Fluid dynamic turbulence is one of the most challenging computational physics problems because of the extremely wide range of time and space scales involved, the strong nonlinearity of the governing equations, and the many practical and important applications. While most linear fluid instabilities are well understood, the nonlinear interactions among them makes even the relatively simple limit of homogeneous isotropic turbulence difficult to treat physically, mathematically, and computationally. Turbulence is modeled computationally by a two-stage bootstrap process. The first stage, direct numerical simulation, attempts to resolve the relevant physical time and space scales but its application is limited to diffusive flows with a relatively small Reynolds number (Re). Using direct numerical simulation to provide a database, in turn, allows calibration of phenomenological turbulence models for engineering applications. Large eddy simulation incorporates a form of turbulence modeling applicable when the large-scale flows of interest are intrinsically time dependent, thus throwing common statistical models into question. A promising approach to large eddy simulation involves the use of high-resolution monotone computational fluid dynamics algorithms such as flux-corrected transport or the piecewise parabolic method which have intrinsic subgrid turbulence models coupled naturally to the resolved scales in the computed flow. The physical considerations underlying and evidence supporting this monotone integrated large eddy simulation approach are discussed.

New insights into large eddy simulation

Usage of large-eddy simulation (LES) methods for calculation of turbulent flows has increased substantially in recent years. This paper attempts to 1) provide an assessment of the current capabilities of LES, 2) outline some recommended practices for using LES, and 3) identify future research needs. The assessment considers flow problems for which LES can be successfully applied today and flow problems for which LES still has limitations. The availability of LES and hybrid Reynolds-averaged Navier-Stokes (RANS)/LES in general-purpose codes is discussed. Several important issues for which the LES community has not yet reached a consensus are discussed. These include grid sensitivity studies, application of unstructured grid methods, upwind-biased solvers, and turbulence (subgrid) modeling including continuous hybrid RANS/LES approaches. A section on recommended practices and key considerations tries to provide guidance on some of the important items that need to be addressed in using LES. The paper concludes with a discussion of future research directions, with a focus on work needed to advance the capabilities and reliability of LES for analysis of turbulent flows.

/pdf/large-eddy-simulation-current-capabilities-recommended-4h3svw0l4f.pdf

Large-Eddy Simulation: Current Capabilities, Recommended Practices, and Future Research

High-order compact finite-difference methods on general overset grids

This article provides a critical review of aero-optics with an emphasis on recent developments in computational predictions and the physical mechanisms of flow-induced optical distortions. Following a brief introduction of the fundamental theory and key concepts, computational techniques for aberrating flow fields and optical propagation are discussed along with a brief survey of wave-front sensors used in experimental measurements. New physical understanding generated through numerical and experimental investigations is highlighted for a number of important aero-optical flows, including turbulent boundary layers, separated shear layers, and flow over optical turrets. Approaches for mitigating aero-optical effects are briefly discussed.

/pdf/physics-and-computation-of-aero-optics-4yulorrc4n.pdf

Physics and Computation of Aero-Optics

https://www3.nd.edu/~sgordeye/ReviewTurretsFinal.pdf

Fluid dynamics and aero-optics of turrets

An Implicit LES Approach Based on High-order Compact Differencing and Filtering Schemes (Invited)

A High-Order Compact Finite-Difference Scheme for Large-Eddy Simulation of Active Flow Control

Large-Eddy Simulation of Airfoil Flows

Hybrid Reynolds Averaged Navier-Stokes/implicit Large Eddy (hybrid RANS/ILES) simulations were performed for flow over a turret to compare the effectiveness of three forms of flow control: oscillatory blowing/suction, steady suction through a slot, and lowvelocity suction through a porous surface. The hybrid RANS/ILES computations were obtained using a well-validated high-order solver employing a 4-order compact spatial discretization in conjunction with a 6-order low-pass spatial filter operator that regularized the evolving solution by selectively removing unresolved high-frequency content. The turret configuration consisted of a half-foot radius hemisphere mounted on a 4.5′′ tall circular cylinder attached to the wind tunnel wall. The flow conditions were M∞ = 0.4 and ReD = 2.4× 10. Numerical simulations were performed on both a 14 million and 23 million point mesh. Results from the numerical simulations are compared with previous numerical and experimental data for a baseline case without flow control. The simulations demonstrate that steady suction through both the slot and the porous surface is more effective in delaying flow separation from the turret dome than oscillatory blowing and suction for the frequency and amplitude chosen. Delaying separation significantly reduced turbulent kinetic energy and small scale flow structures in the turret dome wake which degrade and distort optical wavefronts. Both types of steady suction flow control generated attached flow over the entire dome of the turret. Steady suction through the porous turret also significantly reduced the size of the wake vortices. Additional research is needed to improve the effectiveness of the oscillatory flow control.

http://enu.kz/repository/2009/AIAA-2009-574.pdf

Numerical Simulations Investigating Control of Flow Over a Turret

This investigation explores the flow over a turret atM1 0:4 and ReD 2:4 10 using both hybrid Reynoldsaveraged Navier–Stokes/implicit large-eddy simulation (RANS/ILES) and k-based unsteady Reynolds-averaged Navier–Stokes (URANS) simulations. Additional hybrid RANS/ILES simulations were performed to compare the effectiveness of two steady-suction flow control approaches. The hybrid RANS/ILES computations were obtained using awell-validated high-orderNavier–Stokes flow solver employing a fourth-order compact spatial discretization in conjunction with a sixth-order low-pass spatial filter. The URANS simulations were performed using a secondorder version of the flow solver and kturbulence model. The turret configuration consisted of a half-foot radius hemisphere atop a 4.5-in.-tall circular cylinder base. Both steady suction through a slot and a leeward porous turret shell were explored as forms of flow control. Time-mean hybrid RANS/ILES results, obtained on a 23 10-point mesh, compared reasonably well to experimental pressure coefficient and velocity profiles for the baseline flow. The separation angle was predicted to within 3 deg of experimental observations. The instantaneous hybrid RANS/ILES solutions display complex three-dimensional flow phenomena in the wake of the turret that the kURANS model was unable to resolve. Both steady-suction flow control approaches successfully attached the flow over most the turret dome and significantly reduced the size of the wake.

Hybrid Reynolds-Averaged Navier-Stokes/Large-Eddy Simulation Investigating Control of Flow over a Turret

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