26,499 research outputs found
Stochastic boundary conditions for molecular dynamics simulations
In this paper we develop a stochastic boundary conditions (SBC) for
event-driven molecular dynamics simulations of a finite volume embedded within
an infinite environment. In this method, we first collect the statistics of
injection/ejection events in periodic boundary conditions (PBC). Once
sufficient statistics are collected, we remove the PBC and turn on the SBC. In
the SBC simulations, we allow particles leaving the system to be truly ejected
from the simulation, and randomly inject particles at the boundaries by
resampling from the injection/ejection statistics collected from the current or
previous simulations. With the SBC, we can measure thermodynamic quantities
within the grand canonical ensemble, based on the particle number and energy
fluctuations. To demonstrate how useful the SBC algorithm is, we simulated a
hard disk gas and measured the pair distribution function, the compressibility
and the specific heat, comparing them against literature values.Comment: 24 pages, 16 figure
On transparent boundary conditions for the high--order heat equation
In this paper we develop an artificial initial boundary value problem for the
high-order heat equation in a bounded domain . It is found an unique
classical solution of this problem in an explicit form and shown that the
solution of the artificial initial boundary value problem is equal to the
solution of the infinite problem (Cauchy problem) in .Comment: 9 page
Transparent boundary conditions based on the Pole Condition for time-dependent, two-dimensional problems
The pole condition approach for deriving transparent boundary conditions is
extended to the time-dependent, two-dimensional case. Non-physical modes of the
solution are identified by the position of poles of the solution's spatial
Laplace transform in the complex plane. By requiring the Laplace transform to
be analytic on some problem dependent complex half-plane, these modes can be
suppressed. The resulting algorithm computes a finite number of coefficients of
a series expansion of the Laplace transform, thereby providing an approximation
to the exact boundary condition. The resulting error decays super-algebraically
with the number of coefficients, so relatively few additional degrees of
freedom are sufficient to reduce the error to the level of the discretization
error in the interior of the computational domain. The approach shows good
results for the Schr\"odinger and the drift-diffusion equation but, in contrast
to the one-dimensional case, exhibits instabilities for the wave and
Klein-Gordon equation. Numerical examples are shown that demonstrate the good
performance in the former and the instabilities in the latter case
Unsteady adjoint of pressure loss for a fundamental transonic turbine vane
High fidelity simulations, e.g., large eddy simulation are often needed for
accurately predicting pressure losses due to wake mixing in turbomachinery
applications. An unsteady adjoint of such high fidelity simulations is useful
for design optimization in these aerodynamic applications. In this paper we
present unsteady adjoint solutions using a large eddy simulation model for a
vane from VKI using aerothermal objectives. The unsteady adjoint method is
effective in capturing the gradient for a short time interval aerothermal
objective, whereas the method provides diverging gradients for long
time-averaged thermal objectives. As the boundary layer on the suction side
near the trailing edge of the vane is turbulent, it poses a challenge for the
adjoint solver. The chaotic dynamics cause the adjoint solution to diverge
exponentially from the trailing edge region when solved backwards in time. This
results in the corruption of the sensitivities obtained from the adjoint
solutions. An energy analysis of the unsteady compressible Navier-Stokes
adjoint equations indicates that adding artificial viscosity to the adjoint
equations can potentially dissipate the adjoint energy while potentially
maintain the accuracy of the adjoint sensitivities. Analyzing the growth term
of the adjoint energy provides a metric for identifying the regions in the flow
where the adjoint term is diverging. Results for the vane from simulations
performed on the Titan supercomputer are demonstrated.Comment: ASME Turbo Expo 201
Simulation of Polymer Flow Using Smoothed Particle Hydrodynamics Method
Reactive rotational molding (RRM) is a process to manufacture hollow plastic articles. Comparing to rotational molding of thermoplastics, it decreases the process cycle time due to the reactivity of the system. However, the number of influent parameters is relatively high and optimization of the process is complex. During RRM, the viscosity is one of the key parameters and varies according to the polymer molecular weight due to chemical reactions. Simulation is a way to optimize this process. Prediction of the reactive flow is of great interest to optimize process conditions and wall thickness distribution of the molded part. We developed a solver based on smoothed particle hydrodynamics method. This Lagrangian meshfree method is well adapted to simulate free surface flows like those occurring in RRM. First, we validated the code comparing the simulation results to analytical Couette flow solution and experimental measurements of dam break problem. Then, we performed two-dimensional (2D) and 3D simulations to observe the influence of the change of viscosity on the flow, due to the chemical reactions. Adhesion of the polymer on the mold surface is modeled by new boundary conditions.Contract grant sponsor : RAIGI society for providing us the reactive materials and the Single Interministerial Fund (FUI)-SAGANE
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