LES and RANS Simulation of the Turbulent Mixing of High-Density-Ratio Axisymmetric Jets
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The turbulent mixing of axisymmetric jet flows is investigated using large-eddy simulation (LES) and Reynolds-averaged Navier-Stokes (RANS) computational fluid dynamics (CFD). The flowfield of interest consists of a round jet and a surrounding, coaxial annular jet ejected into a quiescent free stream. This flow situation arises, for example, in the flow of hydrogen and oxygen in rocket combustion systems. In such cases, the jets have significantly different densities, which has been found experimentally to strongly influence the downstream turbulent mixing dynamics. The goal of the present study is to evaluate the capacity of the LES and RANS methodologies to accurately predict jet mixing without the complicating effects of combustion. Simulations were performed for test cases matching an experimental study that has been documented in the open literature. Both air-air and hydrogen-air jet combinations were investigated, yielding density ratios ranging from 1.46 to 0.08. The LES and RANS results were compared to one another and to experimental measurements in terms of centerline decay and radial distribution of mean velocity and concentration. The results highlight the strengths and weaknesses of the two computational approaches for this class of problems.Keywords:
Large-Eddy Simulation
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Large-Eddy Simulation
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The paper presents 1) the numerical results of RANS (Reynolds Averaging Navier-Stokes) simulations for two versions of the premixed combustion GE10 burners: the old one with non-premixed and modified one with swirled premixed pilot flames; and 2) the numerical results of joint RANS/LES (Large Eddy Simulation) modelling of the ONERA model burner and a simplified GE10 combustor. The original joint RANS/LES approach is based on using the Kolmogorov theory for modelling sub-grid turbulence and combustion intensity and using RANS numerical results for closure the LES model equations. The main conclusion is that developed joint RANS/LES approch is the efficient timesaving tool for simulations both the average and instantaneous fields of parameters in gas turbine and boiler burners with premixed combustion.
Large-Eddy Simulation
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Large-Eddy Simulation
Detached-Eddy Simulation
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We examine and benchmark the emerging idea of applying the large-eddy simulation (LES) formalism to unconventionally coarse grids where RANS would be considered more appropriate at first glance. We distinguish this idea from very-large-eddy-simulation (VLES) and detached-eddy-simulation (DES), which require switching between RANS and LES formalism. LES on RANS grid is appealing because first, it requires minimal changes to a production code; second, it is more cost-effective than LES; third, it converges to LES; and most importantly, it accurately predicts flows with separation. This work quantifies the benefit of LES on RANS-like grids as compared to RANS on the same grids. Three canonical cases are considered: periodic hill, backward-facing step, and jet in cross flow. We conduct direct numerical simulation (DNS), proper LES on LES grids, LES on RANS-quality grids, and RANS. We show that while the LES solutions on the RANS-quality grids are not grid converged, they are twice as accurate as the RANS on the same grids.
Large-Eddy Simulation
Detached-Eddy Simulation
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Coaxial
Matrix (chemical analysis)
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Coaxial
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SUMMARY Numerous comparisons between Reynolds‐averaged Navier–Stokes (RANS) and large‐eddy simulation (LES) modeling have already been performed for a large variety of turbulent flows in the context of fully deterministic flows, that is, with fixed flow and model parameters. More recently, RANS and LES have been separately assessed in conjunction with stochastic flow and/or model parameters. The present paper performs a comparison of the RANS k − ε model and the LES dynamic Smagorinsky model for turbulent flow in a pipe geometry subject to uncertain inflow conditions. The influence of the experimental uncertainties on the computed flow is analyzed using a non‐intrusive polynomial chaos approach for two flow configurations (with or without swirl). Measured quantities including an estimation of the measurement error are then compared with the statistical representation (mean value and variance) of their RANS and LES numerical approximations in order to check whether experiment/simulation discrepancies can be explained within the uncertainty inherent to the studied configuration. The statistics of the RANS prediction are found in poor agreement with experimental results when the flow is characterized by a strong swirl, whereas the computationally more expensive LES prediction remains statistically well inside the measurement intervals for the key flow quantities.Copyright © 2012 John Wiley & Sons, Ltd.
Large-Eddy Simulation
Inflow
Mean flow
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Large-Eddy Simulation
Detached-Eddy Simulation
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Water header is the most common structure in the design of flow system for energy and power system. The complex flow structure could result in some problems when CFD simulation is applied in the whole system analysis. The rapid change in velocity distribution of the flow field leads to difficulties to create suitable boundary-layer mesh, and the complex flow structure will also make residuals hard to reach convergence criteria. Large eddy simulation is promising to promote these studies, it is more accurate than RANS method and can capture many non-steady-state characteristics those RANS method cannot obtain. In this study a typical water header flow structure is investigated by RANS and large eddy simulation methods. By comparing the detailed flow structures in the results of two methods, the deficiency of RANS method was found. The results of large eddy simulation can be used to guide the establishment of meshes and the application of time-averaged turbulence models to improve efficiency in engineering. The asymmetric Reynolds stresses may induce asymmetric flow field in symmetric geometry.
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Header
Detached-Eddy Simulation
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A numerical study is made of the effects of both axisymmetric and non-axisymmetric disturbances on the stability of spiral flow between rotating cylinders. If we let Ω 1 and Ω 2 be the angular speeds of the inner and outer cylinders, and R 1 and R 2 be their respective radii, then for fixed values of η = R 1 / R 2 and μ = Ω 2 / Ω 1 , the onset of instability depends on both the Taylor number T and the axial Reynolds number R . Here R is based on the gap width between the cylinders and the average axial velocity of the basic flow, while T is based on the average angular speeds of the cylinders. Using the compound matrix method, we have computed the complete stability boundary in the R , T -plane for axisymmetric disturbances with η = 0.95 and μ = 0. We find that, for sufficiently high Reynolds numbers, there are two distinct axisymmetric modes corresponding to the usual shear and rotational instabilities. We have also obtained the stability boundaries for non-axisymmetric disturbances for R ≼ 6000 for η = 0.95 and 0.77 with μ = 0. These last results are found to be in substantial agreement with the experimental observations of Snyder (1962, 1965), Nagib (1972) and Mavec (1973) in the low and moderate axial Reynolds number régimes.
Taylor number
Taylor–Couette flow
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