<p dir="ltr">This thesis examined the effect of variations in the geometry of the juncture of a cylinder at a flat plate. The effect on the pressure and skin friction on the face and surroundings were examined. When compared to the experimental data obtained under similar conditions, the computational cases had a slightly higher pressure, with a qualitatively similar profile. Four cases were considered: a simple baseline configuration, a pedestal, a gap, and a fairing. The results of the pedestal case displayed this behavior to an extreme, exaggerating all the jumps and dips in the experiment. The RMS pressure was examined to investigate the shock foot locations and again the experiment and computation matched very closely. When looking at the flow visualizations and spectra, the gap case showed a larger concentration of skin friction magnitude at the base as well as the highest intensity of the low frequencies at separation and reattachment, as well as an observed higher frequency activity like Liu observed in his computations. For the new fairing case that was introduced, very similar properties to the pedestal case were observed when looking at the pressure, skin friction, and even spectra, but the flow visualization in the wake showed that it was much closer in structure to the baseline case. The small differences between the computational and experimental data could be attributed to the turbulence model used as well as the uncertainty in the pressure sensitive paint technique used in the experiments. In this thesis it was found that the gap case had higher fluctuations and skin friction, the new fairing case was very similar to the pedestal and baseline case, and the experimental data matched well for most of the computations.</p>

<p><br></p> <p>A  combustion simulation involves various physiochemical processes, such as molecular and turbulent diffusion, smoke and soot formation, thermal radiation, chemical reaction mechanisms, and kinetics. In the last decade, computational fluid dynamics (CFD) has been increasingly used in combustion modeling. It is critically important to improve and enhance the predictive capabilities of combustion models. This work presents an analysis of two types of diffusion flames: the momentum-dominant jet flames and buoyancy-controlled pool fires. The gap between the existing knowledge of differential molecular diffusion in turbulent high momentum jet flow and the practical applications has been reduced. The importance of mixing modeling in pool fire simulations has been revealed, and enhancement for predicting fire extinction limits has been proposed.</p> <p><br></p> <p>Modeling differential molecular diffusion in turbulent non-premixed combustion remains a great challenge for flamelet models. The laminar flamelet is a key component of a flamelet model for turbulent combustion. One significant challenge that has not been well addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion problems. Laminar flamelet is generated typically based on two conceptual burner configurations, the opposed jet burner, and the Tsuji burner. They are commonly considered equivalent when dealing with the description of laminar flamelet structures. A difference between them is revealed in this work for the first time when they are used to represent differential molecular diffusion. The traditionally opposed jet burner yields an almost fixed equal diffusion location in the mixture fraction space for the transport of different elements. The Tsuji burner can produce a continuous variation of the equal diffusion location in the mixture fraction space with a slight extension. This variation of the equal diffusion location is shown to be an essential characteristic of turbulent non-premixed combustion, as demonstrated in a laminar jet mixing layer problem, a turbulent jet mixing layer problem, and a turbulent jet non-premixed flame. The Tsuji burner is thus potentially a more suitable choice than the opposed jet burner for laminar flamelet generation that can be consequently used in flamelet modeling of differential molecular diffusion for turbulent non-premixed combustion.</p> <p><br></p> <p>Capturing fire extinction limits in simulations is essential for developing predictive capabilities for fire. In this work, the combined large-eddy simulation (LES) and transported probability density function (PDF) methods are assessed for the predictions of fire extinction. The University of Maryland line burner is adopted as a validation test case. The NIST Fire Dynamics Simulator (FDS) code for LES is combined with an in-house PDF code called HPDF for the fire simulations. The simulation results were verified by using the available experimental data. The combustion efficiency under the different oxygen depletion levels in the oxidizer is analyzed. Fire extinction occurs when the oxygen depletion level reduces to a certain level. The model’s capability to capture this extinction limit is assessed by using the experimental data. Different mixing models and model parameters are examined. It is found that the fire extinction limit is very sensitive to the different mixing models and mixing parameters. The level of sensitivity is higher than in momentum-driven turbulent flames, which suggests the importance of mixing modeling in fire simulations. The existing mixing models need further enhancement for predicting fire extinction. </p> <p><br></p>

<p>There is growing interest in the use of nanosecond surface dielectric barrier discharge (ns-SDBD) actuators for high-speed (supersonic/hypersonic) flow control. A plasma discharge is created using a nanosecond-duration pulse of several kilovolts, and leads to a rapid heat release and a complex three-dimensional flow field. Past work has been limited to qualitative visualizations such as schlieren imaging, and detailed measurements of the induced flow are required to develop a mechanistic model of the actuator performance. </p><p><br></p><p></p><p>Background-Oriented Schlieren (BOS) is a quantitative variant of schlieren imaging and measures density gradients in a flow field by tracking the apparent distortion of a target dot pattern. The distortion is estimated by cross-correlation, and the density gradients can be integrated spatially to obtain the density field. Owing to the simple setup and ease of use, BOS has been applied widely, and is becoming the preferred density measurement technique. However, there are several unaddressed limitations with potential for improvement, especially for application to complex flow fields such as those induced by plasma actuators. </p><p></p><p>This thesis presents a series of developments aimed at improving the various aspects of the BOS measurement chain to provide an overall improvement in the accuracy, precision, spatial resolution and dynamic range. A brief summary of the contributions are: </p><p>1) a synthetic image generation methodology to perform error and uncertainty analysis for PIV/BOS experiments, </p><p>2) an uncertainty quantification methodology to report local, instantaneous, a-posteriori uncertainty bounds on the density field, by propagating displacement uncertainties through the measurement chain,</p><p>3) an improved displacement uncertainty estimation method using a meta-uncertainty framework whereby uncertainties estimated by different methods are combined based on the sensitivities to image perturbations, </p><p>4) the development of a Weighted Least Squares-based density integration methodology to reduce the sensitivity of the density estimation procedure to measurement noise.</p><p>5) a tracking-based processing algorithm to improve the accuracy, precision and spatial resolution of the measurements, </p><p>6) a theoretical model of the measurement process to demonstrate the effect of density gradients on the position uncertainty, and an uncertainty quantification methodology for tracking-based BOS,</p><p>Then the improvements to BOS are applied to perform a detailed characterization of the flow induced by a filamentary surface plasma discharge to develop a reduced-order model for the length and time scales of the induced flow. The measurements show that the induced flow consists of a hot gas kernel filled with vorticity in a vortex ring that expands and cools over time. A reduced-order model is developed to describe the induced flow and applying the model to the experimental data reveals that the vortex ring's properties govern the time scale associated with the kernel dynamics. The model predictions for the actuator-induced flow length and time scales can guide the choice of filament spacing and pulse frequencies for practical multi-pulse ns-SDBD configurations.</p>

Direct Numerical Simulations (DNS) are performed to investigate laminar-turbulent transition in a boundary layer on a sharp cone at Mach 6. The main objective of this dissertation research is to explore which nonlinear breakdown mechanisms may be dominant in a broad--band "natural" disturbance environment and then use this knowledge to perform controlled transition simulations to investigate these mechanisms in great detail. Towards this end, a "natural" transition scenario was modeled and investigated by generating wave packet disturbances. The evolution of a three-dimensional wave packet in a boundary layer has typically been used as an idealized model for "natural" transition to turbulence, since it represents the impulse response of the boundary layer and, thus, includes the interactions between all frequencies and wave numbers. These wave packet simulations provided strong evidence for a possible presence of fundamental and subharmonic resonance mechanisms in the nonlinear transition regime. However, the fundamental resonance was much stronger than the subharmonic. In addition to these two resonance mechanisms, the wave packet simulations also indicated the possible presence of oblique breakdown mechanism. To gain more insight into the nonlinear mechanisms, controlled transition simulations were performed of these mechanisms. Several small and medium scale simulations were performed to scan the parameter space for fundamental and subharmonic resonance. These simulations confirmed the findings of the wave packet simulations, namely that, fundamental resonance is much stronger compared to the subharmonic resonance. Subsequently a set of highly resolved fundamental and oblique breakdown simulations were performed. In these DNS, remarkable streamwise arranged "hot'' streaks were observed for both fundamental and oblique breakdown. The streaks were a consequence of the large amplitude steady longitudinal vortex modes in the nonlinear régime. These simulations demonstrated that both second--mode fundamental breakdown and oblique breakdown may indeed be viable paths to complete breakdown to turbulence in hypersonic boundary layers at Mach 6.

<div>Bladeless fans are well known for their unique shape and efficient performance, which have a great impact on the fan industry. At present, there are few studies on the bladeless fan and the research on the improvement of fan design is a lack. Therefore, the study on the performance of the bladeless fan with different design is the main purpose of this thesis. </div><div>In the present study, a bladeless fan prototype is created and studied by numerical simulations. When characterizing the aerodynamic and aeroacoustic performances of the bladeless fan, the entire fan prototype, including wind channel, base, rotor and stator, is adopted; when investigating the influence of the wind channel's geometric parameters, only wind channel is considered in simulations. The influence of the slit width, the height of the cross-section, the slit location and the profile of the cross-section are studied. </div><div><br></div><div>It is found that the flow outside the bladeless fan consists of the air blown out from the wind channel and entrained from the back and side of the fan. The air entrained from the side is the main source of flow rate increase. As for the aeroacoustic performance, the rotor and stator inside the base are the predominated source of the noise generated by the bladeless fan. </div><div>The performances of the bladeless fan are very sensitive to the geometric details of the wind channel. The generated noise always increases as the wind strength improves. The slit width of the wind channel has the greatest impact. With the slit moves away from the leading edge, the wind produced by the bladeless fan becomes more powerful and the noise becomes louder. The cross-sectional height of 4cm has the best aerodynamic performance but the generated noise is a little larger than other designs. The profile of the cross-section shows insignificant influence on the performances. </div>

<div>To reduce the risk of using suction in a hypersonic quiet-tunnel nozzle design, this project tested micro-perforated suction sections to remove the boundary layer on an axisymmetric model in the Boeing/AFOSR Mach-6 Quiet Tunnel. The model was a cone-flare geometry tested at 0° angle of attack. The turn from the 7° half-angle cone to the flare was designed to prevent flow separation. The flare was designed to amplify the Görtler instability.</div><div><br></div><div>Five suction sections were designed with different perforation patterns and porosities. Four were successfully manufactured, but only the first of the four sections has been tested so far. The first suction section has pores drilled along straight lines with a nominal 5% porosity.</div><div><br></div><div>Measurements were made with temperature-sensitive paint and oil-flow visualization on a non-perforated blank to measure the baseline development of Görtler vortices on the flare. Although the signal-to-noise ratio of the measurement techniques were insufficient to measure the vortices, it was confirmed that the boundary layer is laminar for the entire model. Measurements with suction also did not show the Görtler vortices.</div><div><br></div><div>Surface pressure fluctuations were measured on the flare. Apparent second-mode waves were detected. The suction measurements showed a slight increase in second-mode peak frequency over the baseline results, as expected.</div><div><br></div><div>Concerns had been raised about acoustic noise that might be radiated from the suction section. Thus, fluctuations above the suction section were measured using a pitot probe and using focused-laser differential interferometry. The measurements during suction showed no noticeable increase in fluctuations compared to the baseline results.</div>

No description available.

Wind energy is one of the fastest-growing renewable energy technologies, and horizontal axis wind turbines (HAWT) have been the most common device to convert wind kinetic energy into electrical energy. As the capacities of wind turbines and scales of wind farm constructions are rapidly increasing over time, environmental impacts of wind energy are becoming more relevant and raising more attention than ever before. One of the major environmental concerns is noise emission from wind energy facilities, especially low-frequency noise and infrasound that allegedly cause so-called wind turbine syndrome. Therefore, a numerical simulation program capable to predict low-frequency noise and infrasound emission from wind turbines is a useful tool to aid future wind energy development. In this study of this thesis, a computer program named TDRIP (Time Domain Rotor Infrasound Prediction) is developed based on acoustic analogy theories. Farassat’s formulation 1A, a solution to Ffowcs Williams-Hawkings (FW-H) equation, is implemented in the TDRIP program to compute aerodynamically generated sound. The advantage of this program is its capability to simultaneously compute infrasound emission of multiple wind turbines in time domain, which is a challenging task for other aerodynamic noise prediction methods. The developed program is validated against results obtained from computational fluid dynamics (CFD) simulations. The program is then used to compute aerodynamic noise emitted from wind turbine rotors. The effects of wind direction, wind turbine siting, and phase of wind turbine rotation on consequent aerodynamic noise are investigated. Results of aerodynamic noise computation imply that wind turbine siting configuration or wind turbine phase adjustment can help reducing noise level at certain locations, which make the program ideal to be integrated into wind farm siting or control tools.

<p dir="ltr"> The need to develop reliable hypersonic capabilities is of critical import today. Among the most prominent tools used in recent efforts to overcome the challenges of developing hypersonic vehicles are NASA's Configuration Based Aerodynamics (CBAERO) and surrogate modeling techniques. This work presents the development of a tool, CBAERO Surrogate (CBAS), which leverages the advantages of both CBAERO and surrogate models to create a simple and streamlined method for building an aerodynamic database for any given vehicle geometry. CBAS is capable of interfacing with CBAERO directly and builds Kriging or Co-Kriging surrogate models for key aerodynamic parameters without significant user or computational effort. Two applicable geometries representing hypersonic vehicles have been used within CBAS and the resulting Kriging and Co-Kriging surrogate models evaluated against experimental data. These results show that the Kriging model predictions are accurate to CBAERO's level of fidelity, while the Co-Kriging model predictions fall within 0.5%-5% of the experimental data. These Co-Kriging models produced by CBAS are 10%-50% more accurate than CBAERO and the Kriging models and offer a higher fidelity solution while maintaining low computational expense. Based on these initial results, there are promising advancements to obtain in future work by incorporating CBAS to additional applications.</p>

Combustion research is at the forefront of development of clean and efficient IC engines, gas turbines, rocket propulsion systems etc. With the advent of faster computers and parallel programming, computational studies of turbulent combustion is increasing rapidly. Many turbulent combustion models have been previously developed based on certain underlying assumptions. One of the major assumptions of the models is the regime it can be used for: either premixed or non-premixed combustion. However in reality, combustion systems are multi-regime in nature, i.e.,\ co-existence of premixed and non-premixed modes. Thus, there is a need for development of multi-regime combustion models which closely follows the physics of combustion phenomena. Much of previous modeling efforts for multi-regime combustion was done using flamelet-type models. As a first, the current study uses the highly robust transported Probability Density Function (PDF) method coupled with Large Eddy Simulation (LES) to develop a multi-regime model. The model performance is tested for Sydney Flame L, a piloted methane-air turbulent flame. The concept of flame index is used to detect the extent of premixed and non-premixed combustion modes. The drawbacks of using the traditional flame index definition in the context of PDF method are identified. Necessary refinements to this definition, which are based on the species gradient magnitudes, are proposed for the multi-regime model development. This results in identifying a new model parameter beta which defines a gradient threshold for the calculation of flame index. A parametric study is done to determine a suitable value for beta, using which the multi-regime model performance is assessed for Flame L by comparing it against the widely used non-premixed PDF model for three mixing models: Modified Curl (MCurl), Interaction by Exchange with Mean (IEM) and Euclidean Minimum Spanning Trees (EMST). The multi-regime model shows a significant improvement in prediction of mean scalar quantities compared to the non-premixed PDF model when MCurl mixing model is used. Similar improvements are observed in the multi-regime model when IEM and EMST mixing models are used. The results show potential foundation for further multi-regime model development using PDF model.