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).

Anechoic Jet Laboratory

Heated Jet Rig:

The jet rig is supplied air from a 1100 hp Ingersoll-Rand Centac compressor capable of 5000 SCFM of dry (–40 °F) air at 125 psia allowing for continuous operation at desired test conditions.  An 800 cu.ft. receiver tank inline between the compressor and the control valves serves to dampen the fluctuations caused by the compressor and dryer components. Computer controlled valves are operated with closed-loop control to maintain the exit Mach number constant within 1%, and a final muffler downstream of the valves removes any excess valve noise.  Heat is provided using a gaseous propane burner system housed well upstream of the nozzle and is followed by a ceramic flow straightener and settling chamber upstream of the main contraction.  Nozzle parts are typically machined from Inconel alloy allowing continuous high-temperature operation.  Alumina particles (0.3 micron) are injected through seeding ports enabling PIV imaging of the jet plume (see Ukeiley et al., 2007 and Murray & Seiner, 2008).

Nozzle Design Capabilities:

Nozzle exit conditions are paramount in determining jet development and noise production.  The NCPA is uniquely suited to address these issues by employing in-house, state-of-the-art machining capabilities to produce a wide array of both axisymmetric and non-axisymmetric nozzle shapes.  Past work has utilized nozzle configurations from simple 1.4 inch round convergent nozzles to a twin-jet configuration with centerbody sections and provision for chevron/tab/corrugation attachment.  As an example, the figure below shows a complex model nozzle assembly that includes a centerbody, a bypass flow region where cool air is forced into the boundary layer, and a smooth bore conical nozzle with chevron-type extensions.  Nozzles have also been produced with a faceted shape to mimic the variable exit nozzle (VEN) seals typical of military engines.  Interchangeable parts allow for effects of the centerbody and/or facets to be measured versus a simple conic nozzle of the same pressure/temperature ratio.

Acoustics Measurements:

For acoustics measurements, the facility includes a far-field arc array of 1/4-inch B&K microphones.  There are 12 microphones covering angles from 20 to 130 degrees from the jet axis with higher polar angle resolution in the peak noise directions.  The following figure shows the arc array layout for a 1.97-inch jet diameter. 

Other Jet Laboratory Setups and Capabilities:

A number of other experiments have been performed in the Anechoic Jet Laboratory including the evaluation of acoustics produced by a twin-jet, measurement of the impingement of a supersonic heated jet using pressure sensitive paint, and external combustion of a supersonic plume.  The twin jet rig provides for azimuthal rotation allowing far-field measurements at any azimuthal angle from the symmetry plane.  Both nozzles have a centerbody installed, and the nozzle exits are canted together slightly to mimic realistic installation effects.  The jet impingement study used a geometrically scaled jet-blast deflector and ground plane attached to a fixture.  Pressure sensitive paint was used to measure the mean pressure distribution at the impingement plane.

To study supersonic combustion, a Mach 2.0 nozzle was modified with an annulus through which a H2/N2 mixture was forced.  The combustion process was initiated with a focused laser.  Once established, the combustion process in the Mach 2.0 combusting flame was measured using Spontaneous Raman Spectroscopy.

Propane Burner - Jet Rig

Faceted Nozzle

Twin Jet Setup

Impinging Jet with Pressure Sensitive Paint

Supersonic External Combustion

Annular Nozzle for Combustion Study

Far-Field Arc Array Setup