Sowing the seeds


Located at the Arnold Air Force Base in Tennessee, USA, the Arnold Engineering Development Complex (AEDC) is the largest and most advanced complex of flight simulation test facilities in the world. Since 1961, AEDC’s propulsion wind tunnel (PWT) facility has been devoted to the aerodynamic and propulsion integration testing of large-scale aircraft models, providing customers in both the military and private sectors with complete testing and analysis capabilities.

AEDC has used the best available particle seeder technology in the 4ft transonic wind tunnel, 4T, at the complex. This has happened despite the system’s inability to direct flow seeding of known size distribution into a desired region of the tunnel, the excess humidity, and the resulting difficulty in collecting acceptable test data. A new seeding system was needed to address these drawbacks, as well as to provide simple operator-control. Tyler Neale, project manager for the PWT ground test complex, explains, “AEDC needed a seeding system that would enable it to complement laser-based flow measurement and diagnostic techniques. This would enable flow fields around regions of interest of test articles to be visualized. This would be a relatively new capability.”

When the US Air Force solicited proposals through the Small Business Innovation Research (SBIR) program, several proposals were received. Fred Heltsley, senior engineer at AEDC, explains, “This solicitation was submitted to enable innovative designers to apply modern materials and technologies to develop an elusive ‘simple’ solution to an old industry need, knowing full well that the result would probably be complex, possibly prohibitively expensive, or maybe even unobtainable.” An SBIR Phase 2 contract was ultimately awarded to Advanced Projects Research Inc. (APRI), a California-based company that specializes in aero propulsion, power system engineering and laser diagnostics. AEDC said it was impressed with the hardware that APRI produced for the Phase 1 contract, with Heltsley calling it “well designed and precisely constructed”. As it turns out, in its selection of APRI, the US Air Force hadn’t found just a contractor, it found an empathetic partner.

The way it works

On a basic level, most smoke generators work the same way. A seed fluid is vaporized when heated to a temperature above its boiling point. When this vapor is injected into the cooler airstream of a wind tunnel, the fluid condenses into tiny droplets that make up what is commonly referred to as ‘smoke’. Technically, these droplets are called ‘seed particles’.

With no viable, commercial options available, the contractor commenced development of a custom-designed solution to offer a robust, reliable and easy-to-use system with precise controls and a high degree of automation. Describing APRI’s new system, Joe Wehrmeyer, engineer at AEDC, says, “The equipment is computer-controllable, so each of the six ‘seeders’ can be individually and remotely controlled using a desktop computer.” Thomas Sobota, CTO at APRI, explains, “Even with just one injector, the old technology demanded a high degree of human attention. Though configured with six injectors for the 4T wind tunnel, the new system has a potential of up to 100 injectors in a larger tunnel, so it was critical that we engineer a system with the utmost control and automation.”

At the top of AEDC’s wish list was the ability to direct a singular high-density stream of seed particles within larger flow with lower density seeding. Engineers at APRI approached this request from both a user’s and a contractor’s point of view. By devising independently controlled, multiple injection points, the size of the particle stream is minimized, while still meeting test requirements. Additionally, each injector in the new design includes mass flow control, enabling the manipulation of seed particle densities.

Innovation provider

Other APRI innovations include heating control of the working fluid up to the tip for each injector, as well as the metering of co-flowing air jets to control the mixing and spreading of the injected droplets. Safety and transient control were essential, leading to minimal settling times, no burned fluids, no burned-out heaters, no sputtering and no leaking – deficiencies well documented in older technology. The new system’s features eliminated these frustrations and now present a huge step forward in particle seeding technology.

“APRI’s proposed conceptual designs for the full-scale seeder positioning system were innovative and designed to minimize installation time and to reduce the adverse impact of tunnel downtime,” Heltsley explains. That need to reduce downtime was critical to both AEDC and APRI.

Turn it on/off

Another key issue was the ability to turn the seed stream on and off quickly. In response, APRI’s team engineered a means by which to efficiently control and minimize the amount of seed material introduced into the tunnel. Among other potential benefits of the new APRI system, AEDC’s Heltsley lists: lack of impact

on tunnel humidity; extended Mach number operation range; more precise adjustment and monitoring of system operation; controllable stream tube size, shape/position; as well as constant and quantifiable particle size density. Final testing will be conducted at AEDC when the new seeder is installed into the 4T wind tunnel for the combined pressure, flow rate, and temperature conditions. Jay Marsh, vice president of engineering at APRI, expects a positive outcome: “APRI anticipates success with this final testing based on validation tests conducted on a thermal heat transfer test model in an altitude test chamber.” Before delivery to AEDC, the entire six-injector system array was functionally tested at APRI at ambient pressure and room temperature conditions.

Additionally, a single injector module was delivered to AEDC for mounting into a low-pressure test system and was successfully tested for functionality, including ambient flow rates. The system also demonstrated no leakage in low-pressure operations.

If final testing approval is granted for this initial array in the 4T wind tunnel, APRI’s goal is that a larger configuration of up to 100 injectors can be installed in the larger 16T wind tunnel. If there is an opportunity for it to be installed, APRI’s new particle seeder should set a new standard for a heightened, state-of-the-art aerodynamics testing experience. Delivering greater precision and more detailed information for AEDC customers, as well as ease of use and simple clean-up for test engineers, this new seeder system promises to help advance the technology of wind tunnel testing well into the 21st century.

Based in Los Angeles, California, Seth E Bass is a writer for multiple platflorms including film, television, magazines and website content

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