Donghyun You

A dynamic global-coefficient subgrid-scale eddy-viscosity model for large-eddy simulation in complex geometries

An improvement of the dynamic procedure of Park et al. [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 et al. Like the Vreman model with a fixed coefficient and the dynamic-coefficient model of Park et al., 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 et al., whi...

Large-eddy simulation analysis of mechanisms for viscous losses in a turbomachinery tip-clearance flow

The 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.

Computational Methodology for Large-Eddy Simulation of Tip-Clearance Flows

A 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-tipclearance flow. The overall objective of these simulations is to determine the underlying mechanisms for lowpressure 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 nondissipative 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.

Grid-independent large-eddy simulation using explicit filtering

The 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 Reτ=180, 395, and 640. By using explicit filtering, turbulent statistics and energy spectra are shown to be independent of the mesh resolution used.

Effects of tip-gap size on the tip-leakage flow in a turbomachinery cascade

The effects of tip-gap size on the tip-leakage vortical structures and velocity and pressure fields are investigated using large-eddy simulation, with the objective of providing guidelines for controlling tip-leakage cavitation and viscous losses associated with the tip-leakage flow. The effects of tip-gap size on the generation and evolution of the end-wall vortical structures are discussed by investigating their evolutionary trajectories and the mean velocity field. The tip-leakage jet and tip-leakage vortex are found to produce significant mean velocity gradients, leading to the production of vorticity and turbulent kinetic energy. Inside the cascade passage, the peak streamwise velocity deficit and magnitudes of vorticity and turbulent kinetic energy in the tip-leakage vortex are reduced as the tip-gap size decreases. The present analysis indicates that the mechanisms for the generation of vorticity and turbulent kinetic energy are mostly unchanged by the tip-gap size variation. However, larger tip-ga...

Short note: A high-order Padé ADI method for unsteady convection-diffusion equations

A 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].

Discrete conservation principles in large-eddy simulation with application to separation control over an airfoil

An unstructured-grid large-eddy simulation (LES) technique is used to investigate the turbulent flow separation over an airfoil with and without synthetic-jet control. Numerical accuracy and stability on arbitrary shaped mesh elements at high Reynolds numbers are achieved using a finite-volume discretization of the incompressible Navier–Stokes equations based on higher-order conservation principles—i.e., in addition to mass and momentum conservation, kinetic energy conservation in the inviscid limit is used to guide the selection of the discrete operators and solution algorithm. Two different stall configurations, which consist of flow over a NACA 0015 airfoil at 16.6° and 20° angles of attack, are simulated at Reynolds number of 896 000 based on the airfoil chord length and freestream velocity. In the case of 16.6° angle of attack where flow separates around a midchord location, LES results show excellent agreement with the experimental data for both uncontrolled and controlled cases. LES confirms the ex...

Effects of hydrophobic surfaces on the drag and lift of a circular cylinder

Effects of hydrophobic surfaces on the drag and lift of a circular cylinder at Reynolds numbers of 300 and 3900 are investigated using numerical simulations. A cylinder of which the entire surface is no-slip, a cylinder of which the entire surface is hydrophobic, and cylinders with alternating circumferential bands of slip and no-slip conditions are considered. The width of the alternating bands ranges from 0.5λz to 2λz, where λz is a spanwise characteristic wavelength in the near wake. At Reynolds number 300, the hydrophobic surface consisting of alternating slip and no-slip bands of width λz is found to be most effective in enhancing wake instability, thereby decreasing the base suction, drag, and rms lift coefficients. At Reynolds number 3900, hydrophobic surface treatments are found to delay flow separation, thereby decreasing the drag and rms lift.

An implicit ghost-cell immersed boundary method for simulations of moving body problems with control of spurious force oscillations

A fully-implicit ghost-cell immersed boundary method for simulations of flow over complex moving bodies on a Cartesian grid is presented. The present immersed boundary method is highly capable of controlling the generation of spurious force oscillations on the surface of a moving body, thereby producing an accurate and stable solution. Spurious force oscillations on the surface of an immersed moving body are reduced by alleviating spatial and temporal discontinuities in the pressure and velocity fields across non-grid conforming immersed boundaries. A sharp-interface ghost-cell immersed-boundary method is coupled with a mass source and sink algorithm to improve the conservation of mass across non-grid conforming immersed boundaries. To facilitate the control for the temporal discontinuity in the flow field due to a motion of an immersed body, a fully-implicit time-integration scheme is employed. A novel backward time-integration scheme is developed to effectively treat multiple layers of fresh cells generated by a motion of an immersed body at a high CFL number condition. The present backward time-integration scheme allows to impose more accurate and stable velocity vectors on fresh cells than those interpolated. The effectiveness of the present fully-implicit ghost-cell immersed boundary method coupled with a mass source and sink algorithm for reducing spurious force oscillations during simulations of moving body problems is demonstrated in a number of test cases.

Large-Eddy Simulation of Flow over a Wall-Mounted Hump with Separation Control

Large-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 synthetic jet oscillations cause a reduction of the reattachment length by about 12.8 and 7.3 %, respectively, compared with the uncontrolled case.