
 The spectral ratio between horizontal and vertical components (H/V ratio) of microtremors measured at the ground surface has been used to estimate fundamental periods and amplification factors of a site, although this technique lacks theoretical background. The aim of this article is to formulate the H/V technique in terms of the characteristics of Rayleigh and Love waves, and to contribute to improve the technique. The improvement includes use of not only peaks but also troughs in the H/V ratio for reliable estimation of the period and use of a newly proposed smoothing function for better estimation of the amplification factor. The formulation leads to a simple formula for the amplification factor expressed with the H/V ratio. With microtremor data measured at 546 junior high schools in 23 wards of Tokyo, the improved technique is applied to mapping site periods and amplification factors in the area.

Ground Motion Characteristics Estimated from Spectral Ratio between Horizontal and Verticcl Components of Mietremors.

Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

New insights into large eddy simulation

Design, Preparation and Properties of Carbon Fiber Reinforced Ultra-High Temperature Ceramic Composites for Aerospace Applications: A Review

应用DES(Detached-Eddy Simulation)方法数值模拟了3种不同失速类型的翼型的升力特性.DES方法结合了RANS(Reynolds-averaged Navier-Stokes)和LES(Large Eddy Simulation approaches)的优点.基于Spalart-Allmaras湍流模型,在近壁面DES体现为RANS模型的特点而在远离物面处又具有LES的亚格子模型的特性.对此模型使用了LU-SGS隐式格式求解.通过和实验结果对比,显示这种方法可以有效地预测翼型的失速特性.

Detached—Eddy Simulation方法模拟不同类型翼型的失速特性

A comprehensive theoretical/numerical framework is established and validated to study the chemical erosion of carbon-carbon/graphite nozzle materials in solid-rocket motors at practical operating conditions. The formulation takes into account detailed thermofluid dynamics for a multicomponent reacting flow, heterogeneous reactions at the nozzle surface, condensed-phase energy transport, and nozzle material properties. Many restrictive assumptions and approximations made in the previous models have been relaxed. Both metallized and nonmetallized AP/HTPB composite propellants are treated. The predicted nozzle surface recession rates compare well with three different sets of experimental data. The erosion rate follows the trend exhibited by the heat-flux distribution and is most severe in the throat region. H 2 O proved to be the most detrimental oxidizing species in dictating nozzle erosion, followed by much lesser contributions from OH and CO 2 , in that order. The erosion rate increases with increasing chamber pressure, mainly due to higher convective heat transfer and enhanced heterogeneous surface reactions. For nonmetallized propellants, the recession rate is dictated by heterogeneous chemical kinetics because the nozzle surface temperature is relatively low. For metallized propellants, the process is diffusion-controlled due to the high surface temperature. The erosion rate decreases with increasing aluminum content, a phenomenon resulting from reduced concentrations of oxidizing species H 2 O, OH, and CO 2 . The transition from the kinetics-controlled to diffusion-controlled mechanism occurs at a surface temperature of around 2800 K.

Chemical Erosion of Carbon-Carbon/Graphite Nozzles in Solid-Propellant Rocket Motors

A study is conducted to predict graphite/carbon–carbon nozzle erosion behavior in solid rocket motors for wide variations of propellant formulations. The numerical model considers the solution of Reynolds-averaged Navier– Stokes equations in the nozzle, heterogeneous chemical reactions at the nozzle surface, variable transport and thermodynamic properties, andheat conduction in the nozzlematerial. Twodifferent ablationmodels are considered and compared: a surface equilibriumapproach and afinite-ratemodel. Results show that the erosion rate is diffusion limited for metallized propellants, ensuring sufficiently high wall temperatures, and it is kinetic limited for nonmetallized propellants. For low surface temperatures, the twomodels are consistent with each other and predict the same erosion rate, while the surface equilibrium model overpredicts the recession at low surface temperatures. The calculated results show an excellent agreement with the experimental data from the ballistic test and evaluation system motor firings, and the finite-rate model actually improves the predictions when the kinetic-limited regime is approached.

Thermochemical erosion analysis for graphite/carbon-carbon rocket nozzles

equilibriumablationwithsurfacemassandenergybalancesfullycoupledwiththenumericalsolverandcanaccount for both surface oxidation and sublimation. The surface temperature is obtained from the steady-state ablation approximation. This numerical procedure can predict aerothermal heating, chemical species concentrations, and carbon material ablation rate over the heat-shield surface of reentry vehicles. Two-dimensional axisymmetric simulations have been performed to numerically reproduce the ablation of a graphite sphere cone that has been tested in the Interaction Heating Facility at the NASA Ames Research Center. The freestream conditions of the selected test case are typical for Earth reentry from a planetary mission. The predicted ablation rate and surface temperatureassumingfrozenchemistryinthe flowshowagoodagreementwiththeavailableexperimentaldata.The agreementisfurtherimprovedfreezingthenitrogenrecombinationreactionatthesurfacetobemoreconsistentwith experimental observation, which has shown nitrogen atom recombination not to occur at the graphite surface.

Navier-Stokes Simulations of Hypersonic Flows with Coupled Graphite Ablation

A study is conducted to predict C/C nozzle recession behavior in solid rocket motors for broad variations of propellant formulations and motor operating conditions. The numerical model considers the turbulent flow in the nozzle, heterogeneous chemical reactions at the nozzle surface, variable transport and thermodynamic properties, and heat conduction in the nozzle material. Results show that the recession rate is largely determined by the diffusion of the major oxidizing species (H2O, CO2, OH) to the nozzle surface. Both the concentration of the major oxidizing species -affected by the aluminum content of the propellant- and the chamber pressure exert a strong influence on the recession rate. The erosion rate increases almost linearly with chamber pressure and decreases with propellants with higher aluminum content. The calculated results show a very good agreement with the experimental data from the BATES motor firings.

Coupled Analysis of Flow and Surface Ablation in Carbon-Carbon Rocket Nozzles

Dual-bell nozzles represent a possible solution to improve the performance of large liquid rocket engines for launcher first stages. The present paper studies the role of the second bell shape on the side loads that can occur during the transition between the two main operating modes of the nozzle. In particular, the design of the second bell profile is critically discussed on the basis of results obtained from suitable test cases. The analysis of performance and behavior during the transition is carried out by a validated turbulent Navier-Stokes solver. Geometries were generated by the method of characteristics. Results show that slightly different geometries designed by the method of characteristics yield modest performance changes but severe differences in behavior during the transition phase.

Role of Wall Shape on the Transition in Axisymmetric Dual-Bell Nozzles

Dual-bell nozzles permit two different operating modes, which provide higher performance than conventional bell nozzles when applied to rocket engines operating from sea level During the low-altitude mode, the flow is separated and the separation line is located near the inflection of the nozzle wall, in a region characterized by a negative value of the wall pressure gradient, as in conventional nozzles, and in which side loads may occur The characteristics of this region in hot full-scale applications are addressed by means of numerical simulations, and scaling laws are sought for cold subscale side-load experiments Moreover, the flow behavior during the transition between the two operating modes is analyzed by time-accurate simulations

Emanuele Martelli

Papers

Thermochemical erosion analysis for graphite/carbon-carbon rocket nozzles

Navier-Stokes Simulations of Hypersonic Flows with Coupled Graphite Ablation

Coupled Analysis of Flow and Surface Ablation in Carbon-Carbon Rocket Nozzles

Role of Wall Shape on the Transition in Axisymmetric Dual-Bell Nozzles

Numerical Parametric Analysis of Dual-Bell Nozzle Flows