1,720,973 research outputs found
Predicting transition in two- and three-dimensional separated flows
No full textThis paper is concerned with the numerical prediction of two- and three-dimensional transitional separated flows of turbomachinery interest. The recently proposed single-point transition model based on the use of a laminar kinetic energy transport equation is considered, insofar as it does not require to evaluate any integral parameter, such as boundary-layer thickness, and is thus directly applicable to three-dimensional flows. A well established model, combining a transition-onset correlation with an intermittency transport equation, is also used for comparison. Both models are implemented within a Reynolds-averaged Navier–Stokes solver employing a low-Reynolds-number k–omega turbulence model. The performance of the transition models have been evaluated and tested versus well-documented incompressible flows past a flat plate with semi-circular leading edge, namely: tests T3L2, T3L3, T3L5, and T3LA1 of ERCOFTAC, with different Reynolds numbers and free-stream conditions, the last one being characterized by a non-zero pressure gradient. In all computations, the first model has proven as adequate as or superior to the second one and has been then applied with success to two more complex test cases, for which detailed experimental data are available in the literature, namely: the two- and three-dimensional flows through the T106 linear turbine cascade
A flamelet/progress-variable approach for the simulation of turbulent combustion of real gas mixtures
No full textThe industrial and scientific communities are devoting major research efforts to identify and assess critical technologies for new advanced propulsive concepts: combustion at high pressure has been assumed as a key issue to achieve better propulsive performance and lower environmental impact, as long as the replacement of hydrogen with a hydrocarbon, to reduce the costs related to ground operations (propellant handling, infrastructure and procedures) and increase flexibility. For the class of engines of interest in this work, namely liquid-propellant rocket engines, the pressure is always supercritical, whereas the temperature could be either sub- or super-critical; however, propellants are typically injected into an environment that exceeds the critical temperature and pressure for both fuel and oxidizer, therefore a fast transition to a supercritical state is observed. In such a condition, it is possible to neglect the liquid phase and treat the liquid as a "dense" gaseous jet. However, the ideal gas equation of state is not suitable for computing the correct p-v-T relationship for oxygen and fuel at the operating pressure and temperature typical of LOx/HC rocket combustion chambers. Therefore, a suitable equation of state together with adequate model equations for the transport properties are employed. Starting from this background, the current work provides a model for the numerical simulation of high-pressure turbulent combustion employing detailed chemistry description, embedded in a Reynolds averaged Navier-Stokes equations solver with a Low Reynolds number k-ε turbulence model
A RANS flamelet-progress-variable method for computing reacting flows of real-gas mixtures
No full textThis paper provides an efficient numerical method for solving reacting flows of industrial interest in the presence of significant real-gas effects. The method combines a state-of-the-art solver of the Reynolds-averaged Navier-Stokes equations - equipped with the low-Reynolds number k - ω turbulence closure - with a combustion flamelet-progress-variable approach. A real-gas model as well as a detailed kinetic scheme are used to generate the flamelet library. The method is tested versus several applications chosen to demonstrate the importance of the real-gas effects and of the kinetic scheme for computing high-pressure combustion. The major contribution of the paper is to provide a single-phase approach which solves turbulent reacting real-gas flows at a computational cost comparable with that of the simulation of a non-reacting flow thanks to the use of the flamelet library
Analysis of a low-emission combustion strategy for a highperformance trans-atmospheric aircraft engine
No full textAn extended SMLD approach for presumed probability density function in flamelet combustion model
No full textAn evaluation of bypass transition models for turbomachinery flows
No full textThis paper provides a thorough analysis of six laminar-to-turbulent bypass transition models. Five models are obtained by combining a transition-onset correlation with an intermittency-factor models, whereas the sixth one is a single-point model based on the use of a laminar kinetic energy transport equation. All models have been embedded in a Reynolds averaged Navier-Stokes solver employing a low Reynolds number k-ω turbulence model. The performance of the transition models have been evaluated by computing three well documented incompressible flows past flat plates, namely, tests T3A, T3B, and T3C2 of ERCOFTAC SIG 10, with different free-stream conditions, the last one being characterized by a non-zero pressure gradient. Finally, a more complex test case has been considered, for which detailed experimental data are available in the literature, namely, the two-dimensional flow through a turbine cascad
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Numerical Investigation of High Enthalpy Flows
Get PDFThis work deals with fluid dynamic simulations of high enthalpy flows. Thermochemical non-equilibrium, typical of such flows, was modelled by using the well known multi-temperature model developed by Park. The non-equilibrium model was implemented in a 2D finite volume solver of the Euler equations and was assessed by comparing the results with available experimental measurements. Several test cases concerning 2D and axisymmetric expansion nozzles were performed by varying gas composition and stagnation temperature
The role of presumed probability density functions in the simulation of nonpremixed turbulent combustion
Get PDFFlamelet-Progress-Variable (FPV) combustion models allow the evaluation of all thermochemical quantities in a reacting flow by computing only the mixture fraction Z and a progress variable C. When using such a method to predict turbulent combustion in conjunction with a turbulence model, a probability density function (PDF) is required to evaluate statistical averages (e.g., Favre averages) of chemical quantities. The choice of the PDF is a compromise between computational costs and accuracy level. The aim of this paper is to investigate the influence of the PDF choice and its modeling aspects to predict turbulent combustion. Three different models are considered: the standard one, based on the choice of a beta-distribution for Z and a Dirac-distribution for C; a model employing a beta-distribution for both Z and C; and the third model obtained using a beta-distribution for Z and the statistically most likely distribution (SMLD) for C. The standard model, although widely used, does not take into account the interaction between turbulence and chemical kinetics as well as the dependence of the progress variable not only on its mean but also on its variance. The SMLD approach establishes a systematic framework to incorporate informations from an arbitrary number of moments, thus providing an improvement over conventionally employed presumed PDF closure models. The rational behind the choice of the three PDFs is described in some details and the prediction capability of the corresponding models is tested vs. well-known test cases, namely, the Sandia flames, and H-2-air supersonic combustion
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