The Experts below are selected from a list of 143916 Experts worldwide ranked by ideXlab platform
Kevin J. Hanley - One of the best experts on this subject based on the ideXlab platform.
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Improving Constant-Volume simulations of undrained behaviour in DEM
Acta Geotechnica, 2020Co-Authors: Joel Keishing, Kevin J. HanleyAbstract:In order to simulate undrained conditions using the discrete element method, a Constant sample Volume is often assumed. There are well-recognised problems with these Constant-Volume triaxial simulations, particularly of dense samples, which inhibit quantitative comparison with laboratory experiments. In this paper, four possible explanations for these problems with conventional Constant-Volume simulations of ideal spherical particles are explored, each of which has a physical basis: particle crushing, the presence of highly compressible air within the sample, or the reduction in stiffness due to particle surface asperities or non-spherical particle shapes. These options are explored independently and in combination through implementation in the open-source LAMMPS code. In situations where a significant amount of particle crushing occurs, it is important to incorporate this in the simulations so that stresses are not over-estimated. There is experimental evidence that irregular particles have lower Young’s moduli than the Hertzian spheres often used in DEM. In the absence of particle crushing, the most effective method to achieve more realistic stress–strain responses is to reduce the particle shear modulus substantially. This approach has the added computational benefit of enabling an increase in the simulation time-step.
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Challenges of simulating undrained tests using the Constant Volume method in DEM
2013Co-Authors: Kevin J. Hanley, Xin Huang, Catherine O'sullivan, Fiona C. Y. KwokAbstract:Liquefaction during earthquakes can cause significant infrastructural damage and loss of life, motivating a fundamental study of undrained sand response using discrete element modeling (DEM). Two methods are widely used in DEM for simulating the undrained response of soil. One approach is to numerically couple the DEM code with a fluid model. Alternatively, if the soil is fully saturated and water is assumed to be incompressible, the Volume of the sample can be held Constant to simulate an undrained test. The latter has the advantage of being computationally straightforward, but the assumption of a Constant Volume can cause some issues which are discussed in this paper. Depending on the contact model selected, extremely high deviatoric stresses and pore water pressures can be generated for dense samples using the Constant Volume approach which are not observed in corresponding laboratory tests. Furthermore the results of these Constant Volume simulations tend to be sensitive to the strain rate selected. T...
Ronald K Hanson - One of the best experts on this subject based on the ideXlab platform.
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shock tube ignition delay time measurements in propane o2 argon mixtures at near Constant Volume conditions
Proceedings of the Combustion Institute, 2011Co-Authors: Zekai Hong, David F Davidson, Ronald K HansonAbstract:Abstract Shock tube measurements of ignition delay times with high activation energies are strongly sensitive to variations in reflected shock temperatures. At longer shock tube test times, as are needed at low reaction temperatures, small gradual increases in pressure (and simultaneous increases in temperature) that result from incident shock wave attenuation and boundary layer growth can significantly shorten measured ignition delay times. To obviate this pressure increase, we made use of a recently developed driver-insert method of Hong et al. [8] that allows generation of near-Constant-Volume test conditions for reflected shock measurements. Using this method, we have measured propane ignition delay times in a lean mixture (0.8% C 3 H 8 /8% O 2 /Ar) over temperatures between 980 and 1400 K and nominal pressures of 6, 24 and 60 atm, under both conventional shock tube operation (with post-shock fractional pressure variation d P 5 /d t ∼ 1–7%/ms) and near-Constant-Volume operation (with d P 5 /d t ∼ 0%/ms). The near-Constant-Volume ignition delay times provide a database for low-temperature propane model development that is independent of non-ideal fluid flow and heat transfer effects. Comparisons of these near-Constant-Volume measurements with predictions using the JetSurF v1.0 mechanism of Sirjean et al. [10] and the Curran et al. mechanism of NUI Galway [5] were performed. Ignition delay times measured with d P 5 /d t ∼ 1–7%/ms were found to be significantly shorter (about 1/3 of the near-Constant-Volume values) at the lowest temperatures and highest pressures studied. However, these ignition times are successfully simulated using the JetSurF v1.0 mechanism when an appropriate gasdynamic model that accounts for changes in pressure and temperature is used.
Zhen Huang - One of the best experts on this subject based on the ideXlab platform.
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Autoignition of n-Hexane, Cyclohexane, and Methylcyclohexane in a Constant Volume Combustion Chamber
Energy & Fuels, 2019Co-Authors: Dong Han, Jiaqi Zhai, Zhen HuangAbstract:The autoignition behaviors of n-hexane, cyclohexane, and methylcyclohexane were experimentally determined and compared in a heated Constant Volume combustion chamber. The combustion pressures, heat...
Bastien Boust - One of the best experts on this subject based on the ideXlab platform.
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Experimental and numerical study of cyclic variations in a Constant Volume Combustion chamber
Combustion and Flame, 2016Co-Authors: Laure Labarrere, Antoine Dauptain, Florent Duchaine, Marc Bellenoue, Bastien BoustAbstract:This paper describes a joint experimental and numerical study of a Constant Volume Combustion (CVC) chamber for propulsion engines. Combustion takes place in a Constant Volume vessel where gases are injected from a pressurized air inlet using valves. They are ignited using a spark and exhaust through a second set of valves. Like piston engines, CVC combustion raises multiple questions linked to the Volumetric efficiency of the valves, the heat losses to the chamber walls, the ignition in a strongly turbulent flow, the influence of residual gases. These issues can compromise the potential gains associated to Constant Volume combustion. They are investigated in an experimental setup and compared to a full compressible LES. The major conclusion is the existence of significant cyclic variations which are observed in the experiment and analyzed in the LES: the local flow velocity at spark timing and the level of residuals are the major factors leading to cyclic variations. Cycles also appear to be coupled: combustion during cycle N directly affects cycle (N+1), more than in a piston engine.
Zekai Hong - One of the best experts on this subject based on the ideXlab platform.
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shock tube ignition delay time measurements in propane o2 argon mixtures at near Constant Volume conditions
Proceedings of the Combustion Institute, 2011Co-Authors: Zekai Hong, David F Davidson, Ronald K HansonAbstract:Abstract Shock tube measurements of ignition delay times with high activation energies are strongly sensitive to variations in reflected shock temperatures. At longer shock tube test times, as are needed at low reaction temperatures, small gradual increases in pressure (and simultaneous increases in temperature) that result from incident shock wave attenuation and boundary layer growth can significantly shorten measured ignition delay times. To obviate this pressure increase, we made use of a recently developed driver-insert method of Hong et al. [8] that allows generation of near-Constant-Volume test conditions for reflected shock measurements. Using this method, we have measured propane ignition delay times in a lean mixture (0.8% C 3 H 8 /8% O 2 /Ar) over temperatures between 980 and 1400 K and nominal pressures of 6, 24 and 60 atm, under both conventional shock tube operation (with post-shock fractional pressure variation d P 5 /d t ∼ 1–7%/ms) and near-Constant-Volume operation (with d P 5 /d t ∼ 0%/ms). The near-Constant-Volume ignition delay times provide a database for low-temperature propane model development that is independent of non-ideal fluid flow and heat transfer effects. Comparisons of these near-Constant-Volume measurements with predictions using the JetSurF v1.0 mechanism of Sirjean et al. [10] and the Curran et al. mechanism of NUI Galway [5] were performed. Ignition delay times measured with d P 5 /d t ∼ 1–7%/ms were found to be significantly shorter (about 1/3 of the near-Constant-Volume values) at the lowest temperatures and highest pressures studied. However, these ignition times are successfully simulated using the JetSurF v1.0 mechanism when an appropriate gasdynamic model that accounts for changes in pressure and temperature is used.