Large eddy simulation of turbulent buffet forces in flow induced vibration

The pressure pulse filter and Smagorinsky subgrid stress model of the Large Eddy Simulation (LES) are introduced. The fluid field in the annular plenum between the pressure vessel and the core barrel of the1:5 model in the second phase of Qinshan Nuclear Power Plant is simulated, and the distribution of the total pressure in the space and time domains is obtained. The results show that the Power Spectrum Density (PSD) of LES from the calculation and the test are in the same quantity order. Thus, the pressure of LES can be a load to stimulate the barrel vibration. __________ Translated from Nuclear Power Engineering, 2007, 28(5): 14–17 [译自: 核动力工程]     

Large eddy simulation of turbulent buffet forces in flow induced vibration

The pressure pulse filter and Smagorinsky sub-grid stress model of the Large Eddy Simulation (LES) are introduced. The fluid field in the annular plenum between the pressure vessel and the core barrel of the1:5 model in the second phase of Qinshan Nuclear Power Plant is simulated, and the distribution of the total pressure in the space and time domains is obtained. The results show that the Power Spectrum Density (PSD) of LES from the calculation and the test are in the same quantity order. Thus, the pressure of LES can be a load to stimulate the barrel vibration.     

Large eddy simulation of turbulent channel flow with mass transfer at high-Schmidt numbers

Large eddy simulation (LES) of turbulent channel flow with mass transfer has been performed to investigate the effect of the Schmidt number on the turbulence behaviors. The three-dimensional filtered Navier-Stokes equations and the concentration equation are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stresses and mass fluxes are employed to closure the governing equations. The objectives of this study are to examine the reliability of the LES technique for predicting the turbulent mass transfer at high-Schmidt numbers and to analyze the behavior of turbulent mass diffusion from a solid boundary to the adjacent shear flow at different Schmidt numbers. Fully developed turbulent channel flows with constant difference of concentrations imposed on different walls are calculated for a wide range of the Schmidt number from 0.1 up to 200 and the Reynolds number 13 800 based on the centerline mean velocity and the half-width of the channel. To show the effects of Schmidt number on the turbulent mass transfer, mean and fluctuating resolved concentrations, mass transfer coefficient, turbulent mass fluxes, and some instantaneous flow and concentration fields are exhibited and analyzed.     

Large eddy simulation of micro-particles transport with different mass flow rate in turbulent planar jet flow

The motion of micro-particles with different mass flow rate in the planer turbulent jet flow has been simulated,using LES method to obtain the flow vorticity evolution and Lagrangian method to track micro-particles.The results showed that when the flow rate is small,the particles more likely to present in the vortex periphery,the distribution pattern is similar to the flow pattern.When the flow rate is high,some particles will escape from the motion region to the original static region,so that in the jet region,particles are relatively evenly distributed.When the flow field is full developed,the particles average concentration in the y direction affected by the mass flow rate relative slightly,the normalized mean particles concentrations at different flow rate were similar to Gaussian shape.     

Large eddy simulation of a lifted turbulent jet flame

The flame index concept for large eddy simulation developed by Domingo et al. [P. Domingo, L. Vervisch, K. Bray, Combust. Theory Modell. 6 (2002) 529–551] is used to capture the partially premixed structure at the leading point and the dual combustion regimes further downstream on a turbulent lifted flame, which is composed of premixed and nonpremixed flame elements each separately described under a flamelet assumption. Predictions for the lifted methane/air jet flame experimentally tested by Mansour [M.S. Mansour, Combust. Flame 133 (2003) 263–274] are made. The simulation covers a wide domain from the jet exit to the far flow field. Good agreement with the data for the lift-off height and the mean mixture fraction has been achieved. The model has also captured the double flames, showing a configuration similar to that of the experiment which involves a rich premixed branch at the jet center and a diffusion branch in the outer region which meet at the so-called triple point at the flame base. This basic structure is contorted by eddies coming from the jet exit but remains stable at the lift-off height. No lean premixed branches are observed in the simulation or and experiment. Further analysis on the stabilization mechanism was conducted. A distinction between the leading point (the most upstream point of the flame) and the stabilization point was made. The later was identified as the position with the maximum premixed heat release. This is in line with the stabilization mechanism proposed by Upatnieks et al. [A. Upatnieks, J. Driscoll, C. Rasmussen, S. Ceccio, Combust. Flame 138 (2004) 259–272].     

Large eddy simulation of heated vertical annular pipe flow in fully developed turbulent mixed convection

The goal of the present study was to perform a large eddy simulation of vertical turbulent annular pipe flow under conditions in which fluid properties vary significantly, and to investigate the effects of buoyancy on the turbulent structures and transport. Isoflux wall boundary conditions with low and high heating are imposed. The compressible filtered Navier-Stokes equations are solved using a second order accurate finite volume method. Low Mach number preconditioning is used to enable the compressible code to work efficiently at low Mach numbers. A dynamic subgrid-scale stress model accounts for the subgrid-scale turbulence. Comparisons were made with available experimental data. The results showed that the strong heating and buoyant force caused distortions of the flow structure resulting in reduction of turbulent intensities, shear stress, and turbulent heat flux, particularly near the wall.     

Large eddy simulation of turbulent flow and heat transfer in outward transverse and helically corrugated tubes

Turbulent flow and heat transfer in outward transverse and helically corrugated tubes are performed with large eddy simulation by the ANSYS Fluent software. The prediction accuracy is validated by comparison with experimental data and empirical correlations for a wavy surface wall and smooth tube, respectively. The turbulent flow patterns, local heat transfer, and friction factor are discussed. The results show that the secondary and turbulent eddies are inhibited by the spiral flow. Otherwise, the flow impact of the wall is the key factor for heat transfer enhancement, and the spiral flow has of small effect on heat transfer performance, however it can decrease the flow resistance significantly. The overall heat transfer performance for the helical corrugated tube is 1.23, which is superior to the value of 1.18 for the transverse corrugated tube.     

Large eddy simulation of spark ignition in a turbulent methane jet

Large eddy simulation (LES) is used to compute the spark ignition in a turbulent methane jet flowing into air. Full ignition sequences are calculated for a series of ignition locations using a one-step chemical scheme for methane combustion coupled with the thickened flame model. The spark ignition is modeled in the LES as an energy deposition term added to the energy equation. Flame kernel formation, the progress and topology of the flame propagating upstream, and stabilization as a tubular edge flame are analyzed in detail and compared to experimental data for a range of ignition parameters. In addition to ignition simulations, statistical analysis of nonreacting LES solutions is carried out to discuss the ignition probability map established experimentally.     

Large eddy simulation of particle response to turbulence along its trajectory in a backward-facing step turbulent flow

In order to understand particle response to turbulence along its path, properties of the particle phase and gas phase are compared and analyzed for a turbulent flow over a backward-facing step. The turbulent gas phase is simulated numerically using large eddy simulation and the particle phase is modeled by means of Lagrangian methods. The particle Stokes number ranges from 0.01 to 111.18 and the Reynolds number is 18,400, based on the step height and the inlet mean velocity. Particle velocities, fluid velocities and vorticity along particle trajectory as well as particle dispersion are obtained so as to provide information on particle response to turbulence. Results show that particle behavior depends heavily on the local fluid turbulence along its path, especially for small particles. Particles follow a path along which the gas-phase vorticity is small. However, large particles maintain the inertia along their trajectories without responding to the fluid fluctuations.     

Large eddy simulation of soot formation in a turbulent non-premixed jet flame

A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108-126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log-normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed.     

Numerical simulation of turbulent fluid flow and heat transfer characteristics in heat exchangers fitted with porous media

Numerical simulations have been carried out to investigate the turbulent heat transfer enhancement in the pipe filled with porous media. Two-dimensional axisymmetric numerical simulations using the k–? turbulent model is used to calculate the fluid flow and heat transfer characteristics in a pipe filled with porous media. The parameters studied include the Reynolds number (Re = 5000–15,000), the Darcy number (Da = 10^?1–10^?6), and the porous radius ratio (e = 0.0–1.0). The numerical results show that the flow field can be adjusted and the thickness of boundary layer can be decreased by the inserted porous medium so that the heat transfer can be enhanced in the pipe. The local distributions of the Nusselt number along the flow direction increase with the increase of the Reynolds number and thickness of the porous layer, but increase with the decreasing Darcy number. For a porous radius ratio less than about 0.6, the effect of the Darcy number on the pressure drop is not that significant. The optimum porous radius ratio is around 0.8 for the range of the parameters investigated, which can be used to enhance heat transfer in heat exchangers.     

Large eddy simulation of flow and scalar transport in a round jet

Large eddy simulation (LES) was performed for a spatially developing round jet and its scalar transport at four steps of Reynolds number set between 1200 and 1,000,000. A simulated domain, which extends 30 times the nozzle diameter, includes initial, transitional, and established stage of jet. A modified version of convection outflow condition was proposed in order to diminish the effect of a downstream boundary. Tested were two kinds of subgrid scale (SOS) models: a Smagorinsky model (SM) and a dynamic Smagorinsky model (DSM). In the former model, parameters are kept at empirically deduced constants, while in the latter, they are calculated using different levels of space filtering. Data analysis based on the decay law of jet clearly presented the performance of SGS models. Simulated results by SM and DSM compared favorably with existing measurements of jet and its scalar transport. However, the quantitative accuracy of DSM was better than that of SM at a transitional stage of flow field. Computed parameters by DSM, coefficient for SGS stresses, C_R and SGS eddy diffusivity ratio, Γ_SGS, were not far from empirical constants of SM. Optimization of the model coefficient was suggested in DSM so that coefficient C_R was nearly equal in the established stage of jet but it was reduced in low turbulence close to the jet nozzle. © 2004 Wiley Periodicals, Inc. Heat Trans Asian Res, 33(3): 175–188, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20001     

Large eddy simulation of passive scalar in complex turbulence with flow impingement and flow separation

In order to reveal unknown characteristics of complex turbulent passive scalar fields, large eddy simulations in forced convection regimes have been performed under several strain conditions, including flow impingement and flow separation. By using the simulation results, relations between the dynamic and scalar fields are carefully examined. It is then confirmed that the scalar is transported by a large vortex structure near the examined regions wherever the mean shear vanishes, although in the high‐shear regions, the scalar transport is governed by a coherent structure due to the high shear strain. In addition, a priori explorations are attempted by processing the data, focusing on the derivation of a possible direction for modeling algebraically the passive scalar transport in a complex strain field. The a priori tests suggest that an expanded form of the GGDH model introducing a quadratic product of the Reynolds stresses is promising for general flow cases. © 2001 Scripta Technica, Heat Trans Asian Res, 30(5): 402–418, 2001     

Large eddy simulation of mean and oscillating flow in a side-dump ramjet combustor

Ramjet flows are very sensitive to combustion instabilities that are difficult to predict using numerical simulations. This paper describes compressible large eddy simulation on unstructured grids used to investigate nonreacting and reacting flows in a simplified twin-inlet ramjet combustor. The reacting flow is compared to experimental results published by ONERA in terms of mean fields. Simulations show a specific flow topology controlled by the impingement of the two air jets issuing from the twin air inlets and by multiple complex recirculation zones. In a second part, all unsteady modes appearing in the reacting LES are analyzed using spectral maps and POD (proper orthogonal decomposition) tools. A Helmholtz solver also computes the frequencies and structures of all acoustic modes in the ramjet. Pure longitudinal, transverse and combined modes are identified by all three diagnostics. In addition, a mode-by-mode analysis of the Rayleigh criterion is presented thanks to POD. This method shows that the most intense structure (at 3750 Hz) is the first transverse acoustic mode of the combustor chamber and the Rayleigh criterion obtained with POD illustrates how this transverse mode couples with unsteady combustion.     

Large eddy simulation of flow and heat transfer in a rotating ribbed channel

The internal cooling passage of a gas turbine blade equipped with ribs is modeled as a rotating ribbed channel. The flow and heat transfer in the ribbed channel have been investigated by conducting large eddy simulations with a dynamic subgrid-scale model. The Reynolds number considered is 30,000 and rotation numbers are 0, 0.1 and 0.3. The time-averaged results show good agreement with the experimental data. By comparing the present data with those of the smooth channel, it is observed that the vortices shed from the rib induce strong wall-normal motions, and they are augmented on the trailing-wall side by the rotation, resulting in a significant increase in the heat transfer due to rotation. It is also shown that the similarity between the streamwise velocity and temperature is significantly destroyed by both the rotation and the rib itself.     

On a subgrid-scale heat flux model for large eddy simulation of turbulent thermal flow

A non-linear subgrid-scale (SGS) heat flux model is introduced in large eddy simulation for turbulent thermal flows. Unlike the linear isotropic eddy diffusivity model, the proposed model accounts for the SGS heat flux in terms of the large-scale strain-rate tensor and the temperature gradients. This is equivalent to using a tensor diffusivity. The model is to some extent similar to a scale-similarity model subjected to a Taylor expansion for the filtering operation. The formulation leading to the present proposal is discussed. The model is examined in LES for a buoyant flow in an infinite vertical channel with two differentially heated side walls. It is shown that the proposed model reproduces reasonable results as compared with the isotropic SGS diffusivity model and DNS data.     

Large eddy simulation of stably stratified turbulent open channel flows with low- to high-Prandtl number

Large eddy simulation of thermally stratified turbulent open channel flows with low- to high-Prandtl number is performed. The three-dimensional filtered Navier-Stokes and energy equations under the Boussinesq approximation are numerically solved using a fractional-step method. Dynamic subgrid-scale (SGS) models for the turbulent SGS stress and heat flux are employed to close the governing equations. The objective of this study is to reveal the effects of both the Prandtl number (Pr) and Richardson (Ri_τ) number on the characteristics of turbulent flow, heat transfer, and large-scale motions in weakly stratified turbulence. The stably stratified turbulent open channel flows are calculated for Pr from 0.1 up to 100, Ri_τ from 0 to 20, and the Reynolds number (Re_τ) 180 based on the wall friction velocity and the channel height. To elucidate the turbulent flow and heat transfer behaviors, some typical quantities, including the mean velocity, temperature and their fluctuations, turbulent heat fluxes, and the structures of the velocity and temperature fluctuations, are analyzed.     

Analysis of turbulent combustion in inert porous media

The objective of this paper is to present an extension of a simplified reaction kinetics model that, combined with a thermo-mechanical closure, entails a full-generalized turbulent combustion model for flow in porous media. In this model, one explicitly considers the intra-pore levels of turbulent kinetic energy. Transport equations are written in their time-and-volume-averaged form and a volume-based statistical turbulence model is applied to simulate turbulence generation due to the porous matrix. The rate of fuel consumption is described by an Arrhenius expression involving the product of the fuel and oxidant mass fractions. These mass fractions are double decomposed in time and space and, after applying simultaneous time-and-volume integration operations to them, distinct terms arise, which are here associated with the mechanisms of dispersion and turbulence. Modeling of these extra terms remains an open question and the derivations herein might motivate further development of models for turbulent combustion in porous media.     

Large eddy simulation of laser ignition and compressible reacting flow in a rocket-like configuration

The control of ignition in a rocket engine is a critical problem for combustion chamber design. Delayed ignition may lead to high-amplitude pressure fluctuations that can damage the burner (strong ignition), whereas early ignition may fail. This paper describes a numerical study of a strong ignition sequence observed in a laboratory-scale single-injector rocket chamber ignited by a laser and fueled with gaseous oxygen and hydrogen. OH-emission images, Schlieren pictures, and pressure measurements make it possible to follow the flame propagation experimentally. The present large eddy simulation (LES) approach includes shock treatment, a six species-seven reaction chemical scheme for H₂-O₂, and a model for the energy deposition by a laser. Flame/turbulence interaction is modeled with the thickened flame concept. LES is used to compute both the filling phase (during which the gaseous hydrogen and oxygen mix) and the ignition phase. The flame location and structure, as well as the temporal evolution of the chamber pressure obtained numerically, are in good agreement with the experiment. The use of complex chemistry in the computation also allows the comparison of LES data with experimental OH-images and shows that the sensitivity of the CCD camera used to record the spontaneous emission of the OH^∗ radical is not high enough to properly locate the flame front in rich regions. The combined experimental and numerical results lead to a more detailed analysis of the ignition processes and its coupling with flow rate oscillations in the H₂ and O₂ feeding lines.     

Large eddy simulation of heat and mass transport in turbulent flows. Part 1: Velocity field

Localized residual or subgrid-scale (SGS) models are presented for use in large eddy simulation of heat and mass transport in turbulent flows. In part (1) (this paper), we discuss the SGS stress models for the velocity field. The models for the scalar field are presented in part (2). The new SGS stress closures are compared with the dynamic-Smagorinsky model (DSM) and the dynamic two-parameter mixed model (DTMM). All models are applied “locally” and their performances are assessed via both a priori and a posteriori analyses with detailed comparisons against data obtained from direct numerical simulation of homogeneous isotropic, homogeneous shear and temporal mixing layer flows. The results of a priori assessments indicate that the new closures predict the SGS stresses better than both DSM and DTMM in all simulated flows. The results of a posteriori assessments also show that the SGS stresses and the statistics of the filtered velocity are more accurately predicted with the new models.