2D: GPIT (GV Pod Inlet Test) Project

The NCAR GV aircraft has the capability of carrying two large underwing instrument pods. The pods are 20 inches in diameter and 158 inches long, and hang about 3 feet below the lower surface of the wing on a structural pylon.  Both the pylon and the pod can be removed from the aircraft when not needed for research. These pods are currently not fully utilized and offer a unique opportunity to develop chemistry instrumentation for the measurement of highly reactive compounds.

Long and bent inlet lines, required for analytical instrumentation mounted within the main cabin of the aircraft, can be detrimental to the measurement of reactive gases and sticky compounds such as complex oxygenated VOC, acids, ammonia, or radicals such as halogen oxides. A pod instrument can be mounted much closer to a straight inlet, minimizing potential wall interactions.

As part of the ACCORD (Atmospheric Chemistry Center for Observational Research and Data) initiative, NSF special funds were granted to NCAR ACOM and two university partners (University of Denver and Clarkson University) to design and build next-generation inlets for the NSF/NCAR research aircraft.  As one part of this project, a new design for a reactive gas inlet fo the GV wingpods was constructed and evaluated during the ARISTO (Airborne Research Instrumentation Testing Opportunity) flights in February of 2017. The assembly is shown in Figure 1. The inlet consisted of a centerline 3-inch internal diameter tube, mated to an elliptical profile nose cone tip, which was mounted at the leading edge of the pod.  The nose cone tip was mated to 33 inches of straight aluminum tubing followed by a section of curved exhaust tubing ending in a flush mounted vent incorporated into one of the side access plates on the NCAR pod.  The first 24 inches of the inlet tube was modular in design to allow for reconfiguring pressure/flow/temperature/heating instrumentation between flights.  Our research focused on the detection of forced convective heating within the inlet, as an analog for sample gas interactions with the inlet walls (Brune et al, 1988). Restrictors at the exhaust port were mounted during some flights reducing the flow through the tube by approximately a factor of 2 and 9.  By manipulating both wall heating and inlet geometry, thermistor arrays were used to determine the profile of the air core inside the inlet, which did not have any contact with the inlet walls, as a function of flight conditions such as altitude, attack and banking angles, wind direction and speed, temperature, and other flight parameters.

Test flights were successful and demonstrated that the core flow is larger than 1.5” diameter up to nine aerodynamic diameters from the inlet lip, thus allowing sampling of air that has not been affected by wall interactions, under all flight conditions. Based on these results, we have started to design of a nested, 3-stage inlet system, which will be suitable for use with a future instrument, such as a CIMS.

The goal of Gulfstream Pod Inlet Test (GPIT I)  was to see if a large bore/centerline inlet and radial dump assembly would be practical on the 20 inch pod. GPIT I was successfully flown on the ARISTO-2017 program during February of 2017. A summary of the GPIT I test results were presented by Frank Flocke at the ICARE conference in Munich in July of 2017. We were also able to integrate an education and outreach component into the GPIT I project by having an Engineering Intern from a local High School work closely with Steve Shertz during the development and deployment phase.

The following main conclusions were derived from the GPIT I data:

  • The design of the  GPIT I inlet/exhaust structure allowed us to reach interior duct velocities up to 80% of the aircraft TAS. Due to the optimized geometry of the GPIT I design, 80% TAS is presumed to be a best-case scenario for future designs that utilize the GPIT I inlet and exhaust geometry.
  • The flow through the inlet was nominally unaffected by normal variations in Angle of Attack and Sideslip.
  • The flow through the inlet was turbulent throughout the majority of the length of the duct structure.
  • Even though the flow was turbulent, the core flow had little wall interaction as long as the axial velocity of the sample gas was much greater than the radial diffusion velocity from the wall.
  • With the thermal diffusion method, we did not observe any contamination of the central core flow until the exit flow was restricted to yield a velocity approximately 10% of TAS.

 

Figure 1. Engineering drawing of the inlet test setup.
Figure 1: Engineering drawing of the inlet test setup.

 

Figure 2: Steve Shertz (ACOM) and Kaley Barnes (Broomfield Legacy High School Engineering Intern with ACOM)
Figure 2: Steve Shertz (ACOM) and Kaley Barnes (Broomfield Legacy High School Engineering Intern with ACOM) with the Wing Pod mounted on the GV, ready for the 2017 ARISTO mission.

In collaboration with Clarkson University we are currently working on the second generation, nested inlet design that will be suitable as a fully functional instrument inlet. We plan to fly this design during the next ARISTO project on the GV aircraft.

Reference

Brune et al, “Mid latitude ClO below 22km altitude: Measurements with a new aircraft borne Instrument”, Geophysical Research Letters, Vol 15, No 2, pp 144-147, Feb 1988.