Browsing by Author "Dr. Hassan A. Hassan, Committee Member"

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Advancements in Aerodynamic Technologies for Airfoils and Wings
(2006-12-08) Jepson, Jeffrey Keith; Dr. Jeffrey A. Joines, Committee Member; Dr. Charles E. Hall, Committee Member; Dr. Hassan A. Hassan, Committee Member; Dr. Ashok Gopalarathnam, Committee Chair
Although aircraft operate over a wide range of ﬂight conditions, current fixed geometry aircraft are optimized for only a few of these conditions. By altering the shape of the aircraft, adaptive aerodynamics can be used to increase the safety and performance of an aircraft by tailoring the aircraft for multiple light conditions. Of the various shape adaptation concepts currently being studied, the use of multiple trailing-edge ﬂaps along the span of a wing offers a relatively high possibility of being incorporated on aircraft in the near future. Multiple trailing-edge ﬂaps allow for effective spanwise camber adaptation with resulting drag beneﬁts over a large speed range and load alleviation at high-g conditions. The research presented in this dissertation focuses on the development of this concept of using trailing-edge ﬂaps to tailor an aircraft for multiple liight conditions. One of the major tasks involved in implementing trailing-edge ﬂaps is in designing the airfoil to incorporate the ﬂap. The ﬁrst part of this dissertation presents a design formulation that incorporates aircraft performance considerations in the inverse design of low-speed laminar-ﬂow adaptive airfoils with trailing-edge cruise ﬂaps. The beneﬁt of using adaptive airfoils is that the size of the low-drag region of the drag polar can be effectively increased without increasing the maximum thickness of the airfoil. Two aircraft performance parameters are considered: level-ﬂight maximum speed and maximum range. It is shown that the lift coefficients for the lower and upper corners of the airfoil low-drag range can be appropriately adjusted to tailor the airfoil for these two aircraft performance parameters. The design problem is posed as a part of a multidimensional Newton iteration in an existing conformal-mapping based inverse design code, PROFOIL. This formulation automatically adjusts the lift coefficients for the corners of the low-drag range for a given ﬂap deﬂection as required for the airfoil-aircraft matching. Examples are presented to illustrate the ﬂapped-airfoil design approach for a general aviation aircraft and the results are validated by comparison with results from post-design aircraft performance computations. Once the airfoil is designed to incorporate a TE ﬂap, it is important to determine the most suitable ﬂap angles along the wing for different flight conditions. The second part of this dissertation presents a method for determining the optimum flap angles to minimize drag based on pressures measured at select locations on the wing. Computational ﬂow simulations using a panel method are used "in the loop" for demonstrating closed-loop control of the ﬂaps. Examples in the paper show that the control algorithm is successful in correctly adapting the wing to achieve the target lift distributions for minimizing induced drag while adjusting the wing angle of attack for operation of the wing in the drag bucket. It is shown that the "sense-and-adapt" approach developed is capable of handling varying and unpredictable inﬂow conditions. Such a capability could be useful in adapting long-span ﬂexible wings that may experience signiﬁcant and unknown atmospheric inﬂow variations along the span. To further develop the "sense-and-adapt" approach, the method was tested experimentally in the third part of the research. The goal of the testing was to see if the same results found computationally can be obtained experimentally. The North Carolina State University subsonic wind tunnel was used for the wind tunnel tests. Results from the testing showed that the "sense-and-adapt" approach has the same performance experimentally as it did computationally. The research presented in this dissertation is a stepping stone towards further development of the concept, which includes modeling the system in the Simulink environment and flight experiments using uninhabited aerial vehicles.
Calibration Methods for a Constant Voltage Anemometer-Operated Hot-Wire in a Hypersonic Flow
(2003-09-17) Reimann, Craig Aaron; Dr. Hassan A. Hassan, Committee Member; Dr. Harvey J. Charlton, Committee Member; Dr. Ndaona Chokani, Committee Chair
The calibration of measurements made with a CVA-operated hot-wire, in a quiet hypersonic wind tunnel, are examined. Two calibration methods are evaluated. The first calibration method is based on the ratio of the power dissipated in the hot-wire and the difference between the hot- and cold-wire resistances. The second method is based on the traditional semi-empirical heat transfer relation for a heated wire in supersonic/hypersonic flow. The mean mass flux and total temperature derived from the calibration methods show overall good agreement with Navier-Stokes predictions. In general, the first method gives better results across the boundary layer. However, limitations in the calibration data meant no definitive assessment of the capability to yield fluctuation measurements could be made. The recently derived sensitivity coefficients for a CVA-operated hot-wire are used to obtain fluctuation measurements. Also, the fluctuating diagram method is used to evaluate the performance of the NASA Langley Mach 6 quiet tunnel and to detail the characteristics of the fluctuations in the laminar hypersonic boundary layer. Within the boundary layer, profiles of the measured rms fluctuations and the fluctuation diagrams show that the intensityof the boundary layer disturbance fluctuations are most intense in the vicinity of the critical layer. The static pressure fluctuations derived from the fluctuation diagrams verify that the fluctuations are less than 0.2%. This observation confirms previous qualitative observations of the performance of the Mach 6 quiet nozzle.
An Investigation of the Influence of Nozzle Geometry in the Hydroentangling Process
(2007-08-18) Anantharamaiah, Nagendra; Dr. Hooman Vahedi Tafreshi, Committee Co-Chair; Dr. Behnam Pourdeyhimi, Committee Co-Chair; Dr. Hassan A. Hassan, Committee Member; Dr. Timothy Clapp, Committee Member; Dr. William Oxenham, Committee Member
Large-Eddy Simulation of Particulate Resuspension and Transport Under Influences of Human-Body Motion in an Indoor Setting
(2007-05-15) Oberoi, Roshan C.; Dr. Jack R. Edwards, Committee Chair; Dr. Hassan A. Hassan, Committee Member; Dr. Pierre A. Gremaud, Committee Member
A methodology is presented for simulating particulate resuspension and transport under influences of human-body motion in an indoor setting. The simulations in this study mirror experiments performed by the U.S. Environmental Protection Agency (EPA), which funded the present study, and the Research Triangle Institute (RTI) at the EPA test facility in Cary, NC. A large-eddy simulation (LES) framework is implemented to obtain the time-dependent flow field within a room. An artificial compressibility method with low-diffusion upwinding and weighted essentially non-oscillatory (WENO) variable extrapolation is employed to obtain an incompressible Navier-Stokes solution. Unresolved fluctuations are accounted for by a Smagorinsky sub-grid scale stress model. A human body is modeled as an immersed boundary within the Cartesian grid domain. This body is comprised of several immersed components, representing separate body parts. Interpolation methods force the fluid and particle properties near the immersed surface to respond to the motion of the bodies, which is governed by prescribed rate laws. The particle phase is assumed to be dilute, and thus, does not affect the solution of the carrier fluid. An Eulerian viewpoint is taken to model the particle fields, requiring separate solutions for each size of particle simulated. Size classes are determined by taking sectional averages of a lognormal probability density function, extracted from experimental data. The motion of the particle fields, subject to hydrodynamic drag forces, is determined by solving mass and momentum conservation equations for each size class. A second-order TVD upwind scheme is used for the advection of particle fields, and a point-implicit sub-iteration method is used for time-advancement. The present simulations involve a human body walking and stamping its feet for about 20 seconds — causing particles initially contained within a carpet to resuspend — then standing still for the remainder of the simulation. In order to account for the porous structure of the carpet, Darcy-type resistance terms are applied to the solution of the carrier fluid. Micro-scale surface effects acting on the particles, such as van der Waals and electrostatic forces, are modeled by applying a size-dependent sticking force to particles contained by the carpet. This sticking force is approximated in a parametric fashion by comparing simulated particle emission factors with those obtained experimentally. Effects of an HVAC system are also modeled by applying inflow boundary conditions of measured velocity at two known vent locations. These simulations are performed on a computational domain of approximately 5.4 million grid points and are mapped to 36 Intel Xeon processors on an IBM Blade Center Linux Cluster using the MPI message passing standard. The simulations produced similar levels of particulate mass resuspension to those observed experimentally. Results indicated that a large majority of the particles resuspended originated from regions of the carpet very near where the immersed-body "feet" penetrated, while particles elsewhere in the room were mostly undisturbed. Despite the fact that most of the mass resuspended was due to large particles, much more small-particle mass remained airborne over the duration of the 7-minute simulations due to much lower settling rates. A relatively "well-mixed" state was achieved in the room after about 3 minutes of physical time. This made it possible to identify steady particle-decay trends over the last few minutes of the simulations in order predict concentrations in the room beyond this extent of time.
Numerical Simulation of Injection of Supercritical Ethylene/Methane into Nitrogen
(2005-12-13) Star, Ana Maria; Dr. Jack R. Edwards, Committee Chair; Dr. Richard D. Gould, Committee Member; Dr. Hassan A. Hassan, Committee Member
The present study investigates the physical mechanisms of supercritical fluid injection for pure ethylene and ethylene/methane mixtures, as well as onset of condensation upon fluid expansion. These mechanisms are considered a key enabling technology in the design of hydrocarbon-fueled scramjet engines. The numerical method combines a solution of the compressible Navier-Stokes equations for the supercritical fluid with two different approaches for condensate growth: one based on a homogeneous equilibrium assumption and the other on classical aerosol dynamics. The thermodynamic behavior of the supercritical fluid is described using the Peng-Robinson equation of state. Computational results are compared with shadowgraph and direct-lighting imaging data, mass flow measurements, mole fraction measurements and temperature measurements in the jet mixing zone, and pressure distributions within a three-dimensional injector geometry. Qualitative results involving jet structure, the appearance of a condensed phase, and the general effects of back pressure and injectant temperature are in good agreement with experimental results for pure ethylene injection. Quantitative results also display reasonable agreement with experimental results but do indicate the need for improving the model. Qualitative trends for ethylene/methane mixture injection are in moderate agreement with the experimental data, suggesting that the thermodynamic interaction between ethylene and methane as modeled by the chosen mixing rules are not sufficiently accurate. For conditions where both are applicable, a finite-rate (nucleation/growth) phase-transition model presents essentially the same bulk fluid response as a homogeneous equilibrium model with additional predictions of number density and average droplet size.
Numerical Simulation of the Internal Two-Phase Flow within an Aerated-Liquid Injector and its Injection into the Corresopnding High-speed Crossflows.
(2002-09-17) Tian, Ming; Dr. Jack R. Edwards, Committee Chair; Dr. Semyon V. Tsynkov, Committee Member; Dr. Hassan A. Hassan, Committee Member
Aerated-liquid atomization, which is produced by the introduction of gas directly into a liquid flow immediately upstream of the injector exit orifice to generate a two-phase flow, has been shown to produce well-atomized sprays in a quiescent environment with only a small amount of aerating gas at relatively low injection pressures. A time-derivative preconditioning method using the Low-Diffusion Flux-Splitting Scheme (LDFSS) has been extended to a 'mixture' model of two-phase flow and applied to simulate the structure of internal two-phase flow for aerated-liquid injectors, with each phase governed by its own equation of state. The Continuum Surface Force (CSF) model of Brackbill, et al. is adapted to model compressible fluid flow influenced by interfacial surface tension. A sub-iterative time integration method based on a planar Gauss-Seidel partitioning of the system matrix is used with implicit source terms as a means of solving the three-dimensional, time-dependent form of the governing equations. The calculations are parallelized using domain-decomposition and Message-Passing Interface (MPI) methods, and are optimized for operation on the 720 processor IBM SP-2 at the North Carolina Supercomputing Center (NCSC). Simulation results for 2-D aerated-liquid injector flowfields at gas-to-liquid (GLR) mass ratios of 0.08% and 2.45% are discussed. In accord with experimental visualization data, the results for GLR = 0.08% indicate a combination of slugging and core-annular two-phase flow in the injector. Results at GLR = 2.45% indicate that a core-annular flow mode dominates, again in agreement with experimental results. The effects of the choice of reference velocity and the level of surface tension on the injector flowfield solutions are also examined.
Numerical Simulation of the Internal Two-Phase Flow within an Aerated-Liquid Injector and its Injection into the Corresponding High-speed Crossflows
(2005-08-16) Tian, Ming; Dr. Hassan A. Hassan, Committee Member; Dr. D. Scott McRae, Committee Member; Dr. Zhilin Li (Dept. of Mathematics), Committee Member; Dr. Jack R. Edwards, Committee Chair
The current study investigates the flow structures within an aerated-liquid (barbotage) injector, which is designed to facilitate the rapid breakup of a hydrocarbon fuel jet prior to its entering a scramjet combustor, and the spray structures in the corresponding crossflow. Simulations of the transient, three-dimensional, two-phase flow within the "out-in" injector operating at different gas-to-liquid (GLR) mass ratios and in the corresponding crossflow domain have been performed, and the results compared with experimental pressure measurements of the injector and shadowgraph images of the crossflow. The numerical method solves a "mixture" model of two-phase flow using a preconditioning strategy. High-order spatial accuracy and good interface-capturing properties are facilitated by the use of shock-capturing schemes combined with second order TVD methods. Also, an immersed boundary method is used to investigate the probe effects, and a droplet transport model is used in the crossflow simulations to get more details about effect of droplet size. The injector simulation results highlight the effects of mesh refinement and turbulence model on the predicted solutions. The pressure drop across the injector is predicted reasonably well by the computational methodology, and the trend of increasing injector pressure with increasing GLR is captured properly. Predictions of the absolute pressure level within the injector show some discrepancies in comparison with experimental data but agree well with theoretical estimates. The results of the injector simulations with plenum included are consistent with the results of the discharge tube cases. If the centerline pressure is close to the experimental data, the gas mass flow rate at outlet will approach a value below the experimental data. If the gas mass flow rate at outlet approaches the experimental data, then the centerline pressure will be higher than the experimental data, but agrees well with theoretical analyses. The intrusion of the probe has little effect on the flowfield if the probe is contained wholly within the liquid core, but does affect the flowfield if the probe tip is in the two-phase mixing region, instead of the liquid core. The results of crossflow show that the two-phase flow injects into the crossflow, bends towards the streamwise direction, disperses into a spray plume, and initiates a horse-shoe shape structure of the jet in the cross-sectional planes. The result based on the previous injector simulation at a higher inlet gas pressure shows best penetration height prediction among all freestream Mach 0.3 cases. Including the droplet transport model gives a similar spray structure in the X-Z centerplane as that of the mixture model, but gives a different spray structure in the cross-sectional planes. The horse-shoe shaped structure fades away with increases in the droplet diameter size, and the liquid mass accumulates to the X-Z centerplane.
Particle Flow, Agglomeration, Mixing, Chemical and Physical Absorption in Circulating Fluidized Bed Absorbers
(2004-01-06) Mao, Deming; Dr. H. Henry Lamb, Committee Member; Dr. Hassan A. Hassan, Committee Member; Dr. Andrey V. Kuznetsov, Committee Co-Chair; Dr. Jack R. Edwards, Committee Chair
Coal-utilization for energy production poses considerable environmental concerns as it results in emission of sulfur dioxide (SO2), nitrogen oxide (NOx), fine particulate matter (PM), and trace heavy metals such as mercury vapor (Hg) during coal-combustion. Circulating Fluidized Bed Adsorbers (CFBAs) are regarded as a potentially effective technology to capture some of the above pollutants. In particular, one could use limestone to remove sulfur dioxide by chemical adsorption, and activated carbon to remove elemental mercury by physical adsorption. Also, sorbent particles could be used to capture fine PM or promote formation of clusters of larger PM. In order to analyze CFBA systems in detail, a new approach has been developed for solving the Navier-Stokes equations for a gas-mixture/solids-mixture system. Sub-models are also developed to be combined with the gas/solids hydrodynamics model to simulate capture of multiple pollutants. Specific tasks accomplished include the following. 1. A model for fine particle agglomeration in CFBAs has been developed. It can model the influence of different factors on agglomeration, such as the geometry of a CFBAs, the superficial gas velocity, initial particle size distribution (PSD), and type of agglomeration mechanism. It is found that the Brownian agglomeration mechanism can be neglected compared to agglomeration by mean shear and turbulence. Sorbent particles are shown to capture fine particles effectively for certain conditions. A simplified version of this model has been developed for coupling with the hydrodynamics model. 2. A mixing model based on a core-annulus model of a CFBA has been developed to simulate the particle residence time distribution (RTD). Thus, macrochemical reaction can be simulated by combining microchemical reaction dynamics with the particle RTD. This has been applied to simulate SO_2 removal by chemical adsorption onto dry lime. 3. A 'gas mixture' and ``solids mixture' model has been developed to simulate fine particle agglomeration onto sorbent particles, sulfur dioxide removal through chemical adsorption with lime, and mercury vapor removal through physical adsorption with activated carbon. The 'gas mixture' is composed of fine PM, sulfur dioxide, mercury vapor, oxygen and inert gas; while 'solids mixture' is composed of solids-1 and solids-2. Solids-1 is composed of lime (CaO) and CaSO_4, and solids-2 is activated carbon. These equations are similar to continuity equations appearing in the gas-solids hydrodynamics system and are integrated fully coupled with that system. 4. A new approach for solving the Navier-Stokes equations governing gas/solids two-phase flow with chemical reaction has been developed. The approach combines a time-derivative preconditioning strategy for a gas/solids two-phase flow model with extensions of low-diffusion flux-splitting upwinding techniques. The combined framework is used to simulate jet-induced bubble formation within a minimally fluidized bed, flow within a circulating fluidized bed without chemical reaction, a downward fludized bed, and gas mixture/solids mixture flow within a prototype CFBA device. For bubble bed simulation, the model gives good results for bubble formation, growth and burst. For three-dimensional CFBAs, it can give good results compared with experimental data and can capture details of solids clustering phenomena. For downward fluidized bed, the model gives plausible results regarding the influence of superficial gas velocity on particle flow and mixing. Finally, simulations of a ``bench-scale' CFBA reactor with combined SO_2, Hg and fine PM capture give reasonable results for gas species and solids species, but further validation is needed.
Reynolds-Averaged Navier-Stokes Analysis of the Flow through a Model Rocket-Based Combined Cycle Engine with an Independently-Fueled Ramjet Stream
(2003-08-18) Bond, Ryan Bomar; Dr. Jack R. Edwards, Committee Co-Chair; Dr. D. Scott McRae, Committee Co-Chair; Dr. Hassan A. Hassan, Committee Member; Dr. C. Timothy Kelley, Committee Member
A new concept for the low speed propulsion mode in rocket based combined cycle (RBCC) engines has been developed as part of the NASA GTX program. This concept, called the independent ramjet stream (IRS) cycle, is a variation of the traditional ejector ramjet (ER) design and involves the injection of hydrogen fuel directly into the air stream, where it is ignited by the rocket plume. Experiments and computational fluid dynamics (CFD) are currently being used to evaluate the feasibility of the new design. In this work, a Navier-Stokes code valid for general reactive flows is applied to the model engine under cold flow, ejector ramjet, and IRS cycle operation. Pressure distributions corresponding to cold-flow and ejector ramjet operation are compared with experimental data. The engine response under independent ramjet stream cycle operation is examined for different reaction models and grid sizes. The engine response to variations in fuel injection is also examined. Mode transition simulations are also analyzed both with and without a nitrogen purge of the rocket. The solutions exhibit a high sensitivity to both grid resolution and reaction mechanism, but they do indicate that thermal throat ramjet operation is possible through the injection and burning of additional fuel into the air stream. The solutions also indicate that variations in fuel injection location can affect the position of the thermal throat. The numerical simulations predicted successful mode transition both with and without a nitrogen purge of the rocket; however, the reliability of the mode transition results cannot be established without experimental data to validate the reaction mechanism.
Time Accurate Unstructured Grid Adaption in Two and Three Dimensions
(2007-12-07) Carpenter, James Givens, V; Dr. D. Scott McRae, Committee Chair; Dr. Jack R. Edwards, Committee Member; Dr. Hassan A. Hassan, Committee Member; Dr. C. T. Kelley, Committee Member
The adaption algorithm of Benson et al is extended to three dimensional unstructured grids, building on the previous extension to two dimensional unstructured grids. R-refinement grid adaption is performed using a center of mass equation constructed from a weight function computed from solution gradients. Solution variables are updated using a coupled approach where the flux interface for each cell face is adjusted by the local grid velocity. Modifications to the integration scheme are incorporated to account for volume changes due to grid adaption through the introduction of an unsteady residual term which is resolved using sub-iterations at each timestep. The previous structured grid definition of grid velocity is shown to be inadequate for unstructured grid motion, and a new conservation based grid velocity equation is constructed from the local face displacement, which is designed to capture the volume change and preserve geometric conservation. Time accuracy is demonstrated for two and three dimensions using a shock tube simulation. Implementation for three dimensions is accomplished using a parallel, point implicit commercial flow solver. Incorporation of the gridspeed terms in the flux interface equations is presented along with the modifications to the implicit integration scheme required to account for the volume change as the grid is displaced. Extension to three dimensions required development of smoothing routines designed to preserve or recapture grid quality for arbitrary tetrahedral grids based on a geometric quality definition. A wing section under a prescribed sinusoidal motion is presented as a demonstration case to show the efficacy of the method. Computational results are compared to experimental data and solutions obtained using CFL3D.