bluuebunny/crossref_metadata_2025 huggingface dataset:
Rotordynamic Force Prediction of a Shrouded Centrifugal Pump Impeller—Part II: Stability Analysis
Authors: Kim Eunseok , Palazzolo Alan
Published: February 2016

In Paper I, some test cases of centrifugal pump impellers which showed unconventional impedances curves were reviewed and possible sources of the bump and dip in the impedance curves were investigated by simulating a wear-ring seal pump impeller. In this paper, the unconventional impedances determined in Paper I are converted into a form for inclusion in rotordynamic stability and forced response analyses. First of all, a finite element (FE) rotor model is considered to investigate the influence of the bump and dip in the impedance curves on the rotordynamic stability. With the same FE model, speed-dependent impedances are calculated to obtain unbalance frequency response. Finally, a new linear curve-fit approach is developed to model the fluctuating impedances since the unconventional impedance cannot be expressed by the second-order polynomials with the rotordynamic coefficients (stiffness, damping, and mass). In order to validate the newly developed method, a Jeffcott rotor model with the impeller forces is considered and rotordynamic stability analysis is implemented. The results of the analysis show that the existence of the bump and dip in the impedance curves may further destabilize the rotor system.
Multi-Objective Optimization of a Steam Surface Condenser Using the Territorial Particle Swarm Technique
Authors: Mirzabeygi Pooya , Zhang Chao
Published: February 2016

The multi-objective territorial particle swarm optimization (MOTPSO) technique is proposed in this work for the optimal design of steam surface condensers. The main objective of this work is to maximize the condensation rate in a condenser while the pressure loss is minimized. Various design parameters, such as the tube outside diameter, thickness, and pitch, are considered to find the optimal ones for shell and tube heat exchangers considered in this study. The two-dimensional computational fluid dynamics (CFD) analysis is performed to solve the fluid flow and heat transfer in the condenser to assess the performance of different designs.
The Current Trends in Conventional Power Plant Technology on Two Continents From the Perspective of Engineering, Procurement, and Construction Contractor and Original Equipment Manufacturer
Authors: Stein-Brzozowska Gosia , Bergins Christian , Kukoski Allan , Wu Song , Agraniotis Michalis , Kakaras Emmanouil
Published: February 2016

In terms of CO2 emissions, the year 2030 has been addressed as a very crucial deadline for both European Union (EU) and the U.S. Whereas the U.S. Clean Power Plan proposes the reduction of national CO2 emissions from the existing power stations by 30% with respect to 2005, the EU aims at cutback by 40% from their levels in 1990. Due to the restricted emission goals dictated by the European and U.S. energy policies, both energy markets witness currently drastic changes. Whereas the U.S. wants to shift away from coal, the EU shifts away from gas due to high natural gas prices in Europe while drastically increasing the feed-ins from renewable energy sources (RES). In some of the European countries constantly growing installation of renewable energy plants is superseding natural gas-fired power plants and thus causing the electrical grid stabilization to be overtaken by coal fired power stations. On the contrary, the U.S. market due to increasing extraction of shale gas and low natural gas prices puts the gas power plants in favor and poses increasing pressure on closing some coal fired plants. A solution that uses the potential of the existing site and reduces overall emissions is converting from coal into gas-fired power plants, so-called fuel switch. Whereas for the U.S. market the later solution is relevant, in the vast majority of EU Member States the focus is on increasing the flexibility of coal fired power plants. The challenges and technical solutions developed and applied according to the demands of the market in both EU and U.S. are addressed in this paper. Both currently applied technologies and technologies under development are shortly presented.
An Experimental Investigation of the Performance Impact of Swirl on a Turbine Exhaust Diffuser/Collector for a Series of Diffuser Strut Geometries
Authors: Xue Song , Guillot Stephen , Ng Wing F. , Fleming Jon , Todd Lowe K. , Samal Nihar , Stang Ulrich E.
Published: February 2016

A comprehensive experimental investigation was initiated to evaluate the aerodynamic performance of a gas turbine exhaust diffuser/collector for various strut geometries over a range of inlet angle. The test was conducted on a 1/12th scale rig developed for rapid and accurate evaluation of multiple test configurations. The facility was designed to run continuously at an inlet Mach number of 0.40 and an inlet hydraulic diameter-based Reynolds number of 3.4 × 105. Multihole pneumatic pressure probes and surface oil flow visualization were deployed to ascertain the effects of inlet flow angle and strut geometry. Initial baseline diffuser-only tests with struts omitted showed a weakly increasing trend in pressure recovery with increasing swirl, peaking at 14 deg before rapidly dropping. Tests on profiled struts showed a similar trend with reduced recovery across the range of swirl and increased recovery drop beyond the peak. Subsequent tests for a full diffuser/collector configuration with profiled struts revealed a rising trend at lower swirl when compared to diffuser-only results, albeit with a reduction in recovery. When tested without struts, the addition of the collector to the diffuser not only reduced the pressure recovery at all angles but also resulted in a shift of the overall characteristic to a peak recovery at a lower value of swirl. The increased operation range associated with the implementation of struts in the full configuration is attributed to the deswirling effects of the profiled struts. In this case, the decreased swirl reduces the flow asymmetry responsible for the reduction in pressure recovery attributed to the formation of a localized reverse-flow vortex near the bottom of the collector. This research indicates that strut setting angle and, to a lesser extent, strut shape can be optimized to provide peak engine performance over a wide range of operation.
Development of a Computational Tool to Simulate Foil Bearings for Supercritical CO2 Cycles
Authors: Qin Kan , Jahn Ingo , Gollan Rowan , Jacobs Peter
Published: February 2016

The foil bearing is an enabling technology for turbomachinery systems, which has the potential to enable cost efficient supercritical CO2 cycles. The direct use of the cycle's working fluid within the bearings results in an oil-free and compact turbomachinery system; however, these bearings will significantly influence the performance of the whole cycle and must be carefully studied. Moreover, using CO2 as the operating fluid for a foil bearing creates new modeling challenges. These include highly turbulent flow within the film, non-negligible inertia forces, high windage losses, and nonideal gas behavior. Since the flow phenomena within foil bearings is complex, involving coupled fluid flow and structural deformation, use of the conventional Reynolds equation to predict the performance of foil bearings might not be adequate. To address these modeling issues, a three-dimensional flow and structure simulation tool has been developed to better predict the performance of foil bearings for the supercritical CO2 cycle. In this study, the gas dynamics code, eilmer, has been extended for multiphysics simulation by implementing a moving grid framework, in order to study the elastohydrodynamic performance of foil bearings. The code was then validated for representative laminar and turbulent flow cases, and good agreement was found between the new code and analytical solutions or experiment results. A separate finite difference code based on the Kirchoff plate equation for the circular thin plate was developed in Python to solve the structural deformation within foil thrust bearings, and verified with the finite element analysis from ansys. The fluid-structure coupling algorithm was then proposed and validated against experimental results of a foil thrust bearing that used air as operating fluid. Finally, the new computational tool set is applied to the modeling of foil thrust bearings with CO2 as the operating fluid.
Second-Law Heat Release Modeling of a Compression Ignition Engine Fueled With Blends of Palm Biodiesel
Authors: Mattson Jonathan , Reznicek Evan , Depcik Christopher
Published: February 2016

Modeling of engine-out heat release (HR) is of great importance for engine combustion research. Variations in fuel properties bring about changing combustion behavior within the cylinder, which may be captured by modeling of the rate of heat release (RHR). This is particularly true for biodiesel fuels, where changes in fuel behavior are linked to viscosity, density, and energy content. HR may also be expanded into an analysis using the second law of thermodynamics, which may ascertain the pathways through which availability is either captured as useful work, unused as thermal availability of the exhaust gas, or wasted as heat transfer. In specific, the second-law model identifies the period of peak availability, and thus the ideal period to extract work, and is of use for power optimization. A multizone (fuel, burned, and unburned) diagnostic model using a first law of thermodynamics analysis is utilized as a foundation for a second-law analysis, allowing for a simultaneous energy and exergy analysis of engine combustion from a captured pressure trace. The model calibrates the rate and magnitude of combustion through an Arrhenius equation in place of a traditional Wiebe function, calibrated using exhaust emission measurements. The created model is then utilized to categorize combustion of diesel and palm biodiesel fuels as well as their blends. The second-law analysis is used to highlight the effects of increasing biodiesel usage on engine efficiency, particularly with respect to fuel viscosity and combustion temperature. The second-law model used is found to provide a more clear understanding of combustion than the original first-law model, particularly with respect to the relationships between biodiesel content, viscosity, temperature, and diffusion-dominated combustion.
Linearized J2 and Atmospheric Drag Model for Control of Inner-Formation Satellite System in Elliptical Orbits
Authors: Cao Lu , Li Hengnian
Published: February 2016

A new set of linearized differential equations governing relative motion of inner-formation satellite system (IFSS) is derived with the effects of J2 as well as atmospheric drag. The IFSS consists of the “inner satellite” and the “outer satellite,” this special configuration formation endows its some advantages to map the gravity field of earth. For long-term IFSS in elliptical orbit, the high-fidelity set of linearized equations is more convenient than the nonlinear equations for designing formation control system or navigation algorithms. In addition, to avoid the collision between the inner satellite and the outer satellite, the minimum sliding mode error feedback control (MSMEFC) is adopted to perform a real-time control on the outer satellite in the presence of uncertain perturbations from the system and space. The robustness and steady-state error of MSMEFC are also discussed to show its theoretical advantages than traditional sliding mode control (SMC). Finally, numerical simulations are performed to check the fidelity of the proposed equations. Moreover, the efficacy of the MSMEFC is performed to control the IFSS with high precision.
The Exponentiated Hencky Strain Energy in Modeling Tire Derived Material for Moderately Large Deformations
Authors: Montella Giuseppe , Govindjee Sanjay , Neff Patrizio
Published: February 2016

This work presents a hyperviscoelastic model, based on the Hencky-logarithmic strain tensor, to model the response of a tire derived material (TDM) undergoing moderately large deformations. The TDM is a composite made by cold forging a mix of rubber fibers and grains, obtained by grinding scrap tires, and polyurethane binder. The mechanical properties are highly influenced by the presence of voids associated with the granular composition and low tensile strength due to the weak connection at the grain–matrix interface. For these reasons, TDM use is restricted to applications involving a limited range of deformations. Experimental tests show that a central feature of the response is connected to highly nonlinear behavior of the material under volumetric deformation which conventional hyperelastic models fail in predicting. The strain energy function presented here is a variant of the exponentiated Hencky strain energy, which for moderate strains is as good as the quadratic Hencky model and in the large strain region improves several important features from a mathematical point of view. The proposed form of the exponentiated Hencky energy possesses a set of parameters uniquely determined in the infinitesimal strain regime and an orthogonal set of parameters to determine the nonlinear response. The hyperelastic model is additionally incorporated in a finite deformation viscoelasticity framework that accounts for the two main dissipation mechanisms in TDMs, one at the microscale level and one at the macroscale level. The new model is capable of predicting different deformation modes in a certain range of frequency and amplitude with a unique set of parameters with most of them having a clear physical meaning. This translates into an important advantage with respect to overcoming the difficulties related to finding a unique set of optimal material parameters as are usually encountered fitting the polynomial forms of strain energies. Moreover, by comparing the predictions from the proposed constitutive model with experimental data we conclude that the new constitutive model gives accurate prediction.
Drag and Turbulent Characteristics of Mobile Bed Channel With Mixed Vegetation Densities Under Downward Seepage
Authors: Devi Thokchom Bebina , Daga Rishabh , Mahto Sumit Kumar , Kumar Bimlesh
Published: February 2016

The present study addresses the drag owing to the presence of vegetation and turbulent characteristics in a mobile bed channel, characterized by fully submerged vegetation formed by nonuniform vegetation densities. The influence of seepage on the velocity profiles, Reynolds stress, and turbulence intensities is discussed. Experimental results show that vegetation density is one of the important parameters that affect the flow resistance. It is found that higher vegetation density when placed at the downstream side leads to a reduction in velocity, Reynolds stress, and turbulent intensities. Downward seepage increases the near bed velocity, Reynolds stress, and turbulent intensities. Moment analysis shows that there is an increase in the inrush of flow, and sediment particles are transported more toward the streamwise direction with the application of seepage. The dominance of sweep events over ejection events increases more sediment transport. However, high vegetation density when placed at the downstream portion slightly decreases the dominance of sweep event. Drag coefficient decreases near the vegetation top and increases near the bed. Downward seepage decreases the effect of drag offered by the vegetation stems. The reduction in flow characteristics, viz., velocity, Reynolds stress, turbulent intensities, in the downstream portion of lesser spacing vegetation stems is attributed an increased drag coefficient.
Analysis of Heliostats' Rotation Around the Normal Axis for Solar Tower Field Optimization
Authors: Leonardi Erminia , Pisani Lorenzo
Published: February 2016

The design of a solar field is one of the crucial aspects when a solar tower system is realized. In general, shading and blocking effects, which are the main causes of solar power losses, are minimized displacing the heliostats each other quite distant, with typical land coverage less than 20%, and thus, strongly limiting the construction of these plants to low value lands. A new method is proposed here to improve the collected energy for solar tower systems with high land coverage (greater than 30%), based on the chance for each heliostat to rotate about the normal passing through the center of its surface. Then, shading and blocking are minimized by optimization of the relative orientations. To this aim, a small solar field composed of 150 rectangular flat heliostats has been considered, and its performances with and without the proposed optimization have been computed and compared for a wide variety of cases. In particular, a systematic analysis is presented to study the effect of the shape of the heliostats on the solar field performance: in a series of simulations, maintaining constant the area of each heliostat, the ratio between its two sides has been varied in a range between 1 (squared heliostats) and 3 (very stretched heliostats), and optimized and nonoptimized systems have been compared. Also, the total energy collected by the solar field has been calculated for optimized and nonoptimized heliostats' orientations, considering towers of different heights. Finally, the real PS10 solar plant has been considered, demonstrating that also for an optimized, very low coverage plant (about 14%), heliostats rotation can still improve the energy collection efficiency by a non-negligible amount.
Active Insulation Technique Applied to the Experimental Analysis of a Thermodynamic Control System for Cryogenic Propellant Storage
Authors: Mer Samuel , Thibault Jean-Paul , Corre Christophe
Published: February 2016

A technological barrier for long-duration space missions using cryogenic propulsion is the control of the propellant tank self-pressurization (SP). Since the cryogenic propellant submitted to undesired heat load tends to vaporize, the resulting pressure rise must be controlled to prevent storage failure. The thermodynamic vent system (TVS) is one of the possible control strategies. A TVS system has been investigated using on-ground experiments with simulant fluid. Previous experiments performed in the literature have reported difficulties to manage the thermal boundary condition at the tank wall; spurious thermal effects induced by the tank environment spoiled the tank power balance accuracy. This paper proposes to improve the experimental tank power balance, thanks to the combined use of an active insulation technique, a double envelope thermalized by a water loop which yields a net zero heat flux boundary condition and an electrical heating coil delivering a thermal power Pc∈[0−360] W, which accurately sets the tank thermal input. The simulant fluid is the NOVEC1230 fluoroketone, allowing experiments at room temperature T ∈ [40–60] °C. Various SP and TVS experiments are performed with this new and improved apparatus. The proposed active tank insulation technique yields quasi-adiabatic wall condition for all experiments. For TVS control at a given injection temperature, the final equilibrium state depends on heat load and the injection mass flow rate. The cooling dynamics is determined by the tank filling and the injection mass flow rate but does not depend on the heat load Pc.
Feasibility Study of a Residential Hybrid Ground Source Heat Pump System
Authors: Balasubramanian Siddharth , Gaspredes Jonathan L. , Moon Tess J. , Masada Glenn Y.
Published: February 2016

A residential hybrid ground source heat pump (HGSHP) model is presented, which integrates a compact cooling tower with a GSHP. The base case GSHP model is for a single story, 195 m2 house with a 14 kW heat pump and four 68.8 m deep vertical boreholes and uses Austin, TX weather data. The GSHP model was run for a range of supplemental heat rejection (SHR) capacities of an unidentified device located between the heat pump outlet and ground loop inlet, and estimates of improved heat pump performance and ground temperature effects are presented. Then, a compact closed wet cooling tower (CWCT) model is presented and coupled to the GSHP model. The tower's 7 kW capacity represents the smallest commercially available cooling tower. Each of the four HGSHP boreholes was reduced to 26.5 m. The operational and economic performance of the HGSHP is compared to a GSHP alone. Metrics include estimates of initial and lifetime operational costs, ground temperature effects, and heat pump efficiency. Simulations for ten years of operation show that adding the compact CWCT is cost effective, extends the lifetime of the borehole system, and maintains high heat pump efficiencies.
Comparison Between Analytical and Finite Element Calculation for Pressurized Container
Authors: Iancu Otto Theodor , Otremba Frank , Sklorz Christian
Published: February 2016

The prediction of the plastic collapse load of cylindrical pressure vessels is very often made by using expensive Finite Element Computations. The calculation of the collapse load requires an elastic-plastic material model and the consideration of non-linear geometry effects. The plastic collapse load causes overalls structural instability and cannot be determined directly from a finite element analysis. The ASME (2007) code recommends that the collapse load should be the load for which the numerical solution does not converge. This load can be only determined approximately if a expensive nonlinear analysis consisting of a very large number of sub steps is done. The last load sub step leading to a convergent solution will be taken as the critical load for the structure. In the instability regime no standard finite element solution can be found because of the lack of convergence of the numerical procedure. Other methods for the calculation of the allowable pressure proposed by the ASME code are the elastic stress analysis and the limit load analysis. In the present paper the plastic collapse load for a cylindrical pressure vessel is determined by an analytical method based on a linear elastic perfectly plastic material model. When plasticity occurs the material is considered as incompressible and the tensor of plastic strains is parallel to the stress deviator tensor. In that case the finite stress-strain relationships of Henkel can be used for calculating the pressure for which plastic flow occurs at the inside of the vessel wall or in the case of full plasticity in the wall. The analytical results are fully confirmed by finite element predictions both for axisymmetric and high costs three dimensional models. The analytical model can be used for fast predictions of the allowable load for the design of a large variety of pressure vessels under safety considerations. The accuracy of the predicted collapse load largely depends on the quality of the temperature dependent wall material data used both in the analytical and numerical calculations.
Optimisation of a Laminated Composite Cylindrical Shell With Curvilinear Fibre Paths Using a Surrogate-Based Approach
Authors: Luersen M. A. , Steeves C. A. , Nair P. B.
Published: February 2016

Conventional design and manufacturing techniques of fibre-reinforced laminated materials keep the fibre orientation angle constant within each ply. However, with the development of advanced tow-placement technology it is now feasible to produce composites with curved fibres. This offers more flexibility to tailor the mechanical properties and improve the performance of laminated structures. In this paper, fibre path optimisation of a laminated cylindrical shell is studied. Curvilinear variations for the fibre orientations are adopted in the circumferential and longitudinal directions of the shell. In order to reduce the computational cost a surrogate-based optimisation strategy is proposed to pursue the optimum design. The laminated shell is subjected to bending and torsion loads and the maximum displacement magnitude is minimised while a constraint on the buckling load is imposed. Numerical studies are presented for two cases. First, only circumferential variation in the fibre orientations is considered. Then, circumferential and longitudinal variations are assumed.
Analytical Investigation and Parametric Study of Lateral Impact Behavior of Pressurized Pipelines and Influence of Internal Pressure
Authors: Dou Yangqing , Liu Yucheng
Published: February 2016

This paper provides a combined computational and analytical study to investigate the lateral impact behavior of pressurized pipelines and inspect all the parameters such as the outside diameter and internal pressure affects such behavior. In this study, quartic polynomial functions are applied to formulate the maximum crushing force (F), maximum permanent displacement (W), and absorbed energy (E) of the pressurized pipelines during the impact problem. The effects of the diameter and pressure on F, W, and E are therefore illustrated through analyzing those functions. Response surfaces are also plotted based on the generated quartic polynomial functions and the quality (accuracy) of those functions are verified through several techniques.
Analysis of Mechanical Response of Aluminum Honeycomb Subjected to Indentation
Authors: Ashab A. S. M. , Ruan Dong , Lu Guoxing , Wong Yat Choy
Published: February 2016

Aluminium honeycombs is a lightweight cellular material and a good energy absorber. In different engineering applications, it is usually used as structural components. Comprehensive study has been conducted to analyse the compressive behaviour of aluminium honeycombs. However, research related to mechanical response of aluminium honeycombs material subjected to different type of loadings, such as indentation, is still limited. In this paper, experimental and numerical studies were conducted to investigate the deformation mechanism and energy dissipation of a HEXCELL® aluminium honeycomb subjected to dynamic indentation. A high speed INSTRON machine was used to conduct dynamic tests at velocities of 0.5 m/s and 5 m/s. Numerical analysis was conducted using ANSYS LS-DYNA at velocities of 5 m/s, 15 m/s and 25 m/s. The simulation results were in good agreement with the experimental results in terms of stress-strain curve profile and deformation mode. In the experiment, it was found that with the increase of velocity (strain rate) the average plateau stress of indentation also increases which was validated in the numerical analysis. The deformation of aluminium honeycombs under indentation showed that the compression of hexagonal honeycomb cells under the indenter and also tearing of honeycomb cell walls along the four edges of the indenter. The dissipation of energy in compression and tearing was calculated and discussed. The effect of loading velocity on the plateau stress and energy absorption was also analyzed.
Extension of Non-Associated Hill48 Model for Characterizing Dynamic Mechanical Behavior of a Typical High-Strength Steel Sheet
Authors: Zhu Juner , Xia Yong , Gu Gongyao , Zhou Qing
Published: February 2016

Sheet metals usually exhibit a certain degree of plastic anisotropy because of the rolling effect. To characterize the anisotropic behavior in simulations related to large deformation, strain-rate independent phenomenological models are frequently used in quasi-static conditions. Two functions are generally included in such a model, i.e. the yield function and the plastic potential. The former limits the stress state within the yield surface while the latter determines the direction of the plastic strain increment.

Traditional plasticity models mostly assume associated flow rule, in which the two functions mentioned above are identical. With the enhanced demand of accuracy, the forms of the associated models become too complex with more and more parameters to achieve an easy calibration procedure. Alternatively, in the past decade the non-associated models were increasingly used for sheet metals. Separate functions for the two aspects of plasticity lead to efficient characterization and convenient calibration.

In numerical study of dynamic loading cases, how to characterize strain-rate dependence of plasticity is an important issue. Some visco-plastic models were developed to take the rate effect into account, e.g. Johnson-Cook and Cowper-Symonds models, where the isotropic J2 flow theory was commonly used. However, when the material is severely anisotropic, this approach is very likely to be insufficient, and a model including both anisotropy and rate dependence would be needed. Extending a non-associated anisotropic model to be rate-dependent is a promising approach which has not been published in open literature to the best knowledge of the authors.

Objective of the present study is to develop an applicable model for characterizing dynamic mechanical behavior of a typical high-strength steel sheet. Two steps are performed. The material is investigated under quasi-static loading firstly. Tensile test results show an obvious anisotropy which cannot be described by traditional associated models. So the non-associated Hill48 model is chosen and calibrated. Accuracy of the model is verified by a quasi-static punching test. Thereafter the dynamic material properties are obtained by conducting tensile tests at quite a few strain-rate levels covering 0.0004–1200s−1. To characterize the positive strain-rate effect in strength, the non-associated Hill48 model is extended to be visco-plastic after checking two rate-dependence formulations in existing isotropic models. With implementing the extended model into a user subroutine of ABAQUS/explicit, simulations of the dynamic tension tests are run and compared to the real experiments. A good agreement between the simulated and the experimental result is achieved using the VUMAT.
Numeric Investigation of Fluid Solid Interaction and Performance Analysis of Pre-Bent Wind Turbine Blade
Authors: Hossain Mohammad A. , Ridwan Shakerur , Soiket Mohammad Ikthair Hossain , Hossain Sarzina
Published: February 2016

The current work is focused on numeric investigation of aerodynamic load developed on a wind turbine blade and its effects on aeroelastic characteristics of a wind turbine blade. In order to do that proper turbulent model along with appropriate assumptions need to be determined. Geometry is modeled with actual blade data for both twist and tapper. The blade tip is not considered during the modeling. Validation is done by NREL phase VI wind turbine blade data as well as other published data. Finally the aerodynamic load obtained from the CFD simulation is transferred to perform the structural analysis. It has been found that the load distribution along the blade span is not linear. It varies with the span length and it also varies along the chord of the blade airfoil. Due to this varying load the stresses developed in the blade are dissimilar which dictates the skin thickness of the blade and the shape of the spur inside the blade. It has also been observed that the aerodynamic characteristics such as lift coefficient (CL) and pressure coefficient (CP) changes with the deflection of the blade which affects the power output of the wind turbine. Finally a pre-bent blade model has been analyzed and the effect due to the bent on the performance of the wind turbine has been observed and presented. It has been found that the pre-bent blade has better Cp distribution than deflected blade and the deviation of Cp from the actual straight blade reduce significantly in pre-bent blade compare to deflected blade. The pressure distribution along the chord of the blade airfoil at different locations have been observed and presented.
Energy Absorption of Thin Walled Members Under Axial Compressive Loading
Authors: Ohioma Eboreime , Ali Muhammad , Alam Khairul
Published: February 2016

This study was conducted to investigate the effects of cross-sectional geometry on thin wall axial crushing members for the purpose of improved energy absorption. A total of five geometrically equivalent shapes (same wall thickness area, material, and length) were analyzed namely, triangle, rectangle, square, pentagon, and circle. The deformation modes and energy absorption of the members were studied under compressive loads and compared using ABAQUS/Explicit module, finite element analysis software. The simulations revealed that for the five geometrically equivalent cross sections under equal loading conditions, the pentagon shaped member absorbed the highest amount of energy. As compared to baseline rectangle member, the pentagon member absorbed approximately 25–28% more energy.
Viscoelastic Contact of a Half-Plane Sliding Over a Slightly Wavy Rigid Surface
Authors: Menga N. , Putignano C. , Contursi T. , Carbone G.
Published: February 2016

In this paper, the sliding contact of a rigid sinusoid over a viscoelastic halfplane is studied by means of an analytical procedure that reduced the original viscoelastic system to an elastic equivalent one, which has been already solved in [1]. In such a way, the solution of the original viscoelastic contact problem requires just to numerically solve a set of two integral equations. Results show the viscoelasticity influence on the solution by means of a detailed analysis of contact area, pressure and displacement distribution. A particular attention is paid to the transition from full contact to partial contact conditions.
Fabrication and Fracture Test of Functionalized Graphene-PETI 5 Composite
Authors: McLaughlin Adam , Kim Byungki
Published: February 2016

The development of a composite cryogenic fuel tank is desirable for the creation of a reusable single-stage launch vehicle. The cyclic loading and temperature changes experienced during launch and re-entry conditions result in the microcracking of conventional composites. To increase the fracture strength of this composite, a property often limited by the matrix, the nanoplatelet known as graphene or exfoliated graphite, has been introduced. Three nanocomposites were produced using graphene and Phenylethynyl Terminated Imide oligomer (PETI-5). The nanocomposites were machined in to flexure samples and tested at room temperature. Results from these tests indicate that the ideal concentration of graphene in our PETI-5 nanocomposite is 0.08%.
Enhanced Charge Carrier Concentration of SiC/CNT With N- and P-Type Doping Agents
Authors: Edwards Kyle , Khan Mujibur , Quirino Rafael , Beckler Brenda , Absar Saheem
Published: February 2016

Single-walled Carbon nanotubes (SWCNTs) have been shown to have excellent conductive properties. SWCNTs were dispersed in a SiC nanoparticle matrix to form a homogeneous mixture that is both mechanically durable and conductive. The SWCNT amount has been varied. SiC/SWCNT mixtures were then doped with various N- and P-type agents, and the resulting samples were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). Raman spectra of the samples were also measured for evidence of structural changes. Seebeck coefficients were measured for the doped samples demonstrating the change in thermoelectric properties. Shifts in the G peak (1580.6 cm-1) of the Raman spectra of the samples provides evidence of an increase in charge carrier concentration in the doped samples, correlating well with the Seebeck coefficient results.
Verification Study of an FEA Hydroelastic Modeling Method
Authors: Cunningham S.
Published: February 2016

The analysis and design of fluid-filled structures is a common problem in mechanical engineering. The fluid will add mass, stiffness, and damping properties to the structure that are not present in the dry configuration, and that alter the dynamic response of the structure. Understanding the fluid-structure dynamics can be an essential part of designing a fluid container, particularly if large structural deflections or dynamic motion are expected. For many structures, it is important to appreciate the modal harmonics of the structure and how the fluid will influence them. For instance, the modal frequencies of the tank change with the fluid level, requiring a study of frequency response for different fluid levels. Actual testing to confirm these effects can be expensive and impractical so it is useful to have an accurate model to supplement or replace physical data. Finite element analysis (FEA) codes are well suited to modeling the hydroelastic coupling between the fluid and container. FEA is commonly used to model the structural dynamics of complex systems and to generate the modal frequencies and shapes. One such application is the design of the fuel tank on a flight vehicle, where pressure fluctuations in the fuel line can lead to oscillations in thrust at the engine and high vibrations in the structure. If the frequency of these vibrations matches a natural frequency of the fluid-tank structure, a feedback loop forms coupling the structural response to the fluid oscillations through the engine thrust. This phenomenon is often referred to as “pogo” instability in flight vehicles. The resulting vibrations can cause the vehicle to break apart. The fuel tank must be designed to avoid natural frequencies in the range likely to be excited vehicle vibrations. This report describes the use and verification of hydroelastic modeling capabilities using FEA software to model a fluid-filled fuel tank. Test data from previous experimental work is used to validate the natural frequencies and mode shapes of the tank at different fluid fill levels. The FEA model compares very well to experimental data and theoretical calculations, providing good validation for the modeling technique. The FEA modeling method is thus authenticated as a valuable tool for predicting the dynamics of fluid-tank systems.
Nanotexture Change Caused by Strain-Induced Anisotropic Diffusion During Creep of Ni-Base Superalloy
Authors: Suzuki Ken , Ochi Motoyuki , Miura Hideo
Published: February 2016

In order to make clear the mechanism of the directional coarsening (rafting) of γ′ phases in Ni-base superalloys under uni-axial tensile strain, molecular dynamics (MD) analysis was applied to investigate dominant factors of strain-induced anisotropic diffusion of Al atoms and nanotexture change of fine dispersed γ′ precipitates. In this study, a simple interface structure model corresponding to the γ/γ′ interface, which consisted of Ni as γ and Ni3Al as γ′ structure, was used to analyze the effect of alloying element on diffusion properties. The diffusion constants of Al atoms were changed drastically by the dopant elements and their contents. When the lattice constant of the γ phase was increased and its melting point was decreased by the addition of Cr or Al atoms, the strain-induced anisotropic diffusion of Al atoms in the γ′ phase was accelerated. On the other hand, the addition of Co decreased the diffusion significantly. Therefore, changes of lattice constant and melting point depending on the chemical composition of the γ/γ′ interface are the dominant factors controlling the strain-induced anisotropic diffusion of Al atoms in the Ni-base superalloy.
Structural Performance of a Glass/Polyester Composite Wind Turbine Blade With Flatback and Thick Airfoils
Authors: Chen Xiao , Qin Zhi Wen , Zhao Xiao Lu , Xu Jian Zhong
Published: February 2016

A 10.3 m wind turbine (WT) blade has been designed to improve structural efficiency of rotor blades. The blade was featured with glass/polyester composites, flatback in the inboard region, thick airfoils in the mid-span region and transversely stepped spar cap thickness. This paper provided an overview of static bending test performed on the blade. Deflections, strains, load-carrying capacity, and failure behavior of the blade were investigated. Finite element (FE) analysis was carried out to complement test and to provide more insights into structural performance of the blade. The blade exhibited linear behavior in spar caps and aft panels at the maximum chord, and it continued to withstand applied loads well beyond the occurrences of local buckling of the shear web and the flatback at the maximum chord. The inboard region showed exceptional load-carrying capacity with failure loads larger than 420% test loads. Through this study, potential structural advantages by applying proposed structural features to large composite blades for multi-megawatt (MW) wind turbines were addressed.
Improved Compliance Solutions for C(T), SE(B) and Clamped SE(T) Specimens Including Side-Grooves, Varying Thicknesses and 3-D Effects
Authors: Donato Gustavo Henrique B. , Moreira Felipe Cavalheiro
Published: February 2016

Fracture toughness and Fatigue Crack Growth (FCG) experimental data represent the basis for accurate designs and integrity assessments of components containing crack-like defects. Considering ductile and high toughness structural materials, crack growing curves (e.g. J-R curves) and FCG data (in terms of da/dN vs. ΔK or ΔJ) assumed paramount relevance since characterize, respectively, ductile fracture and cyclic crack growth conditions. In common, these two types of mechanical properties severely depend on real-time and precise crack size estimations during laboratory testing. Optical, electric potential drop or (most commonly) elastic unloading compliance (C) techniques can be employed. In the latter method, crack size estimation derives from C using a dimensionless parameter (μ) which incorporates specimen’s thickness (B), elasticity (E) and compliance itself. Plane stress and plane strain solutions for μ are available in several standards regarding C(T), SE(B) and M(T) specimens, among others. Current challenges include: i) real specimens are in neither plane stress nor plane strain - modulus vary between E (plane stress) and E/(1-ν2) (plane strain), revealing effects of thickness and 3-D configurations; ii) furthermore, side-grooves affect specimen’s stiffness, leading to an “effective thickness”. Previous results from current authors revealed deviations larger than 10% in crack size estimations following existing practices, especially for shallow cracks and side-grooved samples. In addition, compliance solutions for the emerging clamped SE(T) specimens are not yet standardized. As a step in this direction, this work investigates 3-D, thickness and side-groove effects on compliance solutions applicable to C(T), SE(B) and clamped SE(T) specimens. Refined 3-D elastic FE-models provide Load-CMOD evolutions. The analysis matrix includes crack depths between a/W=0.1 and a/W=0.7 and varying thicknesses (W/B = 4, W/B = 2 and W/B = 1). Side-grooves of 5%, 10% and 20% are also considered. The results include compliance solutions incorporating all aforementioned effects to provide accurate crack size estimation during laboratory fracture and FCG testing. All proposals revealed reduced deviations if compared to existing solutions.
A Structurally Motivated Constitutive Model for Bat Wing Skin
Authors: Skulborstad Alyssa J. , Goulbourne N. C.
Published: February 2016

Unique among animal flyers, bats have highly flexible and stretchable thin wing membranes. The connection between the structural constituents of bat wing skin, its material behavior, and flight abilities is not yet known. In this work we propose a structurally motivated constitutive model for the wing skin. Within a continuum mechanics framework, the proposed strain energy function for the wing skin is the sum of contributions due to the matrix and two mesoscopic fiber families, one oriented primarily spanwise consisting of elastin fiber bundles and the other family oriented chordwise consisting of muscle fibers. While the fibers are flat and straight when the wing is somewhat open, the matrix exhibits corrugations due to compressive loading from the pre-stretched spanwise fibers. This mismatch in the natural configurations of components is accounted for in the model by a decomposition of the deformation gradient of the spanwise fibers. The material parameters are fit with a procedure motivated by the underlying deformation mechanisms of the tissue corresponding to the regions of the j-shaped constitutive curves. The proposed model is fit to the first set of biaxial experimental stress-strain data for bat wing skin and captures the general features of the tissue response well.
Explicit and Implicit Lifetime Assessment Methods of 9Cr-1Mo Steel Under Combined Creep and Fatigue Loads Using a Strip Yield Model
Authors: Andrews B. , Potirniche G. P.
Published: February 2016

Growing demand for clean, affordable energy has driven the power industry towards generation plants with higher thermal efficiency and higher operating temperatures. ASTM Grade 91 is a high chromium (9Cr-1Mo) creep resistant steel commonly used in high temperature pressure vessel and piping applications. These service conditions often involve a combination of stationary and cyclic loads at elevated temperatures. Lifecycle assessments of components under such conditions require modeling of both creep and fatigue behaviors. This paper develops two approaches to modeling mixed creep and fatigue crack growth for lifetime assessment of high service temperature components. Both approaches model fatigue crack growth using the Paris law integrated over the number of lifetime cyclic reversals to obtain crack extension. A strip yield model is used to characterize the crack tip stress-strain fields.

The first approach employed an explicit method to approximate creep crack growth using C* as a crack tip parameter characterizing creep crack extension. The Norton power law was explicitly solved to model the primary and secondary stages of creep.

The second approach used an implicit method to solve a set of constitutive equations based on properties of the material microstructure to model all creep stages. Constitutive equations were fit to experimental data collected at stresses 10–60% of yield and temperatures 550–650°C. These methods were compared to published experimental data under purely stationary loads, purely cyclic loads and mixed loading. Both models showed good agreement with experimental data in the stress and temperature conditions considered.
A Two-Parameter Fatigue Crack Growth Correlation Using ΔK and KMAX Parameters
Authors: Sree Phani C. R. , Kujawski Daniel
Published: February 2016

The paper presents a correlation of fatigue crack growth rate (FCGR) behavior for different load ratios R (= min load/max load). The proposed method of correlation is explained in detail using comprehensive experimental FCGR data from 2524 aluminum alloy. The consistency of the proposed method has been verified using test data taken from literature for more than 10 different materials including aluminum, steel, titanium and other alloys. For all the materials studied, a transition load ratio Rt was found, which marks the transition between a dominant influence of ΔK or Kmax on FCGR behavior. For R>Rt the FCGR is dominated by ΔK. On the other hand, for R<Rt it is found that Kmax is to be the dominating parameter. Two equations, in terms of ΔK or Kmax, have been developed to represent FCGR curves for various load ratios. ΔKdriving is used to represent FCGR behavior for R>Rt and Kmax driving for R<Rt. The study reveals that the FCGR curves for different R-ratios can be collapsed into two narrow scatter bands, where each band is influence by either ΔK or Kmax. The final correlation was further simplified to a single equation, which represents “the master curve” corresponding to the transition load ratio, Rt. Thus by knowing FCGR data for one load ratio and using the proposed method, FCGR curves for any other load ratio may be predicted within a narrow scatter band.