1,720,971 research outputs found
A high order Pade-ADI method for unsteady convection-diffusion equations
No full textA high-order alternating direction implicit (ADI) method for computations of unsteady convection-diffusion equations is proposed. By using fourth-order Pade schemes for spatial derivatives, the present scheme is fourth-order accurate in space and second-order accurate in time. The solution procedure consists of a number of tridiagonal matrix operations which make the computation cost effective. The method is unconditionally stable, and shows higher accuracy and better phase and amplitude error characteristics than the standard second-order ADI method [D.W. Peaceman, H.H. Rachford Jr., The numerical solution of parabolic and elliptic differential equations. Journal of the Society of Industrial and Applied Mathematics 3 (1959) 28-41] and the fourth-order ADI scheme of Karaa and Zhang [High order ADI method for solving unsteady convection-diffusion problem, Journal of Computational Physics 198 (2004) 1-9]. (c) 2005 Elsevier Inc. All rights reserved.X114143sciescopu
A dynamic global-coefficient subgrid-scale model for large-eddy simulation of turbulent scalar transport in complex geometries
No full textThe dynamic global-coefficient subgrid-scale eddy-viscosity model by You and Moin [Phys. Fluids 19, 065110 (2007)] is generalized for large-eddy simulation of turbulent flow with scalar transport. The model coefficient for subgrid-scale scalar flux which is constant in space but varies in time is dynamically determined based on the "global conservation" of the transport equation for scalar variance. Large-eddy simulations of turbulent flow with passive scalar transport through a channel and over a backward-facing step show that the present model has a similar predictive capability as the dynamic Smagorinsky model. The present dynamic model is especially suitable for large-eddy simulation of turbulent flow with scalar transport in complex geometries since it does not require any spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and requires only a single-level test filter. The present model is not more complicated in implementation and not more expensive in terms of computational cost than the dynamic Smagorinsky model.open112227sciescopu
A dynamic global-coefficient subgrid-scale eddy-viscosity model for large-eddy simulation in complex geometries
No full textAn improvement of the dynamic procedure of Park [Phys. Fluids 18, 125109 (2006)] for closure of the subgrid-scale eddy-viscosity model developed by Vreman [Phys. Fluids 16, 3670 (2004)] is proposed. The model coefficient which is globally constant in space but varies in time is dynamically determined assuming the "global equilibrium" between the subgrid-scale dissipation and the viscous dissipation of which utilization was proposed by Park Like the Vreman model with a fixed coefficient and the dynamic-coefficient model of Park , the present model predicts zero eddy-viscosity in regions where the vanishing eddy viscosity is theoretically expected. The present dynamic model is especially suitable for large-eddy simulation in complex geometries since it does not require any ad hoc spatial and temporal averaging or clipping of the model coefficient for numerical stabilization and more importantly, requires only a single-level test filter in contrast to the dynamic model of Park , which employs two-level test filters. (c) 2007 American Institute of Physics.open1159110sciescopu
Grid-independent large-eddy simulation using explicit filtering
No full textThe governing equations for large-eddy simulation are derived from the application of a low-pass filter to the Navier-Stokes equations. It is often assumed that discrete operations performed on a particular grid act as an implicit filter, causing results to be sensitive to the mesh resolution. Alternatively, explicit filtering separates the filtering operation, and hence the resolved turbulence, from the underlying mesh distribution alleviating some of the grid sensitivities. We investigate the use of explicit filtering in large-eddy simulation in order to obtain numerical solutions that are grid independent. The convergence of simulations using a fixed filter width with varying mesh resolutions to a true large-eddy simulation solution is analyzed for a turbulent channel flow at Re-tau=180, 395, and 640. By using explicit filtering, turbulent statistics and energy spectra are shown to be independent of the mesh resolution used. (C) 2010 American Institute of Physics. [doi:10.1063/1.3485774]open114355sciescopu
Large-eddy simulation of flow over a wall-mounted hump with separation control
No full textLarge-eddy simulation with a dynamic subgrid-scale model and nondissipative numerics is employed to predict the turbulent flow separation over a wall-mounted hump and its control. Large-eddy simulation results for the baseline (no control), steady suction, and oscillatory-jet control cases are compared with the results of experimental measurements and previous computational predictions using large-eddy simulation with a constant coefficient Smagorinsky model and dissipative numerics, implicit large-eddy simulation, detached eddy simulation, and unsteady Reynolds-averaged Navier-Stokes simulation. The present large-eddy simulation is shown to be consistently more accurate than the previous numerical approaches in predicting the experimentally measured flow quantities such as the pressure coefficient, reattachment length, mean velocity, and turbulence statistics. It is shown that steady suction and syntheticjet oscillations cause a reduction of the reattachment length by about 12.8 and 7.3 respectively, compared with the uncontrolled case.X112034sciescopu
Computational methodology for large-eddy simulation of tip-clearance flows
No full textA large-eddy-simulation-based flow solver that combines an immersed-boundary technique with a curvilinear structured grid has been developed to study the temporal and spatial dynamics of an incompressible rotor-tip-clearance flow. The overall objective of these simulations is to determine the underlying mechanisms for low-pressure fluctuations downstream of the rotor near the end wall. Salient features of the numerical methodology, including the mesh topology, the immersed boundary method, the treatment of numerical instability for nondissipalive schemes on highly skewed meshes, and the parallelization of the code for shared memory computer platforms, are discussed. The computational approach is shown to be capable of capturing the evolution of the highly complicated flowfield characterized by the interaction of distinct blade-associated vortical structures with the turbulent end-wall boundary layer. Simulation results are compared with experiments, and qualitative as well as quantitative agreement is observed.X1152sciescopu
Control of flow-induced noise behind a circular cylinder using splitter plates
No full textLaminar vortex sheddings behind a circular cylinder with and without splitter plates attached to the cylinder base at the Reynolds numbers of 100 and 160 are simulated by solving the unsteady two-dimensional incompressible Navier-Stokes equations. The Strouhal number, lift, and drag rapidly change with the length of the splitter plate. Acoustic source functions are obtained from the computed near-field velocity and pressure using the Curie's solution of the Lighthill acoustic analogy. In the case of no splitter plate, the volume quadrupole noise is small at a low Mach number, compared with the surface dipole noise from the cylinder. When a splitter plate is attached to the cylinder, there are significant modifications of the dipole and quadrupole sources. Changes in the noise sources at Re = 100 are very different from those at Re = 160, and their differences are closely related to the secondary vortex generated at the tip of the splitter plate, Scattering effects at the edge of the splitter plate are also considered.X1128sciescopu
Study of tip-clearance flow in turbomachines using large-eddy simulation
No full textA powerful computational technique, large-eddy simulation, helps researchers study the detailed flow dynamics in the tip-gap region of hydraulic turbomochines. LES also helps researchers investigate ways to mitigate undesirable effects, such as cavitation, which con lead to reduced performance, increased noise, and structural vibration and erosion.X119sciescopu
Large-eddy simulation analysis of mechanisms for viscous losses in a turbomachinery tip-clearance flow
No full textThe tip-leakage flow in a turbomachinery cascade is studied using large-eddy simulation with particular emphasis on understanding the underlying mechanisms for viscous losses in the vicinity of the tip gap. Systematic and detailed analysis of the mean flow field and turbulence statistics has been made in a linear cascade with a moving endwall. Gross features of the tip-leakage vortex, tip-separation vortices, and blade wake have been revealed by investigating their revolutionary trajectories and mean velocity fields. The tip-leakage vortex is identified by regions of significant streamwise velocity deficit and high streamwise and pitchwise vorticity magnitudes. The tip-leakage vortex and the tip-leakage jet which is generated by the pressure difference between the pressure and suction sides of the blade tip are found to produce significant mean velocity gradients along the spanwise direction, leading to the production of vorticity and turbulent kinetic energy. The velocity gradients are the major causes for viscous losses in the cascade endwall region. The present analysis suggests that the endwall viscous losses can be alleviated by changing the direction of the tip-leakage flow such that the associated spanwise derivatives of the mean streamwise and pitchwise velocity components are reduced.X116671sciescopu
Nonlinear dynamics and synthetic-jet-based control of a canonical separated flow
No full textA novel flow configuration devised for investigation of active control of separated airfoil flows using synthetic jets is presented. The configuration consists of a flat plate, with an elliptic leading edge and a blunt trailing edge, at zero incidence in a free stream. Flow separation is induced on the upper surface of the airfoil at the aft-chord location by applying suction and blowing On the top boundary of the computational domain. Typical separated airfoil flows are generally characterized by at least three distinct frequency scales corresponding to the shear layer instability, the unsteadiness of the separated region and the vortex shedding in the wake, and all these features are present in the current flow. Two-dimensional Navier Stokes simulations of this flow at a chord Reynolds number of 6 x 10(4) have been carried out to examine the nonlinear dynamics in this flow and its implications for synthetic-jet-based separation control. The results show that there is a strong nonlinear coupling between the various features of the flow, and that the uncontrolled as well as the forced flow is characterized by a variety of 'lock-on' states that result from this nonlinear coupling. The most effective separation control is found to occur at the highest forcing frequency for which both the shear layer and the separated region lock on to the forcing frequency. The effects of the Reynolds number on the scaling of the characteristic frequencies of the separated flow and its subsequent control are studied by repeating some of the simulations at a higher Reynolds number of 1 x 10(5).X112433sciescopu
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