Dinesh K. Prabhu

Analysis and Model Validation of Shock Layer Radiation in Air

This paper analyzes the shock layer radiative heating environment for a large entry vehicle on a lunar return trajectory. Modeling results show that much of the shock layer plasma is in local thermodynamic equilibrium (LTE) and is not optically thin. The ionization level is generally high (15%) and the air is almost fully dissociated. A significant amount of vacuum ultraviolet (VUV) radiation is produced due to bound-bound and bound-free transitions of N and O atoms. The sensitivity of total radiation to Stark broadening, which dominates over other line broadening mechanisms, is quantified. The latter part of this paper reports the status of ongoing validation of the current radiation models with measurements in the Electric-Arc Shock Tube (EAST) facility at NASA Ames Research Center. Model predictions are compared with the calibrated radiation spectra measured in the equilibrium portion of the shock layer at 0.3 Torr. The reasons for discrepancy between model and measurements are also discussed with possible hypotheses presented for further investigation.

Assessment of Laminar, Convective Aeroheating Prediction Uncertainties for Mars-Entry Vehicles

DOI: 10.2514/1.A32257An assessment of computational uncertainties is presented for numerical methods used by NASA to predictlaminar, convective aeroheating environments for Mars-entry vehicles. A survey was conducted of existingexperimental heat transfer and shock-shape data for high-enthalpy reacting-gas CO

Analysis of Apollo command module afterbody heating. Part I: AS-202

Computational-fluid-dynamics simulations are performed to simulate the wake flow and afterbody heating environment of the Apollo AS-202 command module reentry. Fifteen three-dimensional simulations that cover the majority of the high-heating portion of the flight are computed. Computed surface oil flow plots predict that the flow remained attached on the windward side of the afterbody throughout the entry

CFD Analysis Framework for Arc-Heated Flowfields, II: Shear Testing in Arc-jets at NASA ARC

A three-dimensional framework has been developed for application of tools of computational fluid dynamics to simulate arc-heated flows expanded in convergent- divergent nozzles. Methods that make up the process for prediction of pressure, shear stress, and heat flux on the face of a blunt wedge and a swept cylinder are presented, along with modeling assumptions. The framework is utilized in simulations of the Interaction Heating Facility at NASA Ames Research Center. Results of numerical simulations are compared against experimental data acquired in test entries from the Mars Science Laboratory program. Recommendations are made for possible improvements to the process.

Absolute Radiation Measurement in Venus and Mars Entry Conditions

Comparisons of experimental characterization and model predictions of entry radiation relevant to Mars and Venus exploration are presented. Characterization is performed in the recently upgraded Ames Electric Arc Shock Tube (EAST) facility. Tests are performed in Mars (96% CO2, 4% N2) and Venus (96.5% CO2, 3.5% N2) simulant gases at downstream pressures and incident velocities spanning from 0.1-2.0 Torr and 3-12 km/s. Velocity and pressure conditions were chosen based on expected flight conditions (direct entry or aerocapture) in Mars and Venus atmospheres. The absolute radiance data are spatially and spectrally resolved and span the vacuum ultraviolet (VUV) through mid-IR (120-1650 nm, 35 µm). Resolved spectra of the CO 4 th positive band in the VUV are reported for the first time. Measurements of CO2 molecular vibrational radiation is also attempted at low velocity conditions. Radiation modeled under equilibrium assumption with NEQAIR code compares favorably to measured radiation under some, but not all, conditions.

A new PNS code for chemical nonequilibrium flows

A new parabolized Navier-Stokes (PNS) code has been developed to compute the hypersonic laminar flow of a multicomponent, chemically reacting mixture of thermally perfect gases over two-dimensional and axisymmetric bodies. The new PNS code solves the gas dynamic and species conservation equations in a coupled manner using a noniterative, implicit, space-marching finite-difference method. The conditions for well-posedness of the space-marching method have been derived from an eigenvalue analysis of the governing equations. The code has been used to compute hypersonic laminar flow of chemically reacting air over wedges and cones. The results of these computations are in good agreement with the results of reacting boundary-layer calculations.

Current grid-generation strategies and future requirements in hypersonic vehicle design, analysis and testing

Abstract Recent advances in computational power enable computational fluid dynamic modeling of increasingly complex configurations. A review of grid-generation methodologies implemented in support of the computational work performed for the X-38 (Experimental Crew Return Vehicle) and X-33 hypersonic vehicles are presented. In strategizing topological constructs and blocking structures, the factors considered are geometric configuration, optimal grid size, numerical algorithms, accuracy requirements, physics of the problem at hand, computational expense and available computer hardware. Also addressed are grid-refinement strategies, the effect of wall spacing and convergence. The significance of grids is demonstrated through a comparison of computational and experimental results of the aeroheating environment experienced by the X-38 vehicle. Special topics on grid-generation strategies are also addressed to model control surface deflections and material mapping.

Numerical solution of Space Shuttle Orbiter flowfield including real-gas effects

The hypersonic, laminar flow around the Space Shuttle Orbiter has been computed for bath an ideal gas (y = 1.2) and equilibrium air using a real-gas, parabolized Navier-Stokes code. This code employs a generalized coordinate transformation; hence, it places no restrictions on the orientation of the solution surfaces. The initial solution in the nose region was computed using a 3-D, real-gas, time-dependent Navier-Stokes code. The thermo- dynamic and transport properties of equilibrium air were obtained from either approximate curve fits or a table look-up procedure. Numerical results are presented for flight conditions corres- ponding to the STS-3 trajectory. The computed surface pressures and convective heating rates are compared with data from the STS-3 flight.

Adaptive Deployable Entry and Placement Technology (ADEPT): A Feasibility Study for Human Missions to Mars

The present paper describes an innovative, semi-rigid, mechanically deployable hypersonic decelerator system for human missions to Mars. The approach taken in the present work builds upon previous architecture studies performed at NASA, and uses those findings as the foundation to perform analysis and trade studies. The broad objectives of the present work are: (i) to assess the viability of the concept for a heavy mass (landed mass ≈40 mT) Mars mission through system architecture studies; (ii) to contrast it with system studies previously performed by NASA; and (iii) to make the case for a Transformable Entry System Technology. The mechanically deployable concept at the heart of the proposed transformable architecture is akin to an umbrella, which in a stowed configuration meets launch requirements by conforming to the payload envelope in the launch shroud, and when deployed in earth orbit forms a large aerosurface designed to provide the necessary aerodynamic forces upon entry into the Martian atmosphere. The aerosurface is a thin skin draped over high-strength ribs; the thin skin or fabric with flexible material serves as the thermal protection system, and the ribs serve as the structure. A four-bar linkage mechanism allows for a reorientation of the aerosurface during aerocapture or during the entry and descent phases of atmospheric flight, thus providing a capability to navigate and control the vehicle and make possible precision landing. The actuators and mechanisms that are used to deploy the aerosurface are multi-functional in that they also allow for reorienting the

Upwind parabolized Navier-Stokes code for chemically reacting flows

A new upwind, parabolized Navier-Stokes (PNS) code has been developed to compute the hypersonic, viscous, chemically reacting flow around two-dimensional or axisymmetric bodies. The new code is an extension of the upwind (perfect gas) PNS code of Lawrence, Tannehill, and Chaussee. The upwind algorithm is based on Roes flux-difference splitting scheme, which has been modified to account for real gas effects. The algorithm solves the gasdynamic and species continuity equations in a loosely coupled manner. The new code has been validated by computing the Moo = 25 laminar flow of chemically reacting air over a wedge and a cone. The results of these computations are compared with the results from a centrally differenced, fully coupled, nonequilibrium PNS code. The agreement is excellent, except in the vicinity of the shock wave, where the present code exhibits superior shock-capturing capabilities.