Large Eddy Simulation Turbulence Model Research Articles

Flow Asymmetry in Symmetric Multiple Impinging Jets: A Large Eddy Simulation Approach

A numerical study on in-line arrays of multiple turbulent round impinging jets on a flat heated plate was conducted. The Large Eddy Simulation turbulence model was used to capture details of the instantaneous and mean flow fields. The Reynolds number, based on the jets diameter, was equal to 20,000. In addition to flow features known from single jets, the interaction between the neighboring jets was successfully elucidated. Symmetry boundary conditions were imposed to reduce the computational domain to only a quarter. In accordance with previous numerical and experimental works, the asymmetry in the velocity field near to the impingement plate was also found to exist. LES showed oval imprints of the Nusselt number similar to experiments but with some discrepancies on the symmetry boundaries. The asymmetry, observed in previous experimental and numerical results, in the horizontal planes, parallel and close to the impingement wall, was confirmed. The recirculation zone responsible for asymmetry, known to develop due to the wall jets interaction, was seen in only one side of the diagonal formed by the central and the farthest jets.

Reynolds Study of the Flow Induced Noise

2D Large Eddy Simulation turbulence model coupled with Ffowcs-Williams-Hawkins model and Boundary Element Method is used to perform a study of the effect of the Reynolds number on the broadband noise emitted due to the incompressible fluid structure interaction. The method is then used to calculate the broadband noise emitted by the aerodynamic flow over different shape bodies having same cross-sectional area with the intent to know the most suitable cross sectional geometry of the fuel manifolds in the aircraft engine afterburner in terms of minimum noise emission. The results obtained suggests that a 5dB benefit in sound emission could be obtained by the halving the diameter of the circular cross-section used for the fuel manifolds, to make them elliptical keeping cross sectional area constant.

Numerical Study of a Flap Rudder Based on Turbulence Model LES

The flap rudder flow field at Re 1.4×106is calculated using computational fluid dynamics (CFD) method. Large eddy simulation (LES) turbulence model is adopted. The results shows that boundary layer separation phenomenon appears in flap after the gap, and as angle of flap (AOF) is increased, the separation tends to asymmetry, the pressure fluctuation range is becomes higher. There is no eddy in gap when AOF is 0°, but as AOF increases, the eddy in gap appears because of the pressure difference between two sides of the rudder.

Three-dimensional numerical simulation of intake model with cross flow

The hydrodynamics of a pump sump consisting of a main channel, pump sump, and intake pipe is examined using Truchas, a three-dimensional Navier-Stokes solver, with a Large Eddy Simulation (LES) turbulence model. The numerical results of streamwise velocity profiles and flow patterns are discussed and compared with experimental data of Ansar and Nakato. Fairly good agreement is obtained. Furthermore, unlike Ansar et al.'s inviscid solution, the proposed numerical model includes the effect of fluid viscosity and considers more realistic simulation conditions. Simulation results show that viscosity affects the prediction of flow patterns and that the streamwise velocity can be better captured by including cross flow. The effects of the submergence Froude number on the free surface and streamwise velocity are also examined. The free surface significantly fluctuates at high submergence Froude number flows and the corresponding distribution of streamwise velocity profiles exhibits a trend different from that obtained for low submergence Froude number flows.

Lattice Boltzmann large eddy simulation of subcritical flows around a sphere on non-uniform grids

In this work, the suitability of the lattice Boltzmann method is evaluated for the simulation of subcritical turbulent flows around a sphere. Special measures are taken to reduce the computational cost without sacrificing the accuracy of the method. A large eddy simulation turbulence model is employed to allow efficient simulation of resolved flow structures on non-uniform computational meshes. In the vicinity of solid walls, where the flow is governed by the presence of a thin boundary layer, local grid-refinement is employed in order to capture the fine structures of the flow. In the test case considered, reference values for the drag force in the Reynolds number range from 2000 to 10 000 and for the surface pressure distribution and the angle of separation at a Reynolds number of 10 000 could be quantitatively reproduced. A parallel efficiency of 80% was obtained on an Opteron cluster.

Measurement and analysis of turbulent liquid metal flow in a high-power spallation neutron source—EURISOL

The European Isotope Separation On- Line (EURISOL) design study completed in 2009 examined means of producing exotic nuclei for fundamental research. One of the critical components identified in the study was a high-power neutron spallation source in which a target material is impacted by a proton beam producing neutrons by a process known as spallation. Due to the high heat power deposition, liquid metal, in this case mercury, is the only viable choice as target material. Complex issues arise from the use of liquid metal. It is characterised by an unusually low Prandtl number and a higher thermal expansivity than conventional fluids. The turbulence structure in LM is thereby affected and still an object of intense research, hampered in part by measurement difficulties. The use of Computational Fluid Dynamics (CFD) allowed a satisfactory design for the neutron source to be found rapidly with little iteration. However it was feared that the development of the boundary layer and associated turbulence would not be correctly represented by the CFD since the codes were developed for standard fluids. Therefore, an experiment and associated measurements were carried out to assess the reliability of the computations and to validate the design of the target. System level measurements such as cavitation noise, target structural vibrations and cover gas pressure were recorded as local pressure fluctuations in the fluid. In this manner, both the macroscopic effect of the turbulence on the target structure, and the liquid metal turbulence itself could be measured and serve as a benchmark for assessing the reliability of the computations. The current work shows how the Shear Stress Transfer (SST) and Large Eddy Simulation (LES) turbulence models were used to simulate the liquid metal flow. LES was proved more accurate than SST in predicting large and small turbulent structures in the liquid. The prediction of the turbulence intensity generated by large eddy structures in the fluid proved to be in agreement with experimental data at frequencies lower than 40 Hz and at speeds up to 6 m/s in the liquid metal.

A Numerical Simulation of a Tidal Bore Flow

A numerical model was developed based on the Cubic-Interpolated Pseudo-particle (CIP) Combined Unified Procedure (CIP-CUP or C-CUP) method equipped with a Large Eddy Simulation (LES) model and a re-initialization method. The model was validated and applied to the tidal bore flow with a weak breaking front. In the tidal bore flow, the model equipped with a LES turbulence model reproduced accurately the large deformation of the free surface during the bore formation by rapid closure of a downstream gate. The free- surface profile and surge front celerity data were in good agreement with the experimental data of [Koch and Chanson 2009, J. Hyd. Res. IAHR]. At a fixed sampling location, the numerical results showed the existence of some short-lived flow reversal next to the bed immediately after the bore front passage. This flow feature was documented by [Koch and Chanson 2009] and [Chanson 2009, Eur. J. Mech. B. Fluids].

CFD Modeling of Dense Medium Cyclone

A number of empirical models are in existence for the dense medium cyclone (DMC) and in recent years this subject has been broached with computational fluid dynamics (CFD). The dense medium presents a centrifugal field within the cyclone body. The coal particles separate in this field due to various forces acting on them. Hence, CFD is ideally suited for the modeling of the DMC. The Large Eddy Simulation (LES) method for resolving the turbulence was used in the CFD simulation of a 76 mm dense medium cyclone. In particular, the magnetite was modeled as three granular fluids. In the simulation the diameter of the vortex finder and spigot are varied to compare with the experimental data of P. A. Verghese and T. C. Rao. The results obtained using LES turbulence model is found to be accurate in terms of the cut density and the slope of the distribution curves. Thus, the three granular fluid modeling of the magnetite stream is a computationally simpler method for the analysis of DMC.

Numerical simulation of flow‐induced noise using LES/SAS and Lighthill's acoustic analogy

AbstractWe present a finite element (FE) formulation of Lighthill's acoustic analogy for the hybrid computation of noise generated by turbulent flows. In the present approach, the flow field is computed using large eddy simulation and scale adaptive simulation turbulence models. The acoustic propagation is obtained by solving the variational formulation of Lighthill's acoustic analogy with the FE method. In order to preserve the acoustic energy, we compute the inhomogeneous part of Lighthill's wave equation by applying the FE formulation on the fine flow grid. The resulting acoustic nodal loads are then conservatively interpolated to the coarser acoustic grid. Subsequently, the radiated acoustic field can be solved in both time and frequency domains. In the latter case, an enhanced perfectly matched layer technique is employed, allowing one to truncate the computational domain in the acoustic near field, without compromising the numerical solution. Our hybrid approach is validated by comparing the numerical results of the acoustic field induced by a corotating vortex pair with the corresponding analytical solution. To demonstrate the applicability of our scheme, we present full 3D numerical results for the computed acoustic field generated by the turbulent flow around square cylinder geometries. The sound pressure levels obtained compare well with measured values. Copyright © 2009 John Wiley & Sons, Ltd.

Investigation of flow and temperature patterns in direct contact condensation using PIV, PLIF and CFD

In this study, experiments have been performed for the steam injected centrally at the bottom of a vertical rectangular water vessel. Instantaneous velocity and temperature field near the plume as well as in the downstream have been measured in a vertical plane through the central axis. For this purpose, Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF) have been employed. The velocity profile of the region above the condensation region was found to be self-similar with a small downward velocity near the wall due to recirculation. The instantaneous (20-ns integration time) thermal images obtained from PLIF had a spatial resolution of 100 μm with a field of view of 100×100 mm. The time averaged velocity and temperature profiles are computed from an ensemble of 100 velocity/temperature images. The present work is also concerned with CFD simulation by employing k- ε and large eddy simulation (LES) turbulence models. All the measurements and simulations were carried out by varying nozzle upstream pressure in the range of 0.3–0.35 MPa (corresponding nozzle velocities were in the range of 286–304 m/s) with the nozzle diameter of 1 mm.

An Analytical/Computational Approach in Assessing Vortex-Induced Vibration of a Variable Tension Riser

The transverse vibratory response of a long, slender vertical top tension riser, subject to an ocean current, is studied. The problem is treated as a coupled fluid flow/vibration problem, which is solved numerically. The fluid flow part is represented by the 2D Navier–Stokes equations, with large-eddy simulation turbulence modeling and strip theory, which are solved numerically to obtain the flow field and determine the vortex-shedding behavior in the flow. The approach flow is a shear flow ranging in Reynolds number from 8000 to 10,000. Given the flow field and vortex-shedding behavior, the transverse fluid forcing function can be determined at a given instant, which becomes the input to the Euler–Bernoulli beam equation to calculate the displacement of the riser, using a technique that involves the Wentzel–Kramers–Brillouin (WKB) method and modal decomposition. The boundary conditions for the fluid flow equations are updated each time step as the cylinder moves. The natural frequency of the riser is tension dominated, not bending-stiffness dominated. With the decrease in tension with increasing depth, the natural frequency is affected. Therefore, the solution will be influenced by the depth-dependent tension. This study has indicated some interesting features regarding the vortex-induced vibration of a variable-tension riser. The vibrational response is greater for a variable-tension riser than for a constant-tension riser, when the variable-tension riser is assumed to have the same top tension as the constant-tension riser. Thus, this is one reason why it is important to take into account the variable tension when estimating fatigue failures of marine risers.

Large eddy simulation of hot and cold fluids mixing in a T-junction for predicting thermal fluctuations

Temperature fluctuations in a mixing T-junction have been simulated on the FLUENT platform using the large eddy simulation (LES) turbulent flow model and a sub-grid scale Smagorinsky-Lilly model. The normalized mean and root mean square temperatures for describing time-averaged temperature and temperature fluctuation intensity, and the velocity are obtained. The power spectrum densities of temperature fluctuations, which are key parameters for thermal fatigue analysis and lifetime evaluation, are analyzed. Simulation results are consistent with experimental data published in the literature, showing that the LES is reliable. Several mixing processes under different conditions are simulated in order to analyze the effects of varying Reynolds number and Richardson number on the mixing course and thermal fluctuations.

Long-Term Stability Estimates and Existence of a Global Attractor in a Finite Element Approximation of the Navier–Stokes Equations with Numerical Subgrid Scale Modeling

Variational multiscale methods lead to stable finite element approximations of the Navier–Stokes equations, dealing with both the indefinite nature of the system (pressure stability) and the velocity stability loss for high Reynolds numbers. These methods enrich the Galerkin formulation with a subgrid component that is modeled. In fact, the effect of the subgrid scale on the captured scales has been proved to dissipate the proper amount of energy needed to approximate the correct energy spectrum. Thus, they also act as effective large-eddy simulation turbulence models and allow one to compute flows without the need to capture all the scales in the system. In this article, we consider a dynamic subgrid model that enforces the subgrid component to be orthogonal to the finite element space in the $L^2$ sense. We analyze the long-term behavior of the algorithm, proving the existence of appropriate absorbing sets and a compact global attractor. The improvements with respect to a finite element Galerkin approximation are the long-term estimates for the subgrid component, which are translated to effective pressure and velocity stability. Thus, the stabilization introduced by the subgrid model into the finite element problem does not deteriorate for infinite time intervals of computation.

A stress transport equation model for simulating turbulence at any mesh resolution

Prior work has demonstrated the effectiveness of using two-equation closures as the basis for universal, self-adapting turbulence models that are effective at any mesh resolution (Perot and Gadebusch in Phys. Fluids 19:115105, 2007). In order to demonstrate the broad applicability of the fundamental approach, the same behavior is now demonstrated for a second-moment closure (SMC). The SMC has the advantage over the earlier two-equation universal closure of being more accurate in the coarse mesh limit and of having a natural mechanism for backscattering energy from the modeled to the resolved turbulent fluctuations. The mathematical explanation for why Reynolds averaged (RANS) transport equation closures are applicable at any mesh resolution, including the large eddy simulation (LES) regime, is reviewed. It is demonstrated that for the problem of isotropic decaying turbulence, the SMC model produces good predictions at any mesh resolution and with arbitrary initial conditions. In addition, it is shown that the proposed model automatically adapts to the mesh resolution provided. The self-adaptive nature of the method is clearly observed when different initial conditions are used. It is shown that classic RANS models (often thought to produce steady and smooth solutions) can produce three-dimensional, unsteady, and chaotic solutions when generalized correctly and when provided with sufficient mesh resolution. The implications of these observations on the fundamental theories of RANS and LES turbulence modeling are discussed.

Three-Dimensional Numerical Simulations of Flows Past Smooth and Rough/Bare and Helically Straked Circular Cylinders Allowed to Undergo Two Degree-of-Freedom Motions

We simulate the flow past smooth and rough rigid circular cylinders that are either bare or outfitted with helical strakes. We consider operating conditions that correspond to high Reynolds numbers of 105 and 106, and allow for two degree-of-freedom motions such that the structure is allowed to respond to flow-induced cross-flow and in-line forces. The computations are performed using a parallelized Navier–Stokes in-house solver using overset grids. For smooth surface simulations at a Reynolds number of 105, we use a Smagorinsky large eddy simulation turbulence model and for the Reynolds number cases of 106 we make use of the unsteady Reynolds-averaged Navier–Stokes equations with a two-layer k-epsilon turbulence model. The rough surface modifications of the two-layer k-epsilon turbulence model due to Durbin et al. (2001, “Rough Wall Modification of Two-Layer k-Epsilon,” ASME J. Fluids Eng., 123, pp. 16–21) are implemented to account for surface roughness effects. In all our computations we aim to resolve the boundary layer directly by using adequate grid spacing in the near-wall region. The predicted global flow parameters under different surface conditions are in good agreement with experimental data, and significant vortex-induced vibration suppression is observed when using helically straked cylinders.

Contribution to the modeling of the interaction between a plasma flow and a liquid jet

A numerical simulation of the interaction between a plasma flow and a liquid jet was investigated, leading to the proposal of a compressible model, based on augmented Lagrangian, Large Eddy Simulation (LES) turbulence modeling and Volume of Fluid (VOF) approaches, capable of managing incompressible two-phase flows as well as turbulent compressible motions. The VOF method utilized volume markers to advect the local concentration of gas and liquid in a Eulerian manner. The numerical model was validated on single-phase plasma configurations as well as two-phase cross flow liquid jet interactions. Finally, an example of the first simulations of the interactions between a liquid jet and a plasma is presented. However improvements should be realized as to increase the speed of the VOF-SM algorithm or add specific subgrid models related to jet fragmentation and phase change.

Diesel combustion modelling using LES turbulence model with detailed chemistry

Diesel spray combustion and emissions are modelled in this study using large eddy simulation (LES) turbulence models coupled with spray breakup and detailed chemistry models. The objective of this study is to develop numerical models that can be used to predict the diesel spray combustion process. The LES filtering procedure yields unknown subgrid scale terms that need to be modelled. A one-equation, non-viscosity dynamic structure model is used to model the subgrid stress tensor and the gradient method is used to model the subgrid scalar flux. The model uses a tensor coefficient determined by a dynamic procedure and the sub-grid kinetic energy that is to enforce a budget on the energy flow between the resolved and unresolved scales. A skeletal n-heptane reaction mechanism is used to simulate the diesel fuel chemistry. A reduced NO x mechanism is incorporated into the n-heptane mechanism to simulate NO x formation, and the soot emission process is simulated by a phenomenological model that uses a competing formation and oxidation rate formulation. The present models were validated by comparing predicted results against experimental data of a diesel engine. The predicted and measured cylinder pressure history and heat release rate data are in good agreement. Trends of NO x and soot emissions are also captured with respect to different injection timings and exhaust gas recirculation (EGR) levels. Results indicated that the present models can capture the overall combustion process and can be further developed into a useful tool for engine combustion simulations.

Computational model of airflow in upper 17 generations of human respiratory tract

Computational fluid dynamics (CFD) studies of airflow in a digital reference model of the 17-generation airway (bronchial tree) were accomplished using the FLUENT ® computational code, based on the anatomical model by Schmidt et al. [2004. A digital reference model of the human bronchial tree. Computerized Medical Imaging and Graphics 28, 203–211]. The lung model consists of 6.744×10 6 unstructured tetrahedral computational cells. A steady-state airflow rate of 28.3 L/min was used to simulate the transient turbulent flow regime using a large eddy simulation (LES) turbulence model. This CFD mesh represents the anatomically realistic asymmetrical branching pattern of the larger airways. It is demonstrated that the nature of the secondary vortical flows, which develop in such asymmetric airways, varies with the specific anatomical characteristics of the branching conduits.

Fire Dynamics Simulator (Version 4.0) Simulation for Tunnel Fire Scenarios with Forced, Transient, Longitudinal Ventilation Flows

In this study, a series of sensitivity analyses were conducted to evaluate a computational fluid dynamic (CFD) model, Fire Dynamics Simulator (FDS) version 4.0, for tunnel fire simulations. A tunnel fire test with a fire size on the order of a 100 MW with forced, time-varying longitudinal ventilation was chosen from the Memorial Tunnel Ventilation Test Program (MTVTP) after considering recent tunnel fire accidents and the use of CFD models in practice. A careful study of grid size and parameters used in the Large Eddy Simulation (LES) turbulence model—turbulent Prandtl number, turbulent Schmidt number, and Smagorinsky constant—was conducted. More detailed analyses were performed to refine the smoke layer prediction of FDS, especially on backflow (i.e., a reversed smoke flow near the ceiling). Also, energy conservation was checked for this scenario in FDS. A simple guideline is given for smoke layer simulations using FDS for similar tunnel fire scenarios.

Comparison of Computational Fluid Dynamics Approaches for Simulating Weapons Bay Flow

Comparison of Computational Fluid Dynamics Approaches for Simulating Weapons Bay Flow