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Terashima H.,Japan Aerospace Exploration Agency | Terashima H.,University of Tokyo | Kawai S.,Stanford University | Kawai S.,Center for Turbulence Research | And 2 more authors.
AIAA Journal | Year: 2011

A high-resolution methodology using a high-order compact differencing scheme with localized artificial diffusivity is introduced with the aim of simulating jet mixing under supercritical pressure environments. The nonlinear localized artificial diffusivity provides the stability to capture different types of discontinuity, such as shock wave, contact surface, and material interface, whereas the high-order compact difference scheme resolves broadband scales in the rest of the domain. The present method is tested on several one-dimensional discontinuity-related problems under super/transcritical conditions and a comparatively more illustrative two-dimensional lowtemperature planar jet problem under a supercritical pressure condition. The localized artificial diffusivity, especially artificial thermal conductivity for temperature gradients, effectively suppresses numerical wiggles near the interfaces. The effects of the artificial thermal conductivity on numerical stability and accuracy are examined. Comparisons between the present method and a conventional low-order scheme demonstrate the superior performance of the present method for resolving a wide range of flow scales while successfully capturing large density/temperature variations at interfaces. Copyright © 2011 by the American Institute of Aeronautics and Astronautics, Inc.

Nicoud F.,Montpellier University | Toda H.B.,French Institute of Petroleum | Cabrit O.,European Center for Research and Advanced Training in Scientific Computation | Bose S.,Center for Turbulence Research | Lee J.,Seoul National University
Physics of Fluids | Year: 2011

An eddy-viscosity based, subgrid-scale model for large eddy simulations is derived from the analysis of the singular values of the resolved velocity gradient tensor. The proposed σ-model has, by construction, the property to automatically vanish as soon as the resolved field is either two-dimensional or two-component, including the pure shear and solid rotation cases. In addition, the model generates no subgrid-scale viscosity when the resolved scales are in pure axisymmetric or isotropic contraction/expansion. At last, it is shown analytically that it has the appropriate cubic behavior in the vicinity of solid boundaries without requiring any ad-hoc treatment. Results for two classical test cases (decaying isotropic turbulence and periodic channel flow) obtained from three different solvers with a variety of numerics (finite elements, finite differences, or spectral methods) are presented to illustrate the potential of this model. The results obtained with the proposed model are systematically equivalent or slightly better than the results from the Dynamic Smagorinsky model. Still, the σ-model has a low computational cost, is easy to implement, and does not require any homogeneous direction in space or time. It is thus anticipated that it has a high potential for the computation of non-homogeneous, wall-bounded flows. © 2011 American Institute of Physics.

El-Asrag H.A.,Massachusetts Institute of Technology | Pitsch H.,Center for Turbulence Research | Kim W.,Stanford University | Do H.,Stanford University | Mungal M.G.,Stanford University
Combustion Science and Technology | Year: 2011

Afterburners (or augmentors) are used to increase thrust in aircraft engines. Static flame stability, or the robustness to flame blowoff, is an important metric in the performance assessment of combustion in aircraft engine afterburners, where bluff-body-type flame holders are typically used to stabilize the flame. The design of such flame holders is complicated by the operating conditions, which involve flows at high speed, high temperature, and low pressure. In this paper, experimental and computational studies of Damkohler (Da) number similarity are presented with relevance to augmentor flame stability. The Da number describes the ratio of flow and chemical time scales. Hence, as long as a reference Da number is kept constant, similar characteristics of static stability should be expected of the bluff-body stabilized flame at low and high speeds. Flame stability in high-speed vitiated flows could then be studied at low speed if the chemical time scale is reduced, for instance by lowering the oxidizer flow temperature. However, each chemical reaction has its own Da number and not all can be kept constant at the same time. Since different stabilization mechanisms are governed by different chemical reactions, it is not necessarily clear what the relevant Da number is. Here, a consistent method for defining the Da number is provided based on the analysis of the modeled governing equations. Numerical simulations are performed for three different velocities with the inflow temperature adjusted to keep the Da number constant. Results are validated by comparison with experimental PIV data and the reported flame liftoff height. For the same characteristic Da number and constant momentum ratio between the fuel jet and the vitiated cross flow, the three flames show similar mean features for the recirculation zone and the flame shape. The flow field is found to exhibit von Karman vortex shedding with the same Strouhal number for all cases. The average nondimensional flame liftoff height is also found to be the same for all cases. These results suggest a method to properly define the Da number to test augmentor stability features in low-speed test facilities under the conditions of similarity. Copyright © 2011 Taylor & Francis Group, LLC.

Scalo C.,Center for Turbulence Research | Bodart J.,Higher Institute of Aeronautics and Space
Physics of Fluids | Year: 2015

We have performed large-eddy simulations of isothermal-wall compressible turbulent channel flow with linear acoustic impedance boundary conditions (IBCs) for the wall-normal velocity component and no-slip conditions for the tangential velocity components. Three bulk Mach numbers, Mb = 0.05, 0.2, 0.5, with a fixed bulk Reynolds number, Reb = 6900, have been investigated. For each Mb, nine different combinations of IBC settings were tested, in addition to a reference case with impermeable walls, resulting in a total of 30 simulations. The adopted numerical coupling strategy allows for a spatially and temporally consistent imposition of physically realizable IBCs in a fully explicit compressible Navier-Stokes solver. The IBCs are formulated in the time domain according to Fung and Ju ["Time-domain impedance boundary conditions for computational acoustics and aeroacoustics," Int. J. Comput. Fluid Dyn. 18(6), 503-511 (2004)]. The impedance adopted is a three-parameter damped Helmholtz oscillator with resonant angular frequency, Ωr, tuned to the characteristic time scale of the large energy-containing eddies. The tuning condition, which reads Ωr = 2πMb (normalized with the speed of sound and channel half-width), reduces the IBCs' free parameters to two: the damping ratio, ζ , and the resistance, R, which have been varied independently with values, ζ = 0.5, 0.7, 0.9, and R = 0.01, 0.10, 1.00, for each Mb. The application of the tuned IBCs results in a drag increase up to 300% for Mb = 0.5 and R = 0.01. It is shown that for tuned IBCs, the resistance, R, acts as the inverse of the wall-permeability and that varying the damping ratio, ζ , has a secondary effect on the flow response. Typical buffer-layer turbulent structures are completely suppressed by the application of tuned IBCs. A new resonance buffer layer is established characterized by large spanwise-coherent Kelvin-Helmholtz rollers, with a well-defined streamwise wavelength λ x, traveling downstream with advection velocity cx = λ x Mb. They are the effect of intense hydro-acoustic instabilities resulting from the interaction of high-amplitude wall-normal wave propagation (at the tuned frequency fr = Ωr/2π = Mb) with the background mean velocity gradient. The resonance buffer layer is confined near the wall by structurally unaltered outer-layer turbulence. Results suggest that the application of hydrodynamically tuned resonant porous surfaces can be effectively employed in achieving flow control. © 2015 AIP Publishing LLC.

Kim J.,Cornell University | Kim J.,Center for Turbulence Research | Pope S.B.,Cornell University
Combustion Theory and Modelling | Year: 2014

A turbulent lean-premixed propane-air flame stabilised by a triangular cylinder as a flame-holder is simulated to assess the accuracy and computational efficiency of combined dimension reduction and tabulation of chemistry. The computational condition matches the Volvo rig experiments. For the reactive simulation, the Lagrangian Large-Eddy Simulation/Probability Density Function (LES/PDF) formulation is used. A novel two-way coupling approach between LES and PDF is applied to obtain resolved density to reduce its statistical fluctuations. Composition mixing is evaluated by the modified Interaction-by-Exchange with the Mean (IEM) model. A baseline case uses In Situ Adaptive Tabulation (ISAT) to calculate chemical reactions efficiently. Its results demonstrate good agreement with the experimental measurements in turbulence statistics, temperature, and minor species mass fractions. For dimension reduction, 11 and 16 represented species are chosen and a variant of Rate Controlled Constrained Equilibrium (RCCE) is applied in conjunction with ISAT to each case. All the quantities in the comparison are indistinguishable from the baseline results using ISAT only. The combined use of RCCE/ISAT reduces the computational time for chemical reaction by more than 50%. However, for the current turbulent premixed flame, chemical reaction takes only a minor portion of the overall computational cost, in contrast to non-premixed flame simulations using LES/PDF, presumably due to the restricted manifold of purely premixed flame in the composition space. Instead, composition mixing is the major contributor to cost reduction since the mean-drift term, which is computationally expensive, is computed for the reduced representation. Overall, a reduction of more than 15% in the computational cost is obtained. © 2014 © 2014 Taylor & Francis.

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