Vortex. Vortices. Swirls of air trailing behind wings as they produce lift. You know from the movies what happens to a jet flying through another jet's wake, but what happens to a missile's tail fins passing through the wake of the missile's wings? Designers must understand these aerodynamic forces to predict their effect on missile flight.
To that end, Josh Doyle and Chris Rosema, missile aerodynamicists in the Aerodynamics Technology Branch of the System Simulation and Development Directorate, have developed a vortex cloud model and integrated it into Missile Data Compendium, a semi-empirical aerodynamic prediction code originally developed by the Air Force in the 1980s. Missile DATCOM, which had been improved several times over the next two decades, is still in use throughout the defense community for conceptual design and early design trade analysis and for rapid database development.
Rosema began making enhancements to Missile DATCOM in 2005. Doyle joined the effort in 2007. They worked together to improve the model for wing-shed vortices propagating downstream.
They produce downwash, an effective downward airflow, on the tails, Rosema explained.
"That's why airplanes have to wait a minute or so in between takeoffs on a runway," he said. "Each plane creates this vortex-generated downwash that is proportional to the lift it is generating. This downwash will have an adverse effect on the runway flow field for the airplane taking off behind it. The vortices dissipate over time due to air viscosity, so once the vortices become sufficiently weak, the next plane can take off."
Similarly, when missile wings or forward control surfaces produce lift, they create vortices that act on the tail surfaces.
Last year, Doyle and Rosema began the follow-on task to develop a higher fidelity model for vortices shed by the body of a missile. Previously, Missile DATCOM had relied on empirical data and a single vortex pair to model the leeward side of a missile body.
Integration of the resulting vortex cloud would then model the actual flow field more accurately. The model accounts for the interactions of body-shed vortices with each other and with wing-shed vortices.
"We're taking the concept, using high fidelity computational fluid dynamics for the empirical component of the model and then implementing into Missile DATCOM for the first time," Rosema said.
This new capability provides a more accurate representation of the vortices' cumulative aerodynamic effects on downstream control surfaces and ultimately, missile flight.
The incorporation of this vortex cloud model in the next version of Missile DATCOM represents a potential cost and schedule savings; the new vortex models really aid in the design process, Rosema said. The more accurate code can lead to fewer design iterations and reduce the need for the costly CFD analysis and wind tunnel testing conducted during the latter stages of design.
Use of the improved Missile DATCOM may result in faster, more affordable development of missile systems and has the potential to benefit systems such as the Counter Rocket Artillery Mortar Program Directorate Accelerated Improved Intercept Initiative and the AMRDEC-developed Extended Area Protection System.
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