule. When designing low Mach number nozzles for this class of facility, the effect on the desired test flow from the reduction in nozzle-supply pressure and altered tailoring must be considered.

自由活塞驱动反射激波隧道的操作很大程度上遵循了经验法则，即喷嘴喉部直径不应超过激波管直径的三分之一。这是为了确保排水不会在波浪效应之前提前终止测试时间。为了在较低马赫数下成功进行测试，需要更高的喷嘴喉部与出口面积比，从而导致较小的测试流量和实验模型，或者需要更大的喷嘴喉部。这项工作通过实验检验了在一系列操作条件下扩大喷嘴喉部超过 T4 反射激波隧道中广泛观察到的三分之一限制的效果。这是在测试设计具有 0.421 的喉部与冲击管直径比的新 6 马赫喷嘴的可行性的背景下进行的。结果表明，由扩大的喷嘴喉部引起的测试气体排放增加会降低反射冲击的速度和喷嘴供应压力。恒压测试时间也减少了 2.3%，尽管这完全符合正常的实验变异性。准一维模拟证实了这些结果，并且还表明反射冲击强度的降低改变了条件的调整。虽然这会导致接触面向下游传播，但喷嘴出口处的皮托管测量表明，在感兴趣的低焓条件下，驱动气体污染不是问题。这些结果表明，可以在超过三分之一规则的喉部直径下运行自由活塞驱动的反射激波隧道。