Russell M. Cummings

Fifty years of hypersonics: where we've been, where we're going

Abstract Hypersonic flight has been with us since 22 September 1963, when Robert M. White flew the North American X-15 at 4520 mph at an altitude of 354,200 ft —a Mach number of 6.7! This remarkable achievement was accomplished over six decades due to intensive research and development by a large number of scientists and engineers. In spite of that momentous achievement, designers have found the hypersonic environment to be harsh and non-forgiving. New programs since the 1960s have often uncovered the unknown unknowns, usually the hard way—early flights of new systems have often revealed problems of which the designers were unaware. Such problems include: the ineffectiveness of the body flap for the Space Shuttle Orbiter, the viscous/inviscid interactions produced by the umbilical fairings that damaged the conical section tile protection system of the Gemini Capsule, and the shock/shock interaction that damaged the X-15A-2 when it carried the hypersonic ramjet experiment. In order to continue to make advances in hypersonic flight a sustained and visionary effort is essential to generate required knowledge and technology. In order to better prepare for future developments in hypersonic flight, this article reviews the advances made within the past 50 years and then looks into the future, not just for new technological developments, but for new ways of thinking about the unknown challenges that lie ahead.

Detached-Eddy Simulation With Compressibility Corrections Applied to a Supersonic Axisymmetric Base Flow

Detached-eddy simulation is applied to an axisymmetric base flow at supersonic conditions. Detached-eddy simulation is a hybrid approach to modeling turbulence that combines the best features of the Reynolds-averaged Navier-Stokes and large-eddy simulation approaches. In the Reynolds-averaged mode, the model is currently based on either the Spalart-Allmaras turbulence model or Menter’s shear stress transport model; in the largeeddy simulation mode, it is based on the Smagorinski subgrid scale model. The intended application of detached-eddy simulation is the treatment of massively separated, highReynolds number flows over complex configurations (entire aircraft, automobiles, etc.). Because of the intented future application of the methods to complex configurations, Cobalt, an unstructured grid Navier-Stokes solver, is used. The current work incorporates compressible shear layer corrections in both the Spalart-Allmaras and shear stress transport-based detached-eddy simulation models. The effect of these corrections on both detached-eddy simulation and Reynolds-averaged Navier-Stokes models is examined, and comparisons are made to the experiments of Herrin and Dutton. Solutions are obtained on several grids—both structured and unstructured—to test the sensitivity of the models and code to grid refinement and grid type. The results show that predictions of base flows using detached-eddy simulation compare very well with available experimental data, including turbulence quantities in the wake of the axisymmetric body. @DOI: 10.1115/1.1517572#

Computational Challenges in High Angle of Attack Flow Prediction

Abstract Aircraft aerodynamics have been predicted using computational fluid dynamics for a number of years. While viscous flow computations for cruise conditions have become commonplace, the non-linear effects that take place at high angles of attack are much more difficult to predict. A variety of difficulties arise when performing these computations, including challenges in properly modeling turbulence and transition for vortical and massively separated flows, the need to use appropriate numerical algorithms if flow asymmetry is possible, and the difficulties in creating grids that allow for accurate simulation of the flowfield. These issues are addressed and recommendations are made for further improvements in high angle of attack flow prediction. Current predictive capabilities for high angle of attack flows are reviewed, and solutions based on hybrid turbulence models are presented.

Wake Integration for Three-Dimensional Flowfield Computations: Theoretical Development

This paper examines the analytical, experimental, and computational aspects of the determination of the drag acting on an aircraft in flight, with or without powered engines, for subsonic/transonic flow. Using a momentum balance approach, the drag is represented by an integral over a crossflow plane at an arbitrary distance behind the aircraft. Asymptotic evaluation of the integral shows the drag can be decomposed into three components corresponding to streamwise vorticity and variations in entropy and stagnation enthalpy. These are related to the established engineering concepts of induced drag, wave drag, profile drag, and engine power and efficiency. This decomposition of the components of drag is useful in formulating techniques for accurately evaluating drag using computational fluid dynamics calculations or experimental data.

An Integrated Computational/Experimental Approach to UCAV Stability & Control Estimation: Overview of NATO RTO AVT-161

A comprehensive research program designed to investigate the ability of computational methods to predict stability and control characteristics of realistic flight vehicles has been undertaken. The integrated approach to simulating static and dynamic stability characteristics for a generic UCAV and the X-31 configuration was performed by NATO RTO Task Group AVT-161. The UCAV, named SACCON (Stability and Control Configuration), and the X-31 are the subject of an intensive computational and experimental study. The stability characteristics of the vehicles are being evaluated via a highly integrated approach, where CFD and experimental results are being used in a parallel and collaborative fashion. The results show that computational methods have made great strides in predicting static and dynamic stability charactersitics, but several key issues need to be resolved before efficient, affordable, and reliable predictions are available.

Integrated Computational/Experimental Approach to Unmanned Combat Air Vehicle Stability and Control Estimation

A comprehensive research program designed to investigate the ability of computational methods to predict stability and control characteristics of a generic unmanned combat air vehicle has been undertaken. The integrated approach to simulating static and dynamic stability characteristics was performed by the NATO Research and Technology Organization Task Group AVT-161. The vehicle named Stability and Control CONfiguration (SACCON) was the subject of an intensive computational and experimental study. The stability characteristics of the vehicle were evaluated via a highly integrated approach, where computational fluid dynamics and experimental results were used in a parallel and collaborative fashion. The results show that computational methods have made great strides in predicting static and dynamic stability characteristics, but several key issues need to be resolved before efficient, affordable, and reliable predictions are available.

Analysis of Delta-Wing Vortical Substructures Using Detached-Eddy Simulation

An understanding of the vortical structures that comprise the vortical flowfield around slender bodies is essential for the development of highly maneuverable and high-angle-of-attack flight. This is primarily because of the physical limits these phenomena impose on aircraft and missiles at extreme flight conditions. Demands for more maneuverable air vehicles have pushed the limits of current computational fluid dynamics methods in the high-Reynolds-number regime. Simulation methods must be able to accurately describe the unsteady, vortical flowfields associated with fighter aircraft at Reynolds numbers more representative of full-scale vehicles. It is the goal here to demonstrate the ability of detached-eddy simulation (DES), a hybrid Reynolds-averaged Navier-Stokes/large-eddy-simulation method, to accurately model the vortical flowfield over a slender delta wing at Reynolds numbers above one million. DES has successfully predicted the location of the vortex breakdown phenomenon, and the goal of the current effort is to analyze and assess the influence of vortical substructures in the separating shear layers that roll up to form the leading-edge vortices. Very detailed experiments performed at ONERA using three-dimensional laser-Doppler-velocimetry measurement will be used to compare simulations utilizing DES turbulence models. The computational results provide novel insight into the formation and impact of the vortical substructures in the separating shear layers on the entire vertical flowfield.

Computational evaluation of an airfoil with a Gurney flap

A 2D numerical investigation was performed to determine the effect of a Gurney flap on a NACA 4412 airfoil. A Gurney flap is a flat plate on the order of 1 to 3 percent of the airfoil chord length, oriented perpendicular to the airfoil chord line and located at the trailing edge of the airfoil. An incompressible Navier Stokes code, INS2D, was used to calculate the flow field about the airfoil. The fully turbulent results were obtained using the Baldwin-Barth one-equation turbulence model. Gurney flap sizes of 0.5 , 1, 1.25, 1.5, 2, and 3 percent of the airfoil chord were studied. Computational results were compared with experimental results where possible. The numerical solutions show that the Gurney flap increases airfoil lift coefficient with only a slight increase in drag coefficient. Use of a 1.5 percent chord Gurney flap increases the maximum lift coefficient by approximately 0.3 and decreases the angle of attack for a given lift coefficient by more than 3 deg. The numerical solutions exhibit detailed flow structures at the trialing edge and provide a possible explanation for the increased aerodynamic performance.

Numerical Investigation of an Airfoil with a Gurney Flap

A two-dimensional numerical investigation was performed to determine the effect of a Gurney flap on a NACA 4412 airfoil. A Gurney flap is a flat plate on the order of 1–3% of the airfoil chord in length, oriented perpendicular to the chord line and located on the airfoil windward side at the trailing edge. The flowfield around the airfoil was numerically predicted using INS2D, an incompressible Navier–Stokes solver, and the one-equation turbulence model of Baldwin and Barth. Gurney flap sizes of 0.5%, 1.0%, 1.25%, 1.5%, 2.0%, and 3.0% of the airfoil chord were studied. Computational results were compared with available experimental results. The numerical solutions show that some Gurney flaps increase the airfoil lift coefficient with only a slight increase in drag coefficient. Use of a 1.5% chord length Gurney flap increases the airfoil lift coefficient by ΔCl≈0.3 and decreases the angle of attack required to obtain a given lift coefficient by ΔαL=0>−3°. The numerical solutions show the details of the flow structure at the trailing edge and provide a possible explanation for the increased aerodynamic performance.

Computational Investigation into the Use of Response Functions for Aerodynamic-Load Modeling

The generation of reduced-order models (ROM) for the evaluation of unsteady and nonlinear aerodynamic loads are investigated. The ROM considered is an indicial theory based on the convolution of step functions with the derivative of the input signal. The step functions are directly calculated using the results of RANS simulations and a grid movement tool. Results are reported for a two dimensional airfoil and a UCAV configuration. Wind tunnel data are first used to validate the prediction of static and unsteady coefficients at both low and high angles of attack, with good agreement obtained for all cases. The generation of the aerodynamic models is described. The focus of the paper shifts to assess the validity of studied ROMs with respect to new maneuvres. This is accomplished by comparison of the model output with time-accurate CFD simulations. The results show that the ROMs can accurately model the unsteady loads in response to slow and fast pitch and plunge motions.