Anechoic Jet Laboratory:
The evaluation of temperature effects on supersonic jets was in fact the purpose for which the NCPA facility was designed. Using years of expertise and knowledge gained through the development of test facilities at NASA, Dr. John M. Seiner envisioned the current NCPA facility to overcome shortcomings of previous facilities. Specifically, the anechoic facility was designed with upstream and downstream “stagnation” chambers through which ambient air is pulled by a 10,000 SCFM fan. The air is allowed to percolate into the 19-by-20-by-8 foot chamber (within the wedges) through 50% porosity sliding panels achieving approximately 1 ft/s in the anechoic section (when the jet is off). This aspiration of the test chamber results in a very even temperature distribution throughout the room while maintaining an acoustically anechoic environment. This novel feature was found to be necessary following work in the NASA Langley Small Anechoic Jet Facility (SAJF) which involved the addition of an open duct surrounding the nozzle assembly with a 4000 SCFM blower in the exhaust (typical of many current heated jet facilities). Even with the additional air flow the SAJF was unable to reach jet total temperatures in excess of 300 °F without serious localized heating problems. By aspirating the entire anechoic chamber, the temperature distribution in the NCPA facility resembles that of a warm day even in the corners of the room. These temperatures can be maintained under continuous operation of a Mach 1.56 jet at 1376 °F (average conditions measured during the experiments presented by Baars et al., 2011). This environment is critical in avoiding adverse refraction effects and obtaining accurate acoustic far-field measurements. Further design details for the NCPA facility are discussed in Ponton et al. (2001) and Ukeiley et al. (2007).
