Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session LM: Supersonic/Hypersonic II |
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Chair: Nicholas Kevlahan, McMaster University Room: 200B |
Monday, November 23, 2009 3:35PM - 3:48PM |
LM.00001: Shock generated vorticity in the interstellar medium and the origin of the stellar initial mass function Nicholas Kevlahan, Ralph Pudritz Observations of the interstellar medium (ISM) and molecular clouds suggest these astrophysical flows are strongly turbulent. The main observational evidence for turbulence is the power-law energy spectrum for velocity fluctuations, $E(k)\propto k^\alpha$, with $\alpha\in [-1.5,-2.6]$. The Kolmogorov scaling exponent, $\alpha=-5/3$, is typical. At the same time, the observed probability distribution function (PDF) of gas densities in both the ISM as well as in molecular clouds is a log-normal distribution. In this paper we examine the density and velocity structure of interstellar gas traversed by curved shock waves in the kinematic limit. We demonstrate mathematically that just a few passages of curved shock waves generically produces a log-normal density PDF. This explains the ubiquity of the log-normal PDF in many different numerical simulations. We also show that subsequent interaction with a spherical blast wave generates a power-law density distribution at high densities, qualitatively similar to the Salpeter power-law for the IMF. Finally, we show that a focused shock produces a {\em downstream\/} flow with energy spectrum exponent $\alpha=-2$. Subsequent shock passages reduce this slope, achieving $\alpha\approx -5/3$ after a few passages. These results suggest that fully-developed turbulence may {\em not\/} be required to explain the observed energy spectrum and density PDF. [Preview Abstract] |
Monday, November 23, 2009 3:48PM - 4:01PM |
LM.00002: Surface Catalysis Modeling of Air-SiO2 Systems Under Hypersonic Conditions Using ReaxFF MD Simulation Paul Norman, Tom Schwartzentruber, Ioana Cozmuta The high-speed entry of a blunt body into Earth's atmosphere brings about the dissociation of diatomic nitrogen and oxygen molecules via the shockwave formed in front of the body. Through surface catalysis, these dissociated atoms can recombine on the heat shield of the body, increasing its overall heating. The goal of this project is to study surface catalysis on amorphous silicon-dioxide (SiO2), a significant component in the reusable thermal protection system used on the Space Shuttle. Specifically, our objective is to determine the rates of recombination of monatomic N and O for the range of temperatures and pressures experienced by a heat shield during Earth re-entry. Additionally, we aim to determine the rates of specific reaction mechanisms on a SiO2 surface, including adsorption, desorption, surface diffusion, and various recombination processes. This is accomplished by performing large reactive molecular dynamics simulations using the ReaxFF force field, which naturally allows bond formation/breaking to occur during the course of a molecular dynamics simulation. Several methods for speeding up the equilibration and collection of rates for low-pressure gas-surface systems (typical of re-entry conditions) where events become infrequent will also be discussed. [Preview Abstract] |
Monday, November 23, 2009 4:01PM - 4:14PM |
LM.00003: A Tightly Coupled Solver for Hypersonic Ablation Problems Nathan Mullenix, Alex Povitsky Ablation is a process of rapid material removal from a solid surface by chemical reactions, sublimation and other erosive processes, absorbing large quantities of heat, and is one of the techniques used for thermal protection on hypersonic vehicles. It consists of several coupled sub-processes including gas dynamics, heat transfer, and ablative mechanisms at the surface. The past state of the art models include only a subset of these and generally ignore transient phenomena involving shape changes (i.e. formation of cavities). The current study presents the development of a solution methodology for the ablation problem in which the model for each sub-process are linked at the point of their development, the solution of each is tightly coupled to the solution of the others, and shape changes effects are intrinsically included. Starting from first principles, the Reynolds Transport Theorem is used to derive a set of governing equations that takes into account the movement of the ablating surface and the resulting mass transfer. Existing explicit-in-time finite-volume numerical methods are modified for this set, and a reactive-Riemann solver is derived for ablative fluxes. Methods for avoiding numerical artifacts such as plumes of ablated material are described. Results are provided for graphite ablation in hypersonic flow, and are compared to relevant experiments, and their sensitivity to particular parameters will also be presented. [Preview Abstract] |
Monday, November 23, 2009 4:14PM - 4:27PM |
LM.00004: ABSTRACT WITHDRAWN |
Monday, November 23, 2009 4:27PM - 4:40PM |
LM.00005: Chemistry-Vibration Coupling in CO$_2$ system for High Enthalpy Nozzle Flows Sriram Doraiswamy, Daniel Kelley, Graham Candler The present work investigates the complex process of vibrational relaxation and its subsequent coupling with the chemical processes in high enthalpy nozzle flows. High enthalpy CO$_2$ nozzle expansion in reflected shock tunnels shows significant difference in shock standoff distance between computational and experimental results. CO$_2$ being a linear triatomic molecule has three modes of vibration - bending, symmetric stretch and antisymmetric stretch modes. To better model the vibrational relaxation, the bending and the symmetric stretch modes were coupled into one mode due the fact that these modes are strongly coupled through Fermi resonance. Furthermore, to simplify the analysis, this coupled mode was assumed to be in equilibrium with the translational mode. For the CO$_2$ only the antisymmetric mode is considered. A vibrational state-specific model was devised by considering the first few vibrational states of the diatomic species in a CO$_2$ system, i.e. CO$_2$, CO and O$_2$. The rate constants for the vibrational relaxation processes were obtained from experimental data. This vibrational model is then coupled with a chemistry model to run the full flowfield nozzle simulation, and also to obtain the shock standoff distance. [Preview Abstract] |
Monday, November 23, 2009 4:40PM - 4:53PM |
LM.00006: Comparison of CFD and Theoretical Post-Shock Gradients in Hypersonic Flow Graham Candler In recent work of Hornung, expressions for the gradients of flow properties immediately behind a curved shock wave were obtained for a reacting gas. In this work, I use the expressions derived by Hornung to compare with inviscid computational fluid dynamics simulations of a Mach 8 flow over a cylinder. A finite-rate vibrational relaxation model is used to simplify the comparisons with theory. The shape of the bow shock wave is extracted from the CFD results, fitted with a polynomial, and then used to compute the post-shock gradients of the main flow variables. It is found that in general the CFD results are in very good agreement with the theory for both perfect gas and vibrationally relaxing flows. There are some notable differences, mostly centered on the location of the change in sign of the post-shock density gradient; this quantity is found to be very sensitive to the relaxation rate of the gas. The theoretical post-shock gradients provide a rigorous test of CFD and suggest possible experiments that would be very sensitive test of the models of finite-rate vibrational and chemical processes. [Preview Abstract] |
Monday, November 23, 2009 4:53PM - 5:06PM |
LM.00007: Accelerated Molecular Dynamics Simulation of Hypersonic Flow Features in Dilute Gases Thomas Schwartzentruber, Paolo Valentini Accurate simulation of high-altitude hypersonic flows requires advanced physical models capable of predicting the transfer of energy between translational, rotational, vibrational, and chemical modes of a gas in strong thermochemical non-equilibrium. A combined Event-Driven / Time-Driven (ED/TD) Molecular Dynamics (MD) algorithm is presented that greatly accelerates the MD simulation of dilute gases. The goal of this research is to utilize advances in computational chemistry to study thermochemical non-equilibrium processes in hypersonic flows. The ED/TD MD method identifies impending collisions (including multi-body collisions) and advances molecules directly to their next interaction, however, then integrates each interaction accurately using an arbitrary interatomic potential via conventional MD with small timesteps. First, the ED/TD MD algorithm and efficiency will be detailed. Next, ED/TD MD simulations of normal shock waves in dilute argon will be validated with experiment and direct simulation Monte Carlo simulations employing the variable-hard-sphere collision model. Profiling of the code reveals that the relative computational time required for the MD integration of collisions is extremely low and the potential for incorporating advanced classical and first-principles interatomic potentials within the ED/TD MD method will be discussed. [Preview Abstract] |
Monday, November 23, 2009 5:06PM - 5:19PM |
LM.00008: On the Structure of Plasma Liners for Plasma Jet Induced Magneto Inertial Fusion Lingling Wu, Roman Samulyak 3D simulations of the formation and evolution of plasma liners for the Plasma Jet Induced Magneto Inertial Fusion (PJMIF) have been performed. In the PJMIF concept, a plasma liner, formed by merging of a large number of radial, highly supersonic plasma jets, implodes on the target in the form of two compact plasma toroids, and compresses it to conditions of the nuclear fusion ignition. The propagation of a single jet with Mach number 60 from the plasma gun to the merging point was studied using the front tracking code FronTier. The simulation result was used as input to the jet merger problem. The merger of 144 jets and the formation and heating of plasma liner by oblique shock waves was studied and compared with recent theoretical predictions. The main result of the study is the prediction of the average Mach number reduction and the description of the liner structure and properties. [Preview Abstract] |
Monday, November 23, 2009 5:19PM - 5:32PM |
LM.00009: Coupled computational fluid-thermal investigation of hypersonic flow over a quilted dome surface Christopher Ostoich, Daniel Bodony, Philippe Geubelle The hypersonic environment is characterized by the high temperatures that are generated in the fluid at a vehicle surface. In the effort to enable the operation of lightweight, reusable hypersonic vehicles, flexible, thin thermal protection panels have been considered to mitigate thermal loads. High surface temperatures create through-the-thickness thermal gradients which cause the panels to bow, resulting in changes to the external flow field and leading to a fully coupled fluid-thermal-structural problem. Certain aspects of the fluid-thermal (no structural) coupling were examined in a 1980s NASA Langley experiment of a Mach 5.74 laminar boundary past an array of spherical domes. We reexamine this case computationally using a high-fidelity Navier-Stokes solver coupled with a thermal solver to investigate the effects on the flow and resulting heat load on the structure due to the bowed panels. Specifically the surface temperature, surface heat flux, and downstream boundary developments are reported, and compared with experiment. [Preview Abstract] |
Monday, November 23, 2009 5:32PM - 5:45PM |
LM.00010: Penetrator Nose Drag Measurements in Supersonic Flows Joseph Holland, Phillip Schinetsky, Yesenia Tanner, Semih Olcmen, Stanley Jones In the current study, a rigid body penetrator nose shape that is optimized for minimum penetration drag (Jones et al., 1998) has been tested to determine the aerodynamic drag of such a penetrator in comparison to three additional nose shapes. Other nose shapes tested were an ogive cylinder, a 3/4 power series nose, and a standard cone. Fineness ratio for the studied nose geometries was chosen as l/d = 1 to maximize variation of the aerodynamic drag forces acting on the nose shapes. The experiments were carried out in the University of Alabama's 6'' x 6'' supersonic wind tunnel, using a 4 component force balance system. Each of the nose shapes were tested at nine different Mach numbers ranging from 1.99 to 3.65. Results show that the nose shape optimized for penetration has the lowest drag coefficient of all the shapes at each Mach number within an uncertainty of 5.75 {\%}. [Preview Abstract] |
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