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Daniel Guggenheim School of Aerospace Engineering
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ItemComputational fluid dynamics simulation of three-dimensional parallel jets(Georgia Institute of Technology, 2018-12-11) Liu, ZhihangHigh-speed air jets are often used in industry for manufacturing thin fibers through a process known as melt-blowing (MB). In melt blowing, high-velocity gas streams impinge upon molten strands of polymer to produce fine filaments. For a very high quantity of fibers to be produced, many small-scale jets placed side by side are needed, these jets draw the air from the same compressed air storage tank, so the fiber formation is critically dependent on the aerodynamics of the impingement jet flow field. However, the real-word MB devices always have complicate internal structures such as mixing chambers and air channels between air tank and die tip, which may cause instability and cross flow in the jet flow filed and had a significant impact on the formation of fibers and non-woven webs with small scale jets. The purpose of this study was inspired by the necessity to understand the effect of the internal geometry on the jet flow filed and tried to prevent the flow instability with fluctuation reduction devices. The MB process in this study was modeled as a pair of two jets placed at an angle of approximately 60 degrees to each other, and when there are many such jet pairs, a stream so that multiple streams of fibers may be simultaneously produced. All internal structures of the MB device were modeled based on US Patent 6,972,104 B2 by Haynes et al. The flow field resulting from the two similar converging-plane jet nozzles was investigated using a computational fluid dynamics approach. The case in which there are flow fluctuation reduction devices installed and the case without the devices installed were studied. The k-ω turbulence model was used, and the model parameters were calculated according to the inlet conditions of the air flow. This study consists of three parts: (a) a baseline case without any flow fluctuation reduction devices was studied to understand the mechanism of the instability and to investigate the details of the internal flow filed; (b) a wired mesh screen was placed between the air plates and the die tip, to study the effect on both the velocity and pressure distribution across the screen; (c) a honeycomb installed near the exit of last mixing chamber trying to reduce the velocity across the flow direction and turbulent intensity. Finally, the effect of the two different flow fluctuation reduction devices was compared in detail using time series measurements and time average flow contours.
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ItemA hybrid Navier Stokes/vortex particle wake methodology for modeling helicopter rotors in forward flight and maneuvers(Georgia Institute of Technology, 2018-04-11) Battey, Luke SterlingManeuvering flight and high-speed flight are critical design points in any rotorcraft’s operating envelope. These conditions give complex flow phenomena, creating high stresses and vibrations. To accurately predict the flow properties over the relatively flexible rotor blades, coupling between computational fluid dynamics (CFD) and computational structural dynamics (CSD) is required. In this work, GT-Hybrid, a hybrid wake rotorcraft CFD code that is coupled to DYMORE, is used. A vortex particle method has been implemented, in place of the existing lattice wake methodology, that has been anticipated to better emulate a convecting wake of a rotor while providing some computational benefits. Several UH-60A flight conditions, including high-speed steady level flight as well as diving-turn and pull-up maneuvers, are simulated using the vortex particle method. Results are compared with those using the traditional wake method and available experimental data both qualitatively and quantifiably. The quantifiable comparison, which consists of a linear regression analysis, shows the vortex particle method improves prediction accuracy for maneuvers and has only minor effects on steady forward flight when compared to the lattice method results. Additionally, computational efficiency is improved by using the vortex particle method and time savings exist in every simulation.
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ItemAssessment of the icing characteristics of single and coaxial rotors(Georgia Institute of Technology, 2016-12-06) Obayashi, NanaIcing on blade surfaces adversely affects the aerodynamic performance and safety of helicopters through loss of lift, loss of power, increase in drag, decrease in stall angle and dangerous ice shedding events. Equipping rotor blades against the effects of icing increases the helicopter cost and puts higher demand on the power plant. In the field of CFD, efforts have focused on modeling the effects of icing, including the resulting rotor performance degradation. Single rotor helicopters have been the primary focus of existing models for ice accretion, leaving an opportunity to expand modeling efforts to other types of helicopters, such as coaxial rotors. Although the coaxial rotor has a number of advantages attributed to its symmetric aerodynamic environment in any flight direction, additional work is needed using physics-based models, in order to analyze the complex flow interactions between the upper and lower blades. An in-house ice accretion model was improved upon prior work by implementing a 3-D Eulerian approach integrated into the CFD flow solver, GT-Hybrid, in order to solve for water droplet collection efficiency on the surface of the rotor blade. This model implements an extended Messinger model with the Stefan condition at the ice/water interface in order to predict ice accretion based on droplet collection and establishment of a thermodynamic balance for phase shift. These improvements have allowed this model to reduce the limitations and empiricism inherent in existing models. The model has been validated based on a limited number of cases with promising predictive power compared to the industry standard ice accretion model by NASA, called LEWICE. The present work contributes to the efforts behind the in-house ice accretion model in two ways. First, ice shape prediction using the in-house model is validated against existing experimental ice accretion data for a single rotor configuration in three different flight conditions. An analysis of the simulated and experimental results presented shows promising evidence of the model’s predictive power, especially at the inboard blade locations where the ice is predominantly rime. Second, the in-house model is adapted for application to a coaxial rotor configuration. In order to validate the flow solution, performance analysis is completed for a coaxial rotor in hover using GT-Hybrid and Star-CCM+ in the absence of ice accretion. Then, ice accretion is simulated for the same rotor for three collective pitch angles and the ice shapes are presented. Finally, the performance degradation of the coaxial rotor due to ice is estimated.
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ItemIn-cloud ice accretion modeling on wind turbine blades using an extended Messinger model(Georgia Institute of Technology, 2015-05-15) Ali, Muhammad AntthoWind turbines often operate under cold weather conditions where icing may occur. Icing causes the blade sections to stall prematurely reducing the power production at a given wind speed. The unsteady aerodynamic loads associated with icing can accelerate blade structural fatigue and creates safety concerns. In this work, the combined blade element-momentum theory is used to compute the air loads on the baseline rotor blades, prior to icing. At each blade section, a Lagrangian particle trajectory model is used to model the water droplet trajectories and their impact on the blade surface. An extended Messinger model is next used to solve the conservation of mass, momentum, and energy equations in the boundary layer over the surface, and to determine ice accretion rate. Finally, the aerodynamic characteristics of the iced blade sections are estimated using XFOIL, which initiate the next iteration step for the computation of air loads via combined blade element theory. The procedure repeats until a desired exposure time is achieved. The performance degradation is then predicted, based on the aerodynamic characteristics of the final iced blades. The 2-D ice shapes obtained are compared against experimental data at several representative atmospheric conditions with acceptable agreement. The performance of a generic experimental wind turbine rotor exposed to icing climate is simulated to obtain the power loss and identify the critical locations on the blade. The results suggest the outboard of the blade is more prone to ice accumulation causing considerable loss of lift at these sections. Also, the blades operating at a higher pitch are expected to accumulate more ice. The loss in power ranges from 10% to 50% of the rated power for different pitch settings under the same operating conditions.
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ItemSmall wind turbines mounted to existing structures(Georgia Institute of Technology, 2010-05-20) Duffy, Michael JamesSmall wind turbines, and especially urban-mounted turbines which require no dedicated pole, have garnered great public enthusiasm in recent years. This enthusiasm has fueled widespread growth among energy conservationists, and estimates predict that the power produced nationally by small wind will increase thirty-fold by 2013. Unfortunately, most of the wind resources currently available have been designed for larger, rural-mounted turbines; thus, they are not well suited for this nascent market. A consequence of this is that many potential urban small wind turbine owners over-predict their local wind resource, which is both costly and inefficient. According to a recent study published by Encraft Ltd., small wind turbines mounted to buildings far underperformed their rural pole mounted counterparts. As a proposed solution to this problem, this project introduces the concept of a Web-based Wind Assessment System (WWAS). This system combines all the necessary resources for potential urban small wind turbine customers into a single web-based tool. The system also presents the concept of a modular wind measurement system, which couples with the WWAS to provide real-time wind data measurements. The benefits of the system include its ease of use, flexibility of installation, data accessibility from any web browser, and expert advice. The WWAS prevents potential clients from investing in a system that may not be viable for their location. In addition, a small wind turbine is designed in this project, which has a unique modular mounting system, allowing the same baseline wind turbine to attach to various structures using interchangeable mounting hardware. This includes such accessible urban structures as street lights, building corners, flag poles, and building walls, among others. This design also utilizes concepts that address some of the challenges associated with mounting small wind turbines to existing urban structures. These concepts include: swept tip blades and lower RPM to reduce noise; vibration suppression using rubber shims; a netted duct to protect wildlife; and a direct-drive permanent magnet generator to ensure low starting torque. Finally, the cost of this system is calculated using off-the-shelf components, which minimize testing and certification expense. This small wind turbine system is designed to be grid-connected, has a 6 foot diameter rotor, and is rated at 1 kW. This design features a unique modular interchangeable mounting system. The cost for this complete system is estimated to be $2,050. If a users' site has an average wind speed of 14 mph (6.5 m/s), this system will generate a return on investment in 8.5 years, leaving over 10 years of profit. The profit for this system, at this sample average wind speed, yields over $4,000 during its 20-year design life, which is a two-fold return on investment. This project has implications for various stakeholders in the small wind turbine market, including designers, engineers, manufacturers, and potential customers. Equally important is its potential role in guiding our future national--even global--energy agenda.
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ItemModeling dynamic stall of SC-1095 airfoil at high mach number(Georgia Institute of Technology, 2010-01-26) Clark, BrianIn this thesis, the Leishman-Beddoes method of determining airloads for an airfoil undergoing dynamic stall is studied over a range of Mach numbers. To validate the method for conditions where little experimental data is available, a computational fluid dynamics solver is utilized to provide airload predictions for comparison to the Leishman-Beddoes results. It is found that even for high Mach numbers the Leishman-Beddoes method provides reliable predictions for lift coefficient. However, at the higher Mach numbers pitching moment is sometimes overpredicted at high angle of attack. This is seemingly due to an inability to accurately determine the center of pressure in the high speed unsteady flow environment.