Browsing by Subject "Turbulence"
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Item A localized dynamic model for large-eddy simulation of the neutrally buoyant atmospheric boundary layer(2007-08) Anderson, William C.; Basu, Sukanta; Letchford, Christopher W.The combination of Geostrophic forcing and an ABL height of O(1000 m) in the atmospheric boundary layer (ABL) leads to a Reynolds number (Re) of O(10^8). Re's of this magnitude indicate turbulence with an excessive number of scales of motion, or degrees of freedom. The computational power required for explicit representation of all scales (to the Kolmogorov scale) in such flows is far beyond that which is currently available (even when massively parallel computing facilities are employed); from this, the method of large-eddy simulation has emerged. In this methodology, a filtering operation separates the scales of motion into resolved- (R-) and subgrid-scale (SG-S) motions. The R-S motions are typically large and anisotropic (owing to their interaction with the boundary conditions), whilst the SG-S motions are small. The R-S motions are solved explicitly using the filtered Navier-Stokes (N-S) equations -- SG-S motions are parameterized. Parameterization of the SG-S fluid motions has been, and remains, the topic of a considerable research effort. A new SG-S model is presented, namely the LDTKE model (dynamic point-to-point computation of SG-S turbulent kinetic energy with a 1-equation model). 1-equation refers to a prognostic equation for TKE (Turbulent Kinetic Energy). The model is applied to LES of the neutrally buoyant ABL. Many highly sophisticated dynamic (tuning-free) SG-S models have recently been developed, however we still observe adhoc averagaing/clipping. TKE-based SG-S parameterizations have been extensively used, although often they're based on constant coefficients -- the varaint presented here combines a completely dynamic modeling procedure with point-to-point computations. The only constraints imposed on LDTKE is that the eddy-viscosity may not be negative (attempts to include TKE backscatter in LDTKE caused numerical instabilities).Item A numerical simulation of a turbulent two-dimensional wake utilizing a vortex street model(Texas Tech University, 1984-08) Sisco, Lori KatherinaNot availableItem A numerical study of buoyant turbulent flows using low-Reynolds number k-e model(Texas Tech University, 2001-05) Seo, Eung RyeolNumerical computation has been performed to determine the influence of buoyant eflects on convective flows with the standard k-e and the low-Reynolds number k-s models. The present study was motivated by the need to overcome the shortcoming of the standard k-e model in separating and reattaching flows because of the wall-function approach employed in the model. The low-Reynolds number k-e model is considered to be an appropriate model for recirculating flows because the model does not employ wall functions. Results of the two different models are compared against the available experimental data and direct numerical simulation (DNS) data. In this work, Kolmogorov velocity, ut=(ve), is introduced instead of shear velocity, ut=Twlp, to avoid the singularity that appears at the separating and reattaching point for both thurbulence models. QUICK differencing scheme is employed for the convective terms. Eddy-diffusivity concept is used in modeling buoyant term. Momentum equation for the velocity field and the energy equation for the temperature field are solved altematively because of the strong coupling that exists between temperature and the velocity fields in a buoyant flow. Turbulent Prandtl number was allowed to vary in the low-Reynolds number k-e model to mimic the experimental data.Item A study of the thermal law of the wall for separated flow caused by a backward facing step(Texas Tech University, 1999-08) Pak, JongyounA thermal law of the wall provides important information to predict the surface temperature of an object in turbulent flow. Surface temperature predictions for an Radial Jet Reattachment (RJR)nozzle were overpredicted particularly inside the reattachment dome using the conventional thermal law of the wall. As a result, the convective heat transfer coefficient was underpredicted in that region. The present research was motivated to more accurately predict the surface temperature of a heated surface subject to turbulent recirculating flow caused by a sudden geometry change using a new thermal law of the wall. To determine a new thermal law of the wall, thermal and hydrodynamic characteristics of turbulent airflow inside the recirculation region caused by a sudden change in geometry with no external pressure gradient were investigated experimentally. A backward facing step was utilized to predict the recirculation region over a heated flat surface. Temperature and velocity data were measured inside the recirculation region using a thermocouple and an X hot-film probe respectively for step heights of 0.043, 0.057, and 0.072 m and Reynolds numbers of 4452, 6678, and 8904. The turbulent Prandtl number was calculated inside the recirculation region and was found to be 0.25. Using the maximum backflow velocity inside the recirculation region as the reference velocity and the measured experimental data, a new thermal law of the wall for separated flows caused by sudden geometry changes was obtained by using the mixing length theory. The new thermal law of the wall was utilized in a Computational Fluid Dynamics (CFD) code to predict surface temperature distribution and air temperature distribution within the recirculation region of a backward facing step geometry. The average error from the wall temperature predictions using the new thermal law of the wall decreased by 77.4% when compared to predictions that used the conventional thermal law of the wall.Item Advanced turbulence models for recirculating flows(Texas Tech University, 1994-12) Yang, HaodongThe standard k-e model has been widely used due to the simplicity and realistic predictions it makes. However, the standard k-e model has its disadvantage. It can only be applied in the high Reynolds number region of the flow where the viscous effect is less important. For flow close to a wall, where local Reynolds number is low and viscous effect dominates, the standard k-e model will not perform satisfactorily. The drawback has limited the first grid adjacent to the wall to be placed in the fully turbulent region which is not close enough to the wall to predict the wall effect on the flow. To tackle this near wall region, the common practice is to use an empirical wall function to approximate turbulence quantities for the grid adjacent to the wall and the standard k-e model for the rest of the grids. The wall function is a logarithmic profile correlated from boundary layer data. Moreover, most of the success of the standard k-e model is limited to simple turbulent flows, such as boundary layer flows and flows in a plane duct with no recirculation. However, industrially, the most important flow, the turbulent flow, is separated in the presence of adverse pressure gradient, creating a recirculating region. In recirculating flows where the flow at a point is influenced by flow conditions upstream and downstream, the standard k-e model is known for underpredicting the recirculation region. This drawback in the standard k-e model has led researchers to look for a better turbulence model to predict recirculating flow. In this study, some efforts were made to move from the standard k-e model to the higher level turbulence models that can predict recirculating flows. Basically, there are two ways to approach this purpose. Both approaches were pursued in this study. The first way is an easier approach, and this approach modifies the standard k-e model so that it works on both viscous and fully turbulent regions. The second way is to get rid of the unrealistic eddy-viscosity concept and study the multi-equation Reynolds stress model. A new Reynolds stress model, the SSG model of Speziale, Sarkar and Gatski [8], was investigated in this study. The original version of SSG model cannot predict recirculating flow. After a simple modification was made, it can work for predicting recirculating flow. However, the modified SSG model still has very poor performance in predicting recirculating flow. Further efforts are made to improve the SSG model, and a new SSG model is proposed in this study.Item An observational study of the South Plains nocturnal low-level jet(2005-08) Giammanco, Ian Matthew; Peterson, Richard E.; Schroeder, John L.; Chang, Chia-Bo; Swift, Andrew H. P.The presence of nocturnal low-level jet streams across the Great Plains of the United States has been well documented over the last fifty years. These features are the main source of moisture transport across this region of the country. The evolution of these features has become a significant area of interest as more structures come in contact with the layer occupied by the low-level jet. The major wind resource in the United States lies in the Great Plains, the same region, which experiences the highest frequency of low-level jet streams. Low-level jets occur throughout the year but exhibit the highest frequency during the summer months as shown by Whiteman et al. [1]. The development of wind turbines that extend up to heights near two hundred meters has resulted in these structures coming into contact with the layer just beneath the jet maximum. This layer is characterized by stable, stratified flows and intense wind shear values. The stratified layers are also responsible for the formation of coherent turbulent structures, such as Kelvin-Helmholtz waves. The turbine itself is now subjected to potentially damaging turbulence. The goal of this work is to present three low-level jet cases, utilizing data from Texas Tech University’s 200 meter instrumented tower and radar wind profiler. This experiment examines the turbulent structure of the three low-level jet features in order to gain an understanding of the motions in the nocturnal boundary layer. This experiment focused on warm season jets, specifically May through July. Three low-level jet events were examined in detail. All three events exhibited a jet maximum greater than 20 ms-1 below a height of one kilometer. The shear generated beneath the jet maximum greatly exceeded the International Electrotechnical Commission’s standard shear exponent value of 1/7. Two of the three low-level jet events produced substantial turbulence. The magnitude of turbulent kinetic energy during the established jet increased with height during the periods of significant turbulence. The Richardson number for each case, for the layer between 46 and 158 meters, fell below the critical value of 0.25 during the lifetime of the low-level jet event. The 2 June 2004 low-level jet exhibited a jet maximum below 200 meters in altitude. This would place any wind energy system in direct contact with the layer just beneath the jet maximum in a region of high shear. The 25 May 2004 low-level jet event illustrates the generation of significant turbulence due to the interaction between the low-level jet and a thunderstorm outflow boundary. Prolonged turbulence was generated lasting over two hours as the boundary passed the instrumented tower. This experiment focused on describing all aspects of the nocturnal low-level jet and the ability of the Texas Tech 200 meter tower to provide high resolution observations of the nocturnal boundary layer. The driving force behind this work is to examine the motions associated with low-level jets in the layer occupied by wind energy systemsItem Analysis of Instabilities and Their Impact on Friction Factor in Hole-Pattern Seals(2012-11-21) Sekaran, Aarthi 1985-The determination of the leakage and consequently the friction factor is an important part of analyzing the flow through a seal. This is done experimentally by means of a flat plate tester, which allows for the simplified representation of the seal pattern on a flat plate surface tested under a range of clearances and pressure drops. The setup mounts a smooth plate opposite a second plate which may be smooth or have a roughened surface while the separation between plates is held constant. The present study analyzes the phenomenon of friction factor 'upset' ? wherein it was seen that as the pressure drop across the parallel plates is increased, there is a sudden increase in the friction factor (i.e. a decrease in flow rate) at a certain Reynolds number and for any further increase in the pressure differential, the friction factor shows the expected trend and decreases slowly. This phenomenon was initially believed to be an anomaly in the rig and was attributed to choking at an upstream flow control valve. The present author differs from that view and hypothesized that the reason for the abrupt change is linked to the flow mechanics of the system and the current study analyzes the same. Preliminary analysis of available data has established that the cause for the 'upset' was not related to the switch from a normal mode resonance driven by the Helmholtz frequency of the cavities on the stator to a shear layer instability, as was seen earlier by Ha. The friction factor jump for this case is therefore proposed to be due to a change of the instability modes as the fluid passes over the cavities in the plate. A detailed analysis of the physics of the flow will be carried out via a numerical simulation using a Large Eddy Simulation (LES) model from ANSYS Fluent. Results will be validated through comparisons with experimental data from the flat plate test rig.Item Analysis of the Reactor Cavity Cooling System for Very High Temperature Gas-cooled Reactors Using Computational Fluid Dynamics Tools(2011-08-08) Frisani, AngeloThe design of passive heat removal systems is one of the main concerns for the modular Very High Temperature Gas-Cooled Reactors (VHTR) vessel cavity. The Reactor Cavity Cooling System (RCCS) is an important heat removal system in case of accidents. The design and validation of the RCCS is necessary to demonstrate that VHTRs can survive to the postulated accidents. The commercial Computational Fluid Dynamics (CFD) STAR-CCM+/ V3.06.006 code was used for three-dimensional system modeling and analysis of the RCCS. Two models were developed to analyze heat exchange in the RCCS. Both models incorporate a 180 degree section resembling the VHTR RCCS bench table test facility performed at Texas A&M University. All the key features of the experimental facility were taken into account during the numerical simulations. Two cooling fluids (i.e., water and air) were considered to test the capability of maintaining the RCCS concrete walls temperature below design limits. Mesh convergence was achieved with an intensive parametric study of the two different cooling configurations and selected boundary conditions. To test the effect of turbulence modeling on the RCCS heat exchange, predictions using several different turbulence models and near-wall treatments were evaluated and compared. The models considered included the first-moment closure one equation Spalart-Allmaras model, the first-moment closure two-equation k-e and k-w models and the second-moment closure Reynolds Stress Transport (RST) model. For the near wall treatments, the low y+ and the all y+ wall treatments were considered. The two-layer model was also used to investigate the effect of near-wall treatment. The comparison of the experimental data with the simulations showed a satisfactory agreement for the temperature distribution inside the RCCS cavity medium and at the standpipes walls. The tested turbulence models demonstrated that the Realizable k-e model with two-layer all y+ wall treatment performs better than the other k-e models for such a complicated geometry and flow conditions. Results are in satisfactory agreement with the RST simulations and experimental data available. A scaling analysis was developed to address the distortion introduced by the experimental facility and CFD model in simulating the physics inside the RCCS system with respect to the real plant configuration. The scaling analysis demonstrated that both the experimental facility and CFD model give a satisfactory reproduction of the main flow characteristics inside the RCCS cavity region, with convection and radiation heat exchange phenomena being properly scaled from the real plant to the model analyzed.Item Anisotropic hybrid turbulence modeling with specific application to the simulation of pulse-actuated dynamic stall control(2015-12) Haering, Sigfried William; Moser, Robert deLancey; Murthy, Jayathi; Bogard, David G; Ezekoye, Ofodike A; Oliver, ToddExperimental studies have shown pulse actuated dynamic stall control may provide a simple means to significantly increase the performance of lifting surfaces and expand their flight envelope. However, precise information of the complex boundary layer reattachment mechanisms are inaccessible to experimental measurements. Therefore, simulations are necessary to fully understand, optimize, and apply this method. Due to the inherent shortcomings of RANS, computational expense of LES, and deficiencies in current hybrid modeling approaches, a new hybrid modeling framework has been developed. Based in using the two-point second-order structure function to drive a local equilibrium between resolved and modeled turbulence, the new approach addresses issues associated with inhomogeneous and anisotropic grids as well as the treatment of the RANS/LES interface in hybrid simulations. Numerical studies using hybrid RANS/LES modeling approaches of a stalled airfoil with spanwise-uniform actuation regions experiencing single pulse actuated flow reattachment have been performed. The mechanism responsible for reattachment has been identified as a repeating wall-vortex interaction process. The new hybrid framework and anisotropic SGS models developed here are anticipated to be of great benefit well beyond the focus of this work with application to many challenging flow situations of pressing engineering interest.Item Application and analysis of RANS based turbulence models for bluff body aerodynamics(2005-05) Unhale, Sanket A.; James, Darryl; Letchford, Christopher W.; Parameswaran, SivaComputational Wind Engineering (CWE) is becoming more popular in the wind engineering community, for prediction of the wind loads on buildings and structures, by development and validation of numerous turbulence models. In the similar context, five two-equation turbulence models based on Reynolds Averaged Navier Stokes (RANS) modeling approach were tested for the flow around sharp edged building models using Fluent as the solver. The results were analyzed and the effects of different turbulence parameters used in the modeling and analysis were discussed. Each turbulence model was individually reviewed for the correctness of its predictions and the best model in this set was chosen. This model was further applied to different obstacle geometry in different experimental conditions to minimize the effects of unique experimental condition on analysis. One of the major problems encountered in the modeling was the type of near wall treatment used for simulation. It was concluded that the standard approach of wall functions was insufficient to estimating the effects of roughness on the wall boundary on the mean flow. A new rough wall model proposed by Durbin et al [58] was implemented using the user defined functions capability of Fluent. This model was applied to the geometries tested in this project to analyze the effects on the rough wall model used for modeling near wall effects. Overall agreement between the computational predictions and experimental findings was acceptable.Item Application and analysis of RANS based turbulence models for bluff body aerodynamics(Texas Tech University, 2004-10) Unhale, Sanket A.; James, Darryl L.; Letchford, Christopher W.; Parameswaran, SivaComputational Wind Engineering (CWE) is becoming more popular in the wind engineering community, for prediction of the wind loads on buildings and structures, by development and validation of numerous turbulence models. In the similar context, five two-equation turbulence models based on Reynolds Averaged Navier Stokes (RANS) modeling approach were tested for the flow around sharp edged building models using Fluent as the solver. The results were analyzed and the effects of different turbulence parameters used in the modeling and analysis were discussed. Each turbulence model was individually reviewed for the correctness of its predictions and the best model in this set was chosen. This model was further applied to different obstacle geometry in different experimental conditions to minimize the effects of unique experimental condition on analysis. One of the major problems encountered in the modeling was the type of near wall treatment used for simulation. It was concluded that the standard approach of wall functions was insufficient to estimating the effects of roughness on the wall boundary on the mean flow. A new rough wall model proposed by Durbin et al [58] was implemented using the user defined functions capability of Fluent. This model was applied to the geometries tested in this project to analyze the effects on the rough wall model used for modeling near wall effects. Overall agreement between the computational predictions and experimental findings was acceptable.Item The attenuation and reduction of a simulated hot streak due to mainstream turbulence, hot streak pitch position and film cooling(2004) Jenkins, Sean Craig; Bogard, David G.This study investigated the effects of the vane and mainstream turbulence level on a simulated hot streak in a simulated three vane cascade. The effect of film cooling on the reduction of the hot streak was investigated for a fully film-cooled vane. To determine how the showerhead, suction side and pressure side coolant regions contributed to hot streak reduction, these regions were tested individually with and without the hot streak activated. The effect of mainstream turbulence level and coolant density ratio on coolant profiles and hot streak reduction was also investigated. Finally, superposition of coolant profiles and hot streak profiles was compared with measured data to evaluate the capability of additive superposition in predicting hot streak reduction due to film cooling. The effects of mainstream turbulence on the attenuation of a hot streak were found to be significant, with changes in the shape and size of the hot streak. Comparisons between the hot streak impacting the vane at the stagnation line and passing through the mid-passage showed that the peak hot streak temperature was the same for an impinging and non-impinging hot streak. Interaction with the adiabatic vane caused very sharp temperature gradients in the hot streak at the trailing edge of the vane, resulting in an increase or decrease in hot streak peak strength depending on pitch position. Additional attenuation of the hot streak occurred in the stator/rotor axial gap. Results with film cooling indicated that, while full-coverage film cooling had a substantial effect on the hot streak, this effect was primarily due to the showerhead and suction side coolant with a much lesser effect due to the pressure side. It was discovered that coolant profiles at the trailing edge could be scaled by the coolant hole exit temperature, while reduction of the hot streak was less for film cooling at low density ratio. Measurements also showed a much higher degree of coolant spreading under conditions of high mainstream turbulence. Overall, downstream of the vane using high blowing ratios, the hot streak peak was reduced by 83% compared with the peak value upstream of the vane.Item Bursting events in the stable atmospheric boundary layer(2008-05) Phillipson, Julie Ann; Basu, Sukanta; Gilliam, Xiaoning; Leary, ColleenThough recent experiments and field projects regarding the characteristics of the boundary layer have been conducted, there is still an overall lack of understanding about the mechanics of the stable boundary layer, whether it is transitionally stable, such as the nocturnal stable boundary layer in mid-latitudes, or persistently stable, such as the boundary layer in Polar Regions. ISCAT-00 is one such field project conducted in Antarctica, where an instrumented tower was used to record wind speed and temperature information, from which heat and momentum flux transfers can be discerned. Understanding of boundary layer processes is especially important for the stable layer since despite its stable nature, sporadic bursts of turbulence have been observed to occur, indicated by data collected from the ISCAT-00 campaign. These bursts, though short-lived, are responsible for most of the heat and momentum transfer that occurs within the otherwise stable layer. Since these bursts of turbulence disrupt the stable layer, they can not only pose problems for air quality forecasts, but they can also inhibit the performance of wind turbines. Transient loading from these bursting events as rotor blades pass through patches of organized, coherent turbulence can shorten the life span of a wind turbine by 5 to 10 years. However, despite the noted occurrence of turbulence bursting in the stable layer there is still very little known about its origins, causes, or basic properties. In order to explore the characteristics of turbulent bursting events in the stable layer, two methodologies are applied to analyze data obtained from the ISCAT00 campaign. The first methodology applied to discern the occurrence of turbulent bursting is one created by Nakamura & Mahrt (2005). The second methodology is a newly developed adaptive threshold methodology, which is much more robust, and removes much of the subjectivity of other turbulence bursting identification methods. The adaptive technique allows for the reduction of subjectivity in the data analysis phase, and is therefore felt to be more accurate. Also, spectral characteristics of polar turbulence are explored, and are found to have similar properties to spectra observed in mid-latitudes. This research focuses primarily on different methods, both old and newly developed that can help to further understanding about turbulence in the stable boundary layer and the corresponding heat and moisture flux properties.Item CFD predictions of heat transfer coefficient augmentation on a simulated film cooled turbine blade leading edge(2011-05) Beirnaert-Chartrel, Gwennaël; Bogard, David G.; Moser, Robert D.Computations were run to study heat transfer coefficient augmentation with film cooling for a simulated gas turbine blade leading edge. The realizable k-[epsilon] turbulence model (RKE) and Shear Stress Transport k-[omega] turbulence model (SST) were used for the computational simulations. RKE computations completed at a unity density ratio were confirmed to be consistent with experimental measurements conducted by Yuki et al.(1998) and Johnston et al. (1999) whereas SST computations exhibited significant discrepancies. Moreover the effect of the density ratio on heat transfer coefficient augmentation was studied because experimental measurements of heat transfer coefficient augmentation with film cooling are generally constrained to unity density ratio tests. It was shown that heat transfer coefficient augmentation can be simulated using unity density ratio jets, but only when scaled with the momentum flux ratio of the coolant jets.Item Characterization of hurricane gust factors using observed and analytical data(Texas Tech University, 2009-05) Edwards, Rebecca Paulsen; Schroeder, John L.; Gilliam, Kathleen; Smith, Douglas A.The nature of turbulence in the hurricane boundary layer has been the subject of much discussion. Two questions in particular continue to be the source for debate and ongoing research. The first question is whether or not hurricane GFs exhibit the same behavior as GFs from winds generated by extratropical systems (thunderstorms excluded). The second question is whether the structure of the wind, and the resulting gust factors, change at high wind speeds. This study seeks to address those two questions using a variety of data sources and analysis techniques. Observational data were collected from both landfalling tropical cyclones and synoptically generated extratropical wind. Analytical data at a variety of wind speeds were created using an inverse fast Fourier Transform of the universal spectrum for wind in the perturbed terrain. Gust factors and other parameters were computed for both types of data and the results assimilated in a data base. Analysis of these data yielded interesting results. A strong dependence on surface roughness was noted for gust factors from both observed and analytical data. However, once efforts were made to control for this dependency by stratifying the data into roughness regimes using the roughness length, slight differences between the tropical and extratropical gust factor data remained. Analysis of the artificial data, suggest spectral differences between the tropical and extratropical regimes due to the presence of additional low-frequency energy in the tropical regime. A slight decrease of the gust factor with increasing wind speed was noted in the high-speed analytical data. A similar decrease was suggested in the tropical data. It was concluded that the low-frequency spectral differences between the two regimes have less of an effect on the resulting gust factors as the wind speed increases, resulting in better agreement between the two distributions.Item Coherent structures and two-dimensionalization in rotating turbulent flow(2005) Ruppert-Felsot, Jori Elan; Swinney, H. L., 1939-Item Direct numerical simulation (DNS) for incompressible turbulent channel flow at Reτ = 5200(2015-12) Lee, Myoungkyu; Moser, Robert deLancey; Biros, George; Bogard, David G.; Murthy, Jayathi; Oliver, Todd A.Nearly all moving objects on Earth pass through fluids and many of them move at high speed. This makes high Re wall-bounded turbulent flows of great technological impor- tance. To study high Re wall-bounded turbulence, high spatial and temporal resolution is required due to the multi-scale nature of turbulence. Direct numerical simulation (DNS) is a technique for the study of turbulence in which the Navier-Stoke equations, the governing equations of fluid flow, are solved with sufficient resolution to represent all the scales of tur- bulence. Hence, DNS is very expensive and always limited by computational capability. To perform DNS on the most advanced high performance computing systems, extensive code optimization is required. A new turbulence DNS code, PoongBack, was developed for the studies reported here. It shows excellent performance and scalability (∼97%) on upto 786k cores on Mira at Argonne Leadership Computing Facility. We have performed DNS of turbulent channel flow using a Fourier-Galerkin method in the streamwise(x) and spanwise (z) directions and a B-Splines collocation method in the wall-normal (y) direction. The highest Reynolds number based on shear velocity (uτ = √(τw/ρ)), Reτ is approximately 5200. The simulation results exhibit a number of the char- acteristics of high Re wall-bounded turbulent flows. For example, a region where the mean velocity has a logarithmic variation is observed, with von Kármán constant κ = 0.384±0.004. There is also a logarithmic dependence of the variance of the spanwise velocity component, though not the streamwise component. A distinct separation of scales exists between the large outer-layer structures and small inner-layer structures. At intermediate distances from the wall, the one-dimensional spectrum of the streamwise velocity fluctuation in both the streamwise and spanwise directions exhibits 1/k dependence over a short range in wavenum- ber (k). Further, consistent with previous experimental observations, when these spectra are multiplied by k (premultiplied spectra), they have a bimodal structure with local peaks located at wavenumbers on either side of the 1/k range. To study the scale dependence of the dynamics of the Reynolds stress components, we applied a spectral analysis to the terms in the Reynolds stress transport equation (RSTE). It is shown that only the turbulent transport terms show significant Re dependencies. Further- more, the turbulent transport terms can be decomposed into two parts, one that contributes to transport in the wall-normal direction and one that is responsible for transfer between length scales. The results show that the large scale motion in the outer region has direct effects on the flow in the near-wall region through transport of turbulent kinetic energy. Also, a reverse energy cascade from intermediate scales to large scales is observed in the spanwise velocity fluctuations.Item Direct numerical simulation and reaction path analysis of titania formation in flame synthesis(2012-08) Singh, Ravi Ishwar; Ezekoye, Ofodike A.; Raman, VenkatFlame-based synthesis is an attractive industrial process for the large scale generation of nanoparticles. In this aerosol process, a gasifi ed precursor is injected into a high-temperature turbulent flame, where oxidation followed by particle nucleation and other solid phase dynamics create nanoparticles. Precursor oxidation, which ultimately leads to nucleation, is strongly influenced by the turbulent flame dynamics. Here, direct numerical simulation (DNS) of a canonical homogeneous flow is used to understand the interaction between a methane/air flame and titanium tetrachloride oxidation to titania. Detailed chemical kinetics is used to describe the combustion and precursor oxidation processes. Results show that the initial precursor decomposition is heavily influenced by the gas phase temperature field. However, temperature insensitivity of subsequent reactions in the precursor oxidation pathway slow down conversion to the titania. Consequently, titania formation occurs at much longer time scales compared to that of hydrocarbon oxidation. Further, only a fraction of the precursor is converted to titania, and a signi cant amount of partially-oxidized precursor species are formed. Introducing the precursor in the oxidizer stream as opposed to the fuel stream has only a minimal impact on the oxidation dynamics. In order to understand modeling issues, the DNS results are compared with the laminar flamelet model. It is shown that the flamelet assumption qualitatively reproduces the oxidation structure. Further, reduced oxygen concentration in the near-flame location critically a ffects titania formation. The DNS results also show that titania forms on the lean and rich sides of the flame. A reaction path analysis (RPA) is conducted. The results illustrate the di ffering reaction pathways of the detailed chemical mechanism depending on the composition of the mixture. The RPA results corroborate with the DNS results that titania formation is maximized at two mixture fraction values, one on the lean side of the flame, and one on the rich side.Item Direct numerical simulation of particle-laden turbulence in a straight square duct(Texas A&M University, 2004-09-30) Sharma, GauravParticle-laden turbulent flow through a straight square duct at Re? = 300 is studied using direct numerical simulation (DNS) and Lagrangian particle tracking. A parallelized 3-D particle tracking direct numerical simulation code has been developed to perform the large-scale turbulent particle transport computations reported in this thesis. The DNS code is validated after demonstrating good agreement with the published DNS results for the same flow and Reynolds number. Lagrangian particle transport computations are carried out using a large ensemble of passive tracers and finite-inertia particles and the assumption of one-way fluid-particle coupling. Using four different types of initial particle distributions, Lagrangian particle dispersion, concentration and deposition are studied in the turbulent straight square duct. Particles are released in a uniform distribution on a cross-sectional plane at the duct inlet, released as particle pairs in the core region of the duct, distributed randomly in the domain or distributed uniformly in planes at certain heights above the walls. One- and two-particle dispersion statistics are computed and discussed for the low Reynolds number inhomogeneous turbulence present in a straight square duct. New detailed statistics on particle number concentration and deposition are also obtained and discussed.Item The effects of buoyancy on turbulent nonpremixed jet flames in crossflow(2005) Boxx, Isaac G.; Clemens, Noel T.
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