Michael J. Hemsch

Connection between leading-edge sweep, vortex lift, and vortex strength for delta wings

An effort is made to clarify the effect of leading-edge sweep on the vortex lift and leading-edge vortex strength of a slender wing; while it is often assumed that increasing sweep enhances vortex lift and strength, the opposite is the case. The suction analogy is used in association with numerical and experimental data to derive simple formulas yielding the actual relationship for delta wings. The difference between vortex lift and nonlinear lift is highlighted.

Alleviation of Side Force on Tangent-Ogive Forebodies Using Passive Porosity

An experimental investigation to determine the effectiveness of passive porosity for alleviating side forces on forebodies was conducted in the NASA Langley Research Center 7-ft by 10-ft high-speed wind tunnel. Force, moment, and surface pressure data were obtained on solid and porous (22% porosity, 0.020-in. hole diam) tangent-ogive forebodies of fineness ratios 2.5 and 5.0. The solid forebodies were tested with transition grit to simulate fully turbulent conditions, and without transition grit to simulate free transition conditions. The extent of porosity on the forebodies was varied to determine the extent of porosity needed to alleviate side forces. Static longitudinal and lateral-directional stability and surface pressure data were obtained at Mach numbers of 0.2, 0.5, and 0.8, angles of attack from — 5 to 45 deg, and roll angles from — 90 to 180 deg. The solid forebodies exhibited large asymmetric pressure loads at moderate to high angles of attack causing large side forces and yawing moments; the transition grit had minimal effect on the asymmetric characteristics, but had a large effect on the longitudinal characteristics. The porous forebodies exhibited no significant side forces or yawing moments at any angle of attack tested.

Statistical Analysis of Computational Fluid Dynamics Solutions from the Drag Prediction Workshop

A simple, graphical framework is presented for robust statistical evaluation of results obtained from N-version testing of a series of Reynolds averaged Navier-Stokes computational fluid dynamics codes. The solutions were obtained by a variety of code developers and users for the June 2001 Drag Prediction Workshop sponsored by the AIAA Applied Aerodynamics Technical Committee. The aerodynamic configuration used for the computational tests is the DLR, German Aerospace Research Center DLR-F4 wing-body combination previously tested in several European wind tunnels and for which a previous N-version test had been conducted. The statistical framework is used to evaluate code results for 1) a single cruise design point, 2) a drag polar at a single Mach number, and 3) drag rise at three values of lift

Uncertainty in Computational Aerodynamics

J. M. Luckring, M. J. Hemsch , J. H. MorrisonAerodynamics, Aerothermodynamics, and Acoustics CompetencyNASA Langley Research CenterHampton, VirginiaABSTRACTAn approach is presented to treat computationalaerodynamics as a process, subject to the fundamentalquality assurance principles of process control andprocess improvement. We consider several aspectsaffecting uncertainty for the computationalaerodynamic process and present a set of stages todetermine the level of management required to meetrisk assumptions desired by the customer of thepredictions•CA

Statistical Analysis of CFD Solutions from the Third AIAA Drag Prediction Workshop (Invited)

The first AIAA Drag Prediction Workshop, held in June 2001, evaluated the results from an extensive N-version test of a collection of Reynolds-Averaged Navier-Stokes CFD codes. The code-to-code scatter was more than an order of magnitude larger than desired for design and experimental validation of cruise conditions for a subsonic transport configuration. The second AIAA Drag Prediction Workshop, held in June 2003, emphasized the determination of installed pylon-nacelle drag increments and grid refinement studies. The code-to-code scatter was significantly reduced compared to the first DPW, but still larger than desired. However, grid refinement studies showed no significant improvement in code-to-code scatter with increasing grid refinement. The third Drag Prediction Workshop focused on the determination of installed side-of-body fairing drag increments and grid refinement studies for clean attached flow on wing alone configurations and for separated flow on the DLR-F6 subsonic transport model. This work evaluated the effect of grid refinement on the code-to-code scatter for the clean attached flow test cases and the separated flow test cases.

An Uncertainty Structure Matrix for Models and Simulations

Software that is used for aerospace flight control and to display information to pilots and crew is expected to be correct and credible at all times. This type of software is typically developed under strict management processes, which are intended to reduce defects in the software product. However, modeling and simulation (M&S) software may exhibit varying degrees of correctness and credibility, depending on a large and complex set of factors. These factors include its intended use, the known physics and numerical approximations within the M&S, and the referent data set against which the M&S correctness is compared. The correctness and credibility of an M&S effort is closely correlated to the uncertainty management (UM) practices that are applied to the M&S effort. This paper describes an uncertainty structure matrix for M&S, which provides a set of objective descriptions for the possible states of UM practices within a given M&S effort. The columns in the uncertainty structure matrix contain UM elements or practices that are common across most M&S efforts, and the rows describe the potential levels of achievement in each of the elements. A practitioner can quickly look at the matrix to determine where an M&S effort falls based on a common set of UM practices that are described in absolute terms that can be applied to virtually any M&S effort. The matrix can also be used to plan those steps and resources that would be needed to improve the UM practices for a given M&S effort.

Leading-Edge Vortex Breakdown for Wing Planforms with the Same Slenderness Ratio

A dye flow visualization investigation was conducted on two sets of wing planforms to better understand the effects of planform shaping on leading-edge vortex breakdown. The first set of planforms consisted of five cropped planforms, including a 66.3-deg delta, 80/55 double-delta, and three blended 80/55 double-deltas. The second set of planforms consisted of nine planforms of the same slenderness ratio, including a 69.3-deg delta, two gothics, three double-deltas, and three corresponding blended double-deltas. Results of these experiments have indicated that planforms with a common slenderness ratio develop quite similar characteristics for breakdown of the apex vortex at the trailing edge. It was found that planform shaping significantly alters the forward progression of breakdown along the wing as angle of attack is increased. The effect of increasing filleting to double-delta planforms was shown to degrade the apex vortex breakdown characteristics of the planforms. Also, it was found that by moving the kink location of the double-delta aft on the planform, breakdown characteristics are improved.