Computational simulation of starting dynamics in a ram accelerator

The Chemical Oxygen-Iodine Laser, COIL, presents a significant challenge for analytical models due to the complex couplings between fluid dynamics and chemistry that occur in the system. This paper demonstrates the use of the Computational Fluid Dynamics(CFD) code GASP as an analytical tool to model these complexities. A conservative, multi-species diffusion model has been developed and added to GASP for molecular transport, and a ten specie, thirteen reaction finite rate kinetics mechanism is used to describe the non-equilibrium chemistry. The model is used for three-dimensional Navier-Stokes numerical simulations detailing the mixing and reacting flows within COIL devices at 126 torr total pressure operating conditions. Two injection configurations of a COIL device are considered: a baseline injection scheme and a modification to the baseline scheme designed to improve high pressure performance. A comparison between numerical simulations of the baseline injection scheme at 126 torr and 72 torr total pressure shows that less mixing occurs at high pressure. Detailed comparisons with experimental gain data show very good agreement, validating the model and the approach. The mixing information obtained from the high and low pressure comparison is then used to design a simulation with a modified injection scheme for high pressure operation. The simulation is performed at conditions identical to the original injection scheme and is found to produce a more uniform mixed flow, but with decreased penetration. L Introduction The COIL-' is a high-power, short wavelength chemical laser. The COIL uses a series of chemical reactions to produce electronically excited atomic iodine, the source of radiative and stimulated emission. A two phase exothermic reaction between basic hydrogen peroxide (BHP) and C12 produces the singlet delta electronically excited state of molecular oxygen O2(A). This occurs in the OzCA) generator, Figure 1. Oa^A) diffuses from the BHP liquid into a carrier gas flow that travels through the device. Molecular iodine is injected into the gas flow, and through a series of electronic-vibrational collisional energy transfers, I2 is dissociated producing ground-state I, I(PM). A resonant collisional energy transfer from O2('A) to I(POT) produces the electronically excited I(PV2). I(Py2), also denoted as I*, emits a photon at a wavelength of 1.315um to provide the lasing energy in the COIL. The gas phase chemistry that drives the COIL can be summarized as: 02(Z) + 2I(P3/2) 02(Z) + 2l(P3/2) [1] * Student member AIAA. f Member AIAA. * Member, associate fellow AIAA. Copyright © 1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. II. Background The CFD code used in the simulation of the COIL is GASP version 2.2 released by AeroSoft, Inc. GASP discretizes the integral form of the Reynolds Averaged Navier Stokes and species continuity equations using a finite volume formulation. The resulting set of algebraic equations are solved using 3 factor approximate factorization. Coupled to these equations are closure models for molecular transport properties and Arrhenius type finite rate chemical kinetics. The low magnitude 10 to 10 Reynolds numbers of COIL operation create conditions under which diffusive molecular fluxes dominate mixing processes. For this reason, a conservative, detailed diffusion model5 was added to the GASP code to calculate the individual species diffusive fluxes, replacing the existing constant Schmit number based Fick's law model. The model is given by the equation

Computational fluid dynamics (CFD) solutions of the full Navier-Stokes equations, including finite-rate chemical kinetics, are used to numerically simulate the reacting in-bore flowfield for a 90-mm (bore diameter) ram-accelerator projectile propulsion system. In this system a gun tube is filled with a high-pressure gaseous mixture of hydrogen, oxygen, and nitrogen. An axisymmetric projectile is launched into this tube, and rails are attached to the tube wall to ensure projectile centering. The shock system that develops around the projectile, along with boundary layers on the projectile and tube surfaces, ignites the mixture on or near the projectile afterbody. The resulting pressure imbalance on the projectile generates thrust. Numerical predictions of this pressure field can be used to evaluate the thrust efficiency of the system, i.e., the projectile design combined with the chemical composition of the propellant gas. Comparison of the predicted pressure field and the measured pressure data is used to judge the accuracy of the CFD code. The choice of chemical kinetics mechanism used in the CFD code is of critical importance in achieving an accurate numerical simulation of the ram accelerator.

Computational fluid dynamic (CFD) simulations in support of the SHARP-B2 (Slender Hypervelocity Aerothermodynamic Research Probes) program are presented. Pre-flight simulations of the aerothermal flight environment have been made to aid in the design of the flight hardware and instrumentation. Simulations of a critical flight qualification ground test performed in the 20 MW Panel Test Facility (PTF) arc jet, which is equipped with a semi-elliptical nozzle, have also been performed. The arc jet simulations were performed with the same CFD code that is used in the flight calculations. For both the flight and the arc jet simulations, the CFD results are used to provide surface heat transfer coefficients that serve as boundary conditions for an in-depth conduction thermal response code. By using the same CFD tool to compute the aerothermal environments for both the ground test and the flight test environment, useful comparisons are made regarding the traceability of the ground test environment to flight. Introduction The NASA Ames Research Center has been developing new Ultra High Temperature Ceramics (UHTC's) with the goal that these materials will enable sharp leading edges to be used for future space vehicles. Because of their unique thermo-structural properties, these materials are capable of operation at temperatures near 5100 °F without ablation. The materials have been developed and tested in ground-based arc jet facilities, and a flight test program designated SHARP (Slender Hypervelocity Aerothermodynamic Research Probes) has been initiated. The first flight demonstration, SHARP-B1, incorporated a UHTC nosetip mounted on a U.S. Air Force Mkl2A entry vehicle in collaboration with Sandia National Laboratory (SNL), and was successfully flown in May, 1997. The second flight test, SHARP-B2, which incorporated four instrumented UHTC strakes mounted on the side of the entry vehicle, was flown in September 2000. The goal of these flight tests is to assess the performance of the materials under entry conditions. The use of Computational Fluid Dynamic (CFD) simulations for the design of Thermal Protection Systems (TPS) has evolved to an extent that they have been integrated into the preliminary design process of Reusable Launch Vehicle (RLV) programs such as the X-33. Real-gas CFD simulations with finite-rate chemistry and proper boundary conditions for coupling with material thermal response codes are now routinely performed on high-end laboratory workstations. Much of the ground based testing of advanced TPS components is done in arc-heated test facilities such as those located in the Arc Jet Complex at the NASA Ames Research Center. These facilities are capable of simulating the high enthalpy flow environment experienced by entry vehicles for a broad range of test conditions and configurations. Semi-elliptical nozzles are available for the testing of flat panels in a high enthalpy boundary layer environment. CFD is • Aerospace Engineer, Reacting Flow Environments Branch, Member AIAA. t Senior Research Scientist, ELORET, Member AIAA. Copyright © 2001 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental Purposes. All other rights are reserved by the copyright owner. (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Auxiliary power units (APUs) are gas turbine engines that provide high-pressure air and electrical power to aircraft systems. They provide primary power while the aircraft is parked on the ramp, starting services for the main engines, and backup power while the aircraft is in flight. Many APUs employ inlet systems which include a “pop-up” door that allow for the capture of freestream ram pressure during flight. This results in increased inlet recovery and a corresponding improvement in the performance of the APU. This APU door, when open, is exposed to airflow instability inherent in the aircraft boundary layer in the aft section of the fuselage, where APUs are typically housed. Additionally, the pop-up nature of the inlet door produces a large region of separated airflow off of the back side of the door. Systematic vortex shedding is frequently a major component of this separated region. As new APU doors are made with less rigid material to save weight, a need to better understand the unsteady aerodynamic excitations of the flow field around the door has arisen, as these new doors may be more susceptible to vibration during flight. Recent advances in Computational Fluid Dynamics (CFD) meshing tools and transient modeling have enabled a CFD study to be performed which will investigate this time-dependent phenomenon. As transient CFD analysis is still a relatively new field for commercial CFD codes, a calibration was needed to verify the accuracy of the CFD predictions and to form any calibration correction terms. Honeywell Aerospace owns a Boeing 757 flight test vehicle which is normally used to flight test propulsion engines. However, this aircraft also includes a pop-up APU inlet door that is similar to most other APU inlet door styles. This APU door was instrumented using high response pressure transducers placed on the forward and aft sides of the inlet door as well as upstream of the door to measure upstream instability. The aircraft was flown at a variety of flight conditions and APU operating points and the transient data was recorded. After the completion of the flight test, a CFD model was constructed of the B757 flight test vehicle. Because aerodynamic instability can be generated anywhere on the aircraft, the entire airframe from nose to tail was modeled. The APU inlet door geometry was also created, meshed and added to the CFD model. This CFD model was run in a transient mode to simulate the exact same flight conditions and APU operating points as were tested during the flight test. Dynamic results in the time and frequency domains predicted by the CFD analyses were compared to flight test data and correlation and calibration factors were derived.

CAPTools - An interactive toolkit for mapping CFD codes onto parallel architectures

In a reactor severe accident, molten jet breakup and solidification are important behaviors after large pours of molten material fall into the coolant in-vessel or ex-vessel. However, heat and mass transfer processes inside melt during jet breakup have not been studied sufficiently. Existing research on jet fragmentation is relatively macroscopic, and the micro interface condensation details are not well studied. In this paper, a two-dimensional multiphase computational fluid dynamics (CFD) code with the Volume of Fluid (VOF) method and solidification model is applied to simulate molten jet breakup with surface solidification. The VOF model is used to capture the interface, study the details, and add the influence of solidification. Solidification and instability can be seen at the interface. In order to simulate melt solidification, an energy equation is modeled using an enthalpy-based formulation, and viscosity variation during phase change is taken into account. The comparative results between the CFD code and jet breakup experiments show that melt jet front position histories, breakup length, and breakup time are in good agreement with the experiments. The simulation results show that crust formation of the jet surface suppresses surface instability and jet breakup behavior. As the interfacial temperature decreases, the droplet cumulative mass fraction decreases, and the solidified metal proportion increases. The simulation results by the CFD code with the solidification model are valuable and important for understanding the molten jet breakup mechanism.

This paper describes the work done to achieve high parallel performance for an unstructured, unsteady turbomachinery computational fluid dynamics (CFD) code. The aim of the work described here is to be able to scale problems to the thousands of processors that current and future machine architectures will provide. The CFD code is in design use in industry and is also used as a research tool at a number of universities. High parallel scalability has been achieved for a range of turbomachinery test cases, from steady-state hexahedral mesh cases to fully unsteady unstructured mesh cases. This has been achieved by a combination of code modification and consideration of the parallel partitioning strategy and resulting load balancing. A sliding plane option is necessary to run fully unsteady multistage turbomachinery test cases and this has been implemented within the CFD code. Sample CFD calculations of a full turbine including parts of the internal air system are presented.

The results of a National Research Council study on the effect that advances in computational fluid dynamics (CFD) will have on conventional aeronautical ground testing are reported. Current CFD capabilities include the depiction of linearized inviscid flows and a boundary layer, initial use of Euler coordinates using supercomputers to automatically generate a grid, research and development on Reynolds-averaged Navier-Stokes (N-S) equations, and preliminary research on solutions to the full N-S equations. Improvements in the range of CFD usage is dependent on the development of more powerful supercomputers, exceeding even the projected abilities of the NASA Numerical Aerodynamic Simulator (1 BFLOP/sec). Full representation of the Re-averaged N-S equations will require over one million grid points, a computing level predicted to be available in 15 yr. Present capabilities allow identification of data anomalies, confirmation of data accuracy, and adequateness of model design in wind tunnel trials. Account can be taken of the wall effects and the Re in any flight regime during simulation. CFD can actually be more accurate than instrumented tests, since all points in a flow can be modeled with CFD, while they cannot all be monitored with instrumentation in a wind tunnel.

Thermal-hydraulic analysis of the TRADE spallation target

A computational fluid dynamics (CFD) code, developed at Argonne national laboratory to simulate turbulent mixing, combustion reaction, radiation heat transfer, and pollutant kinetics of combustion flow, has been used to study various melting furnaces. The code employs an integral approach to incorporate a lumped combustion reaction model in the flow calculation and a separate hybrid technique to perform pollutant kinetics calculations. The CFD code has been validated with experimental data collected from industrial furnaces and then used for a parametric study of various furnace geometries and operation conditions. The furnace configuration greatly effected the flow property distribution as well as the combustion efficiency. The air injection velocity effected the flow penetration and the species mixing. The injection angle also significantly effected the species mixing. And finally, the equivalence ratio effected the temperature and pollutant concentrations. The study demonstrates that CFD can be a useful tool for analyzing the flow field of the combustion space in industrial furnaces.

Accurate prediction of bubble behavior in large diameter pipes is crucial for evaluating the performance of safety systems, steam generators, and heat exchangers in nuclear systems. Bubble behavior in large diameter pipes under two-phase flow significantly differs from that in small pipes. With the increasing use of computational fluid dynamics (CFD) codes, predicting interfacial area concentration (IAC) is critical for understanding multi-dimensional bubble behavior. This study developed a two-group local bubble size model for bubbly, slug, and churn flows under adiabatic conditions. The model includes correlations for void fraction and bubble sizes of two groups, which were implemented into CFD codes and validated against experimental data from large diameter pipes with low-pressure air–water flow. Results show the model's prediction accuracy surpasses existing correlations. The developed correlations are applicable across a range of flow conditions covering pipe diameters in the range 0.05–0.152 m, pressures from atmospheric to 300 kPa, superficial liquid velocities from 0.25 m/s to 2.85 m/s, and superficial gas velocities from 0.04 m/s to 5.48 m/s. The model is expected to enhance the prediction capabilities of CFD codes for the adiabatic two-group two-phase flows in the large diameter pipes.

The Use of Computational Fluid Dynamic Code for Modelling Spray from a Mistblower

Transitional flow has a significant impact on vehicles operating at supersonic and hypersonic speeds. An economic way to simulate this problem is to use computational fluid dynamics (CFD) codes. However, not all CFD codes can solve transitional flows. This paper examines the ability of the Spalart–Allmaras one-equation BCM (SA-BCM) transitional model to solve hypersonic transitional flow, implemented in the open-source CFD code Eilmer. Its performance is validated via existing wind tunnel data. Eight different hypersonic flow conditions are applied. A flat plate model is built for the numerical tests. The results indicate that the existing SA-BCM model is sensitive to the freestream turbulence intensity and the grid size. It is not accurate in all the test cases, though the transitional length can be matched by tuning the freestream intensity. This is likely due to the intermittency term of the SA-BCM model not being appropriately calibrated for high-velocity flow, though if the model can be recalibrated it may be able to solve the general high-velocity flows. Although the current SA-BCM model is only accurate under certain flow conditions after one calibration process, it remains attractive to CFD applications. As a one-equation model, the SA-BCM model runs much faster than multiple-equation flow models.

Modelling aerosol number distributions from a vehicle exhaust with an aerosol CFD model

Simulation of some fluid phenomena associated with Generation IV reactors requires the capability of modeling mixing in two- or three-dimensional flow. At the same time, the flow condition of interest is often transient and depends upon boundary conditions dictated by the system behavior as a whole. Computational fluid dynamics (CFD) is an ideal tool for simulating mixing and three-dimensional flow in system components, whereas a system analysis tool is ideal for modeling the entire system. This paper presents the reasoning which has led to coupled CFD and systems analysis code software to analyze the behavior of advanced reactor fluid system behavior. In addition, the kinds of scenarios where this capability is important are identified. The important role of a coupled CFD/systems analysis code tool in the overall calculation scheme for a Very High Temperature Reactor is described. The manner in which coupled systems analysis and CFD codes will be used to evaluate the mixing behavior in a plenum for transient boundary conditions is described. The calculation methodology forms the basis for future coupled calculations that will examine the behavior of such systems at a spectrum of conditions, including transient accident conditions, that define the operational and accident envelope of the subject system. The methodology and analysis techniques demonstrated herein are a key technology that in part forms the backbone of the advanced techniques employed in the evaluation of advanced designs and their operational characteristics for the Generation IV advanced reactor systems.