ARNOLD AIR FORCE BASE, Tenn. -- Until recently, the last period of sustained activity at von Kármán Gas Dynamics Facility (VKF) Wind Tunnel D at Arnold Air Force Base was more than four decades ago.
Today, Tunnel D is humming once again, as more than 50 checkout runs have been conducted so far this year. The complete resurrection of the long-quiescent tunnel is moving closer to fruition.
Checkout work is nearing completion, after which the reactivated Tunnel D will be characterized and used to support research later this year.
“Bringing wind tunnels back online illustrates the United States Air Force’s dedication to research, development and test in support of our nation’s highest priorities,” said Lt. Col. David Hoffman, director of the Flight Systems Combined Test Force at Arnold AFB and executing authority for the tunnel reactivations.
The Air Force Research Laboratory (AFRL) at Arnold AFB has taken the lead on the Tunnel D reactivation efforts, with this work beginning around two years ago. An extensive renovation of Building 607 where Tunnel D is housed and the tunnel reactivation itself were funded by AFRL, with AEDC contributing to earlier feasibility assessment efforts in 2015.
Members of the AFRL High Speed Experimentation Branch, which is part of the High Speed System Division of the AFRL Aerospace Systems Directorate, will operate, manage and schedule the tunnel. Tunnel D will be primarily used by AFRL as a research facility to explore supersonic and hypersonic aerodynamic phenomena.
Dr. Jerrod Hofferth, the AFRL technical lead on the Tunnel D reactivation project, said returning Tunnel D to an operational state falls in line with original motivations for the establishment of the AFRL High Speed Experimentation Branch at Arnold in 2014-15. The facility will be used to bolster AFRL research efforts.
“One mission of the AFRL High Speed Experimentation Branch is to ‘Collaborate to advance test capabilities at AEDC in support of AFRL research,’ and Tunnel D is an ideal platform for each aspect of this,” Hofferth said.
The first air-on operations at Tunnel D were conducted in February, ending more than 40 years of inactivity. Hofferth said the last evidence for wind tunnel operations in Tunnel D was dated around 1977.
As of mid-April, 42 separate runs had been conducted primarily for validation of control valve and in-line flow heater performance, each varying in duration from 2 to 22 minutes. Prior to on-air operations, offline checkouts on the data systems, control systems and other tunnel subsystems have also been performed.
“Having Tunnel D online will enhance the ability of the Arnold AFB team to develop and validate science and technology for the next generation of high-speed and hypersonic weapon systems,” said AFRL High Speed Experimentation Branch Chief Glenn Liston.
In its final operational state, Tunnel D will operate intermittently in what Hofferth called a “pressure-vacuum blowdown” mode, as it originally did from 1953 to 1977. This means that the tunnel uses a combination of high-pressure air as the upstream supply and a vacuum applied at the exhaust to achieve the pressure ratios across the nozzle necessary for supersonic flow.
The typical maximum run-time in this normal operating mode will be about 4 minutes at a time, occurring once or twice a day. Alternatively, we have the option to do several shorter runs instead.”
Although Tunnel D will be exclusively operated at supersonic Mach numbers once complete, the recent runs have been conducted with subsonic flow in the test section. Hofferth said this is because recent checkouts are not yet using the vacuum side of the system, exhausting through the sphere at atmospheric pressure rather than into the sphere evacuated to low pressures.
“This allows us to focus on the upstream side performance without limitations of short run-times or long turnaround times for evacuation of the vacuum sphere,” Hofferth said. Checkouts of the vacuum system were conducted separately in September 2017.
Otherwise, the facility is being operated as though full supersonic runs are being conducted.
The goal of the checkouts has been to operate the supply-side process air components – the hydraulically-operated control valves and an electric in-line flow heater – for the first time to validate their performance against expectations and specifications. Hofferth said the series of checkouts have been successful in this regard.
“’Success’ is measured by demonstrating the operation of the various VKF Tunnel D subsystems within their design specifications,” Liston said. “By completing the subsystem checkouts, we gain confidence that the tunnel will operate correctly across the full performance envelope that is required for AFRL’s research mission.”
Although the runs have been conducted at subsonic levels up to this point, Hofferth said the Tunnel D checkout campaign has been valuable.
“One of the main benefits of the checkout campaign has been the accumulation of operational experience,” he said. “For all of these runs, we have been using the exact same data systems, controls, procedures, and safety systems and protocols that we intend to use for operating the tunnel in the full supersonic pressure-vacuum blowdown mode. As such, the experience we have gained by operating the facility in subsonic checkout mode will directly translate into reduced risk for our first supersonic runs.”
The subsonic blowdown phase of the checkout campaign is nearly complete, with all primary test objectives for control valve and heater characterizations met. The AFRL-AEDC team is working to evaluate and resolve an unexpected mechanical issue with legacy hardware, after which the return to service will continue toward supersonic operations. Once basic checkouts in supersonic mode are complete, the test section flow quality will be characterized at Mach 4, and the first research activities in Tunnel D will begin.
The operating envelope of the facility will be gradually expanded to other Mach numbers as needed.
Tunnel D is similar to the supersonic VKF Tunnel A. Tunnel D was originally built and operated as the small-scale demonstrator/pilot tunnel for Tunnel A, using the same flexible-plate, variable-Mach number nozzle concept for Mach numbers 1.5 to 5.
The principal differences between Tunnel D and Tunnel A are that Tunnel D is smaller, with a test section of 12 inch-by-12 inch versus the 40 inch-by-40 inch Tunnel A section, and that Tunnel D cannot operate continuously. The result is that Tunnel D can generate similar flow conditions to Tunnel A at a much lower cost and with minimal impact to the base’s utilities scheduling.
“That Tunnel D can generate Tunnel A conditions at low cost and with high flexibility makes it ideal for various types of research and development,” Hofferth said. “AFRL researchers intend to use it as their primary platform for a variety of fundamental and applied research experiments, with a focus on scaling effects and the coupling of high-speed aerodynamics with the deformation of compliant structures.”
In addition to the lower operating cost, Liston said there were other considerations taken into account when the AFRL and AEDC began planning the joint venture that was the reactivation of Tunnel D.
“The specific conditions that Tunnel D can achieve are different from what’s available in other AFRL facilities, so reactivating it adds robustness to AFRL’s research capabilities,” he said. “We recognized that Tunnel D has value for AEDC in that we can cooperatively develop diagnostics and test techniques that can be transitioned to AEDC’s production test facilities.”
“The overall success of this effort is remarkable and is benefiting our large scale super/hypersonic Tunnel 16S return to service,” Hoffman said. “Bringing a wind tunnel back online is an extremely rare event. Having two super/hypersonic wind tunnels return to service within a year is something never before done and clearly shows the nation’s dedication to collecting the data and presenting the information needed in support of our nation’s needs.”
The reactivated Tunnel D will also be used as a diagnostics testbed and calibration platform, both for AFRL in-house activities and those being performed or sponsored by various entities, including the AEDC Technology group, research groups, university research groups, and small businesses under the Small Business Innovation Research program.
“There is also interest in using Tunnel D for workforce development by offering exposure to supersonic wind tunnel operations for new employees as part of a training curricula, to students through internship or fellowship programs, academic faculty on visiting researcher programs, industry professionals, and more,” Hofferth said.
Tunnel D will, in practice, be an AFRL facility, though the wind tunnel and the building containing it will remain AEDC assets in the real property sense. The local AFRL branch will finance and operate Tunnel D, and contractors at Arnold AFB will perform much of the maintenance and assist with test support tasks.
“This renovation at Tunnel D has been a great project,” said VKF Group Manager David Milleville. “We put several folks with widely varying backgrounds together to wake up a tunnel that had been dormant and essentially abandoned for so many years. Tunnel components, original and new, have been coupled with a modern and highly-sophisticated control system to maximize tunnel run time and data quality.
“Every member of the team contributed as we met challenges along the way. Checkouts have progressed enough to confirm that this team has successfully delivered a revitalized and viable test asset at AEDC. This team will continue to support tunnel commissioning toward full test envelope capability.”
Daniel Ogg, a research engineer with the AFRL High Speed Experimentation Branch who managed the programmatic aspects of the reactivation, expressed his appreciation for all of those who helped make the return of Tunnel D possible.
“The past two years have been rather demanding at times, full of surprises to go along with reactivating a tunnel that has sat dormant for 40 years,” Ogg said. “There are a handful of engineers that have gone above and beyond during this reactivation, and we owe a sincere ‘thank you’ to these folks: Troy Wetherholt, for all of his design and analysis work; Tony Medley, for his mechanical expertise; Ken Brown, for his dedicated integration efforts; Sid Ledbetter, for his controls engineering expertise and the expert electricians supporting his work; Scott Sisk, for his controls software development and ongoing support through the checkout phases; and last, but certainly not least, Denise Bryant, who at one point or another has supported almost every phase and aspect of this reactivation effort. Thank you for all of your hard work and dedication these past two years, and thank you to the countless other folks who have supported this project as well.”