Eddy Simulation Of Turbulent Flow Research Articles

Large Eddy Simulation of turbulent flow and heat transfer in a Kenics static mixer

Large Eddy Simulation of turbulent flow and heat transfer in a Kenics static mixer

Direct- and Large Eddy Simulations of Turbulent Flow in CS0 Diffuser on Resolved and Under-resolved Meshes

Direct- and Large Eddy Simulations of Turbulent Flow in CS0 Diffuser on Resolved and Under-resolved Meshes

Large eddy simulations of turbulent flow based on a removal of energy production through nonlinear interactions

In large eddy simulations (LES), nonlinear dispersive subgrid scale (SGS) models are usually found to be not sufficiently dissipative. In this work, a new model is proposed to directly remove the unphysical energy accumulation due to the nonlinear terms. The model can be obtained from two approaches, either based on the energy transfer in multi-level scales or derived from the kinetic energy equation. Moreover, a gradient-type modification is included to ensure Galilean invariance and generate extra dissipative effect, which is found to have negligible impact on the ideal energy removal. We compare the present model with other structural models and regularization techniques both theoretically and numerically. We show that because of the reduction of energy production at desired scales, the collective effect of our new model was able to provide sufficient SGS transfer in energy cascade without the help of extra dissipative terms. The scale separation is facilitated by a smooth low-pass filter, which becomes increasingly more active for higher wavenumbers. Since filtering already takes the grid size into account, the model is capable of consistently produce accurate results for all the test cases of wall-bounded turbulence. We believe our results further emphasized the importance of attenuating the energy pileup in LES and showed that it is possible to overcome the issue through a simple but effective modification of the nonlinear term.

Large eddy simulation of turbulent flow and heat transfer in a turbine disc cavity with impellers

The turbine disc cavity is an important component of a gas turbine, supplying airflow to the turbine blades and the sealing labyrinth. Impellers are installed in the cavity to increase pressure, but their mechanism was unknown in previous studies. Large eddy simulations were used in this study to reveal the effect of the impeller on turbulent flow and heat transfer in a turbine disc cavity. In addition, a nonlinear integral model capable of predicting velocity, pressure, and temperature was developed, which takes into account the heat transfer on the disc walls. This was the first engineering application of nonlinear solution of Ekman layer equations. The large eddy simulation results demonstrated that the vortex development is suppressed by the impeller, and the vortex in the impeller channel flows rapidly to the upstream disc, causing the Nusselt number on the upstream and downstream discs to increase and decrease, respectively. The turbulent heat flux in the central region of the cavity was low due to the destruction of the wall jet. The nonlinear integral model was used to calculate a working condition in <1 s, indicating that it has great potential for engineering applications.

Pore-scale large eddy simulation of turbulent flow and heat transfer over porous media

• Fluid flow and heat transfer in a composite porous-fluid system is explored using LES. • Portion of the fluid entering to the porous region leaks to the non-porous region. • Counter-rotating vortex pairs is observed within and above the porous region. • Contour of temperature shows Kelvin-Helmholtz instabilities above the interface. • In contrast to the solid block, flow laminarisation occurs on the porous block. This paper investigates turbulent fluid flow and heat transfer over a porous medium in a channel using pore-scale large eddy simulation. Special attention is placed on the exchange of heat and flow between the porous and non-porous regions through the interface between the two regions. For this purpose, two different porous systems made of a packed bed of spheres and rectangular rods are analysed and the results are compared against a solid block case of the same size. Flow visualization shows that a significant portion of the fluid entering the porous blocks leaks from the porous region to the non-porous region through the porous-fluid interface. To discuss the effects of this flow leakage on the flow features and heat transfer, discussions are made regarding velocity, pressure, and temperature fields, as well as coherent structures, and turbulence production. The flow pattern inside the porous region indicates that the flow leakage clogs the pore channels inside the porous medium which induces a significant reduction in the streamwise momentum of the pore flow. In addition, coherent structures show that flow leakage leads to the creation of counter-rotating vortex pairs of fluid flow within and above the porous block that results in the formation of organized hairpin structures. Finally, the comparison of turbulence production for the porous and solid cases together with the onset growth of the Kelvin-Helmholtz instability on the porous-fluid interface show a reduction in turbulent kinetic energy above the leading edge of porous blocks. This observation implies that for the porous cases the transition to turbulence is postponed to the downstream of the porous block and it is not achieved as fast as the solid block.

A Differential Subgrid Stress Model and Its Assessment in Large Eddy Simulations of Non-Premixed Turbulent Combustion

We present a new subgrid stress model for the large eddy simulation of turbulent flows based on the solution of transport equations for stress tensor components. The model was a priori term-by-term calibrated against an open DNS database on forced isotropic turbulence (Johns Hopkins University database). After that, it was applied in a large eddy simulation of non-premixed turbulent combustion. To demonstrate the impact of the new subgrid stress model on scalar fields, we excluded the backward effect of heat release on the subgrid stresses, considering an isothermal reaction (i.e., diluted mixture; the density variations associated with chemical heat release can be neglected) and a Burke–Schumann reaction sheet approximation. A periodic box filled with a homogeneous turbulent velocity field and a three-layer top-hat mixture fraction field was studied. Four simulations were performed in which a fixed model for mixture fraction and its variance was combined with either the proposed subgrid stress model or one of the standard models, including Smagorinsky, dynamic Smagorinsky and WALE. Qualitatively correct backscatter was observed in a simulation with the new model. The differences in the statistics of the mixture fraction and reactive component fields caused by the new subgrid stress model were analyzed and discussed. The importance of using an advanced subgrid stress model was highlighted.

Wall‐resolved large eddy simulation of turbulent flows in helically ribbed steam cracker reactors

AbstractHelically ribbed coils are commonly applied in steam cracking furnaces. To fully understand the impact of these ribbed wall modifications on the local heat transfer and associated pressure drop throughout the reactor, detailed experimental, and numerical studies have been performed. Experimental data based on stereo‐particle image velocimetry (S‐PIV) and liquid crystal thermography have been used to validate the numerical results from wall‐resolved large eddy simulations using OpenFOAM. The validation shows an excellent agreement in terms of mean and fluctuating velocities, pressure drop, and heat transfer behavior in a discontinuously ribbed tube. Compared with the pressure drop in a continuously ribbed tube, an approximately 40% lower pressure drop is obtained with a discontinuously ribbed tube, at the cost of a slightly decreased heat transfer enhancement. This makes the discontinuously ribbed tube design particularly interesting for steam cracking applications. The results also show that the nonuniform heat transfer at the wall is inherently linked to the flow reattachment and recirculation zones caused by the rib. Finally, the validated numerical model was used to study comparable designs and propose novel helical rib designs. Based on the results of the study, enlarging the trailing edge of the conventional ribbed geometry will improve the thermal enhancement performance, and is therefore found most promising for steam cracking reactor design.

Invariant data-driven subgrid stress modeling in the strain-rate eigenframe for large eddy simulation

We present a new approach for constructing data-driven subgrid stress models for large eddy simulation of turbulent flows. The key to our approach is representation of model input and output tensors in the filtered strain rate eigenframe. Provided inputs and outputs are selected and non-dimensionalized in a suitable manner, this yields a model form that is symmetric, Galilean invariant, rotationally invariant, reflectionally invariant, and unit invariant. We use this model form to train a simple and efficient neural network model using only one time step of filtered direct numerical simulation data from a forced homogeneous isotropic turbulence simulation. We demonstrate the accuracy of this model as well as the model’s ability to generalize to previously unseen filter widths, Reynolds numbers, and flow physics using a priori and a posteriori tests.

Large eddy simulation of turbulent flow of pseudoplastic and dilatant fluids: turbulence characteristics

Large eddy simulation of turbulent flow of pseudoplastic and dilatant fluids: turbulence characteristics

Large eddy simulation of turbulent flow of pseudoplastic and dilatant fluids: rheological and hydrodynamic behaviour

Large eddy simulation of turbulent flow of pseudoplastic and dilatant fluids: rheological and hydrodynamic behaviour

Large Eddy Simulation of Turbulent Flow and Inclusion Transport during Filtration of Liquid Steel inside Open‐Cell Ceramic Foams

The present numerical investigation is conducted to improve the understanding of the physical processes, encountered during the filtration of steel melt. For this purpose, the process conditions in a pouring experiment are considered. The effects of increasing either the superficial melt velocity or the pore count of the filter are studied. The turbulent flow is modeled using the large eddy simulation in the context of the lattice Boltzmann method and the motion of nonmetallic inclusions is predicted using discrete Lagrangian particle tracking. The simulations show that turbulent flow develops as the melt advances through the filter, with the length scale of the smallest flow structures lying in the same order as the particle size. The pressure drop and the filtration coefficient are found to agree with the previous studies.

Implicit Large Eddy simulations of turbulent flow around a square cylinder at [formula omitted

In this paper, the Implicit Large-Eddy Simulation (ILES) is investigated on the flow around a square cylinder incorporating an unstructured Weighted Essential Non-Oscillatory (WENO) reconstruction method for a Reynolds number of 22,000. Simulations are undertaken in the framework of open-source package OpenFOAM and additional implicit 2nd/3rd-order WENO scheme on the convective term of the viscous incompressible Navier-Stokes Equations. A 2nd-order Euler implicit time integration and Pressure-Implicit Splitting-Operator (PISO) algorithm is used to for the pressure-velocity coupling. Conventional LES with Wall Adapting Local Eddy Viscosity (WALE) model is also carried out as a baseline. The results are compared to high fidelity experiment, DNS data and conventional LES with dynamic Smagorinsky model from previous work. Results show favorable performance for ILES with 3rd-order WENO scheme compared with the conventional LES with dynamic Smagorinsky model and similar performance against LES with WALE model. Results also show acceptable predictions over time-averaged statistics with less computational effort for the ILES of 2nd-order WENO scheme. Shear layer flow analysis suggests that both ILES and LES face similar challenges with small quantities, such as shear stress. Finally, simulations are capturing von Kármán vortex, Kelvin-Helmholtz instability and induced frequency changes.

Extension of an immersed boundary method for large eddy simulation of turbulent flows

In this paper, an immersed boundary method is extended to large eddy simulation of turbulent flows. For the interior nodes in the immediate vicinity of the immersed wall, some of their neighbouring nodes are in the solid phase, and the flow variables at these interior nodes cannot be calculated by solving the governing equations. In the present immersed boundary method, the flow variables at these nodes are determined via an approximate form of solution involving the boundary condition. A wall model based on the simplified turbulent boundary layer equations is introduced to alleviate the requirement of mesh resolution in the near-wall region. The wall shear stress prescribed by the wall modelling technique and the no-penetration condition are enforced at the immersed boundary to evaluate the velocity at an interior node in the immediate vicinity of the wall. A dynamic subgrid-scale model is adopted in the framework of the immersed boundary approach. Several numerical experiments have been conducted to verify the ability of the present method. The predicted results agree well with the published experimental or numerical data.

Large Eddy Simulation of Turbulent Flows Through Several Isosceles Triangle Ducts

Large Eddy Simulation of Turbulent Flows Through Several Isosceles Triangle Ducts

Extension of an immersed boundary method for large eddy simulation of turbulent flows

Extension of an immersed boundary method for large eddy simulation of turbulent flows

Synchronizing subgrid scale models of turbulence to data

Large eddy simulations of turbulent flows are powerful tools used in many engineering and geophysical settings. Choosing the right value of the free parameters for their subgrid scale models is a crucial task for which the current methods present several shortcomings. Using a technique called nudging, we show that large eddy simulations can synchronize with data coming from a high-resolution direct numerical simulation of homogeneous and isotropic turbulence. Furthermore, we found that the degree of synchronization is dependent on the value of the parameters of the subgrid scale models utilized, suggesting that nudging can be used as a way to select the best parameters for a model. For example, we show that for the Smagorinsky model, synchronization is optimal when its constant takes the usual value of 0.16. Analyzing synchronization dynamics puts the focus on reconstructing trajectories in phase space, contrary to traditional a posteriori tests of large eddy simulations where the statistics of the flows are compared. These results open up the possibility of utilizing non-statistical analysis in a posteriori tests of large eddy simulations.

Gabor mode enrichment in large eddy simulations of turbulent flow

Abstract

Investigation of a thin permeable layer effect on turbulent flow and passive scalar transport in a channel

Large eddy simulations of turbulent flow and passive scalar transport in a channel partially filled with an array of spherical particles, mimicking a layer of deposited material over a surface, are carried out to investigate the effect of turbulence mixing on the passive scalar transfer rate from the channel wall. It is observed that large turbulence structures generated at the permeable layer-fluid interface penetrate into the porous media and enhance the scalar transport from the channel wall. The small gap between the layer of particles and the wall also induces a channeling effect that implies turbulence mixing and high velocity near the wall resulting in an enhancement in scalar transport from the channel wall. The influence from the distance between the particles in one layer, particle diameter and the gap thickness on the passive scalar transport is also investigated. It is found that the highest Stanton number is derived for the largest distance between particles since this facilitates the motion of the large turbulent structures into the porous layer. Such motions will affect the scalar transport from the channel wall into the bulk. The channeling effect is observed to be the most important parameter when the particle diameter and the gap thickness are varied, and the highest Stanton number is achieved for the cases with the most effective channeling effect.

A fractional subgrid-scale model for turbulent flows: Theoretical formulation and a priori study

Coherent structures/motions in turbulence inherently give rise to intermittent signals with sharp peaks, heavy-skirt, and skewed distributions of velocity increments, highlighting the non-Gaussian nature of turbulence. This suggests that the spatial nonlocal interactions cannot be ruled out of the turbulence physics. Furthermore, filtering the Navier–Stokes equations in the large eddy simulation of turbulent flows would further enhance the existing nonlocality, emerging in the corresponding subgrid scale fluid motions. This urges the development of new nonlocal closure models, which respect the corresponding non-Gaussian statistics of the subgrid stochastic motions. To this end and starting from the filtered Boltzmann equation, we model the corresponding equilibrium distribution function with a Lévy-stable distribution, leading to the proposed fractional-order modeling of subgrid-scale stresses. We approximate the filtered equilibrium distribution function with a power-law term and derive the corresponding filtered Navier–Stokes equations. Subsequently in our functional modeling, the divergence of subgrid-scale stresses emerges as a single-parameter fractional Laplacian, (−Δ)α(·), α ∈ (0, 1], of the filtered velocity field. The only model parameter, i.e., the fractional exponent, appears to be strictly dependent on the filter-width and the flow Reynolds number. We furthermore explore the main physical and mathematical properties of the proposed model under a set of mild conditions. Finally, the introduced model undergoes a priori evaluations based on the direct numerical simulation database of forced and decaying homogeneous isotropic turbulent flows at relatively high and moderate Reynolds numbers, respectively. Such analysis provides a comparative study of predictability and performance of the proposed fractional model.

Experiments and Stress-Blended Eddy Simulations of turbulent flow over edge-rounded dimples

Experiments and numerical simulations have been conducted to reveal the detailed turbulent flow and heat transfer characteristics over the surfaces with spherical dimples with different edge schemes, including the sharp edge, the fully rounded edge and the upstream rounded edge. The dimples have the same depth-to-diameter ratio of hd/d = 0.2. Steady-state heat transfer experiments were carried out for the turbulent flow over the three dimpled surfaces within the Reynolds number range of 10,000 ≤ ReDh ≤ 60,000, and the globally averaged heat transfer enhancements and friction factor ratios were obtained. Furthermore, transient liquid crystal thermography technique was used to obtain the local heat transfer characteristics on the dimpled surfaces. In addition, numerical simulations were carried out with a newly developed hybrid RANS-LES method of Stress-Blended Eddy Simulation (SBES) and the local heat transfer enhancement, detailed flow structure and turbulence statistics data were obtained and analyzed. The experimental results show that the best thermal performance is achieved by the dimples with upstream rounded edge, indicating an increase of about 11.4% in total heat transfer enhancement and an increase of about 5.2% in pressure loss when compared to the conventional dimples with sharp edge. The numerical results show that the flow over the dimpled surfaces can be divided into five zones: the mainstream zone, the recirculation zone, the Kelvin–Helmholtz vortex zone, the upwash zone and the rear separation zone, and each zone has its unique flow features. It is also found that the lowest pressure is achieved by the dimple with fully rounded edge, which is due to the absence of flow separation behind the rear edge of the dimple. The upstream-edge-rounded dimples show the reduced recirculation flow in the recirculation zone, the enhanced impingement and upwash flow in the upwash zone near the rear edge. These cause the stronger production of turbulent kinetic energy behind the rear edge, which are responsible for the higher thermal performance.