Active control for statistically stationary turbulent premixed flame simulations
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The speed of propagation of a premixed turbulent flame correlates with the intensity of the turbulence encountered by the flame.One consequence of this property is that premixed flames in both laboratory experiments and practical combustors require some type of stabilization mechanism to prevent blow-off and flashback.The stabilization devices often introduce a level of geometric complexity that is prohibitive for detailed computational studies of turbulent flame dynamics.Furthermore, the stabilization introduces additional fluid mechanical complexity into the overall combustion process that can complicate the analysis of fundamental flame properties.To circumvent these difficulties we introduce a simple, heuristic feedback control algorithm that allows us to computationally stabilize a turbulent premixed flame in a simple geometric configuration.For the simulations, we specify turbulent inflow conditions and dynamically adjust the integrated fueling rate to control the mean location of the flame in the domain.We outline the numerical procedure, and illustrate the behavior of the control algorithm on methane flames at various equivalence ratios in two dimensions.The simulation data are used to study the local variation in the speed of propagation due to flame surface curvature.We formulate multifractal models for velocity differences and gradients which describe the full range of length scales in turbulent flow, namely: laminar, dissipation, inertial, and stirring ranges. The models subsume existing models of inertial range turbulence. In the localized ranges of length scales in which the turbulence is only partially developed, we propose multifractal scaling laws with scaling exponents modified from their inertial range values. In local regions, even within a fully developed turbulent flow, the turbulence is not isotropic nor scale invariant due to the influence of larger turbulent structures (or their absence). For this reason, turbulence that is not fully developed is an important issue which inertial range study can not address. In the ranges of partially developed turbulence, the flow can be far from universal, so that standard inertial range turbulence scaling models become inapplicable. The model proposed here serves as a replacement. Details of the fitting of the parameters for the τ p and ζ p models in the dissipation range are discussed. Some of the behavior of ζ p for larger p is unexplained. The theories are verified by comparing to high resolution simulation data.
Multifractal system
Kolmogorov microscales
Intermittency
Turbulence Modeling
Taylor microscale
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Large-Eddy Simulation
Length scale
Laminar flame speed
Intensity
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Experimental data are presented for the case of the onset of dynamic and thermal laminar-turbulent transition (LTT) on the surface of a flat plate under the elevated turbulence level of an external flow. It is shown that the ratio of the end-to-start LTT determined by means of characteristic thicknesses is the same as at Tue ≈ 0.3% (Ree/Re ≈ 2.7). However, the length of LTT increases substantially with external turbulence (Rexe/Rexs ≈ 1.7 at Tue ≈ 0.25% and Rexs/Rexs ≈ 2.8 at Tue ≈ 4.75%).
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We formulate multifractal models for velocity differences and gradients which describe the full range of length scales in turbulent flow, namely: laminar, dissipation, inertial, and stirring ranges. The models subsume existing models of inertial range turbulence. In the localized ranges of length scales in which the turbulence is only partially developed, we propose multifractal scaling laws with scaling exponents modified from their inertial range values. In local regions, even within a fully developed turbulent flow, the turbulence is not isotropic nor scale invariant due to the influence of larger turbulent structures (or their absence). For this reason, turbulence that is not fully developed is an important issue which inertial range study can not address. In the ranges of partially developed turbulence, the flow can be far from universal, so that standard inertial range turbulence scaling models become inapplicable. The model proposed here serves as a replacement.Details of the fitting of the parameters for the $\tau_p$ and $\zeta_p$ models in the dissipation range are discussed. Some of the behavior of $\zeta_p$ for larger $p$ is unexplained. The theories are verified by comparing to high resolution simulation data.
Multifractal system
Kolmogorov microscales
Enstrophy
Intermittency
Turbulence Modeling
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To overcome weak poinks of the standard k- turbulence model when applied to complex turbulent flows, various modified models were proposed. But their effects are confined to special flow fields. They have still some problems. Recently, an anisotropic k- turbulence model was also proposed to solve the drawback of the standard k- turbulence model. This study is concentrated on the evaluation of the anisotropic k- turbulence model by the analysis of axisymmetric swirling turbulent flow. Results show that the anisotropic k- turbulence model has scarecely the fundamentally physical mechanism of predicting the swirling structure of flow.
Turbulence Modeling
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The dynamic structure of turbulence in transient pipe flows was studied experimentally. Two types of stepwise change in the flow rate of a fully developed pipe flow showed apparently different behaviours in this turbulence. With a stepwise increase in the flow rate, the dominant feature was the generation and propagation of a new turbulence. With a stepwise decrease in the flow rate the dominant feature was the decay of an old turbulence. However, a comparison of these two types of stepwise change indicates a coherent structure in the propagation of a new turbulence in both transient pipe flows. The propagation time of new turbulence is determined by the condition at its generation. Moreover, this coherent character is also applicable to the propagation of an old turbulence, and the beginning of the decay of the old turbulence is predicted by the propagation time in initial steady state. On the basis of these facts the dynamic behaviours of turbulence in both these types of transient flow are interpreted consistently. For the decay of the old turbulence, the ''linear'' decay law is applicable and the decay rate is governed by the flow condition during decay, although the propagation time is not affected by the transient flow.
Transient (computer programming)
Pipe flow
Turbulence Modeling
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We formulate multifractal models for velocity differences and gradients which describe the full range of length scales in turbulent flow, namely: laminar, dissipation, inertial, and stirring ranges. The models subsume existing models of inertial range turbulence. In the localized ranges of length scales in which the turbulence is only partially developed, we propose multifractal scaling laws with scaling exponents modified from their inertial range values. In local regions, even within a fully developed turbulent flow, the turbulence is not isotropic nor scale invariant due to the influence of larger turbulent structures (or their absence). For this reason, turbulence that is not fully developed is an important issue which inertial range study can not address. In the ranges of partially developed turbulence, the flow can be far from universal, so that standard inertial range turbulence scaling models become inapplicable. The model proposed here serves as a replacement.Details of the fitting of the parameters for the $\tau_p$ and $\zeta_p$ models in the dissipation range are discussed. Some of the behavior of $\zeta_p$ for larger $p$ is unexplained. The theories are verified by comparing to high resolution simulation data.
Multifractal system
Kolmogorov microscales
Enstrophy
Intermittency
Turbulence Modeling
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The present paper aims at improving the modeling of turbulence for the upward turbulent bubbly flow through the use of experimental databases that contain data on small and large vertical ducts. First, the role of bubble-induced turbulence was analyzed, which indicated the dominant role of the bubble-induced turbulence in the duct center for relatively high void fraction cases. Therefore, the turbulence therein was mainly focused on, which indicated that the stronger turbulence could be induced by bubbles in large ducts with similar void fractions as compared to that in small ducts. Next, the turbulence of upward turbulent bubbly flow near the wall is discussed to understand the interaction between the wall-induced and bubble-induced turbulence. It showed that the existence of a wall could suppress the bubble-induced turbulence given the same void fraction, and the existence of bubbles could also suppress the solely wall-induced turbulence as compared to the single-phase turbulent flow, even though the total turbulence is enhanced. The above characteristics indicated that the current turbulence modeling method needs to be modified, especially when the bubble-induced turbulence plays a dominant role.
Turbulence Modeling
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In order to study turbulence modulation in solid-liquid two-phase flows, in particular effects of slip velocity between the two phases on turbulence structure of the fluid phase, interaction between turbulence generated by settling particles and the oscillating grid turbulence is calculated using the turbulence model based on the one-point closures. The calculated values either for the turbulence due to settling particles or for the oscillating grid turbulence agree well with the experimental data in the previous studies. And the model predicts that mixing of particles decays turbulence of the fluid phase.
Settling
Turbulence Modeling
Reynolds decomposition
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