Fully Polarizable QM/Fluctuating Charge Approach to Two-Photon Absorption of Aqueous Solutions

We present the extension of the quantum/classical polarizable fluctuating charge model to the calculation of single residues of quadratic response functions, as required for the computational modeling of two-photon absorption cross-sections. By virtue of a variational formulation of the quantum/classical polarizable coupling, we are able to exploit an atomic orbital-based quasienergy formalism to derive the additional coupling terms in the response equations. Our formalism can be extended to the calculation of arbitrary order response functions and their residues. The approach has been applied to the challenging problem of one- and two-photon spectra of rhodamine 6G (R6G) in aqueous solution. Solvent effects on one- and two-photon spectra of R6G in aqueous solution have been analyzed by considering three different approaches, from a continuum (QM/PCM) to two QM/MM models (non-polarizable QM/TIP3P and polarizable QM/FQ). Both QM/TIP3P and QM/FQ simulated OPA and TPA spectra show that the inclusion of discrete water solvent molecules is essential to increase the agreement between theory and experiment. QM/FQ has been shown to give the best agreement with experiments

Actuator selection and placement for linear feedback control of compressible flows

Actuator and sensor placement for active control of high-Reynolds number flows is largely based on experience and trial-and-error because of the system’s large dimensionality and complexity. A novel strategy for estimating how to select and place a linear feedback control system using co-located actuator(s)/sensor(s) suitable for affecting the dynamics of compressible, viscous flows is developed. The methodology uses the flow’s gain and receptivity information from the forward and adjoint global modes of the baseflow obtained from direct/large eddy simulations. The baseflow can be an equilibrium (steady-state) or a time-averaged solution of the compressible Navier Stokes equations. The method uses structural sensitivity arguments to determine regions of the flow-field with high dynamical sensitivity, and a search procedure determines effective actuator/sensor locations. The control algorithm is flexible, and different types of control and feedback can be considered. The efficacy of the method is demonstrated with three different flow control problems: flow stabilization in a Mach 0.65 diffuser, noise reduction of an axisymmetric Mach 1.5 jet, and noise reduction of a turbulent Mach 0.9 jet. For the diffuser, global stabilization is achieved for low Reynolds numbers resulting in complete suppression of vortex shedding. For longer domains and higher Reynolds number flows in the diffuser, although significant reduction in growth rates of the unstable modes was achieved, complete stabilization could not be attained. For the axisymmetric Mach 1.5 jet, equilibrium and time-averaged configurations are compared to examine the differences in global stability. The jet’s optimal transient response that leads to the largest pressure fluctuations away from the jet is used to relate the global modes needed for the control methodology to the radiated sound. The spectrum also contains modes that are hydrodynamically bound to the jet, without significant sound field contributions. Direct numerical simulations using the control show significant noise reduction, with additional reduction with increase in control gain. Eigenanalysis of the controlled mean flows reveal fundamental changes in the spectrum at frequencies lower than that used by the control, with the quieter flows having unstable eigenvalues that correspond to eigenfunctions without significant support in the acoustic field. Analysis of the mean flow quantities shows that the control induced mean flow changes only become obvious beyond 15 radii from the nozzle. Reduced order analysis using Proper Orthogonal Decomposition (POD) shows flow regularization in the quieter flows. The active control strategy is then applied to a Mach 0.9 turbulent jet. The global analysis of the time-and-azimuthal averaged baseline flow showed that the flow supports acoustically efficient super-directive and multi-directive global modes. Significant noise reduction was obtained and, similar to the axisymmetric case, the global analysis of the time-and-azimuthal averaged flow for the quiet jet show the existence of an unstable mode at a low Strouhal number, that lacks any significant sound-field support. The variation of mean quantities at the centerline and the lipline for the loud and quiet jets also showed trends similar to the axisymmetric case

Air Force Institute of Technology Research Report 2019

This Research Report presents the FY19 research statistics and contributions of the Graduate School of Engineering and Management (EN) at AFIT. AFIT research interests and faculty expertise cover a broad spectrum of technical areas related to USAF needs, as reflected by the range of topics addressed in the faculty and student publications listed in this report. In most cases, the research work reported herein is directly sponsored by one or more USAF or DOD agencies. AFIT welcomes the opportunity to conduct research on additional topics of interest to the USAF, DOD, and other federal organizations when adequate manpower and financial resources are available and/or provided by a sponsor. In addition, AFIT provides research collaboration and technology transfer benefits to the public through Cooperative Research and Development Agreements (CRADAs). Interested individuals may discuss ideas for new research collaborations, potential CRADAs, or research proposals with individual faculty using the contact information in this document

Poszerzając możliwości DFT: Modelowanie kompleksów złota oraz niekowalencyjnych oddziaływań w układach otwartopowłokowych

Praca doktorska mgra Michała Hapki prezentuje rozwój metodologii oraz zastosowań teorii funkcjonału gęstości elektronowej (DFT). W części poświęconej rozwojowi metodologii wprowadzone zostały dwa nowe sformułowania rachunku zaburzeń o adaptowanej symetrii (SAPT) oparte o opis oddziałujących monomerów metodą Kohna-Shama. Pierwsze z nich wykorzystuje opis monomerów przy pomocy funkcjonałów gęstości elektronowej o poprawnej asymptotyce (LRC od ang. long-range corrected functionals). Wprowadzona metoda LRC-SAPT łaczy w sobie korzyści wynikające z zapewnienia poprawnego asymptotycznego zachowania potencjału korelacyjno-wymiennego, jak również minimalizacji tzw. błędu delokalizacji elektronowej (DE). Drugi z wprowadzonych formalizmów, SAPT(UKS), umożliwia obliczenia energii oddziaływania niekowalencyjnie związanych wysokospinowych dimerów o charakterze otwartopowłokowym w oparciu o opis monomerów przy pomocy nieograniczonej metody Kohna-Shama (UKS, od ang. unrestricted Kohn-Sham). Metoda SAPT(UKS) stanowi atrakcyjną alternatywę dla zaproponowanego przez Żuchowskiego i wsp. [JCP, 129, 084101 (2008)] podejścia SAPT(ROKS). W części poświęconej zastosowaniom DFT przedstawiono badania skupione na wykorzystaniu tej teorii w opisie wybranych aspektów chemii związków złota. Po pierwsze, opracowano metodologię obliczania energii oddziaływania w kompleksach klastrów złota stabilizowanych ligandami. Wykazano, że zaproponowane podejście oparte o minimalizację DE oraz uwzględnienie oddziaływań dyspersyjnych pozwala na wiarygodny opis oddziaływań donorowo-akceptorowych w tej klasie związków. Po drugie, przedstawiono obliczenia absorpcyjnych widm UV klastrów złota Au(n) (n = 4, 6, 8, 12, 20) w ramach zależnej od czasu metody funkcjonału gęstości elektronowej, TD-DFT. W tym celu po raz pierwszy wykorzystano funkcjonały LRC. Wykazano, że zarówno poprawne asymptotyczne zachowanie potencjału korelacyjno-wymiennego jak również obecność dokładnej, krótkozasięgowej wymiany są niezbędne do poprawnego opisu własności optycznych klastrów złota. Ostatnia z prac zaprezentowanych w części aplikacyjnej dotyczyła syntezy oraz charakterystyki szeregu kompleksów złota wykazujących tzw. bezmostkowe oddziaływania aurofilowe. Opisano trzy nowe sole: [Au(H2-mmta)2]3H2O (1), Na3[Au(mmta)2]6H2O (2) oraz Na3[Au(mmta)2]10.5H2O (3), (H2-mmta = kwas 2-merkapto-4-metylo-5-tiazolowy). Struktury otrzymanych związków określone na podstawie analizy krystalograficznej pozwoliły zidentyfikować obecność aurofilowych dimerów [Au2(H2-mmta)4](2+) w przypadku 1 oraz [Au2(mmta)4](6-) w przypadku struktur 2 oraz 3. Analiza z wykorzystaniem szeregu metod DFT pozwoliła na charakterystkę słabych oddziaływań stabilizujących konformacje otrzymanych związków

Importance of Electrostatically Driven Non-Covalent Interactions in Asymmetric Catalysis

Computational chemistry has become a powerful tool for understanding the principles of physical organic chemistry and rationalizing and even predicting the outcome of catalytic and non-catalytic organic reactions. Non-covalent interactions are prevalent in organic systems and accurately capturing their impact is vital for the reliable description of myriad chemical phenomena. These interactions impact everything from molecular conformations and stability to the outcome of stereoselective organic reactions and the function of biological macromolecules. Driven by the emergence of density functional theory (DFT) methods that can account for dispersion-driven noncovalent interactions, there has been a renaissance in terms of computational chemistry shaping modern organic chemistry. DFT Studies of the origins of stereoselectivity in asymmetric organocatalytic reactions can not only provide key information on the mode of asymmetric induction, but can also guide future rational catalyst design. We start with an overview of weak intermolecular interactions and aromatic interactions. Special emphasis is given to the methods that one can use to study these ephemeral interactions. We next provide a brief account how computational chemistry has aided our understanding of chiral phosphoric acid (CPA) catalyzed reactions. Thereafter, three case studies showcasing the importance of non-covalent interactions in chiral NHC catalysis, CPA catalysis, and chiral nucleophilic catalysis has been elaborated. Each of these studies highlights the importance of electrostatically-driven non-covalent interactions in controlling reactivity and selectivity. Moreover, unprecedented activation modes are identified and new predictive selectivity models developed that can be used to rationalize the outcome of future reactions. Studying these reactions using state of art DFT methods, we aimed not only to contribute to the understanding of their selectivity and the importance of noncovalent interactions in catalysis, but also to bring a sound understanding that will enable the design of new reactions and better catalysts. Overall, this dissertation highlights the underappreciated role of electrostatic interactions in controlling reactivity and selectivity in asymmetric catalysis

HOMOGENEOUS AND HETEROGENEOUS SENSORS FOR COMBUSTION SYSTEMS

Due to increasingly stringent emission regulations, it is important to develop clean combustors. Combustion behavior is very complex in almost all practical power plant systems. Measurement of temperature, pressure, local flow, and chemical composition inside the flame provides critical information to develop cleaner combustors. This would result in significant improvement in energy efficiency and reduce the environmental impact. A high density sensor network system would assist in understanding the various ongoing processes occurring within the combustors. This dissertation is focused on how much additional information can be gathered from multiple sensors. Four different time delay estimation methods (using cross correlation, phase transform, generalized cross correlation with maximum-likelihood estimation, and average square difference function) were examined using two sensors. Phase transform offered better results to calculate the time delay between a given pair of microphones. This has the potential to determine local noise generation sources from within flows and flames with the additional information on local noise generation source. As a step towards the development of a sensor network, different sensors were examined. A micro-thermocouple, microphone and microphone probes were utilized to enhance understanding of the flame with detailed information on the various ongoing processes in a premixed swirl flame. High frequency temperature and pressure measurements were used to identify the thermal and acoustic characteristics of the flame and combustor. The local distributions of fluctuating pressure and temperature were measured in different regions, in and around the flame. Pressure fluctuation showed significant variation in different directions for the combustive case relative to non-combustive flow. Also a comparison of the pressure and temperature fluctuations revealed that maximum temperature fluctuations occur mostly near to the visible flame boundary while maximum pressure fluctuation occur further away from the flame. Acoustic data from the premixed swirl combustor showed variation in fuel to air ratio changes the spatial distribution of noise as measured by different sensors placed around the combustor. A comparison of different sensors showed that a single sensor does not capture all the information with changes in fuel to air ratio

Numerical Investigation of The Effect of Inlet Turbulence Intensity on a Bluff-Body Stabilized Flame at Near Blow-off Condition

This thesis investigates the effect of a boundary condition on the dynamics of a bluff-body stabilized f lame operating near blow-off condition. Special emphasis is given to the effect of inlet turbulence intensity. This work is motivated by the understanding that more stringent regulations on fossil fuels generated emissions necessitate the design of combustion systems that operate at very fuel-lean conditions. Combustion at very lean conditions, however, induces flame instability that can ultimately lead to flame fluttering and eventual extinction. The dynamics of the flame at lean conditions can therefore be very sensitive to its boundary conditions. To better understand this, a numerical investigation was needed as experimental research used for our model validation ceased to provide this information. The first stage of the numerical research is based on the experiment conducted in the Volvo Flygmotor AB program. The numerical models are validated by comparing the results with the available experimental data. The near blow-off equivalence ratio is then determined using the validated set of models. The effect of ITI on the flame dynamics is subsequently investigated for a lean flame that is near blow-off condition. For the computational analysis Large Eddy Simulation (LES) method was selected for its accuracy and efficiency. Combustion is accounted for through the transport of chemical species and the turbulence-combustion interaction through laminar finite-rate model. The sensitivity to inlet turbulence is assessed by carrying out simulations at near blow-off condition. The inlet turbulence intensity is varied in increments of 5%. It is observed that while the inlet intensity of 5% causes blow-off, further increase to 10% preserves a healthy flame on account of more heat release arising from greater entrainment of combustible mixtures into the f lame zone just behind the bluff-body. This balance is again lost as the inlet turbulence intensity is further increased to 15%. These conclusions are first obtained using 2D LES and selected cases are verified through 3D LES. Further, the importance of chemical kinetics is addressed by comparative analysis using global and detailed chemical kinetics models. The results jointly highlight strategies that can be used to reduce the required computational costs without loss of critical flow features of near blow-off bluff body turbulent flame