Ted A. Manning

Numerical simulations of shock-vortex interactions in supersonic jet screech

Supersonic jet screech is a form of jet noise which adversely impacts both the environment and the life of aircraft structures. A basic understanding of the mechanisms which generate the screech tone and determine its amplitude is therefore important. In the present study we perform direct numerical simulations (DNS) of the interaction of an oblique shock with instability waves of a finite thickness supersonic shear layer. We thereby retain the basic elements of an isolated jet screech source. The simulations are carried out in two dimensions using a high order accurate spatial scheme with nonreflecting boundary conditions. The unsteady shock motion is resolved with the essentially non-oscillatory (ENO) discontinuity capturing scheme. The shear layer (M = 1.2, Re = 1000 based on initial vorticity thickness) is forced at the most unstable frequency such that the instability waves develop into fully formed vortices upstream of the shock. The interaction of the vortices with the shock causes streamwise oscillations in the shock near its tip. On the subsonic side of the shear layer, the acoustic wave is released as the shock tip deflects upstream through the braid region between vortices. This acoustic field is approximately cylindrical and its directivity is nearly uniform. The acoustic waveform is comprised of a sharp compression followed by a longer expansion. On the supersonic side of the shear layer, a complex wave field is observed. Cases in which the incident shock is replaced by a nearly isentropic compression wave produce qualitatively similar behavior. Acoustic pressure amplitude is found to Copyright ©1998 Ted A. Manning and Sanjiva K. Lele. Published by the American Institute of Aeronautics and Astronautics with permission. talso affiliated with the Department of Mechanical Engineering, Stanford University. scale with compression wave strength. Acoustic wave form is insensitive to compression wave amplitude and profile width for cases examined. This research is directed toward modeling the screech generation process.

A ray tracing study of shock leakage in a model supersonic jet

Recent work has described screech noise from a supersonic jet as being due to leakage of a wave that is otherwise trapped in the jets interior. In that work, the simplest of many techniques used is ray tracing for a single shear-layer modeled as a row of Stuart vortices. In the present work, a lower row of vortices is added to form a plane jet. Instead of plotting ray paths, a technique of visualization analogous to streaklines is used that better corresponds to instantaneous density fields as observed, for instance, by the Schlieren method. This produces striking images that show leakage of waves at each internal reflection resulting in a row of acoustic sources as envisioned since the 1950s. However, the sources are not isotropic and each has a zone of silence in the downstream direction. Leakage creates a fold in the wave pattern internal to the jet which leads to fine scale features. Reported experiments have also observed fine scale features (described as splitting) in the shock-cell pattern; they m...

Near-Earth phase risk comparison of human Mars campaign architectures

A risk analysis of the launch, orbital assembly, and Earth-departure phases of human Mars exploration campaign architectures was completed as an extension of a probabilistic risk assessment (PRA) originally carried out under the NASA Constellation Program Ares V Project [1]. The objective of the updated analysis was to study the sensitivity of loss-of-campaign risk to such architectural factors as composition of the propellant delivery portion of the launch vehicle fleet (Ares V heavy-lift launch vehicle vs. smaller/cheaper commercial launchers) and the degree of launcher or Mars-bound spacecraft element sparing. Both a static PRA analysis and a dynamic, event-based Monte Carlo simulation were developed and used to evaluate the probability of loss of campaign under different sparing options. Results showed that with no sparing, loss-of-campaign risk is strongly driven by launcher count and on-orbit loiter duration, favoring an all-Ares V launch approach. Further, the reliability of the all-Ares V architecture showed significant improvement with the addition of a single spare launcher/payload. Among architectures utilizing a mix of Ares V and commercial launchers, those that minimized the on-orbit loiter duration of Mars-bound elements were found to exceed the reliability of no spare all-Ares V campaign if unlimited commercial vehicle sparing was assumed.

Intermediate failure states in simulation-based launch vehicle risk study

A framework for representing intermediate failure states in an assessment of abort risk during the ascent phase of a crewed space launch vehicle mission is presented. The framework refines a previously established, simulation-based risk assessment approach (Ref. 1) by improving the characterization of vehicle failure prior to the onset of final, potentially catastrophic “loss of crew” (LOC) hazards through the introduction of system-level “loss of mission” (LOM) failure states, or “LOM environments.” The intermediate failure state framework is found to improve the risk analysis with respect to both risk model fidelity and risk model management. While LOM environments primarily provide an incremental point of reference for failure evolution modeling and refinement, they also serve to consolidate the risk analysis in late failure evolution and thereby reduce overall analysis effort. Ultimately, the logical boundary in failure event sequences formed by LOM environments is found to effectively delineate areas of failure analysis responsibility between the teams that provide inputs to the risk assessment, and furthermore, define the data interface between those teams. The merits of this framework are illustrated in a case study concerning the treatment of upper stage liquid fuel engine failures.

Fragment Acceleration Modeling for Pressurized Tank Burst

As part of comprehensive efforts to develop physics-based risk assessment techniques for space systems at NASA, coupled computational fluid and rigid-body dynamic simulations are carried out to inv...