Browsing by Subject "LES"
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Item Advanced simulation and modeling of turbulent sprays(2014-03) Liu, WanjiaoSprays have wide applications in agriculture, pharmaceutical synthesis, engines, ink jet printing and so on. The successful spray applications and the control of spray param- eters require a thorough understanding towards the physical mechanisms. Numerical tools have been developed in the past few years for simulating the multiphase turbu- lent flows like sprays. Several researchers have successfully carried out direct numerical simulations (DNS) to investigate the primary breakup in such flows. DNS is accurate but requires extensive computational resources. In comparison, large eddy simulation (LES) is more practical, resolving only the large-scale flow structures and modeling the small-scale effects. The major difficulty with LES of multiphase turbulent flows is the need to model the interfacial subgrid-scale terms. Subgrid surface tension force, for ex- ample, plays an important role in the small droplet formation process. Subgrid surface tension force is, however, a highly non-linear term and can be difficult to model. In this research, we propose a new approach that combines the filtered density function (FDF) approach with the large eddy simulation. The major advantage of FDF is that the non-linear surface tension force appears in a closed form and thus needs no sub- grid modeling. The FDF transport equation is solved conveniently via a Lagrangian Monte-Carlo method. The Lagrangian approach is attractive in that it facilitates the transport of the liquid-gas interface without the diffusive or dispersive errors found in the Eulerian approaches. The surface tension source term in the momentum equation is closed using a Lagrangian volume of fluid (LVOF) approach. We utilize concepts from the smoothed particle hydrodynamics (SPH) in the LVOF approach to obtain the surface tension source term based on the Lagrangian particles. Several modifications have been made towards the original SPH formulation such that it is more suitable for the large-scale, turbulent multiphase flow simulations. Multiple particles are seeded in each Eulerian cell to achieve higher statistical accuracy, while the original SPH seeds one particle in each cell. What's more, a weighted SPH formula for the color function is adopted and is shown to be capable of handling variable particle number density. Performance assessment is via the rotation of Zalesak's disk and an oscillating elliptical droplet. Results show that the modified approach is able to handle the variable particle number density case appropriately. The simulations of multiphase turbulent flows are then performed with the proposed FDF-VOF methodology. At the same time, results from the simulations are compared with the DNS approach for validation and com- parison. Results show that the FDF-LES based approach can be a promising method, in that it models the flow with lower computational cost than DNS, yet maintaining accuracy in a model-free manor.Item A Cleaning System for Urban Air Pollution Removal(2019-11) Cao, QingfengAir pollution is a severe issue worldwide, which is adversely affecting the health and living environment of urban population. A novel outdoor air cleaner, named Solar-Assisted Large-Scale Cleaning System (SALSCS), was proposed as an innovative approach to facilitate the separation of particulate matter (PM) from atmospheric air by installing pleated filters in the system. This work first proposed the basic concept of SALSCS, which is composed of a flat-plate solar collector, a chimney, a filter bank, and fans if necessary. A three-dimensional numerical model of the system was developed by using the ANSYS Fluent fluid solver. The numerical results indicated that a full-scale system can generate a total flow rate of 2.64 × 105 m3 s-1 with the pressure drop of installed filter bank considered. In addition, the numerical model was applied to design a demonstration unit constructed in Xi’an with a tower of 60 m in height and a solar collector of 43 × 60 m2 in the horizontal directions. Field measurements were conducted and the obtained experimental data was used to validate the numerical model of SALSCS, which was further applied to investigate the performance characteristics of systems in the dimensional range of 10–120 m. The parameters that considerably influence the system performance have been identified. Meanwhile, atmospheric simulations over the terrain of Beijing were carried out by using the Weather Research and Forecasting (WRF) model to investigate the effectiveness of SALSCS for PM2.5 mitigation. A derived tendency term in the potential temperature equation was applied to simulate the buoyancy effect of SALSCS created with solar heating on its nearby atmosphere. PM2.5 pollutant and SALSCS clean air were simulated in the model domain by passive tracer scalars. Simulation conditions with two system flow rates of 2.64 × 105 m3 s-1 and 3.80 × 105 m3 s-1 were tested for seven air pollution episodes of Beijing during the winters of 2015–2017. The numerical results showed that with eight SALSCSs installed along the 6th Ring Road of the city, 11.2% and 14.6% of PM2.5 concentrations were reduced under the two flow-rate simulation conditions, respectively. To further improve the system’s effectiveness, an air cleaner with a reverse-flow configuration was proposed to be directly installed inside city blocks. An open-source large-eddy simulation (LES) model, called PALM, has been utilized to study the nearby atmospheric flow behavior and investigate its effectiveness in reducing air pollution. A method of incorporating the flow pattern of the air cleaner into the surrounding atmospheric flows was developed. A scenario of two systems operating together has also been investigated. The simulations illustrated that there is a clean air plume with higher turbulence intensity arising near the SALSCS providing a cleaner region within the polluted atmospheric flows. The numerical results indicated that as high as 60-100% of the nearby air pollution can be reduced, depending on its operating conditions and urban topographies. The filtration elements in SALSCS is one of the many applications of pleated filters. This dissertation also presents our experimental study on the flow fields of pleated filters by using the Particle Image Velocimetry (PIV) method. Flow patterns and pressure drop across pleated filters with various pleat configurations have been measured under our laboratory setup. It was found that the pleat geometry impacts the downstream flow pattern more significantly than the upstream pattern. The obtained downstream flow distributions indicated lower permeability at the pleat corner regions due to compression of the fibers. We discovered that when pleat geometry stays unchanged, similarity exists among downstream flow structures of pleated filters with different pleat numbers. These four studies below comprise parts of the main body of this dissertation and have already been published. Chapter 2: Q. Cao, D.Y.H. Pui, W. Lipiński. A concept of a novel Solar-Assisted Large-Scale Cleaning System (SALSCS) for urban air remediation. (2015). Aerosol. Air. Qual. Res. 15 (1), 1-10. Chapter 3: Q. Cao, T.H. Kuehn, L. Shen, S.-C. Chen, N. Zhang, Y. Huang, J. Cao, D.Y.H. Pui. Urban-scale SALSCS, part I: Experimental evaluation and numerical modeling of a demonstration unit. (2018). Aerosol. Air. Qual. Res. 18, 2865-2878. Chapter 4: Q. Cao, M. Huang, T.H. Kuehn, L. Shen, W.-Q. Tao, J. Cao, D.Y.H. Pui. Urban-scale SALSCS, part II: A parametric study of system performance. (2018). Aerosol. Air. Qual. Res. 18, 2879-2894. Chapter 5: Q. Cao, L. Shen, S.-C. Chen, D.Y.H. Pui. WRF modeling of PM2.5 remediation by SALSCS and its clean air flow over Beijing terrain. (2018). Sci. Total Environ. 626, 134-46.Item Control of jets in cross.(2010-07) Sau, RajesWe use direct numerical simulations to study control of jets in cross ow by axial pulsing. Our main idea is that pulsing generates vortex rings; the effect of pulsing on jets in crossflow can therefore be explained by studying the behavior of vortex rings in crossflow. A method is proposed that allows optimal values of pulsation frequency, modulation and energy to be estimated a priori. This is accomplished in the following three stages. First, direct numerical simulation is used to study the mixing of a passive scalar by a vortex ring issuing from a nozzle into stationary fluid. The ‘formation number’ (Gharib et al. 1998), is found to be 3.6. Simulations are performed for a range of stroke ratios encompassing the formation number, and the effect of stroke ratio on entrainment, and mixing is examined. When the stroke ratio is greater than the formation number, the resulting vortex ring with trailing column of fluid is shown to be less effective, at mixing and entrainment. As the ring forms, ambient fluid is entrained radially into the ring from the region outside the nozzle exit. This entrainment stops once the ring forms, and is absent in the trailing column. The rate of change of scalar containing fluid is studied for its dependence on stroke ratio. This rate varies linearly with stroke ratio until the formation number, and falls below the linear curve for stroke ratios greater than the formation number. This behavior is explained by considering the entrainment to be a combination of that due to the leading vortex ring, and that due to the trailing column. For stroke ratios less than the formation number, the trailing column is absent, and the size of the vortex ring increases with stroke ratio, resulting in increased mixing. For stroke ratios above the formation number, the leading vortex ring remains the same, and the length of the trailing column increases with stroke ratio. The overall entrainment decreases as a result. Next, direct numerical simulation is used to study the effect of crossflow on the dynamics, entrainment and mixing characteristics of vortex rings issuing from a circular nozzle. Three distinct regimes exist, depending on the velocity ratio and stroke ratio. Coherent vortex rings are not obtained at velocity ratios below approximately 2. At these low velocity ratios, the vorticity in the crossflow boundary layer inhibits roll–up of the nozzle boundary layer at the leading edge. As a result, a hairpin vortex forms instead of a vortex ring. For large stroke ratios and velocity ratio below 2, a series of hairpin vortices are shed downstream. The shedding is quite periodic for very low Reynolds numbers. For velocity ratios above 2, two regimes are obtained depending upon the stroke ratio. Lower stroke ratios yield a coherent asymmetric vortex ring, while higher stroke ratios yield an asymmetric vortex ring accompanied by a trailing column of vorticity. These two regimes are separated by a transition stroke ratio whose value decreases with decreasing velocity ratio. For very high values of the velocity ratio, the transition stroke ratio approaches the ‘formation number’ defined by Gharib et al. (1998). In the absence of trailing vorticity, the vortex ring tilts towards the upstream direction, while the presence of a trailing column causes it to tilt downstream. This behavior is explained. Then, we study the mixing behavior of pulsed jets in crossflow using direct numerical simulations. The pulse is a square wave and the simulations consider several jet velocity ratios and pulse conditions. We study the effects of pulsing, and explain the wide range of optimal pulsing conditions found in experimental studies of the problem. Vortex rings in crossflow exhibit three distinct flow regimes depending on stroke ratio and ring velocity ratio. The simulations of pulsed transverse jets show that at high velocity ratios, optimal pulse conditions correspond to the transition of the vortex rings produced by pulsing between the different regimes. At low velocity ratios, optimal pulsing conditions are related to the natural timescale on which hairpin vortices form. An optimal curve in the space of stroke ratio and velocity ratio is developed. Data from various experiments are interpreted in terms of the properties of the equivalent vortex rings and shown to collapse on the optimal curve. The proposed regime map allows the effects of experimental parameters such as pulse frequency, duty cycle, modulation, and pulse energy to all be predicted by determining their effect on the equivalent stroke and velocity ratios. The thesis also discusses work towards the development of Large Eddy Sim- ulation (LES) methodology to predict mixing in very high Reynolds number turbulent flows. We propose a novel estimation procedure to model the subgrid velocity for LES. The subgrid stress is obtained directly from the estimated subgrid velocity. The model coefficients for the subgrid velocity are obtained by imposing constraints on resulting ensemble-averaged subgrid dissipation and local subgrid kinetic energy. The subgrid dissipation may be obtained through either eddy–viscosity models or a new dynamic model for dissipation. The subgrid kinetic energy may be obtained either from the dynamic Yoshizawa model or a modeled transport equation. We also extend the estimation procedure to LES of passive scalar transport and propose an estimation model for subgrid scalar concentration. The subgrid flux is computed directly from the estimated subgrid velocity and estimated subgrid scalar. The model coefficient for the subgrid scalar is obtained by constraining mean scalar dissipation which is provided by an eddy–diffusivity approach. The velocity and scalar estimation models are applied to decaying isotropic turbulence with an uniform mean scalar gradient and good results are obtained. Realistic backscatter is also predicted. A dynamic model for subgrid scale dissipation is proposed. The dissipation is modeled using invariants of strain–rate tensor. The proposed dynamic approach uses a second level test filter and the model coefficient is obtained using two scalar and propose an estimation model for subgrid scalar concentration. The subgrid flux is computed directly from the estimated subgrid velocity and estimated subgrid scalar. The model coefficient for the subgrid scalar is obtained by constraining mean scalar dissipation which is provided by an eddy–diffusivity approach. The velocity and scalar estimation models are applied to decaying isotropic turbulence with an uniform mean scalar gradient and good results are obtained. Realistic backscatter is also predicted. A dynamic model for subgrid scale dissipation is proposed. The dissipation is modeled using invariants of strain–rate tensor. The proposed dynamic approach uses a second level test filter and the model coefficient is obtained using two scalar identities. We show that this approach can also be used to obtain the Smagorinsky model coefficient for subgrid stress. This is an alternative to Germano’s dynamic procedure where the single model constant is obtained by minimizing the error in a tensor identity, the Germano identity errorItem Modeling and simulation of homogeneous nucleation in turbulent flows: physics, methods and realizable solutions(2013-03) Murfield, Nathan JamesNumerical simulations of nanoparticle nucleation in turbulent shear flows are performed. We consider the homogeneous nucleation of dibutyl-phthalate (DBP) nanoparticles via direct numerical simulation (DNS) and large-eddy simulation (LES). The flows consist of a high-temperature, DBP-laden stream issuing into a low-temperature, faster or slower moving, DBP-free environment. As the flows cool, via molecular and large- scale convective mixing, the DBP vapor becomes highly supersaturated and particles are formed by nucleation. This particle formation takes place in the absence of condensation or coagulation. Classical nucleation theory is used to model particle nucleation and the Navier-Stokes equations are coupled with the scalar transport equations to provide the fluid, thermal, and chemical fields. The effects of large-scale mixing and vapor concentration on homogeneous nucleation rates are investigated via DNS in three-dimensional planar jets. The simulation results provide a demonstration of how nucleation takes place in narrow regions where molecular mixing of the two streams occurs. When maximum nucleation rates occur in conditions where the nucleation rates are sensitive to ambient conditions, islands of nucleation form. There are two possible nucleation events: initial shear layer nucleation, and later nucleation in coherent structures or eddies generated by the velocity difference between the jet and the co-flow. A scatter plot diagram of observed dilution paths in temperature versus condensable vapor concentration space where nucleation rates are superimposed is shown to be a convenient tool for analyzing nucleation events. Convection by large-scale eddies gradually spreads the range of mixing paths in this space towards higher nucleation rates. The results also show that boundary conditions, including inlet concentration and velocity ratio, have both qualitative and quantitative effects on particle nucleation. The effects of Lewis number on the homogeneous nucleation of DBP particles are also studied via DNS. Simulations at two Lewis numbers are performed to investigate the effects of molecular mixing on nucleation. These simulations are also carried out at two co-flow velocities to assess the effects of large-scale mixing. The results show that the Lewis number as well the level of large-scale mixing inherent in the flow have substantial effects on particle nucleation. The effects of the subgrid-scale (SGS) scalar interactions on nanoparticle nucleation are investigated via a priori analysis of DNS data. To assess the effect of SGS scalar interactions on DBP particle nucleation, the temperature and mass-fractions are filtered and the resulting quantities are used to compute the nucleating particle field. Two filter widths are used to obtain varying levels of SGS interactions. Particle size distributions are computed to examine the particle fields produced. This work shows that the SGS interactions' effect on nucleation has two distinct trends. In the proximal region of the flow, the unresolved interactions act to decrease particle formation. However, as the flow transitions or becomes turbulent the effect of the SGS interactions acts to increase particle formation. In the DNS, all relevant length and time-scales are resolved while in LES a closure is used to represent the SGS stress, and fluid-scalar fluxes. We perform the LES at two resolutions to illustrate the effect of "resolving less" and "modeling more". Additionally, to illustrate the effects of the SGS interactions on homogeneous nucleation in turbulent flows, the unresolved scalar-scalar interactions appearing in nucleation source term are neglected. The results again show that nucleation initially occurs in the shear layers where molecular transport dominates and across the span of the wake once the core collapses and the flow transitions to turbulence. Pre-transition, the saturation ratio - representative of the driving force for particle formation - predicted by the LES and DNS is quite similar. Post-transition, the saturation ratios predicted by the LES are significantly greater than those predicted by the DNS, and the discrepancy increases as the filter-width increases (and resolution decreases). This dynamic is also reflected in the nucleation rate. The LES predicts nucleation rates between one and two orders of magnitude greater than the DNS and the discrepancy increases as the resolution decreases. There is also a shift towards the nucleation of smaller nanoparticles in the LES compared to the DNS. The results suggests that the SGS interactions act to decrease the rate of nanoparticle nucleation and increase nuclei size. The compute time between the DNS and LES decreased by three orders of magnitude, suggesting that SGS closures for nucleation would be a significant addition to simulation capabilities and tools. The work concludes with a discussion of a probabilistic method able to resolve these issues, which are inherent to LES.Item On variable-density subgrid effects in turbulent flows(2018-11) GS, SidharthEulerian mass density variations in a flow relate to compressibility and material inhomogeneities in the fluid. These variations can be caused due to high flow speeds, heat transfer, thermo-chemical reactions and/or phase change. From a local perspective, density gradient in space affects the velocity gradient dynamics due to variable inertia, in the presence of pressure-gradient driven acceleration, and therefore indirectly, the dissipation rate of kinetic energy and enstrophy. In turbulent flows, density variations and their effects on the velocity field influences the interscale interactions. Of particular interest is the turbulent dynamics in the presence of large vorticity generation by baroclinic torque. Although these effects are usually transient (in space or time) as turbulent mixing homogenizes the density field, the deviation from constant-density dynamical evolution can be statistically significant, particularly in instability-dominated flows with high sensitivity to initial/boundary conditions. In unsteady reacting flows, sustained chemi-acoustic interactions result in turbulent vorticity dynamics that is markedly different from the well-studied incompressible constant-density turbulence. Large-eddy simulations of high Reynolds number variable-density flows require adequate representation of unresolved small-scale variable-density effects. The present work is an effort to understand subgrid-scale (SGS) variable-density effects to improve the fidelity and accuracy of our simulations in these regimes. The thesis focuses on Reynolds-filtered governing equations to compute the large-scale vorticity dynamics more precisely. A novel equation set for coarse-grained mass, momentum and energy is derived that employs only second order moment based closures, and allows explicit representation of subgrid-scale compressibility and inertial effects. The new form of the filtered equations has terms that represent the SGS mass flux, pressure-gradient acceleration, and velocity-dilatation correlation. We attempt to quantify the dynamical significance of these terms with direct numerical and large eddy simulations.Item Simulations of injection, mixing, and combustion in supersonic flow using a hybrid RANS/LES approach.(2011-09) Peterson, David MichaelThere is a great need for accurate and reliable numerical simulation of injection, mixing, and combustion in supersonic combustion ramjet engines. This study seeks to improve the accuracy and reliability which these flow can be simulated with by investigating the use of recent improvements in turbulence modeling and numerical methods. The present numerical simulations use implicit time integration and low-dissipation flux evaluation schemes in an unstructured grid framework. A hybrid Reynolds-Averaged Navier-Stokes and large-eddy simulation approach is used to model turbulence. The large-scale turbulent structure of the flow is resolved, while the near-wall structure is fully modeled. The effects of numerics, grid resolution, and boundary conditions are investigated. The simulation approach is thoroughly validated against available experimental data at a variety of flow conditions. The simulations focus on the injection of fuel through circular injector ports that are oriented either normal to the supersonic crossflow, or at a low angle with respect to the crossflow. The instantaneous flow structure resolved by the simulations is qualitatively compared to experimental flowfield visualization. Quantitative comparisons are made to mean wall pressure, mean velocity, turbulence quantities, and mean mixing data. The simulations are found to do very well at predicting the mean flowfield as well as fluctuations in velocity and injectant concentration. The simulation approach is then used to simulate the flow within a model supersonic combustor. The focus is on the non-reacting case. The simulation results are found to agree well with experimental measurements of temperature and species concentrations. The flow is examined to improve understanding of the mixing within the model combustor. Preliminary results for a simulation including hydrogen combustion are also presented.