Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session MR: Supersonic and Hypersonic Flows I |
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Chair: Guillaume Bres, Exa Corporation Room: Long Beach Convention Center 203C |
Tuesday, November 23, 2010 8:00AM - 8:13AM |
MR.00001: DNS of high speed boundary layers over ablating surfaces Kalen Braman, Venkat Raman, Rochan Upadhyay, Ofodike Ezekoye Ablation of thermal protection shields is an important design problem in developing reentry vehicles. Development of predictive computational models for this problem will enable optimization of the size and hence weight of the protective layer. In this work, direct numerical simulation (DNS) of a compressible ablating boundary layer is used to understand the modeling issues in the context of Reynolds-averaged Navier Stokes (RANS) equations. The DNS is performed at conditions obtained from a detailed RANS study of a reentry vehicle. The free stream conditions of the two simulations are Mach 0.6, temperature 5940 K, and $Re_{\theta}$ 1000; and Mach 1.2, temperature 5580 K, and $Re_{\theta}$ 2000. The surface ablation of a graphite ablator is modeled using a locally 1-D, quasi-steady state formulation with control volume mass and energy balances over the interior of the ablator. A 10-species gas phase chemistry mechanism is used. A priori studies are used to evaluate scalar flux models and the reaction source term closure in RANS. [Preview Abstract] |
Tuesday, November 23, 2010 8:13AM - 8:26AM |
MR.00002: Direct Numerical Simulation of low-temperature ablation by turbulent flows Ryan Crocker, Yves Dubief, Christopher White The present study is motivated by the understanding and modeling of the dynamic interactions between a turbulent fluid transporting an erosive agent, and an erodible surface. As the erosive agent causes changes in the geometry of the wall-boundary conditions, turbulence may rapidly evolve into a non-equilibrium state and may further accelerate ablation. To investigate this complex process, a direct numerical simulation (DNS) algorithm is designed to simulate the temporal and spatial evolution of a surface subjected to low-temperature ablation caused by turbulent flow. The ablative wall is fully discretized and the interface fluid/wall is modeled by a level-set method combined with flow and thermal immersed boundary methods. After a discussion of numerical challenges and their solutions, low Reynolds turbulent ablation flows are used to illustrate the complexity of the problem with a focus on emerging turbulent and topographical scales as ablation proceeds. [Preview Abstract] |
Tuesday, November 23, 2010 8:26AM - 8:39AM |
MR.00003: Transport of Ablation Products in a Mach 5 Boundary Layer using Naphthalene PLIF Clemens Noel, Bryan Lochman, Zach Murphree, Venkat Narayanaswamy Planar laser-induced fluorescence of sublimated naphthalene was used to visualize the transport of ablation products in a Mach 5 turbulent boundary layer. The naphthalene was molded into a rectangular insert that was mounted flush with the floor of a Mach 5 wind tunnel. The naphthalene fluorescence was excited using a frequency quadrupled Nd:YAG laser (266 nm) and the fluorescence emission between 310 -- 350 nm was collected. The temperature and pressure dependences of the fluorescence were studied in detail. An increase in the fluorescence was observed with increasing temperature for the temperature range tested (300 -- 525K). Fluorescence-lifetime measurements were made in pure-air and nitrogen environments at 300 K over the range 3.3-101.3 kPa. The results in air exhibited the expected Stern-Volmer behavior with decreasing lifetimes at increasing pressure, whereas nitrogen exhibited the opposite trend. Preliminary PLIF images of the sublimated naphthalene were acquired in the Mach 5 turbulent boundary layer. Relatively low signal-to-noise-ratio images were obtained at a stagnation temperature of 345 K, but much higher signal images were obtained at a stagnation temperature of 375 K. [Preview Abstract] |
Tuesday, November 23, 2010 8:39AM - 8:52AM |
MR.00004: Stochastic approach of meteor-generated infrasounds Christophe Millet, Christophe P. Haynes In recent years, numerous bolide sources have been detected by the International Monitoring System infrasound arrays. In the present study, the meteorite fall near Carancas, Peru, on September 15, 2007 has been analyzed through the simulation of the emission and non-linear propagation of shocks. Given that the meteoric body shape and the way infrasounds are generated are generally not well known, the unknown parameters of bolides have been chosen to be random fields. By comparing our analytic and numeric results to recorded data, we aim to show that it might be entirely plausible that an N-wave type signal could have originated from the Carancas meteor scenario. It is shown that, given a random entry diameter, the uncertainty of the ground overpressure increases as the N-wave emission altitude decreases, as a result of bolide near-field effects, except for low-altitudes, where the uncertainties associated with the propagation medium may be dominant. [Preview Abstract] |
Tuesday, November 23, 2010 8:52AM - 9:05AM |
MR.00005: Shock-wave surfing and the separation of meteoroid fragments in the atmosphere Stuart Laurence, Ralf Deiterding Studying the aerodynamic interactions between bodies travelling at highly supersonic speeds is necessary to our understanding of the separation of meteoroid fragments following atmospheric disruption. Here we show that a phenomenon referred to as ``shock-wave surfing,'' in which a body moves in such a way as to follow the shock wave generated by another upstream body, can lead to the accumulation of a significantly higher relative lateral velocity between fragments than would otherwise be possible. The surfing phenomenon is investigated for the canonical cases of interactions between a sphere and a wedge, and between two spheres. Numerical simulations are performed and a simple theoretical model is developed to determine the forces acting on the surfing body. A phase-plane description is employed to elucidate features of the system dynamics in both cases. For the two sphere case, a strong influence of the body radius ratio on the separation process is found and a critical ratio is predicted for initially touching fragments that delineates entrainment of the smaller fragment within the larger fragment's shock from expulsion. It is also shown that a large fraction of the variation in the separation behaviour of meteoroid fragments deduced by previous authors from an analysis of terrestrial crater fields can be explained by a combination of surfing and a modest rotation rate of the parent body. [Preview Abstract] |
Tuesday, November 23, 2010 9:05AM - 9:18AM |
MR.00006: Experimental Investigation of Shock Wave Surfing N.J. Parziale, H.G. Hornung, J.E. Shepherd, S.J. Laurence Shock wave surfing is investigated experimentally in GALCIT's Mach 4.0 Ludwieg Tube. Shock wave surfing occurs when a secondary free-body follows the bow shock formed by a primary free-body; an example of shock wave surfing occurs during meteorite breakup. The free-bodies in the current investigation are nylon spheres. During each run in the Ludwieg tube a high speed camera is used to capture a series of schlieren images; edge tracking software is used to measure the position of each sphere. Velocity and acceleration are had from processing the position data. The radius ratio and initial orientation of the two spheres are varied in the test matrix. The variation of sphere radius ratio and initial angle between the centers of gravity are shown to have a significant effect on the dynamics of the system. [Preview Abstract] |
Tuesday, November 23, 2010 9:18AM - 9:31AM |
MR.00007: Shock Structure in a Supersonic Jet Catalna Stern, Cesar Aguilar We visualize the stationary shock structure of a 1.6mm supersonic jet flow using a shadowgraph. The form and size of the structure can be determined. Through the heterodyne detection of Rayleigh scattering by the flow, we can obtain the instant spatial Fourier Transform for a given wave vector, of the density fluctuations in the flow. The wave vector is related to the size of the fluctuations and indicates their direction of propagation. The spectral density of these fluctuations shows two of the modes described by Monin and Yaglom: The entropic mode is related to entropy fluctuations at constant pressure, the acoustic mode is related to isentropic pressure fluctuations. Besides the two predicted peaks, another one appears close to the shocks, that moves at slow speed. We present preliminary results that show that this slow speed fluctuation appears always close and parallel to the shock. Research is still in progress to understand the dynamics in these regions of the flow. [Preview Abstract] |
Tuesday, November 23, 2010 9:31AM - 9:44AM |
MR.00008: Large eddy simulation of an underexpanded sonic jet Catherine Gorle, Mirko Gamba, Frank Ham Large eddy simulations (LES) for an underexpanded sonic jet in quiescent flow have been performed using the explicit spatially-filtered compressible Navier-Stokes solver Charles. The unstructured finite volume method uses a blended central-upwind scheme in smooth flow regions to minimize artificial damping of resolvable turbulence scales and switches to a third order WENO method and an HLCC approximate Riemann solver to capture discontinuities. Time discretisation is performed with an explicit third order Runge Kutta scheme. The simulations reproduce the conditions of an experiment where single-shot Schlieren imaging of the jet is used to investigate the instantaneous and time-averaged steady-state structure of the barrel shock and the jet far-field growth rate. A pseudo-time sequence of the formation of the barrel shock that tracks the jet injection transient is also constructed. A comparison of the shock structure and time scale of the jet formation obtained from the LES and the experiment is presented, showing a good agreement in the shock structure. Future work includes a similar study for an underexpanded sonic jet in a supersonic cross flow, and will also focus on the investigation of turbulent mixing. [Preview Abstract] |
Tuesday, November 23, 2010 9:44AM - 9:57AM |
MR.00009: Ignition, Flame Structure and Near-Wall Burning in Transverse Hydrogen Jets in Supersonic Crossflow Mirko Gamba, M. Godfrey Mungal, Ronald K. Hanson The work aims at investigating near-wall ignition and flame structure in transverse underexpanded hydrogen jets in high-enthalpy supersonic crossflows generated in an expansion tube. Crossflow conditions are held fixed at $M\mbox{=}2.4$, $p=40\ kPa$ and $T\approx1400\ K$, while jet-to-crossflow momentum flux ratios $J$ in the range $0.3-5.0$ are considered. Schlieren and $OH^{*}$ chemiluminescence imaging are used to characterize flow structure, ignition and flame penetration, while the instantaneous reaction zone is identified with planar laser-induced fluorescence imaging of OH on side- and plan-view planes. The upstream separation length is found to scale as $J^{0.44}D$ ($D$ jet diameter). Similarly, the ignition point $x_{ig}$ strongly depends on $J$: $x_{ig}$ tends to a limiting value of $\sim22D$ as $J\rightarrow0$, and the flame is anchored in the upstream recirculation region and lee-side of the jet for $J>3$. Flame penetration is well described by the traditional form $k\left(x/DJ\right)^{m}$ where both $k$ and $m$ are found to depend on $J$ but these parameters reach a limiting value of $k\approx1$ and $m\approx0.3$ for $J>2$. The roles of the unsteady bow shock, the separation and recirculation regions on the near-wall ignition, stabilization and mixing at large $J$ are discussed. [Preview Abstract] |
Tuesday, November 23, 2010 9:57AM - 10:10AM |
MR.00010: Implementation and Validation of a MILES Solver for the Simulation of Supersonic Jet Flow Andrew Corrigan, Junhui Liu, K. Kailasanath, Ravi Ramamurti A new MILES (Monotonically Integrated Large-Eddy Simulation) solver has been developed for the simulation of supersonic jet flow and its acoustic properties. The solver uses a finite volume discretization together with the FCT (Flux-Corrected Transport) convection scheme. In order to validate the code, a number of benchmark runs have been performed and compared to those obtained using an FCT solver from the finite element code FEFLO [Liu et al, AIAA, 2009]. The runs use identical input flow conditions and geometry, and include a convergent-divergent nozzle with both over- and under-expanded flow. [Preview Abstract] |
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