Robert L. Ash

Application of spectral collocation techniques to the stability of swirling flows

Abstract A Chebyshev spectral collocation method for the temporal and spatial stability of swirling flows is presented. The linearized stability equations in cylindrical coordinates are solved using the method and eigenvalues obtained by employing the QZ routine. The developed algorithm is found to be robust and easily adaptable to various flow configurations, including internal and external flows, with only minor changes in the application of the boundary conditions. The accuracy and efficiency of the spectral method is tested for plane Poiseuille flow, annular flow, rotating pipe flow, and a trailing line vortex.

Effect of compliant wall motion on turbulent boundary layers

A critical analysis of available compliant wall data which indicated drag reduction under turbulent boundary layers is presented. Detailed structural dynamic calculations suggest that the surfaces responded in a resonant, rather than a compliant, manner. Alternate explanations are given for drag reductions observed in two classes of experiments: (1) flexible pipe flows and (2) water−backed membranes in air. Analysis indicates that the wall motion for the remaining data is typified by short wavelengths in agreement with the requirements of a possible compliant wall drag reduction mechanism recently suggested by Langley.

Feasibility of rocket propellant production on Mars

In situ production of rocket propellant to reduce landed mass requirements for Mars return missions has been investigated. The analysis has shown that a system which utilizes atmospheric carbon dioxide and soil moisture to produce liquid methane-oxygen propellant requires a landed mass which is less than half the mass of the ascent vehicle it produces.

Study of aircraft wake vortex behavior near the ground

Aircraft wake vortices have been modeled using an unsteady, two-dimensional laminar (constant eddy viscosity) approximation of the Navier-Stokes equations, to study the influence of ground coupling, stratification, and cross-wind on vortex system behavior and decay. Initialization and boundary conditions are developed and implemented systematically for a nonuniform grid representation of the semi-infinite domain containing a vortex pair. Subsequently, the physics of wake vortex interactions with the ground for different types of surface weather conditions are discussed.

Organized nature of a turbulent trailing vortex

The turbulence structure of a trailing vortex produced at the juncture of a flow aligned cylinder and a pair of oppositely loaded airfoils is analyzed. The freestream turbulence intensity in this study varies from 0.32 to 1.48 percent, the vortex Reynold number varies from 15000 to 25000, and the Rossby number varies from 0.65 to 0.81. Within this parameter range, it is shown that the screens, but not the freestream turbulence level, are able to produce significant variations in the turbulence structure of the vortex, and that the turbulent structure is determined by the Rossby number and not the vortex Reynolds number. It is noted that the core is dynamic and an organized exchange of momentum takes place between the outer flow and the core region of the vortex. The vortex structure in the trailing vortex having the lowest Rossby number is considered.

The structure and dynamics of bubble-type vortex breakdown

A unique discrete form of the Navier-Stokes equations for unsteady, three-dimensional, incompressible flow has been used to study vortex breakdown numerically. A Burgers-type vortex was introduced along the central axis of the computational domain, and allowed to evolve in space and time. By varying the strength of the vortex and the free stream axial velocity distribution, using a previously developed Rossby number criterion as a guide, the location and size of the vortex breakdown region was controlled. While the boundaries of the vortex breakdown bubble appear to be nominally symmetric, the internal flow field is not. Consequently, the mechanisms for mixing and entrainment required to sustain the bubble region are different from those suggested by earlier axisymmetric models. Results presented in this study, for a Reynolds number of 200, are in good qualitative agreement with higher Reynolds number experimental observations, and a variety of plots have been presented to help illuminate the fluid physics.

Recent Concepts in Missions to Mars: Extraterrestrial Processes

This paper presents some recent concepts in unmanned Mars Sample Return (MSR) missions that utilize extraterrestrial resources; the concept has been discussed earlier under in situ propellant production (ISPP). The concepts examined include the power and energy needs of this mission. It is shown that solar energy is not especially attractive. Use of radioisotopic power generators and a Rankine cycle are seen to be viable options. Quantitative estimates, taking into consideration state-of-the-art and projected technologies, indicate that the power/energy per se are not critical to the mission, but the reliability is. Hence, various modern options for the components of the power generation and utilization are discussed. The dramatic savings in shuttle (or other) vehicle launches are quantitatively plotted. The basic system here is the production of hydrocarbon (methane) fuel and oxygen from the Martian atmosphere. For the simplest mission, it is seen that Earth-carried methane burned with oxygen produced on site provides the best system. This conclusion results from the fact that cryogenic storage of hydrogen (despite the higher specific impulse (Isp)| is avoided, and because a large portion of the propellant is really the oxidizer and not the fuel. The paper outlines several simple low-risk experiments that can be tested as a prelude to the MSR

Variational approach to the volume viscosity of fluids

The variational principle of Hamilton is applied to develop an analytical formulation to describe the volume viscosity in fluids. The procedure described here differs from those used in the past in that a dissipative process is represented by the chemical affinity and progress variable (sometimes called “order parameter”) of a reacting species. These state variables appear in the variational integral in two places: first, in the expression for the internal energy, and second, in a subsidiary condition accounting for the conservation of the reacting species. As a result of the variational procedure, two dissipative terms appear in the Navier-Stokes equation. The first is the traditional volume viscosity term, proportional to the dilatational component of velocity; the second term is proportional to the material time derivative of the pressure gradient. Values of the respective volume viscosity coefficients are determined by applying the resulting volume-viscous Navier-Stokes equation to the case of acousti...

Numerical simulations of commercial aircraft wakes subjected to airport surface weather conditions

Using tower flyby data for validation, we have developed a two-dimensional numerical simulation of the influence of surface weather conditions on wake vortex motion and decay for representative commercial aircraft. Our simulations support the conjecture that the ratio of eddy viscosity to kinematic viscosity, appropriate for modeling aircraft wakes, scales linearly with circulation, which yields a nominally constant equivalent Reynolds number for all commercial aircraft sizes. We have tested the constant eddy viscosity approximation for three different aircraft types and six surface weather states, showing the utility of the approach in predicting wake vortex motion and decay. Subsequently, we have shown how data from one aircraft flight test can be used to infer the decay behavior of another, and we have examined the influence of the six surface weather states on the vortex decay behavior predicted for a large commercial aircraft. Based upon these simulations, we have determined that the two-dimensional, constant eddy viscosity approach can be useful in assessing the influence of surface weather conditions on wake vortex decay

Glow‐discharge enhanced permeation of oxygen through silver

The permeation of oxygen through Ag0.05Zr over the temperature range of 300–650 °C under glow‐discharge conditions has been studied and compared to the permeation of thermally dissociated molecular oxygen. A low‐energy dc glow‐discharge in O2 has been employed which produced approximately 10% atoms. The permeation rate during the glow discharge was found to be much higher (a factor of ∼10) than without the glow discharge. The small fraction of oxygen atoms generated appears to dominate the permeation because of much higher solution probabilities. Below 500 °C, the activation energy for the permeation with glow discharge was found to be 15.5 kcal/mol compared to 22.0 kcal/mol without glow discharge (molecular oxygen). Above 500 °C, the enhanced permeation with glow discharge gradually diminishes with increasing temperature and approaches that observed without the glow discharge at high temperature; the reason for this is primarily because of the thermal instability of the supersaturated high‐pressure inter...