Stephen M. Ruffin

Vibrational Relaxation of Anharmonic Oscillators in Expanding Flows

Although the Landau-Teller vibrational model accurately predicts the vihrational excitation process in postshock and compressing flows, it underpredicts the rate of de-excitation in cooling and expanding flows. In the present paper, detailed calculations of the vibrational relaxation process of NI and CO in cooling flows are conducted. A coupled set of vibrational transition rate equations and quasi-one-dimensional fluid dynamic equations is solved. Multiple quantum level transition rates are computed using SSH theory. The SSH transition rate results are compared with available experimental data and other theoretical models. Vibration-vibration exchange collisions are responsible for some vibrational relaxation acceleration in situations of high vibrational temperature and low translational temperature. The present results support the relaxation mechanisms proposed by Bray and by Treanor, Rich and Rehm. Qualitative agreement with experimental results is achieved for the overall vibrational relaxation rate; however, the accuracy of the SSH results for vibration-vibration exchange transitions must be studied further and additional experimental investigations are needed for quantitative agreement.

Single expansion ramp nozzle simulations

The single-expansion-ramp-nozzle (SERN) experiment underway at NASA Ames Research Center simulates the National Aerospace Plane propulsive jet-plume flow. Recently, limited experimental data has become available from an experiment with a generic nozzle/afterbody model in a hypersonic wind tunnel. The present paper presents full three-dimensional solutions obtained with the implicit Navier-Stokes solver, FL3D, for the baseline model and a version of the model with side extensions. Analysis of the computed flow clearly shows the complex 3-D nature of the flow, critical flow features, and the effect of side extensions on the plume flow development. Flow schematics appropriate for the conditions tested are presented for the baseline model and the model with side extensions. The computed results show excellent agreement with experimental shadowgraph and with surface pressure measurements. The computed and experimental surface oil-flows show the same features but may be improved by appropriate turbulence modeling.

Hypersonic single expansion ramp nozzle simulations

The single-expansion ramp-nozzle experiment underway at NASA Ames Research Center simulates the National Aerospace Plane propulsive jet-plume flow. Recently, limited experimental data have become available from an experiment with a generic nozzle/afterbody model in a hypersonic wind tunnel. The present paper presents full three-dimensional solutions obtained with the implicit Navier-Stokes solver, FL3D, for the baseline model and a version of the model with side extensions. Analysis of the computed flow clearly shows the complex three-dimensional nature of the flow, critical flow features, and the effect of side extensions on the plume flow development. Schematics of the flowfield appropriate for the conditions tested are presented for the baseline model and the model with side extensions. The computed results show excellent agreement with experimental shadowgraph and with surface pressure measurements. The computed and experimental surface oil flows show the same features but may be improved by appropriate turbulence modeling.

Computational design aspects of a NASP nozzle/afterbody experiment

This paper highlights the influence of computational methods on design of a wind tunnel experiment which generically models the nozzle/afterbody flow field of the proposed National Aerospace Plane. The rectangular slot nozzle plume flow field is computed using a three-dimensional, upwind, implicit Navier-Stokes solver. Freestream Mach numbers of 5.3, 7.3, and 10 are investigated. Two-dimensional parametric studies of various Mach numbers, pressure ratios, and ramp angles are used to help determine model loads and afterbody ramp angle and length. It was found that the center of pressure on the ramp occurs at nearly the same location for all ramp angles and test conditions computed. Also, to prevent air liquefaction, it is suggested that a helium-air mixture be used as the jet gas for the highest Mach number test case.

Fluid-Structure Analysis of a Clamped Ballute in Titan's atmosphere

This paper presents the ∞uid-structure analysis of a clamped ballute in Titan’s atmosphere. A ballute (balloon + Parachute) is a large in∞atable aerocapture device designed to be used in interplanetary mission. Since a ballute is operated in a high temperature and high altitude environment, such analysis requires a non-equilibrium ∞ow model solver that is coupled with a structure solver that takes into consideration large structure deformation. An unstructured Cartesian grid based ∞ow solver, NASCART-GT, with Titan’s atmospheric composition is used for the ∞ow solver and LS-DYNA is used for the structure solver.

Multi-Fidelity Approach to Estimate Heating for Three-Dimensional Hypersonic Aeroshells

In the early stages of aeroshell shape design, it is important to strike an appropriate balance between analysis fidelity and computational effort. Because the prediction of aerodynamic heating for axisymmetric flows is significantly faster than for three-dimensional flows, it is advantageous to employ an axisymmetric analysis method. The current work couples an equivalent axisymmetric body technique with different axisymmetric analysis methods, using a series of axisymmetric bodies to approximate the three-dimensional heating. Three levels of fidelity are considered: 1)xa0a Newtonian inviscid solution coupled with an axisymmetric integral boundary layer approach, 2)xa0an Euler solution coupled with an axisymmetric integral boundary-layer approach, and 3)xa0a Navier–Stokes solution applied to each equivalent axisymmetric body. Approximate solutions for three-dimensional flows were compared with high-fidelity computational and experimental data, establishing the accuracy for various levels of fidelity. The multi...

A Loosely Coupled Analysis of the Fluid-Structure Interactions for Inflatable Aerodynamic Decelerators

The interaction between fluid and structural dynamics has become an important topic with regards to understanding the overall dynamics of inflatable aerodynamic decelerators. The present work aims to establish the capability to perform loosely coupled, fluid-structure interactions (FSI) through the use of the Cartesian Navier-Stokes solver, NASCART-GT and the finite element analysis (FEA) tool, LS-DYNA. Verification and validations are presented for the computational fluid dynamics (CFD) in order to demonstrate sufficient accuracy and applicability. These include CFD simulations of moving geometries, as well as a stationary analysis of a rigid tension cone. The FSI capability is demonstrated by examining the flow over a wedge with a deformable membrane, as well as flow over a semi-rigid tension cone configuration.

Application of a Method to Estimate Heating for Three-Dimensional Hypersonic Aeroshells

In the early stages of aeroshell shape design it is important to strike the appropriate balance between analysis fidelity and computational effort. Since the prediction of aerodynamic heating in axisymmetric flows is significantly faster than three-dimensional flows, it is desirable to employ an axisymmetric analogue method. The current work couples an equivalent axisymmetric body technique with different axisymmetric heating approaches to approximate three-dimensional heating. Multiple levels of fidelity are investigated in order to gauge the computational effort as a function of analysis fidelity. The three levels of fidelity include: 1) a Newtonian inviscid solution coupled with an axisymmetric integral boundary layer approach, 2) an Euler solution coupled with an axisymmetric integral boundary layer approach, and 3) a Navier-Stokes solution applied to each equivalent axisymmetric body. Heating analyses are presented for two sphere-cones in perfect air and a cylinder in thermochemical nonequilibrium. Results are also presented for a 15 sphere-cone at an angle of attack in perfect air and for an elliptic paraboloid in chemically reacting air. Solutions are compared with high fidelity computational and experimental data.