Modern experimental techniques for high-speed flow measurements

Abstract

ITHIN the last few years there has been a marked increase in interest in supersonic and hypersonic flow regimes. This has been partly as a result of the establishment of a transatmpspheric vehicle and the National Aerospace Plane (NASP) program. Addressing the technology problems for these future systems will provide a significant challenge for the aerospace community. Supersonic and hypersonic airbreathing vehicles are highly integrated systems involving strongly coupled technologies. The conceptual design process for various components requires analytical and experimental tools. The flowfield regimes include the internal combustion of propulsion systems, the external flow of the vehicle, and the overall performance of the system. The rebirth of interest in high-speed flows follows a period of inactivity. During the 1960's, considerable research was conducted on supersonic and hypersonic flows. Much of the research was directed at understanding the problem of re-entry. Test facilities such as supersonic/hypersonic wind tunnels, shock tunnels, and shock tubes were established. The diagnostic instrumentation in these facilities provided limited data. The instrumentation included pressure transducers, heattransfer gages, schlieren and inter ferometry, and gas-sampling probes. The data were sufficient to help understand the flow phenomena. With the advent of computer and laser technologies, the activities in the area of computational fluid dynamics (CFD) and sophisticated instrumentation were accelerated. The design and analysis of re-entry-type vehicles have become increasingly dependent upon numerical schemes for predicting the flowfield.1 The adequacy of CFD as the sole design tool for aerodynamic vehicles in the foreseeable future is doubtful. 2 CFD produces more detailed aerodynamic information faster than previously possible, particularly on simple configurations such as an airfoil section, a fuselage, or even a complete wing. However, reliable modeling of complex complete configurations including vortex interactions, boundary-layer transition, and chemistry are not yet available. Even the detailed physics of the flow near the leading or trailing edge of a wing is very difficult, if not impossible,, to compute within the required accuracy. Experimental aerodynamics, therefore, remain an important component in the design of future transport aircraft. This is even more true in terms of development of the propulsion system. To examine the applicability of various diagnostic techniques, it is necessary to establish the measurement requirements in terms of the parameters and the measurement environments. This in turn requires the examination of the available or proposed design facilities. Figure 1 shows the performance envelope of some existing ground test facilities.3 Also shown is the performance profile for a proposed entry research vehicle (ERV). The research vehicle is proposed for investigation of basic flowfield phenomena for high-altitude/ hypervelocity flights. Nonintrusive measurement techniques are also needed in support of candidate flight experiments. Examination of the performance envelope of existing flow facilities (e.g., Fig. 1) will help determine the required dynamic range of the instrumentation employed in these facilities.4 It is also generally agreed that the environments associated with supersonic and hypersonic test facilities are harsh both inside and outside the test facility. Inside, the measurement environment is characterized by high temperatures, high velocities, and high noise levels. Outside, it is characterized by high noise and vibration levels and by varying temperature and pressure fields, which can adversely affect the alignment and overall performance. For application of optical techniques, this requires that all optical components be mounted independent of