Gavin Tabor

A comparative study of subgrid scale models in homogeneous isotropic turbulence

Recently, a number of studies have indicated that Large Eddy Simulation (LES) models are fairly insensitive to the adopted Subgrid Scale (SGS) models. In order to study this and to gain further insight into LES, simulations of forced and decaying homogeneous isotropic turbulence have been performed for Taylor Re numbers between 35 and 248 using various SGS models, representative of the contemporary state of the art. The predictive capability of the LES concept is analyzed by comparison with DNS data and with results obtained from a theoretical model of the energy spectrum. The resolved flow is examined by visualizing the morphology and by analyzing the distribution of resolved enstrophy, rate of strain, stretching, SGS kinetic energy, and viscosity. Furthermore, the correlation between eigenvalues of the resolved rate of strain tensor and the vorticity is investigated. Although the gross features of the flow appear independent of the SGS model, pronounced differences between the models become apparent whe...

Application of a flame-wrinkling les combustion model to a turbulent mixing layer

The necessity for turbulent combustion modeling in the large-eddy simulation (LES) of premixed turbulent combustion is evident from the computational cost and the complexity of handling flame kinetics reaction mechanisms directly. In this paper, a new flame-wrinkling LES combustion model using conditional filtering is proposed. The model represents an alternative approach to the traditional flame-surface density based models in that the flame distribution is represented by a flame-wrinkle density function and that the effects of flame stretch and curvature are handled through a modeled transport equation for the perturbed laminar flame speed. For the purpose of validating the LES combustion model, LESs of isothermal and reacting shear layers formed at a rearward-facing step are carried out, and the results are compared with experimental data. For the isothermal case, the agreement between LES and the experimental data is excellent. For the reacting case, the evolution and topology of coherent structures is examined, and direct comparisons are made with time-averaged profiles of velocity and its fluctuations. temperature, and reaction products. Good agreement is obtained, to a large extent due to accurate modeling of the flame-wrinkle density but also to the novel treatment of the strain-rate effects on the laminar flame speed of the lean propane-air mixture.

Measurements and large eddy simulations of turbulent premixed flame kernel growth

A combined experimental and large eddy simulation (LES) study of flame kernel growth in isotropic, homogenous turbulence has been carried out. LES calculations using the combustion methodology of Weller were compared with experimental measurements from a fan-stirred bomb for iso-octane and propane air mixtures at various turbulence intensities and pressures. For the purpose of model validation, the mean radius evolution was compared with experimental measurements, obtained from Schlieren photographs. Initially, a small laminar flame kernel was produced that burned at an increasing rate as it grew and was wrinkled by the turbulent flow field. It was also observed that at atmospheric pressure propaneair flames demonstrated less variability between experimental realizations than did iso-octane air flames High-variability combustion events were associated with the convection of the flame kernel away from the spark plug during ignition. Good agreement between experiments and calculations was obtaiend for the full range of conditions investigated in this study, and the LES results were able to reproduce some of the observed variability between experimental realizations as a result of turbulent interactions with the small kernel during ignition. These results provided further validation of the combustion model, though the simple ignitiion treatment did not reproduce the full range of ignition variability due to strain sensitivity.

Large Eddy Simulations of the Flow Around a Square Prism

The aim of this study is to examine unsteady wake flows by means of large eddy simulation (LES). In particular, the flow around a square prism in a channel at a prism-height Reynolds number of 2.14 x 10 4 is studied using different subgrid scale (SGS) models and different grids. Results from first- and second-order statistical moments of the velocity are validated against two sets of experimental data and compared with different Reynolds-average simulations. All LES models correctly reproduce the first- and second-order statistical moments of the resolvable velocity, the global parameters, such as the lift and drag coefficients and their fluctuations, and the Strouhal number, as well as the length of the recirculation region. However, a locally refined grid is necessary to reproduce the maximum velocity within the recirculation region. LES appears virtually independent of the details of the SGS model if it can correctly channel kinetic energy out of eddies close to the cutoff wave number to prevent aliasing provided that the resolution is fine enough to ensure that the cutoff wave number is within the inertial subrange. In addition, phase-averaged flow quantities are compared with experimental data

Modeling flow in collecting lymphatic vessels: one-dimensional flow through a series of contractile elements

The lymphatic system comprises a series of elements, lymphangions, separated by valves and possessed of active, contractile walls to pump interstitial fluid from its collection in the terminal lymphatics back to the main circulation. Despite its importance, there is a dearth of information on the fluid dynamics of the lymphatic system. In this article, we describe linked experimental and computational work aimed at elucidating the biomechanical properties of the individual lymphangions. We measure the static and dynamic mechanical properties of excised bovine collecting lymphatics and develop a one-dimensional computational model of the coupled fluid flow/wall motion. The computational model is able to reproduce the pumping behavior of the real vessel using a simple contraction function producing fast contraction pulses traveling in the retrograde direction to the flow.

Differential subgrid stress models in large eddy simulations

Here we study differential stress equation models for the subgrid scale SGS stress tensor in large eddy simulations of urbulent incompressible flow. A study of the SGS stress equation is performed using the principle of frame indifference and the concept of realizability. Closure models are proposed that satisfy these constraints together with the additional requirement that the modeled balance equation must degenerate into the ordinary balance equation for SGS kinetic energy when contracted. The SGS stress equation model is applied to forced homogeneous isotropic turbulence and fully developed turbulent channel flow. For low Re numbers the differential stress equation model behaves in a similar manner to the linear combination model. At higher Re numbers it behaves increasingly like an eddy-viscosity model, but is better able to handle flow and grid anisotropy than traditional SGS models.

Experimental and numerical investigation of interactions between above and below ground drainage systems

This paper presents the results of the experimental and numerical investigation of interactions between surface flood flow in urban areas and the flow in below ground drainage systems (sewer pipes and manholes). An experimental rig has been set up at the Water Engineering Laboratory at the University of Sheffield. It consists of a full scale gully structure with inlet grating, which connects the 8 m(2) surface area with the pipe underneath that can function as an outfall and is also further connected to a tank so that it can come under surcharging conditions and cause outflow from the gully. A three-dimensional CFD (Computational Fluid Dynamics) model has been set up to investigate the hydraulic performance of this type of gully inlet during the interactions between surface flood flow and surcharged pipe flow. Preliminary results show that the numerical model can replicate various complex 3D flow features observed in laboratory conditions. This agreement is overall better in the case of water entering the gully than for the outflow conditions. The influence of the surface transverse slope on flow characteristics has been demonstrated. It is shown that re-circulation zones can form downstream from the gully. The number and size of these zones is influenced by the transverse terrain slope.

Comparison of LES of Steady Transitional Flow in an Idealized Stenosed Axisymmetric Artery Model With a RANS Transitional Model

In this study, two different turbulence methodologies are investigated to predict transitional flow in a 75% stenosed axisymmetric experimental arterial model and in a slightly modified version of the model with an eccentric stenosis. Large eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) methods were applied; in the LES simulations eddy viscosity subgrid-scale models were employed (basic and dynamic Smagorinsky) while the RANS method involved the correlation-based transitional version of the hybrid k-ε/k-ω flow model. The RANS simulations used 410,000 and 820,000 element meshes for the axisymmetric and eccentric stenoses, respectively, with y(+) less than 2 viscous wall units for the boundary elements, while the LES used 1,200,000 elements with y(+) less than 1. Implicit filtering was used for LES, giving an overlap between the resolved and modeled eddies, ensuring accurate treatment of near wall turbulence structures. Flow analysis was carried out in terms of vorticity and eddy viscosity magnitudes, velocity, and turbulence intensity profiles and the results were compared both with established experimental data and with available direct numerical simulations (DNSs) from the literature. The simulation results demonstrated that the dynamic Smagorinsky LES and RANS transitional model predicted fairly comparable velocity and turbulence intensity profiles with the experimental data, although the dynamic Smagorinsky model gave the best overall agreement. The present study demonstrated the power of LES methods, although they were computationally more costly, and added further evidence of the promise of the RANS transition model used here, previously tested in pulsatile flow on a similar model. Both dynamic Smagorinsky LES and the RANS model captured the complex transition phenomena under physiological Reynolds numbers in steady flow, including separation and reattachment. In this respect, LES with dynamic Smagorinsky appeared more successful than DNS in replicating the axisymmetric experimental results, although inflow conditions, which are subject to caveats, may have differed. For the eccentric stenosis, LES with Smagorinsky coefficient of 0.13 gave the closest agreement with DNS despite the known shortcomings of fixed coefficients. The relaminarization as the flow escaped the influence of the stenosis was amply demonstrated in the simulations, graphically so in the case of LES.

Conifer ovulate cones accumulate pollen principally by simple impaction

In many pine species (Family Pinaceae), ovulate cones structurally resemble a turbine, which has been widely interpreted as an adaptation for improving pollination by producing complex aerodynamic effects. We tested the turbine interpretation by quantifying patterns of pollen accumulation on ovulate cones in a wind tunnel and by using simulation models based on computational fluid dynamics. We used computer-aided design and computed tomography to create computational fluid dynamics model cones. We studied three species: Pinus radiata, Pinus sylvestris, and Cedrus libani. Irrespective of the approach or species studied, we found no evidence that turbine-like aerodynamics made a significant contribution to pollen accumulation, which instead occurred primarily by simple impaction. Consequently, we suggest alternative adaptive interpretations for the structure of ovulate cones.

Inlet Conditions for Large Eddy Simulation of Gas-Turbine Swirl Injectors

In this paper, we present a novel technique for generating swirl inlets for large eddy simulation. The velocity a short distance downstream of the inlet to the main domain is sampled and the flow velocity data are reintroduced backintothedomaininlet,creatinganinletsectionintegratedintothemaindomaininwhichturbulencecandevelop. Additionally, variable artificial body forces and velocity corrections are imposed in this inlet section, with feedback control to force the flow toward desired swirl, mean, and turbulent profiles. The method was applied to flow in an axisymmetric sudden expansion, with and without swirl at the inlet, and compared against experimental and literature large eddy simulation data and against similar results in the literature. The method generates excellent results for this case and is elegant and straightforward to implement.