Frontiers of Engineering: Reports on Leading-Edge Engineering from the 2001 NAE Symposium on Frontiers of Engineering (2002)

JEFFREY W. HAMSTRA AND DANIEL N. MILLER

Lockheed Martin Aeronautics Company

Fort Worth, Texas

Jet engine inlet and exhaust systems will play a major role in determining the configuration and capability of tomorrow’s military air vehicles. To support advances in vehicle design, these systems must deliver higher aerodynamic performance, as well as enhanced functionality (such as thrust vectoring), and at the same time be lighter in weight, less expensive, and smaller than current state-of-the-art systems. Traditionally, the physical laws governing high-speed viscous flow have limited the implementation of the exotic inlet and exhaust flowpaths that will be required for the future. The emerging technology of active flow control (AFC) could provide a breakthrough in aeronautical science that would enable the engineering design of next-generation inlet and exhaust systems.

Combat aircraft have continued to evolve since the introduction of the jet engine during World War II. The F-16, first flown in 1974, the F-22 (1990), and the F-35 (2001), exemplify the last 25 years of this evolution (Figure 1). These aircraft are characterized by a traditional wing/body/tail arrangement, are all commanded by an on-board pilot, and are all driven by the requirement for superb aerodynamic performance. Throughout the evolutionary process, two additional design characteristics, affordability and stealth, have become increasingly important.

The propulsion system for a combat air vehicle is critically important in terms of cost, weight, volume, stealth, performance, and overall configuration integration. Major propulsion system components, such as the engine inlet

FIGURE 1 Today’s combat aircraft are driven by requirements for superb aerodynamic performance. Source: 2002 by Lockheed Martin. Published with permission.

system and the engine exhaust system, are also critical. The inlet system captures outside air and delivers it to the engine. Inlet systems are typically 10 to 20 feet long and weigh on the order of 500 to 1200+ pounds. Major performance figures-of-merit for the inlet system are pressure recovery (a measure of the overall efficiency of the system) and distortion (a measure of the pressure non-uniformity at the inlet/engine interface). Severe distortion can cause stalling or even flameout of the jet engine. The purpose of the exhaust system is to convert the engine’s high-temperature thermodynamic energy into net propulsive force. Performance figures-of-merit for the exhaust system include gross thrust coefficient (a measure of the efficiency of the system) and thrust vector angle (a measure of the system’s ability to divert or steer the exhaust thrust in a nonaxial direction). Thrust vectoring is used in conjunction with the wing and tail flaps to control the air vehicle.

As combat aircraft continue to evolve, they must retain current levels of aerodynamic performance and, at the same time, become more affordable and more stealthy. These improvements will require changes in the design of inlets and exhausts; both systems will have to be shorter and more compact, simpler and lighter in terms of mechanical complexity and moving parts, and shaped to conform to the advanced, all-wing, tailless vehicle configurations of future aircraft (Figure 2). Internal studies at Lockheed Martin (LM) have shown that many of the necessary design characteristics of future inlet and exhaust systems are not achievable with current aerodynamic technology. Researchers in the

FIGURE 2 Advanced combat aircraft must retain high aeroperformance while emphasizing affordability and stealth. Source: 2002 by Lockheed Martin. Published with permission.

government, at LM, at other companies, and at universities are all investigating an emerging technology, AFC, to address these problems. AFC is defined as the ability to control large-scale aerodynamic flow phenomena with very small-scale (or microscale) perturbations to the flow field near the wall.

One application for AFC is in an advanced inlet system that features conformal shaping and extreme serpentine wall curvature (Figure 2) (Anderson et al., 1999; Bender et al., 1999; Hamstra et al., 2000). With current technology, high-speed flow entering the inlet duct is unable to negotiate the extreme internal wall curvature and thus detaches or “separates” from the wall. This behavior, which has its genesis in the very thin “boundary layer” of air next to the wall, results in massive pressure loss and flow distortion at the inlet/engine interface. These flow field characteristics greatly reduce net thrust and, sometimes, even result in engine stall. AFC can be used to energize/restructure the boundary layer in a way that would prevent flow separation and greatly diminish distortion. Key design considerations include using the proper type of actuation/energization device and identifying the proper “receptive zones” for device placement because

even very small perturbations near the wall may cause a global change in the entire flow field.

Numerous design variables must be properly chosen in designing an AFC actuation system. These variables include physical size, orientation, location, and the number of actuators near the inlet wall. Because the size of the AFC design space is so large, researchers at LM have helped pioneer the use of a coupled design of experiments (DOE)/computational fluid dynamics (CFD) process for optimization. This process uses CFD to evaluate each element in the design matrix and repetitive applications of DOE methods to search through the design space. The process is occasionally checked and validated through testing of large-scale (~30 percent) models.

A joint LM/NASA Glenn Research Center (GRC) team has conducted several validation tests at the GRC W1B test facility. The tests featured models fabricated from resin using a “rapid prototyping” laser stereo-lithography process. The results of one test (Figure 3) show that, without flow control, the inlet produces zones of massive pressure loss and resultant high distortion, yielding a pressure contour pattern unacceptable for turbofan engine operation. With flow control, pressure losses were decreased by 40 percent, distortion was decreased by 80 percent, and an acceptable pattern was produced. These tests demonstrate the viability of flow control under realistic conditions on a relevant, large-scale inlet-system configuration.

Continuing research is focused on making the inlet flow-control suite robust across the range of maneuver, speed, and airflow settings envisioned for future inlet systems. Control schemes that incorporate distributed feedback sensors and reactive, closed-loop control logic are also under study. The goal of this research is to produce a flow-control system that allows unprecedented freedom in inlet design while simultaneously optimizing engine inflow conditions across the entire operating range of the aircraft.

A second application for AFC is in the engine exhaust system. Modern exhaust systems incorporate significant complexity to provide thrust vectoring and jet area control (Bender et al., 2000; Miller and Catt, 1995; Miller et al., 1997, 1999, 2001; Vakili et al., 1999; Yagle et al., 2001). These mechanical subsystems incur weight and cost impacts to the vehicle and limit the exhaust system to shapes that can be easily and efficiently mechanized, namely, simple round designs or simple rectangular designs. With AFC, researchers hope to design systems that achieve the same functionality without mechanical flowpath variations, thus reducing cost and weight while enabling more exotic cross-sectional shapes that can conform to the body of an advanced aircraft.

The fundamental approach to flow control in the exhaust system is shown in Figure 4. High-pressure air is injected into the nozzle’s divergent section with

the goal of creating “virtual aerodynamic surfaces” that provide the same functionality as current variable-geometry mechanical flaps. By injecting symmetrically about a given axial location, the minimum flow area of the jet plume can be changed, thereby allowing for changes in the engine power setting. By injecting asymmetrically, the direction of the jet plume can be changed, thereby allowing for thrust vectoring.

The process used to develop the exhaust system flow-control suite is identical to the one used for the inlet system. DOE methods are used to search through the design space, and CFD is used to evaluate the thrust coefficient and vectoring capability of each design element. Large-scale tests are occasionally conducted to validate the design process.

AFC technology is also under development for a number of other air-vehicle system applications. These applications include the control and stabilization of aircraft wakes in the vicinity of directed-energy weapons; augmentation or replacement of conventional aircraft-control effectors, such as trailing edge flaps; and suppression of acoustic loads for internal weapons bays.

AFC is an emerging technology that could enable a breakthrough in traditional aerodynamic design limitations for a wide range of advanced combat aircraft systems. For the engine inlet and exhaust systems, flow-control technology has the potential to enable unprecedented freedom to incorporate exotic flowpath designs, enable optimization of engine inflow conditions regardless of aircraft condition, and provide superior exhaust system functionality with reduced weight and cost.

Anderson, B., D. Miller, P. Yagle, and P. Truax. 1999. A Study on MEMS Flow Control for the Management of Engine Face Distortion in Compact Inlet Systems. ASME Paper No. FEDSM99-6920.

Bender, E., B. Anderson, and P. Yagle. 1999. Vortex Generator Modeling for Navier-Stokes Codes. ASME Paper No. FEDSM99-69219.

Bender, E., D. Miller, B. Smith, P. Yagle, P. Vermeulen, and S. Walker. 2000. Simulation of Pulsed Injection in a Crossflow Using 3-D Unsteady CFD. AIAA Paper No. 2000-2318.

Hamstra, J., D. Miller, P. Truax, B. Anderson, and B. Wendt. 2000. Active inlet flow control technology demonstration. Aeronautical Journal 104(1040):473–480.

Miller, D., and J. Catt. 1995. Conceptual Development of Fixed-Geometry Nozzles Using Fluidic Throat-Area Control. AIAA Paper No. 95-2603.

Miller, D., J. Catt, and S. Walker. 1997. Extending Flow Control of Fixed Nozzles Through Systematic Design: Introducing Assisted Reinjection. ASME Paper No. FEDSM97-3680.

Miller, D., P. Yagle, and J. Hamstra. 1999. Fluidic Throat Skewing for Thrust Vectoring in Fixed-Geometry Nozzles. AIAA Paper No. 99-0365.

Miller, D., P. Yagle, E. Bender, and P. Vermeulen. 2001. A Computational Investigation of Pulsed Injection into a Confined Expanding Cross Flow. AIAA Paper No. 2001-3026.

Vakili, A., S. Sauerwein, and D. Miller. 1999. Pulsed Injection Applied to Nozzle Internal Flow Control. AIAA Paper No. 99-1002.

Yagle, P.J., D.N. Miller, K.B. Ginn, and J.W. Hamstra. 2001. Demonstration of fluid throat skewing for thrust vectoring in structurally fixed nozzles. Journal of Engineering for Gas Turbines and Power 123(3):502–507.