Robert B. Greendyke

A convective and radiative heat transfer analysis for the FIRE II forebody

A Navier-Stokes flowfield solution method (LAURA code) using finite-rate chemistry and two-temperature thermal nonequilibrium was used in combination with two nonequilibrium radiative heat transfer codes to calculate heating for the FIRE II vehicle. An axisymmetric model of the actual body shape was used. One radiative heating code (NEQAIR) was used in uncoupled fashion with the flowfield solvers energy equations, while the other code (LORAN) was used in both coupled and uncoupled variations. Several trajectory points ranging from highly nonequilibrium flow to near-equilibrium flow were used for a study of both convective and radiative heating over the vehicle. Considerable variation in radiative heating was seen at the extremes, while agreement was good in the intermediate trajectory points. Total heat transfer calculations gave good comparison until the peak heating trajectory points were encountered, and returned to good agreement for the last two equilibrium points.

Comparison of vibration-dissociation coupling and radiative transfermodels for AOTV/AFE flowfields

A series of detailed studies comparing various vibration dissociation coupling models, reaction systems and rates, and radiative heating models has been conducted for the nonequilibrium stagnation region of an AFE/AOTV vehicle. Atomic and molecular nonequilibrium radiation correction factors have been developed and applied to various absorption coefficient step models, and a modified vibration dissociation coupling model has been shown to yield good vibration/electronic temperature and concentration profiles. While results indicate sensitivity to the choice of vibration dissociation coupling model and to the nitrogen electron impact ionization rate, by proper combinations accurate flowfield and radiative heating results can be obtained. These results indicate that nonequilibrium effects significantly affect the flowfield and the radiative heat transfer. However, additional work is needed in ionization chemistry and absorption coefficient modeling.

Simulation of Various Turret Configurations at Subsonic and Transonic Flight Conditions Using OVERFLOW

In this work, the ow elds associated with two canonical turret geometries, a fully exposed hemisphere on a at plate and a 50% submerged hemisphere on a at plate, were simulated using the OVERFLOW 2 ow solver. Both turret geometries utilize a at-window aperture with an aperture ratio (ratio of the aperture diameter to the turret diameter) of 0.295 and an elevation angle of 57 . The forward eld of regard was the particular focus in this study, and both symmetric (azimuth angle of 0 ) and asymmetric (azimuth of 45 ) window orientations were examined. Two ight conditions were also studied; a subsonic case with M = 0:45 and ReD = 6:30 10 and a transonic case with M = 0:85 and ReD = 9:53 10. The ow eld was simulated using the Delayed Detached Eddy Simulation capability of OVERFLOW in conjunction with the spatially fth-order Weighted Essentially Non-Oscillatory (WENO) scheme to capture the o -body vortical structures The incoming boundary layer was set a the same height for both geometries, which corresponded to a quarter of the height of the fully exposed hemisphere and half of the height of the submerged turret. The impact of the turret aerodynamics on the performance of the turrets for directed energy applications is inferred through consideration of the ow features, density and pressure uctuations, and forces on the turrets.

CFD Simulation of Laser Ablation Carbon Nanotube Production

The production of carbon nanotubes and fullerenes by laser ablation has been an ongoing process at NASA-Johnson Space Center for several years now. The mechanisms of nanotube production are not well understood and a conventional CFD simulation code (VULCAN) is applied to the study of the flowfield dynamics and chemical kinetics of nanotube production. A simple 12-species, 14-reaction model for the formation of carbon molecules up to C6 has been incorporated into the code. Simulations of the plume resulting from a single 10 ns laser pulse are used for the analysis of flowfield dynamics and chemical concentrations using C6 as an ‘indicator species’ for fullerene production. An additional dual laser pulse simulation was conducted to mimic actual production techniques. Both cases were run to a simulated post ablation time of 8 milliseconds. Thermal profiles appear to exceed those of experimental observations, although this comparison might be erroneous. Flowfield dynamics yield good agreement with the available limited experimental data for most of the simulation. C6 production remained nearly constant despite diffusion of other species into the background argon.

Dynamic Simulation of a Wave Rotor Topped Turboshaft Engine

The dynamic behavior of a wave-rotor-topped turboshaft engine is examined using a numerical simulation. The simulation utilizes an explicit, one-dimensional, multipassage, computational e uid dynamics- (CFD-) based wave-rotor code in combination with an implicit, one-dimensional, component-level dynamic engine simulation code. Transient responses to rapid fuel e ow rate changes and compressor inlet pressurechanges aresimulated and compared with those of a similarly sized, untopped, turboshaft engine. Resultsindicate that the wave-rotor-topped engine responds in a stable and rapid manner. Furthermore, during certain transient operations, the wave rotor actually tends to enhance engine stability. In particular, there is no tendency toward surge in the compressor of thewave-rotor-topped engine during rapid acceleration. In fact, thecompressor actually movesslightly away from the surge line during this transient. This behavior is precisely the opposite to that of an untopped engine. The simulation is described. Issues associated with integrating CFD and component-level codes are discussed. Results from several transient simulations are presented and discussed.

Heating analysis for a Lunar Transfer Vehicle at near-equilibrium flow conditions

A heating analysis for a 15.2 m diameter Lunar Transfer Vehicle (LTV) at 0 and 10.6 deg angle of attack for a nominal trajectory through the earths atmosphere is described. The analysis utilizes the Langley Aerothermodynamic Upwind Relaxation Algorithm (LAURA) with thin-layer, Navier-Stokes, thermochemical nonequilibrium options. Radiative heating levels are calculated using the Langley Optimized RAdiative Nonequilibrium (LORAN) and the Non-EQuilibrium AIr Radiation (NEQAIR) codes. At peak heating, the shock layer is substantially in equilibrium. Comprehensive spatial and spectral grid convergence studies have been implemented to quantify grid effects on the convective and radiative heating levels. Axisymmetric tests including the coupled effects of radiative energy transfer show negligible change to the convective heating and a 20 percent reduction in the radiative heating.

Calculated electron number density profiles for the aeroassist flight experiment

Basic features of a Microwave Reflectometer lonization Sensor (MRIS) as designed for use on the Aeroassist Flight Experiment are described. The MRIS is designed to measure the distances into the shock layer of four critical electron number densities corresponding to four frequencies (20, 44, 95, and 140 GHz). A parametric study of the effects of trajectory and several thermochemical nonequilibrium models for reaction rates, translational and vibrational-electronic energy exchange rates, the average electronic excitation level of atoms, and axisymmetric vs three-dimensional effects is conducted for evaluating MRIS contributions to the code validation process. The parametric study, implemented with the Langley Aerpthermodynamic Upwind Relaxation Algorithm program, reveals a particular sensitivity of the onset and severity of an electron avalanche phenomena associated with changes in these physical models that lead to strong electron-impact ionization. Predicted electron number density profiles are sensitive to reasonable variations in certain kinetic models; consequently, MRIS measurements could assist the code validation process.

Development of a Nonequilibrium Finite-Rate Ablation Model for Radiating Earth Reentry Flows

Thermal protection system design for atmospheric reentry vehicles remains a challenging and complex problem. Recent advances in computational modeling of air–carbon interactions consider competing finite-rate reactions on a limited number of available surface sites. One of the most advanced kinetic models is due to Zhluktov and Abe. However, the Zhluktov and Abe model only describes the oxidation and sublimation of carbon and has no nitridation mechanism. The following study develops several modifications to the Zhluktov and Abe air–carbon model that account for all three reaction mechanisms with the goal of improving cyanogen shock-layer radiation predictions to recent experimental results. First, the study examines two possible paths for carbon nitridation and then assesses the augmented surface reaction model in a representative blunt-body reentry flow. Second, a sensitivity analysis is performed to determine which surface reactions have the most impact on altering cyanogen radiance predictions. It was...

Photogrammetric Measurement of Recession Rates of Low Temperature Ablators in Supersonic Flow

Ablative heat shields have been used to protect hypersonic vehicles during atmospheric reentry during the Apollo missions and could be used for future flight vehicles as well. Advances in computational models enable a large variety of vehicle shapes to be considered. However, it is exceptionally difficult to perform reliable tests at conditions which are representative of flight to validate the models. Historically, tests have been conducted on substitute materials at low temperatures to validate models, but even these tests can pose significant challenges. For example, most previous studies rely on model shape measured before and after tests or have relied on Schlieren photography for measuring changes in model profile only. Only recently has photogrammetry been used to quantify shape change in three dimensions for the ablative models. In our study, the AFIT Mach 3 pressurevacuum wind tunnel was used in combination with models consisting of dry ice to collect ablation data for models of different shapes at stagnation pressures ranging from approximately 0.4 atm to 3 atm and stagnation temperatures equivalent to room temperature. High speed Schlieren photography was used for visualization, and the three dimensional shape change was quantified with sub-millimeter accuracy using laser dot photogrammetry. Results for one shape are compared to those computed using a computational model, which employs a finite-volume approach to solving the (3-D) NavierStokes equations, with the gas assumed to be at equilibrium, while employing an implicit solver accounting for the material response. Increased stagnation pressure led to larger material loss in the stagnation region of the model, as expected.

Parametric analysis of radiative structure in aerobrake shock layers

A broad-spectrum version of the NEQAIR code was modified to account for self-absorption and applied to AFE flowfields calculated by the LAURA code with a variety of kinetic models. The resulting radiative fluxes were obtained in a decoupled fashion from the flowfield solver along the vehicles stagnation streamline. The radiative flux obtained was broken down by causative process to study the radiative structure of the AFEs flowfield for the various kinetic models. In addition, the radiative fluxes for several points on a typical AFE trajectory were analyzed to examine how the radiative structure changes as the vehicle completes its aeropass. Only two radiative processes dominated the stagnation radiative flux, and the flow field conditions near the wal were found to exert considerable influence over the radiative flux to the wall.