David W. Riggins

Blunt-Body Wave Drag Reduction Using Focused Energy Deposition

A parametric computational study of energy deposition upstream of generic two-dimensional and axisymmetric blunt bodies at Mach numbers of 6.5 and 10 is performed utilizing a full Navier-Stokes computational fluid dynamics code. The energy deposition modifies the upstream shock structure and results in large wave drag reduction and very high power effectiveness. Specifically, drag is reduced to values as low as 30% of baseline drag (no energy deposited into flow) and power effectiveness ratios (ratio of thrust power saved to power deposited into the flow) of up to 33 are obtained. The fluid dynamic and thermodynamic bases of the observed drag reduction are examined

THRUST LOSSES IN HYPERSONIC ENGINES PART 1 : METHODOLOGY

Expressions for the thrust losses of a scramjet engine are developed in terms of irreversible entropy increases and the degree of incomplete combustion. A method is developed that allows the calculation of the lost engine thrust or thrust potential caused by different loss mechanisms within a given e owe eld. This method allows the performance-based assessment of the trade between mixing enhancement and resultant increased e ow losses in scramjet combustors. An engine effectiveness parameter for use in optimization of engine components is dee ned in terms of thrust losses.

Vortex Generation and Mixing in Three-Dimensional Supersonic Combustors

The generation and evolution of the flow vorticity established by instream injector ramps in a high Mach number/high enthalpy scramjet combustor flow-field are described in detail for a number of computational cases. Classical fluid dynamic circulation is presented for these cases in order to clarify the spatial distribution and convection of the vorticity. The ability of the simulations to accurately represent Stokes Law of circulation is discussed and shown. In addition, the conservation of swirl (effectively the moment-of-momentum theorem) is presented for these flows. The impact of both turbulent diffusion and the vortex/ramp non-uniformity on the downstream mixing rate is clearly illustrated. A correlation over the length of the combustor between fuel-air mixing and a parameter called the vortex stirring length is demonstrated. Finally, computational results for a representative ramp injector are compared with experimental data. Influence of the stream vorticity on the effective turbulent Prandtl number used in the simulation is discussed.

Hypersonic Drag and Heat-Transfer Reduction Using a Forward-Facing Jet

A two-dimensional numerical study of the effects of a forward-facing jet located at the stagnation point of a blunt body on wave drag, heat transfer, and skin-friction drag is presented for Mach 6.5 flow at 30 km altitude. The full Navier-Stokes equations are used with variable viscosity and thermal conductivity. Upstream injection can significantly modify the flowfield. If the jet conditions are chosen properly, large reductions in drag and heat transfer can be obtained resulting in possible increases in the volumetric efficiency and static stability of aircraft as well as reductions in the heating protection requirements for hypersonic vehicles

Investigation of Scramjet Injection Strategies for High Mach Number Flows

A method for estimating the axial distribution of thrust performance potential in a supersonic combustor is described. A complementary technique for illustrating the spatial evolution and distribution of thrust potential and loss mechanisms in reacting flows is developed. A wall jet case and swept ramp injector case for Mach 17 and Mach 13.5 flight enthalpy inflow conditions, respectively, are numerically modeled and analyzed using these techniques.

Analysis of losses in supersonic mixing and reacting flows

A method for analyzing flow losses and thrust potential in supersonic combustors is presented. This method relies on a complete and consistent one-dimensional representation of a three-dimensional flow-field. Numerical results for flush wall fuel injection into a Mach 3 flow are examined and comparisons are made with experimental measurements of fuel concentration. Mixing results for a swept injection ramp, a straight (unswept) injection ramp, and a thirty degree downstream-directed flush wall jet in the same combustor duct are analyzed. The flow loss/thrust potential of the flush wall jet and the swept ramp are investigated (based on reacting solutions) using computed combustor effectiveness. The wall jet displays slightly higher thrust potential than the swept ramp at the end of the combustor.

Mixing enhancement in a supersonic combustor

Research has been conducted for a number of years at the NASA Langley Research Center to develop a supersonic combustion ramjet (scramjet) capable of propelling a vehicle at hypersonic speeds in the atmosphere or beyond. Recently, that research has been directed toward the optimization of the scramjet combustor, and in particular the efficiency of fuel-air mixing and reaction in the engine. This paper describes a numerical study of fuel-air mixing and reaction in a supersonic combustor, and discusses the analysis of a technique that was used to enhance the mixing processes and overall combustion efficiency in the flow. Based on the results of that study, conclusions are drawn regarding the applicability of the technique to enhance mixing in a scramjet combustor.

A numerical study of mixing enhancement in a supersonic combustor

This investigation describes an application of the Langley Research Center (LaRC) SPARK family of computer codes to swept and unswept ramp fuel injectors in a reacting highly vortical flow. Both mixing and reacting studies are performed. They show substantially higher mixing as well as flow losses for the swept ramp case. Computational results are compared both qualitatively and quantitatively with experimental results.

Thermodynamic Analysis of Dual-Mode Scramjet Engine Operation and Performance

Recent analytical advances in understanding the performance continuum (the thermodynamic spectrum) for air-breathing engines based on fundamental second-law considerations have clarified scramjet and ramjet operation, performance, and characteristics. Second-law based analysis is extended specifically in this work to clarify and describe the performance characteristics for dual-mode scramjet operation in the mid-speed range of flight Mach 4 to 7. This is done by a fundamental investigation of the complex but predictable interplay between heat release and irreversibilities in such an engine; results demonstrate the flow and performance character of the dual mode regime and of dual mode transition behavior. Both analytical and computational (multi-dimensional CFD) studies of sample dual-mode flow-fields are performed in order to demonstrate the second-law capability and performance and operability issues. The impact of the dual-mode regime is found to be characterized by decreasing overall irreversibility with increasing heat release at a given flight Mach number, within the operability limits of the system.

Three-Dimensional Effects in Modeling of Dual-Mode Scramjets

A numerical investigation of an experimental dual-mode scramjet configuration is performed. Both experimental and numerical results indicate significant upstream interaction for this case. Several computational cases are examined: these include the use of jet-to-jet symmetry and entire halfduct modeling. Grid convergence, turbulence modeling, and wall temperature effects are studied in terms of wall pressure predictions and flow-field characteristics. Wall pressure comparisons between CFD and experiment show fair agreement for the jet-to-jet case. However, further computations of the entire half-duct show the development of a large sidewall separation zone extending much further upstream than the separation zone at the duct centerline. This sidewall separation is the dominant feature in the CFD-generated flowfield but is not evident in the experimental data, resulting in a unfavorable comparison between CFD and experimental data. Current work aimed at resolving this issue and at further understanding asymmetric flow-structures in dual-mode flow-fields is discussed.