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Daniel Guggenheim School of Aerospace Engineering

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https://hdl.handle.net/1853/70742

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College of Engineering

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Aerospace Design Group

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Aerospace Systems Design Laboratory (ASDL)

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Air Transportation Laboratory

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Cognitive Engineering Center

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Space Systems Design Laboratory (SSDL)

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Publication Search Results

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Computational fluid dynamics simulation of three-dimensional parallel jets

(Georgia Institute of Technology, 2018-12-11) Liu, Zhihang

High-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.
A 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 Sterling

Maneuvering 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.
Aerodynamics and aeroacoustic sources of a coaxial rotor

(Georgia Institute of Technology, 2018-04-10) Schatzman, Natasha Lydia

Vehicles with coaxial, contra-rotating rotor systems (CACR) are being considered for a range of applications, including those requiring high speed and operations in urban environments. Community and environmental noise impact is likely to be a concern in these applications. Design parameters are identified that effect the fundamental aerodynamics and fluid dynamic features of a CACR in hover, vertical, and edgewise flight. Particular attention is paid to those features affecting thickness, loading, blade vortex interaction (BVI), and high speed impulsive (HSI) noise. Understanding the fluid dynamic features is a precursor to studying the aeroacoustics of a coaxial rotor. Rotor performance was computed initially using Navier-Stokes solver with prescribed blade section aerodynamic properties, the results validated against generic experimental test cases. The fluid dynamics of blade interactions was simplified and broken into a 2-D blade crossing problem, with crossing locations and velocity fields from the rotor results. Two trains of 8 airfoils passing were simulated to understand the effects due to shed vorticity. The airfoils are displaced vertically by a distance equivalent to the typical spacing between the upper and lower rotors of a coaxial system. A 2D potential flow code and 2D OVERFLOW compressible-flow Navier-Stokes solver were used to investigate the complex coaxial rotor system flow field. One challenge of analyzing the CACR is the difficulty in envisioning all the possible interactions and their possible locations as flight conditions and rotor designs change. A calculation tool has been developed to identify time and location of blade overlap. The tool was then integrated with a wake aerodynamics model to identify locations and instances of upper rotor tip vortex interaction with a lower rotor blade. This tool enables rapid identification of different types of BVI based on relative rotor orientation. Specific aerodynamic phenomena that occur for each noise source relevant to CACR are presented, along with computational tools to predict these occurrences.
Assessment of the icing characteristics of single and coaxial rotors

(Georgia Institute of Technology, 2016-12-06) Obayashi, Nana

Icing 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.
A new rotorcraft design framework based on reliability and cost

(Georgia Institute of Technology, 2016-07-26) Scott, Robert C.

Helicopters provide essential services in civil and military applications due to their multirole capability and operational flexibility, but the combination of the disparate performance conditions of vertical and cruising flight presents a major compromise of aerodynamic and structural efficiency. In reviewing the historical trends of helicopter design and performance, it is apparent that the same compromise of design conditions which results in rotorcraft performance challenges also affects reliability and cost through vibration and fatigue among many possible factors. Although many technological approaches and design features have been proposed and researched as means of mitigating the rotorcraft affordability deficit, the assessment of their effects on the design, performance, and life-cycle cost of the aircraft has previously been limited to a manual adjustment of legacy trends in models based on regression of historical design trends. A new approach to the conceptual design of rotorcraft is presented which incorporates cost and reliability assessment methods to address the price premium historically associated with vertical flight. The methodology provides a new analytical capability that is general enough to operate as a tool for the conceptual design stage, but also specific enough to estimate the life-cycle effect of any RAM-related design technology which can be quantified in terms of weight, power, and reliability improvement. The framework combines aspects of multiple design, cost, and reliability models – some newly developed and some surveyed from literature. The key feature distinguishing the framework from legacy design and assessment methods is its ability to use reliability as a design input in addition to the flight conditions and missions used as sizing points for the aircraft. The methodology is first tested against a reference example of reliability-focused technology insertion into a legacy rotorcraft platform. Once the approach is validated, the framework is applied to an example problem consisting of a technology portfolio and a set of advanced rotorcraft configurations and performance conditions representative of capabilities desired in near-future joint service, multirole rotorcraft. The framework sizes the different rotorcraft configurations for both a baseline set of assumptions and a tradespace survey of reliability investment to search for an optimum design point corresponding to the level of technology insertion which results in the lowest life-cycle cost or highest value depending on the assumptions used. The study concludes with a discussion of the results of the reliability trade study and their possible implications for the development and acquisition of future rotorcraft.
Dynamics of Harmonically Forced Nonpremixed Flames

(Georgia Institute of Technology, 2016-04-19) Magina, Nicholas A

This thesis describes the dynamics, both spatio-temporal and heat release, of harmonically excited non-premixed flames. Analytical, numerical, computational, and, experimental analyses were performed, along with combined analyses methods, to study excitation and evolution of wrinkles on the flame front. Explicit expressions for the dynamics were developed. Wrinkle convection at the mean axial flow speed, and wrinkle dissipation and dispersion were analytically identified in the Pe-->∞ and Pe>>1 limits, respectively. Altered inlet mixture fraction profiles and attachment point dynamics were shown to accompany axial diffusion effects. Some physical effects such as axial diffusion, forcing configuration, and anisotropic diffusion altered the wrinkle interference pattern/waveform characteristics, while others, such as confinement, dimensionality, and differential diffusion, altered the dynamics through modifying the mean flame location. Comparisons to established premixed flame dynamics were made throughout. Despite having similar space-time dynamics, the heat release dynamics of the two differed greatly, having different dominant contributions, as well as different asymptotic trends. Experimental results obtained validated previous findings as well as enabled advanced model development, revealing the importance of accurate mixture fraction field capture, particularly in the near burner exit region. Findings shed light onto model and predictive improvements for future works.
Development of a physics based methodology for the prediction of rotor blade ice formation

(Georgia Institute of Technology, 2015-11-10) Kim, Jee Woong

Modern helicopters, civilian and military alike, are expected to operate in all weather conditions. Ice accretion adversely affects the availability, affordability, safety and survivability. Availability of the vehicle may be compromised if the ice formation requires excessive torque to overcome the drag needed to operate the rotor. Affordability is affected by the power requirements and cost of ownership of the deicing systems needed to safely operate the vehicle. Equipment of the rotor blades with built-in heaters greatly increases the cost of the helicopter and places further demands on the engine. The safety of the vehicle is also compromised due to ice shedding events, and the onset of abrupt, unexpected stall phenomena attributable to ice formation. Given the importance of understanding the effects of icing on aircraft performance and certification, considerable work has been done on the development of analytical and empirical tools, accompanied by high quality wind tunnel and flight test data. In this work, numerical studies to improve ice growth modeling have been done by reducing limitations and empiricism inherent in existing ice accretion models. In order to overcome the weakness of Lagrangian approach in unsteady problem such as rotating blades, a water droplet solver based on 3-D Eulerian method is developed and integrated into existing CFD solver. Also, the differences between the industry standard ice accretion analyses such as LEWICE and the ice accretion models based on the extended Messinger model are investigated through a number of 2-D airfoil and 3-D rotor blade ice accretion studies. The developed ice accretion module based on 3-D Eulerian water droplet method and the extended Messinger model is also coupled with an existing empirical ice shedding model. A series of progressively challenging simulations have been carried out. These include ability of the solvers to model airloads over an airfoil with a prescribed/simulated ice shape, collection efficiency modeling, ice growth, ice shedding, de-icing modeling, and assessment of the degradation of airfoil or rotor performance associated with the ice formation. While these numerical simulation results are encouraging, much additional work remains in modeling detailed physics important to rotorcraft icing phenomena. Despite these difficulties, progress in assessing helicopter ice accretion has been made and tools for initial analyses have been developed.Modern helicopters, civilian and military alike, are expected to operate in all weather conditions. Ice accretion adversely affects the availability, affordability, safety and survivability. Availability of the vehicle may be compromised if the ice formation requires excessive torque to overcome the drag needed to operate the rotor. Affordability is affected by the power requirements and cost of ownership of the deicing systems needed to safely operate the vehicle. Equipment of the rotor blades with built-in heaters greatly increases the cost of the helicopter and places further demands on the engine. The safety of the vehicle is also compromised due to ice shedding events, and the onset of abrupt, unexpected stall phenomena attributable to ice formation. Given the importance of understanding the effects of icing on aircraft performance and certification, considerable work has been done on the development of analytical and empirical tools, accompanied by high quality wind tunnel and flight test data. In this work, numerical studies to improve ice growth modeling have been done by reducing limitations and empiricism inherent in existing ice accretion models. In order to overcome the weakness of Lagrangian approach in unsteady problem such as rotating blades, a water droplet solver based on 3-D Eulerian method is developed and integrated into existing CFD solver. Also, the differences between the industry standard ice accretion analyses such as LEWICE and the ice accretion models based on the extended Messinger model are investigated through a number of 2-D airfoil and 3-D rotor blade ice accretion studies. The developed ice accretion module based on 3-D Eulerian water droplet method and the extended Messinger model is also coupled with an existing empirical ice shedding model. A series of progressively challenging simulations have been carried out. These include ability of the solvers to model airloads over an airfoil with a prescribed/simulated ice shape, collection efficiency modeling, ice growth, ice shedding, de-icing modeling, and assessment of the degradation of airfoil or rotor performance associated with the ice formation. While these numerical simulation results are encouraging, much additional work remains in modeling detailed physics important to rotorcraft icing phenomena. Despite these difficulties, progress in assessing helicopter ice accretion has been made and tools for initial analyses have been developed.
In-cloud ice accretion modeling on wind turbine blades using an extended Messinger model

(Georgia Institute of Technology, 2015-05-15) Ali, Muhammad Anttho

Wind 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.
Numerical investigation on the use of multi-element blades in vertical-axis wind turbines

(Georgia Institute of Technology, 2015-01-12) Bah, Elhadji Alpha Amadou

The interest in sustainable forms of energy is being driven by the anticipated scarcity of traditional fossil fuels over the coming decades. There is also a growing concern about the effects of fossil fuel emissions on human health and the environment. Many sources of renewable energy are being researched and implemented for power production. In particular, wind power generation by horizontal- and vertical-axis wind turbines is very popular. Vertical-axis wind turbines (VAWTs) have a relative construction simplicity compared to horizontal-axis wind turbines (HAWTs). However, VAWTs present specific challenges that may hinder their performance. For instance, they are strongly affected by dynamic stall. A significant part of the kinetic energy contained in the oncoming wind is lost in swirl and vortices. As a result, VAWTs have lower power production compared to HAWTs. First, the present work is aimed at the study of the aerodynamics of straight-bladed VAWTs (SB-VAWTs). Empirical calculations are conducted in a preliminary work. Then a two-dimensional double multiple streamtube (DMST) approach supported by a two-dimensional numerical study is implemented. The dynamic stall and aerodynamic performance of the rotor are investigated. A VAWT-fitted dynamic stall model is implemented. Computational fluid dynamics (CFD) simulations are conducted to serve as reference for the DMST calculations. This three-pronged approach allows us to efficiently explore multiple configurations. The dynamic stall phenomenon is identified as a primary cause of performance loss. The results in this section validate the DMST model as a good replacement for CFD analysis in early phase design provided that a good dynamic stall model is used. After having identify the primary cause of performance loss, the goal is to investigate the use to dual-element blades for alleviating the effect of dynamic stall, thereby improving the performance of the rotor. The desirable airfoil characteristics are defined and a parametric analysis conducted. In the present study the parameters consists of the size of the blade elements, the space between them, and their relative orientation. The performance of the rotor is calculated and compared to the baseline. The results highlight the preeminence of the two-element configuration over the single-element provided that the adequate parametric study is conducted beforehand. A performance enhancement is obtained over a large range of tip speed ratios. The starting characteristics and the operation stability are also improved. Finally, an economic analysis is conducted to determine the cost of energy and thus the financial viability of such a project. The Great Coast of Senegal is selected as site of operation. The energy need and sources of this region are presented along with its wind energy potential. The cost evaluation shows the economic viability by comparing the cost of energy to the current energy market prices.
Physics based prediction of aeromechanical loads for the UH-60A rotor

(Georgia Institute of Technology, 2013-04-12) Marpu, Ritu Priyanka

Helicopters in forward flight experience complex aerodynamic phenomena to various degrees. In low speed level flight, the vortex wake remains close to the rotor disk and interacts with the rotor blades to give rise to blade vortex interaction phenomena. In high speed flight, compressibility effects dominate leading to the formation of shocks. If the required thrust is high, the combination of high collective pitch and cyclic pitch variations give rise to three-dimensional dynamic stall phenomena. Maneuvers further exacerbate the unsteady airloads and affect rotor and hub design. The strength and durability of the rotor blades and hub components is dependent on accurate estimates of peak-to-peak structural loads. Accurate knowledge of control loads is important for sizing the expensive swash-plate components and assuring long fatigue life. Over the last two decades, computational tools have been developed for modeling rotorcraft aeromechanics. In spite of this progress, loads prediction in unsteady maneuvers which is critical for peak design loads continues to be a challenging task. The primary goal of this research effort is to investigate important physical phenomena that cause severe loads on the rotor in steady flight and in extreme maneuvers. The present work utilizes a hybrid Navier-Stokes/free-wake CFD methodology coupled to a finite element based multi-body dynamics analysis to systematically study steady level and maneuvering flight conditions. Computational results are presented for the UH-60A rotor for a parametric sweep of speed and thrust conditions and correlated with test data at the NFAC Wind Tunnel. Good agreement with test data has been achieved using the current methodology for trim settings and integrated hub loads, torque, and power. Two severe diving turn maneuvers for the UH-60A recorded in the NASA/Army Airloads Flight Tests Database have also been investigated. These maneuvers are characterized by high load factors and high speed flight. The helicopter experiences significant vibration during these maneuvers. Mean and peak-to-peak structural loads and extensive stall phenomena including an advancing side stall phenomena have been captured by the present analyses.