778 research outputs found
Numerical Boundary Condition Procedures
Topics include numerical procedures for treating inflow and outflow boundaries, steady and unsteady discontinuous surfaces, far field boundaries, and multiblock grids. In addition, the effects of numerical boundary approximations on stability, accuracy, and convergence rate of the numerical solution are discussed
ASHEE: a compressible, equilibrium-Eulerian model for volcanic ash plumes
A new fluid-dynamic model is developed to numerically simulate the
non-equilibrium dynamics of polydisperse gas-particle mixtures forming volcanic
plumes. Starting from the three-dimensional N-phase Eulerian transport
equations for a mixture of gases and solid particles, we adopt an asymptotic
expansion strategy to derive a compressible version of the first-order
non-equilibrium model, valid for low concentration regimes and small particles
Stokes . When the model reduces to the dusty-gas one. The
new model is significantly faster than the Eulerian model while retaining the
capability to describe gas-particle non-equilibrium. Direct numerical
simulation accurately reproduce the dynamics of isotropic turbulence in
subsonic regime. For gas-particle mixtures, it describes the main features of
density fluctuations and the preferential concentration of particles by
turbulence, verifying the model reliability and suitability for the simulation
of high-Reynolds number and high-temperature regimes. On the other hand,
Large-Eddy Numerical Simulations of forced plumes are able to reproduce their
observed averaged and instantaneous properties. The self-similar radial profile
and the development of large-scale structures are reproduced, including the
rate of entrainment of atmospheric air. Application to the Large-Eddy
Simulation of the injection of the eruptive mixture in a stratified atmosphere
describes some of important features of turbulent volcanic plumes, including
air entrainment, buoyancy reversal, and maximum plume height. Coarse particles
partially decouple from the gas within eddies, modifying the turbulent
structure, and preferentially concentrate at the eddy periphery, eventually
being lost from the plume margins due to the gravity. By these mechanisms,
gas-particle non-equilibrium is able to influence the large-scale behavior of
volcanic plumes.Comment: 29 pages, 22 figure
Analysis and mitigation of numerical dissipation in inviscid and viscid computation of vortex-dominated flows
The conservative unsteady Euler equations for the flow relative motion in the moving frame of reference are used to solve for the steady and unsteady flows around sharp-edged delta wings. The resulting equations are solved by using an implicit approximately-factored finite volume scheme. Implicit second-order and explicit second- and fourth-order dissipations are added to the scheme. The boundary conditions are explicitly satisfied. The grid is generated by locally using a modified Joukowski transformation in cross flow planes at the grid chord stations. The computational applications cover a steady flow around a delta wing whose results serve as the initial conditions for the unsteady flow around a pitching delta wing about a large angle of attack. The steady results are compared with the experimental data and the periodic solution is achieved within the third cycle of oscillation
Influence of large-scale motion on turbulent transport for confined coaxial jets. Volume 2: Navier-Stokes calculations of swirling and nonswirling confined coaxial jets
The existence of large scale coherent structures in turbulent shear flows has been well documented. Discrepancies between experimental and computational data suggest a necessity to understand the roles they play in mass and momentum transport. Using conditional sampling and averaging on coincident two-component velocity and concentration velocity experimental data for swirling and nonswirling coaxial jets, triggers for identifying the structures were examined. Concentration fluctuation was found to be an adequate trigger or indicator for the concentration-velocity data, but no suitable detector was located for the two-component velocity data. The large scale structures are found in the region where the largest discrepancies exist between model and experiment. The traditional gradient transport model does not fit in this region as a result of these structures. The large scale motion was found to be responsible for a large percentage of the axial mass transport. The large scale structures were found to convect downstream at approximately the mean velocity of the overall flow in the axial direction. The radial mean velocity of the structures was found to be substantially greater than that of the overall flow
Hydrodynamic/acoustic splitting approach with flow-acoustic feedback for universal subsonic noise computation
A generalized approach to decompose the compressible Navier-Stokes equations
into an equivalent set of coupled equations for flow and acoustics is
introduced. As a significant extension to standard hydrodynamic/acoustic
splitting methods, the approach provides the essential coupling terms, which
account for the feedback from the acoustics to the flow. A unique simplified
version of the split equation system with feedback is derived that conforms to
the compressible Navier-Stokes equations in the subsonic flow regime, where the
feedback reduces to one additional term in the flow momentum equation. Subsonic
simulations are conducted for flow-acoustic feedback cases using a
scale-resolving run-time coupled hierarchical Cartesian mesh solver, which
operates with different explicit time step sizes for incompressible flow and
acoustics. The first simulation case focuses on the tone of a generic flute.
With the major flow-acoustic feedback term included, the simulation yields the
tone characteristics in agreement with reference results from K\"uhnelt based
on Lattice-Boltzmann simulation. On the contrary, the standard hybrid
hydrodynamic/acoustic method with the feedback-term switched off lacks the
proper tone. As the second simulation case, a thick plate in a duct is studied
at various low Mach numbers around the Parker-beta-mode resonance. The
simulations reveal the flow-acoustic feedback mechanism in very good agreement
with experimental data of Welsh et al. Simulations and theoretical
considerations reveal that the feedback term does not reduce the stable
convective flow based time step size of the flow equations.Comment: Submitted to Journal of Computational Physic
Towards Improved Scale-Resolving Modeling and Simulations of Turbulent Flows
Scale-resolving simulations are viewed as powerful means for predicting complex turbulent flows, as often encountered in aeronautical applications. However, since turbulent scales span over a considerable range from the smallest Kolmogorov scales to the largest of equivalence to configuration size, scale-resolving computations are often demanding on computational resources and, furthermore, on the underlying numerical methods used in the simulations. Nonetheless, hybrid RANS (Reynolds-Averaged Navier-Stokes)-LES (Large-Eddy Simulation) techniques are considered computationally accurate and affordable for aeronautical industry applications. This thesis explores and develops numerical methods suitable for hybrid RANS-LES. These methods are implemented in the Computational Fluid Dynamics (CFD) solver M-Edge
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