Paul Batten

Interfacing Statistical Turbulence Closures with Large-Eddy Simulation

Progress toward a general purpose hybrid Reynolds-averaged Navier-Stokes (RANS)/large-eddy simulation (LETS) framework is described, in which large-scale, statistically represented turbulence kinetic energy is converted automatically into resolved-scale velocity fluctuations wherever the local mesh resolution is sufficient to support them. Existing hybrid RANS/LES approaches alter the nature of the local partial differential equations according to the local mesh resolution, but they do not alter the nature of the data on which these equations operate. The implications of this are discussed. Subsequently, a simple mechanism is introduced to transfer statistical kinetic energy into resolved-scale fluctuations in a manner that preserves a given set of space/time correlations and set of second moments. This process, which can appropriately be termed Large-Eddy STimulation (LEST), generates the large-scale eddies needed to form the unsteady boundary conditions at RANS interfaces to LES regions, into which turbulence energy can be deposited either through mean convection or through turbulent transport

Sub-grid turbulence modeling for unsteady flow with acoustic resonance

This paper proposes a novel combination of Reynoldsaveraged Navier-Stokes (RANS) and large-eddy simulation (LES) sub-grid models, which combines the best features of time-averaged and spatially-filtered models, yielding the superior near-wall stress predictions of (algebraic or full-transport) Reynolds-stress models with the ability to override any quasi-steady grid-converged RAN’S model solution in regions of sufficiently high grid density. The proposed hybrid formulation is well suited to the coupled simulation of all flow scales in resonating high-Reynolds number flows and contains no additional empirical constants beyond those appearing in the original RANS and LES sub-grid models.

Reynolds-stress-transport modeling for compressible aerodynamics applications

Progress is reported in the development of a nonlinear Reynolds-stress-transport model for compressible, turbulent flow. The focus is on a variation of a particular cublc model that does not require the usual topography-related parameters, such as normal-to-wall vectors. However, certain wall-proximity corrections that have been used in the model to replace conventional wall-reflection terms display the wrong response to shocks, which are falsoly interpreted as localized regions of strong inhomogeneity. A modified cubic variant is proposed that allows integration across the semiviscous sublayer and incorporates additional constraints to guard against unphysical response of the pressure-strain model in the vicinity of shock waves. The modified model is applied to both two- and three-dimensional compressible flows, involving shock-wave/boundary-layer interaction, and is shown to yield generally favorable results

Broadband Trailing-Edge Noise Predictions - Overview of BANC-III Results

The Third Workshop on Benchmark Problems for Airframe Noise Computations, BANC-III, was held on 14-15 June 2014 in Atlanta, Georgia, USA. The objective of this workshop was to assess the present computational capability in the area of physics-based prediction of different types of airframe noise problems and to advance the state-of-the-art via a combined effort. This documentation summarizes the results from workshop category 1 (BANC-III-1) which focuses on the prediction of broadband turbulent boundary-layer trailing-edge noise and related source quantities. Since the forerunner BANC-II workshop identified some room for improvements in the achieved prediction quality, BANC-III-1 relies on the same test cases, namely 2D NACA0012 and DU96-W-180 airfoil sections in a uniform flow. Compared to BANC-II particularly the scatter among predictions for the DU96-W- 180 test case could be significantly reduced. However, proposed adaptations of previously applied computational methods did not systematically improve the prediction quality for all requested parameters. The category 1 workshop problem remains a challenging simulation task due to its high requirements on resolving and modeling of turbulent boundary-layer source quantities.

Variable Turbulent Schmidt and Prandtl Number Modeling

Abstract: This paper describes variable turbulent Schmidt and Prandtl number formulations, based on Rodi’s (1980) algebraic Reynolds stress model adapted to velocity-scalar correlations. Since the approach is algebraic in nature, it does not introduce transport equations beyond the ones already existing to compute the flow, rendering it a viable engineering tool. The method is scrutinized against several test cases, including a 3D Scramjet combustor problem. Results are encouraging and lend confidence in the proposed approach.

Reconstructed Sub-Grid Methods for Acoustics Predictions at all Reynolds Numbers

This paper considers the concept and practical application of an artificially−generated, or synthetic, unsteady, turbulent flow field, generated using statistics obtained from a quasi−steady, Reynolds−averaged Navier−Stokes (RANS) solution. Two closely−related applications of this synthesis to the extraction of noise from flow fields of arbitrarily−high Reynolds number are considered. The first application is that of hybrid RANS/Large Eddy Simulation (LES), in which the reconstruction enables an energy transfer from the statistical RANS field into the resolved, large−scale (LES) components, thereby automatically providing appropriate boundary conditions for any embedded LES regions. The second application is to a non−linear acoustics solver, in which the reconstruction plays the role of a sub−grid closure, providing the volumetric sources for the unresolved, short−wavelength fluctuations. The synthetic, or artificially− generated, turbulence is constructed so as to mimic certain essential features of the statistical (RANS− represented) turbulence, including spatial and temporal correlations, turbulence energy and anisotropy. This paper considers developments relating to applications in which a limited portion of the turbulence energy spectrum has to be synthesized, and in which the underlying statistical field is both inhomogeneous and time−dependent.

The k-ε-Rt Turbulence Closure

Abstract A turbulence closure, based on transport equations for the turbulence kinetic energy, k, its dissipation rate, ε and the undamped eddy viscosity, R, is presented. The model, which is free of topography-dependent parameters, combines a k-ε closure with the Rt model so that no inflow turbulence decay occurs in external flows, an attribute often sought by aeronautical engineers using CFD for flow computations. The model is shown to revert to the k-ε closure in near-wall flow regions. Two aerodynamic flow cases are presented, comparing the original k-ε closure to the current 3-equation model.

Smart Sub-Grid-Scale Models for LES and Hybrid RANS/LES

This paper considers the implementation of sub-grid scale (SGS) models for large eddy simulation (LES) or hybrid Reynolds-averaged Navier-Stokes (RANS)/LES methods. The paper describes how to reconcile Smagorinsky-type eddy viscosity SGS models with monotonically-integrated large eddy simulation (MILES) approaches by using ‘Smart’ SGS models that understand, and compensate for, the inherent diffusion in the underlying numerical transport algorithm. An improved ‘burst’ model of synthetic turbulence is also introduced for applications to inhomogeneous turbulence.

Reynolds-stress modelling of transonic afterbody flows

Several afterbody flows, involving shock-boundary-layer interaction, are used to evaluate recent developments in a realizable low-Reynolds-number, second-moment closure of turbulence. The model considered is a compressibility-adapted variant of the recent incompressible-flow form of Craft and Launder. This includes a tensorially cubic model for the influential pressure-strain process, o ij , which satisfies the two-component-turbulence limit at the wall, is directly applicable to low-Reynolds-number flow regions and does not rely on or use surface-topography parameters, such as wall-normal distance or direction. Improved predictions for afterbody flows are demonstrated, relative to existing low-Reynolds-number two-equation models and the most elaborate form of Reynolds-stress closure incorporating a linear approximation for the pressure-strain process.

A wall-distance-free version of the SST turbulence model

The present work aims to retain the predictive quality of the 2003 SST model while replacing its dependence on explicit wall distance with a local representation thereof, thereby avoiding the need to repeatedly compute wall distance arrays in flow cases involving nonstationary grids or in those involving multimillion-cell meshes. The approach proposed here is a local, algebraic-based representation of wall distance, using k and ω. Several examples are given, showing that the proposed alternative to explicit wall distance usually enables as good predictions as those of the 2003 SST model and in some cases even better. The paper also provides a proposal to treat free shear flows and includes a jet flow example.