Inside this Issue

• Bioinformatics
• Next Generation Tsunami Modeling
• Space Weather Forecasting
• Navigating Information Space
• Imagining Without Constraints
• Internal Tides of the Ocean
• High-Flying at Mach 8

Story by Leone Thierman

The possibility of flying seven or eight times the speed of sound could turn an eight-hour flight from New York to London into a short hop of less than an hour.

Scientists are working on new ways to fly air vehicles farther and faster by making them smaller and lighter. Conventional rocket propulsion combines liquid fuel with liquid oxygen to create thrust. If the need to carry liquid oxygen were removed by using air in the surrounding atmosphere instead, aircraft could be smaller and lighter, thereby providing a cost-effective method for routine, affordable access to space. However, several other problems arise when flying in air at very high speeds. Mixing and burning fuel in the very short time that the air is in the combustor is a major difficulty. Equally critical is the fact that the vehicle must withstand tremendous heat and drag loads arising from friction and pressure effects.

Solving the issues surrounding the design and development of vehicles capable of flying at sustained high speeds is the work of a group of scientists at the Air Vehicles Directorate, Aeronautical Sciences Division, at Wright Patterson Air Force Base, led by technical area leader, Dr. Datta Gaitonde.

Gaitonde and his colleagues are developing computer simulations to understand the basic physics of air-breathing devices, called scramjets, or Supersonic Combustion Ramjets, as a primary source of propulsion. He hopes these simulations will help them explore high-speed flows and their control with plasma-based methods. Plasma in this context is a medium, such as air, that is ionized and can conduct electricity. Although air is normally a very poor conductor, flights at supersonic speeds occur at extreme altitudes where, with some augmentation, it can be ionized. It is then susceptible to forces and heating through electromagnetic fields, which are exerted by onboard devices such as magnets, coils and electrodes.

Using advanced electrical technology eliminates the need for ineffective mechanical moving parts, such as slats and flaps that can create problems, particularly at high speeds.

A ramjet engine provides a simple, light propulsion system for high-speed flight within the atmosphere, unlike a rocket, which must carry all of its oxygen. The ramjet has only an inlet, a combustor that consists of a fuel injector and flame holder, and a nozzle. The compressor is not needed in a ramjet because “ramming” external air into the combustor using the forward speed of the vehicle produces the necessary high pressure.

For a vehicle traveling supersonically (greater than the speed of sound, or at a Mach number greater than 1), the air entering a ramjet engine must be slowed to subsonic speeds (less than Mach 1) by the aircraft inlet. At speeds beyond Mach 5 however, this process is accompanied by severe losses in propulsion efficiency. The new scramjet (supersonic-combustion ramjets) attempts to solve this problem by performing the combustion in the burner, even as the air is flowing at supersonic speeds.

Scramjets cannot produce thrust when the engine is stationary, only when the vehicle is already moving. As a result, some other propulsion system must be used to boost the vehicle to a speed at which the ramjet begins to produce thrust. An accepted approach to reach scramjet take-over speeds is to attach the scramjet to a rocket booster as a first stage and then operate the scramjet once the rocket has increased the speed sufficiently.

Given the difficulty of reproducing flight conditions in ground-test facilities, computer simulations play an integral role in design and development. Integrating many different disciplines, including fluid dynamics, electromagnetics and chemical kinetics, creates a huge and complex set of equations. Solving those equations requires the use of massive computational resources. Together with other High-Performance Computing (HPC) centers, the Arctic Region Supercomputing Center (ARSC) at the University of Alaska Fairbanks (UAF), is helping Gaitonde’s effort for a Department of Defense (DoD) High Performance Computing Modernization Program (HPCMP) Challenge project.

The focus of the effort is on tip-to-tail simulation of a scramjet operating under the magnetogasdynamic (MGD) energy bypass procedure. In this, energy is removed from the inlet with an MGD generator and reinserted in the nozzle through an MGD accelerator. Although the specific goal of this proposal is on a propulsion problem, the demonstration has profound implications in terms of simulation techniques and scientific analyses of various other areas where plasma fluid interactions are important. This includes low-speed flow-control methods, cooling of nuclear components with liquid metal flows and a variety of biomedical applications. The utility of simulation scales directly with the realism of the model incorporated into the numerical procedure.

The present approach deploys the most advanced high-fidelity numerical schemes together with a blend of first-principles and phenomenological models, to predict coupled aerospace plasma/fluid phenomena and, upon completion, will demonstrate the cost efficiency of simulation-led design and development.

Gaitonde’s project addresses science issues encountered prior to and during the development phase of advanced technology demonstrations. Such an approach reduces development costs by a significant factor through vetting of inefficient and unworkable concepts and providing lessons-learned experience at minimal costs. Furthermore, by reducing the wall-clock time, the high performance simulation strategy provides the versatility needed to foster the generation and testing of out-of-the box concepts and to confront and overcome key physics-based limitations, thus precluding costly technological surprises at advanced stages of the development program.

Results to date have focused on a non-axisymmetric dual-plane compression design. Several conclusions have been drawn from this ongoing effort. In particular, it has been determined that the interaction of shock-waves and boundary layers has a profound effect on the flow field and its interaction with the imposed electromagnetic environment.

Generator operation is generally efficient and is a viable method of extracting power from the incoming flow. However, if the flow separates from the walls, the length along which energy can be extracted is limited. On the other hand, flow distortion severely curtails the efficiency of the MGD-augmented thrust.

Successful and affordable long-range hypersonic flight requires more breakthroughs in the understanding and implementation of revolutionary concepts, such as plasma-based flow control. Therefore, formulations must be extended to include variants of the Maxwell equations and sophisticated plasma models.

Gaitonde feels that it is essential to develop highly accurate physical models, and to couple them with advanced, robust numerical methods that use massively parallel modern computational systems. Broad research is required to develop and implement highly accurate algorithms for a hierarchy of theoretical models of increasing fidelity with and without the continuum approximation. The computational tools developed must continually be employed to investigate a variety of physical phenomena, including direct numerical simulations of supersonic and hypersonic transition and turbulence, plasma behavior in the aerospace environment, and shock/boundary layer interactions. Other key areas of research addressed by this project include development, implementation, and validation of models for state-to-state kinetics, and exploration of drag reduction and thermal protection techniques through flow control.