Ibrahim Yavuz

Index of Resolution Quality for Large Eddy Simulations

In the light of rapidly increasing applications of large-eddy simulations (LES), it is deemed necessary to impose some quality assessment measures for such studies. The verification of LES calculations is difficult because of the fact that both the subgrid scale (SGS) model contribution and numerical discretization errors are functions of the grid resolution. In this study, various indexes of quality measures, hereafter referred to as LES_IQ, are proposed. The recommended LES_IQ is based on the Richardson extrapolation concept. This method has been applied to various cases and the calculated LES_IQ results are compared with the relative total experimental and direct numerical simulation (DNS) error, defined as IQ_EX and IQ_DNS, respectively. It is postulated that in practical applications of LES, numerical dissipation will always be a significant part of the overall dissipation, and it must be accounted for in any assessment of the quality of LES

Large eddy simulations of in-cylinder turbulence for internal combustion engines: A review

Abstract While engineering applications of the large eddy simulation (LES) technique are becoming a common reality in many branches of engineering and science, its application to engine flows has lagged behind due to the relatively more complex nature of both the flow and the geometry relevant to in-cylinder flows. In this paper a review of the limited number of LES applications to engine flows is given, and most significant results from these studies are presented. Also, the LES formulation appropriate for engine applications is briefly described, along with the main characteristics of in-cylinder flows. As expected, engine applications of LES are not of the so-called ‘high-fidelity’ type, but rather they employ formally second-order accurate numerical schemes in conjunction with finite volume formulation. The subgrid scale (SGS) models used are also kept as simple as possible, mostly using a variant of the Smagorinsky model. Nevertheless, this review reveals that even with relatively coarse grids, LES captures much more interesting features of in-cylinder flows, such as the large coherent vortical flow structures developed during the intake stroke. In the opinion of the present authors, a low-resolution LES provides a better solution than RANS (Reynolds averaged Navier-Stokes) with moderate grid resolution because the important features of flow dynamics cannot be reproduced in RANS due to the high level of non-physical diffusion. Of course, some overheads in computational costs must be paid for this benefit obtained from LES.

Prediction of In-Cylinder Turbulence for IC Engines

Abstract This paper presents the preliminary results of some of a few of its kind efforts in large eddy simulation (LES) of engine flows to predict turbulent fluctuations, and the statistics of turbulence quantities inside IC engine cylinders. For this purpose, the well-known engine simulation code, KIVA, is used with special precautions to keep the numerical accuracy at a sufficiently high level, as well as using relatively fine grid resolution. The capabilities of this code are tested against benchmark cases, such as lid-driven cavity flow, and swirling and non-swirling free jet flows. It is then applied to a typical engine geometry under motored conditions. In particular, turbulence generated during the intake stroke, and the instabilities induced by a typical piston-bowl assembly are investigated. The computed velocity fluctuations, correlation coefficients and energy spectra of turbulent fluctuations are compared to experimental results. The predictions seem to extend well into the inertial range of turbulence and depict a good qualitative agreement with measurements. The results also shed light into the mechanisms by which turbulence may be generated by the piston-bowl assembly.

Predicting Worker Exposure—The Effect of Ventilation Velocity, Free-Stream Turbulence and Thermal Condition

Three-dimensional computational fluid dynamics (CFD) simulations were used to predict the flow field and resulting worker exposures when toxic airborne contaminants were released into the wake region of a mannequin that had its back to the airflow while holding the source of airborne contaminants. The effects of ventilation velocity, free-stream turbulence, and various thermal conditions on fluid flow and exposure levels were evaluated. The results showed good agreement between predicted and experimental concentrations at the mouth at a broad range of airflow velocities when the mannequin was both heated and unheated. When the mannequin was unheated, the exposure level decreased as the ventilation velocity increased. The expectation that buoyancy provided by the heat from the mannequin would be most important at very low velocities and decreasingly important at high velocities was proved true for both the predicted and observed exposures. The result was that when the mannequin was heated to normal human body temperatures, exposure levels had an inverted V relationship with velocity. These findings are important, since they call into question the common practice of modeling human exposures with mannequins at ambient temperatures. In addition, free-stream turbulence could be used to reduce worker exposure to airborne pollutants as suggested by the simulations. CFD enabled a detailed investigation of the effect of particular factors for exposure predictions in a cost-effective way.

A Numerical Study of Worker Exposure to a Gaseous Contaminant: Variations on Body Shape and Scalar Transport Model

Abstract Three-dimensional computational fluid dynamics simulations are used to investigate the distribution and level of contaminant concentrations in the true breathing zone (at the nose and mouth) when toxic airborne contaminants are released within an arms length in front of the worker who has his back to the airflow. The effects of different body shapes on fluid flow and concentration patterns around the body in a wind tunnel were evaluated and clarified that a sharp body or a block may not be a good surrogate for the human form in consideration of occupational and environmental health studies. The comparison of the concentration field calculated with the Eulerian and Lagrangian methods revealed that the Eulerian method has a more diffusive nature than the Lagrangian method. The concentrations at different locations were also compared to determine the optimum sampling location. It was found that the concentration at the breathing zone may be significantly different from the one at the chest area. The influence of the heat flux from the body was studied at two different Reynolds numbers. Predictions indicate that the heat flux may have a significant impact on exposure especially when the convection induced by buoyancy dominates the flow.

Index of Quality for Large Eddy Simulations

In the light of rapidly increasing applications of large-eddy-simulations (LES) it is deemed necessary to impose some quality assessment measures for such studies. The validation of LES is difficult because of the fact that both the sub-grid scale (sgs) model contribution and numerical discretization errors are functions of the grid resolution. In this study various index of quality measures, here and after referred to as LES_IQ, are proposed and applied to some case studies. The recommended LES_IQ is based on the concept of Richardson’s extrapolation. It is postulated that in practical applications of LES numerical dissipation will always be a significant part of the overall dissipation and it must be accounted for in any assessment of the quality of LES results. It is further suggested that an LES_IQ of 75% to 85% can be considered adequate for most engineering applications that typically occur at high Reynolds numbers; LES-IQ greater than 90% can be classified as DNS.Copyright

Modeling of Effect of Inflow Turbulence Data on Large Eddy Simulation of Circular Cylinder Flows

A random flow generation (RFG) algorithm for a previously established large eddy simulation (LES) code is successfully incorporated into a finite element fluid flow solver to generate the required inflow/initial turbulence boundary conditions for the three-dimensional (3D) LES computations of viscous incompressible turbulent flow over a nominally two-dimensional (2D) circular cylinder at Reynolds number of 140,000. The effect of generated turbulent inflow boundary conditions on the near wake flow and the shear layer and on the prediction of integral flow parameters is studied based on long time average results. Because the near-wall region cannot be resolved for high Reynolds number flows, no-flip velocity boundary function is used, but wall effects are taken into consideration with a near-wall modeling methodology that comprises the no-slip function with a modified form of van Driest damping approach to reduce the subgrid length scale in the vicinity of the cylinder wall. Simulations are performed for a 2D and a 3D configuration, and the simulation results are compared to each other and to the experimental data for different turbulent inflow boundary conditions with varying degree of inflow turbulence to assess the functionality of the RFG algorithm for the present LES code and, hence, its influence on the vortex shedding mechanism and the resulting flow field predictions.

Effect of Channel Aspect Ratio on Planar SOFC Performance

The optimization process is in general an important issue to show the viability of solid oxide fuel cells (SOFCs) compared to traditional power sources. This optimization process can be done in a faster and cheaper way by making use of numerical simulations. In this study, three-dimensional, non-isothermal, steady state numerical simulations of planar solid oxide fuel cells (SOFC) are performed using the commercial FLUENT software. First, a detailed analysis of grid and iteration-dependent simulations is performed. This analysis predicts a 20% difference between a coarse and fine grid in the velocity magnitude in both anode and cathode gas flow channels, and in the y-component of current density. Then, the performance of a planar SOFC with changing channel aspect ratio is analyzed comparing their V-I curves and critical parameters like temperature, concentration, and current density distributions. The predictions show a 12 degrees difference in temperature at the fuel exhaust between low and high aspect ratio channel simulations. These results suggest that the channel aspect ratio is a significant parameter, worthwhile to be investigated.Copyright

The Effect of Turbulence and Scalar Transport Models on Prediction of Worker Exposure to Aerosols

The present study is concerned with the relative concentrations of contaminants in the breathing-zone (BZ) when toxic airborne contaminants are released within an armlength in front of a worker, a common location that often leads to high exposures. Three-dimensional turbulent flow around a modeled human body in a wind tunnel has been simulated numerically and the results from various models are compared with each other. A model was built using a mesh generator to represent a real human body as closely as possible. Four different turbulence models, namely, standard k-e turbulence model, RNG k-e turbulence model, Reynolds Stress turbulence model (RSM) and Large Eddy Simulation (LES), were used to evaluate their effect on the predicted concentration levels used in assessing the exposure of the worker. Results from Eulerian scalar transport method and Lagrangian particle tracking method are also compared. Concentration levels are calculated at various sampling locations in the vicinity of the human face. It is found that the predicted concentration varies significantly from model to model: at the breathing zone the coefficient of variation for predicted concentration is 30%.Copyright

A Simple Model for Fluid Flow and Particle Motion Inside the Human Larynx

This study aims to develop a simple and quick, but sufficiently accurate solution method for calculating the air flow and tracking the particles in a complex tubular system, where the flow changes its magnitude and direction in a periodic manner. The flow field is assumed to be quasi-two-dimensional and a pressure-correction method is employed to calculate the spetio-temporal variation of the air velocity inside the larynx. Then, the calculated one-dimensional flow distribution is used to reconstruct a two-dimensional flow field is constructed based on the average velocity along the axial direction. The system geometry is taken as close as possible to the actual larynx for an average person with an average glottis opening. For the current study the walls of the larynx is approximated as rigid walls, but different ways to account for compliant walls are proposed within the context of the one-dimensional mode. The 1-D transient model is validated against a two-dimensional model using a verified commercial code. Particles are introduced into the system and tracked during every time fraction of the respiratory cycle. Then, the histograms of particles that come into contact with the larynx are calculated, and regions with a higher probability for particle deposition are identified.Copyright