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T HE operation of an expansion tube, first introduced by Trimpi [1], relies on a series of unsteady shock and expansion waves to generate a high-enthalpy supersonic flow. This type of flow facility has several advantages over alternative ones in its ability to generate a wide range of high-enthalpy flow conditions. However, the range of accessible flow conditions is ultimately limited by a combination of undesired effects, such as flow disturbances, boundary-layer growth, limited core flow size, and short test times. Previous studies [2–6] have attempted to quantify how some of these effects may alter test conditions and times. Paull and Stalker [3] showed that flow disturbances originating in the driver gas can be transmitted into the test gas, and may undergo a frequency focusing process across the secondary expansion wave [7] to give rise to a significant amplification of pressure fluctuations in the test gas. However, noise transmission to the test gas can be minimized under certain conditions of operation. The presence of the secondary diaphragm has also been found to affect test gas conditions by creating a reflected shock wave that alters the test gas [8,9] and introduces limitations in static pressure measurements [10]. However, the use of thin secondary diaphragms or eliminating them altogether [11] can reduce these undesired effects. Here we report observations of nonideal operation of expansion tubes that results in undesired properties in the test gas. The interference effects investigated here result from the interaction of the initial unsteady flow with models located in the test section, and induces a significant increase in test gas pressure and temperature at the beginning of the test time. Thus, the experimental flow conditions are altered from what was desired. The test-model-induced interference effect is postulated to be caused by the secondary shockwave diffracting around the test article, generating a series of compression waves that coalesce into an upstream-propagating shockwave,which we refer to as a disturbance wave. In some respect, it is analogous to test model blockage effects in steady flow facilities [12,13]. However, the effects studied here are different from other flow disturbances discussed previously; they also differ from test gas composition contamination in expansion tubes [6] and reflected shock tunnels [14–16]. Under certain operating conditions with test models mounted inside the test section, a strong distortion of the test gas pressure time history measured in the test section (e.g., on models) has been observed, which effectively reduces the test time and alters the aerothermodynamic flow properties from what was desired. Figure 1 shows the static pressuremeasured in the test section for a typical case where this effect is observed. The static pressurewasmeasuredwith a flush-mounted PCB piezoelectric transducer inserted in a flat plate with the leading edge protruding 2.1 cm into the expansion section. After the initial phase, where the secondary shock (I) and shocked expansion gas arrive (II), there is a rapid increase in pressure at the end of the expansiongas flow,which results inwhatwehere denote as the secondary overshoot. This is then followed by the test gas with an additional rapid increase in pressure (III) to a peak value approximately two times the design flow pressure, here denoted as the primary overshoot. Subsequently, the pressure decays to the nominal design value (IV) until the arrival of the test time termination wave. A generalized space–time diagram summarizing the wave processes responsible for altering the test gas properties is shown in Fig. 2. The labeling of the states follows that of Trimpi [1]. After the secondary shock (W2) arrives in the test section, a disturbance wave (WU) processes the shocked expansion gas and interacts with the contact surface at a distance x̂ from the secondary diaphragm. This results in generating two new waves, a transmitted and a reflected wave. The transmitted wave processes a portion of the test gas and thus alters its aerothermdynamic properties, which causes the primary overshoot in Fig. 1 and is denoted as in Fig. 2. Furthermore, the interaction reduces the speed of the contact surface, which further limits the useful test time. The reflected wave, which processes the shocked expansion gas, is responsible for the secondary overshoot in Fig. 1 and is denoted as in Fig. 2. In this study we investigate the mechanism for the onset of such flow disturbance using a combined experimental, computational, and analytical approach. A series of experiments and computer simulations using ANSYS CFXwas carried out to gain insight on the cause of the pressure overshoots and reach the conclusions summarized in Fig. 2. An analytical model describing the observed effects is formulated and is shown to be a useful tool in predicting the extent of test gas interference. Lastly, we provide a brief discussion on what is required to limit the undesired interference effects.

Test-Model-Induced Interference Effects in Expansion Tube Flows