Investigation of a Stiff-Integrator Scheme for High-Speed Reacting Flows

2000 ◽  
Author(s):  
Lance D. Woolley ◽  
Douglas A. Schwer ◽  
Russell L. Daines
Keyword(s):  
Time Scales ◽  
High Speed ◽  
Reaction Rates ◽  
Reacting Flows ◽  
Operator Technique ◽  
Detailed Chemical Kinetics ◽  
Time Step ◽  
Kinetics Models ◽  
Split Operator ◽  
Air Chemistry

Abstract Improvements in the modeling of high-speed reacting propulsion flowfields are sought through the coupling of a stiff integrator to determine chemical reaction rates with a multidimensional CFD code. Detailed chemical kinetics models usually have significantly shorter reaction time scales than the fluid time scales, resulting in stiff governing equations and robustness issues. The present work investigates the application of a stiff ordinary differential equation solver, coupled to a diagonalized alternating-direction implicit scheme to decouple the governing time scales. This coupled ODE-ADI split-operator technique is applied to two high-speed reacting flows using hydrogen/air chemistry. The results from the stiff integrator method are compared to the traditional coupled approach utilizing 8- and 18-step kinetics models. Time-step choice, robustness, and comparison of results between the different solution methods are discussed, along with CPU times.

Cavity Flame Holding for High Speed Reacting Flows

2010 ◽  
Author(s):  
Onur Tuncer
Keyword(s):  
Mach Number ◽  
High Speed ◽  
Fuel Injection ◽  
Transport Model ◽  
Leading Edge ◽  
Reacting Flows ◽  
Stagnation Temperature ◽  
Detailed Chemical Kinetics ◽  
Flame Holder ◽  
Shear Stress Transport Model

Combustion phenomena in a ramjet combustor with cavity flame-holder is studied numerically. Combustor follows a constant area isolator and comprises of hydrogen fuel injected sonically upstream of the cavity. Secondary fuel injection is performed at the cavity backwall. A diverging section follows the cavity to prevent thermal choking. These concepts are also utilized in practice. Calculations were performed for an entrance Mach number of 1.4. Stagnation temperature is 702 K, corresponding to a flight Mach number of 3.3 at an altitude of 12.5 km. Detailed chemical kinetics are taken into account with a reaction mechanism comprising of 9 species and 25 reaction steps. Turbulence is modeled using Menter’s k–ω shear stress transport model, which is appropriate for high speed internal flows. It is observed that flame anchors at the leading edge of the cavity, and the flame is stabilized in the cavity mode rather than the jet-wake mode. Numerical simulation captures all the essential features of the reacting flowfield.

scholarly journals The Influence of an Integration Time Step on Dynamic Calculation of a Vehicle-Track-Bridge under High-Speed Railway

Mathematics ◽  
2021 ◽  
Vol 9 (4) ◽  
pp. 431
Author(s):  
Junjie Ye ◽  
Hao Sun
Keyword(s):  
High Speed ◽  
Coupled Model ◽  
Short Wave ◽  
Integration Time ◽  
Vertical Acceleration ◽  
Dynamic Calculation ◽  
Time Step ◽  
High Speed Railway ◽  
Track Irregularity ◽  
Track Bridge

In order to study the influence of an integration time step on dynamic calculation of a vehicle-track-bridge under high-speed railway, a vehicle-track-bridge (VTB) coupled model is established. The influence of the integration time step on calculation accuracy and calculation stability under different speeds or different track regularity states is studied. The influence of the track irregularity on the integration time step is further analyzed by using the spectral characteristic of sensitive wavelength. According to the results, the disparity among the effect of the integration time step on the calculation accuracy of the VTB coupled model at different speeds is very small. Higher speed requires a smaller integration time step to keep the calculation results stable. The effect of the integration time step on the calculation stability of the maximum vertical acceleration of each component at different speeds is somewhat different, and the mechanism of the effect of the integration time step on the calculation stability of the vehicle-track-bridge coupled system is that corresponding displacement at the integration time step is different. The calculation deviation of the maximum vertical acceleration of the car body, wheel-sets and bridge under the track short wave irregularity state are greatly increased compared with that without track irregularity. The maximum vertical acceleration of wheel-sets, rails, track slabs and the bridge under the track short wave irregularity state all show a significant declining trend. The larger the vibration frequency is, the smaller the range of integration time step is for dynamic calculation.

Numerical Simulation of Heat Transfer and Fluid Dynamics in Supersonic Chemically Reacting Flows

2010 ◽  
Author(s):  
Alexander M. Molchanov ◽  
Anna A. Arsentyeva
Keyword(s):  
Heat Transfer ◽  
Fluid Dynamics ◽  
High Speed ◽  
Stokes Equations ◽  
Order Approximation ◽  
Reacting Flows ◽  
Supersonic Combustion ◽  
Special Method ◽  
Chemically Reacting Flows ◽  
Chemically Reacting

An implicit fully coupled numerical method for modeling of chemically reacting flows is presented. Favre averaged Navier-Stokes equations of multi-component gas mixture with nonequilibrium chemical reactions using Arrhenius chemistry are applied. A special method of splitting convective fluxes is introduced. This method allows for using spatially second-order approximation in the main flow region and of first-order approximation in regions with discontinuities. To consider the effects of high-speed compressibility on turbulence the author suggests a correction for the model, which is linearly dependent on Mach turbulent number. For the validation of the code the described numerical procedures are applied to a series of flow and heat and mass transfer problems. These include supersonic combustion of hydrogen in a vitiated air, chemically reacting flow through fluid rocket nozzle, afterburning of fluid and solid rocket plumes, fluid dynamics and convective heat transfer in convergent-divergent nozzle. Comparison of the simulation with available experimental data showed a good agreement for the above problems.

scholarly journals White Light Photometry of the Cataclysmic Variable BZ Camelopardalis

1992 ◽  
Vol 151 ◽  
pp. 441-444
Author(s):  
Gabriel Pajdosz ◽  
Stanisław Zoła
Keyword(s):  
Power Spectrum ◽  
Time Scales ◽  
White Light ◽  
High Speed ◽  
Cataclysmic Variable ◽  
Prominent Feature ◽  
Periodic Oscillations ◽  
Fourier Technique ◽  
Orbital Modulation ◽  
Photoelectric Observations

We present high-speed photoelectric observations of a 13th-mag cataclysmic binary BZ Cam (=0623+71), gathered in 1989 and 1990. The data were analyzed by means of the Fourier technique. The most prominent feature detected in the power spectrum is a 3h36m period which we ascribe as being a photometric orbital modulation. This period is significantly different from the periodicity previously known from spectroscopic observations. The power spectrum shows also quasi-periodic oscillations of shorter time-scales but any conclusions about their stability can not be drawn yet.

Three-Dimensional Computation of Gas Turbine Combustors and the Validation Studies of Turbulence and Combustion Models

1997 ◽  
Author(s):  
D. Biswas ◽  
K. Kawano ◽  
H. Iwasaki ◽  
M. Ishizuka ◽  
S. Yamanaka
Keyword(s):  
Gas Turbine ◽  
Validation Studies ◽  
Three Dimensional ◽  
Chemical Species ◽  
Reaction Rates ◽  
Reacting Flows ◽  
Gas Turbine Combustor ◽  
Combustion Models ◽  
Rate Of Dissipation

The main aim or the present work is to explore computational fluid dynamics and related turbulence and combustion models for application to the design, understanding and development of gas turbine combustor. Validation studies were conducted using the Semi-Implicit Method for Pressure Linked Equations (SIMPLE) scheme to solve the relevant steady, elliptical partial differential equations of the conservation of mass, momentum, energy and chemical species in three-dimensional cylindrical co-ordinate system to simulate the gas turbine combustion chamber configurations. A modified version of k-ε turbulence model was used for characterization of local turbulence in gas turbine combustor. Since, in the present study both diffusion and pre-mixed combustion were considered, in addition to familiar bi-molecular Arhenius relation, influence of turbulence on reaction rates was accounted for based on the eddy break up concept of Spalding and was assumed that the local reaction rate was proportional to the rate of dissipation of turbulent eddies. Firstly, the validity of the present approach with the turbulence and reaction models considered is checked by comparing the computed results with the standard experimental data on recirculation zone, mean axial velocity and temperature profiles, etc. for confined, reacting and non-reacting flows with reasonably well defined boundary conditions. Finally, the results of computation for practical gas turbine combustor using combined diffusion and pre-mixed combustion for different combustion conditions are discussed.

Introducing a Novel Reactor Concept: Indirectly Fired Integrated Gasification and Steam Generation System

2010 ◽  
Author(s):  
Monem Alyaser ◽  
Rory Monaghan ◽  
Abdlmonem Beitelmal ◽  
Drazen Fabris
Keyword(s):  
Heat Exchanger ◽  
Plug Flow ◽  
Reaction Rates ◽  
Reacting Flows ◽  
Steam Gasification ◽  
Reacting Flow ◽  
Steam Generation ◽  
Integrated Gasification ◽  
Gasification Reaction ◽  
Char Conversion

This paper introduces a novel gasification reactor that uses steam gasification of carbonaceous feedstock by indirectly heating the reacting flow through a high temperature heat exchanger without the need for partial combustion with oxygen. It demonstrates the importance of gasification as a method for increasing power plant efficiency and reducing emissions. This paper also describes the computational model created to model this novel gasifier and the results of the model that illustrates the efficiency and purity advantages of the new gasifier. The reactor was modeled as a 1D counter-reacting flows heat exchanger, using the effectiveness-number of transfer units (ε-Ntu) method. The heating flow was assumed to be fully combusted at the inlet. The gasification stream was modeled as a plug flow, where the reaction is kinetically controlled. A simplified version of the Random Pore Model (RPM) was used to predict the char consumption. The results indicate that the gasification of coal with steam without partial combustion with oxygen using this new concept is feasible. The gasification reaction rates are found to be slow at temperatures less than 1200°C, but most of the char conversion, which reached about almost 100% completion, occurred at higher than 1200°C.

An Efficient Non-Iterative Hybrid Method for Analyzing Train–Rail–Bridge Interaction Problems

2020 ◽  
pp. 2150029
Author(s):  
Kang Shi ◽  
Xuhui He ◽  
Yunfeng Zou ◽  
Zhi Zheng
Keyword(s):  
Hybrid Method ◽  
Computational Efficiency ◽  
High Speed ◽  
Low Frequency ◽  
Coupled Analysis ◽  
Time Step ◽  
Coupling Vibration ◽  
Current Time ◽  
Railway Bridges ◽  
Frequency Coupling

The dynamic interaction problem for the train–rail–bridge (TRB) systems presents a computational challenge, especially for the analysis of large-size TRB coupling systems. To address this issue, an efficient non-iterative hybrid method (NHM) is proposed. With this method, the integrated TRB system is divided into three subsystems, i.e. the train subsystem, the rail subsystem, and the bridge subsystem. Based on the individual subsystems, a multi-step[Formula: see text] technique is adopted in which a fine time step is used to analyze the high-frequency coupling vibration for the train and rail subsystems, and a coarse time step is adopted to calculate the low-frequency coupling vibration for the rail and bridge subsystem. Additionally, Zhais explicit integral method is used to predict the displacement of the wheelsets and the rail at the current time step before using the Newmark method. The proposed method incorporates the advantages of Zhai’s explicit method and the MS technique to avoid the iteration that may be required for the train–rail coupled analysis. The simulation fidelity and computational efficiency of the proposed method are demonstrated in the analysis of two examples of typical high-speed railway bridges. It was demonstrated that the proposed method can significantly enhance the computational efficiency, while maintaining a higher precision with a larger time step in comparison with other existing methods.

scholarly journals Mechanical Properties and Reaction Characteristics of Al-ZrH2-PTFE Composites under Quasi-Static Compression

Metals ◽  
2019 ◽  
Vol 9 (4) ◽  
pp. 421 ◽  
Author(s):  
Jun Zhang ◽  
Xiang Fang ◽  
Yuchun Li ◽  
Zhongshen Yu ◽  
Junyi Huang ◽  
...  
Keyword(s):  
Mechanical Properties ◽  
Reaction Mechanism ◽  
High Speed ◽  
Calorific Value ◽  
Reaction Rates ◽  
Zirconium Hydride ◽  
Reaction Products ◽  
Static Compression ◽  
Reaction Characteristics ◽  
Quasi Static Compression

To analyze the mechanical properties and reaction characteristics of Al-ZrH2-PTFE (aluminum-zirconium hydride-polytetrafluoroethylene) composites under quasi-static compression, five types of specimens with different ZrH2 contents (0%, 5%, 10%, 20% and 30%) were prepared by molding-vacuum sintering. The true stress-strain curves and reaction rates of the different specimens were measured using quasi-static compression. The specific reaction processes were recorded by a high-speed camera. The corresponding reaction products were characterized by the XRD phase analysis, the calorific value was tested by a Calorimeter, and the reaction mechanism was analyzed. According to the results, the strength of the composites increased first and then decreased with the increase in the content of ZrH2. It reached a maximum of 101.01 MPa at 5%. Violent reaction occurred, and special flames were observed during the reaction of the specimens with 5% ZrH2. With the increase in the content of ZrH2, the chemical reaction was hard to induce due to the reduction in strength and toughness of composites. The reaction mechanism of Al/ZrH2/PTFE reveals that high temperatures at crack tip induced the reaction of Al and PTFE. Subsequently, ZrH2 decomposed to release hydrogen and generate ZrC. Calorimetric experiment shows that the calorific value of Al/ZrH2/PTFE with 20% ZrH2 is higher than that of Al/PTFE. The findings verify the potential of ZrH2 as an energetic additive for the enhancement of strength and release of the energy of the composites.

scholarly journals Imaging localized neuronal activity at fast time scales through biomechanics

2019 ◽  
Vol 5 (4) ◽  
pp. eaav3816 ◽  
Author(s):  
Samuel Patz ◽  
Daniel Fovargue ◽  
Katharina Schregel ◽  
Navid Nazari ◽  
Miklos Palotai ◽  
...  
Keyword(s):  
Time Scales ◽  
Neuronal Activity ◽  
High Speed ◽  
Fast Time ◽  
Regional Patterns ◽  
Cortical Input ◽  
Brain Biomechanics ◽  
Neuronal Processes ◽  
High Level

Mapping neuronal activity noninvasively is a key requirement for in vivo human neuroscience. Traditional functional magnetic resonance (MR) imaging, with a temporal response of seconds, cannot measure high-level cognitive processes evolving in tens of milliseconds. To advance neuroscience, imaging of fast neuronal processes is required. Here, we show in vivo imaging of fast neuronal processes at 100-ms time scales by quantifying brain biomechanics noninvasively with MR elastography. We show brain stiffness changes of ~10% in response to repetitive electric stimulation of a mouse hind paw over two orders of frequency from 0.1 to 10 Hz. We demonstrate in mice that regional patterns of stiffness modulation are synchronous with stimulus switching and evolve with frequency. For very fast stimuli (100 ms), mechanical changes are mainly located in the thalamus, the relay location for afferent cortical input. Our results demonstrate a new methodology for noninvasively tracking brain functional activity at high speed.

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