In this study, a segment of water conveyance system at a chemical manufacturing facility is under investigation. The pipe segment under investigation conveys a daily average flow of five million gallons of water per day (MGD) from the river to a water treatment plant. The exact age of the pipe system is unknown as limited construction or maintenance information exists. The study area is a pipe segment near the treatment plant where three flow restrictions exist within a 30-foot distance bounded by a T-junction and a water filtration plant. These restrictions include two self-actuated butterfly valves and an orifice plate on a 16-inch diameter steel pipe, buried approximately three feet below ground surface. When standing in the study area, heavy vibrations are felt at the ground surface. The valves and orifice plate are to control flowrate and reduce pressure from 80 PSI to 45PSI as the flow enters the water treatment plant. Flow restrictions in close proximity can cause cavitation, water hammer and other flow phenomena within a pipe system. This can result in excessive wear of the pipe’s inner walls and valves which may compromise the structural integrity and/or function of the system. Computational fluid dynamics (CFD) software is a useful tool for determining if the conditions for the various flow phenomena are present in a system. The flow characteristics were numerically calculated in MATLAB then computationally modeled in AFT Fathom. The purpose of the numerical analysis was to describe the stability of the fluid flow at discrete points in the pipe network and identify the network segments with significantly unstable flow profiles. The purpose of the AFT Fathom CFD model purpose was to provide a continuous simulation of the flow stability in the pipe segment and provide a more robust description of the flow profiles in the network. While Fathom cannot explicitly predict cavitation or water hammer, the kinematic parameters produced by the Fathom model and the physical conditions observed in the study indicate that water hammer is likely occurring.

Recent studies have shown that the addition of nanomaterials to fuels can improve combustion characteristics. A downside, however, is that these mixtures are unstable and prone to phase separation. Finding stable nanomaterial-fuel mixtures are required to make these mixtures viable for practical use. Current research studied the stability of Renewable jet fuel combined with multiple nanomaterial additives being acetylene black, graphene nanoparticles, and multiwalled carbon nanotubes, at 1.0% w/w ratio. Results were compared with prior research and it was shown that renewable jet fuel had a similar effect on settling as soy biodiesel and the results indicated that the fuel’s bulk viscosity was not a major factor determining the stability of the nanofuel.

The Taylor cone jet is a well-known electrohydrodynamic flow (EHD), usually produced by applying an external electric field to a capillary liquid. The generation of this kind of flow involves a multi-phase and a multi-physics process and its stability has a specific operation window. This operating window is intrinsically dependent on the flow rate and magnitude of the applied electric voltage. In case high voltages are applied to the jet it can atomize and produce an electrospray. Our work presents a numerical study of the process of atomization of a Taylor cone jet using computational fluid dynamics (CFD). The study intents to assess the limit conditions of operation and the applied voltage needed to stabilize an electrospray. The numerical model was implemented within OpenFOAM, where the multi-phase hydrodynamics equations are solved using a volume-of-fluid (VOF) approach. This method is coupled with the Maxwell equations governing an electrostatic field, in order to incorporate the electric body forces into the incompressible Navier-Stokes equations. The leaky-dielectric model is used and, therefore, the interface between the two phases is subject to the hydrodynamic surface tension and electric stress (Maxwell stress). This allows a leakage of charge though the phase due to ohmic conduction. Thus, the permittivity and conductivity of the phases are taken into consideration. A two-fluid system with relevant electric properties can be categorized as, dielectric-dielectric, dielectric-conducting, and conducting-conducting considering the electrical conductivity and permittivities of the participating phases. Due to the usage of the leaky-dielectric model, it is possible to simulate any of this physical situations. By increasing the applied voltage reaches a value where the cone instability is verified, allowing a discussion on this effect. It is demonstrated that to adequately model the process of atomization a fine grid refinement is needed. The validation of the numerical model is made by comparing against diverse experimental data, for the case of a stable jet. The diameter and velocity of the droplet and the electric current of the jet are the main variables that are compared with previous results. The tests were performed with Heptane. The cone and the jet are strongly affected by the flow rate. The dimensionless diameter, as a function of the dimensionless flow rate, agrees with the scaling laws. The model predicts accurate results over a wide range of flow rates with an accuracy of around 10%. The results are obtained using structured meshes.

This project developed a study on methods to increase downforce on the university’s Formula SAE vehicle by implementing a lightweight, efficient aerodynamic design. The team planned to improve the performance and reduce lap times of the vehicle with an undertray, which grants better wheel traction and stability while handling corners. Upon completion, the aerodynamic component would have allowed the PNW Motorsports team to more effectively compete at the FSAE design competition in the spring of 2020. While reducing drag, an undertray provides the capability to direct the air beneath the vehicle chassis in a way which adds “artificial weight” to the system. A pressure gradient of high magnitude is established between the two sides of the undertray, with a low negative pressure region found beneath the body. This design is based upon the principles of fluid dynamics, in particular the venturi effect through the use of nozzles and diffusers. In this fashion, the vehicle can receive the benefits of a heavier car around corners while maintaining the higher straight-line acceleration of a lighter car. This report describes the use of simulation software in the design of an undertray, as well as the approach to manufacture it. Two-dimensional benchmark cases were performed in the replication of results obtained in a literature search. Subsequently, the undertray model was optimized with CFD and FEA/FEM techniques to obtain a component that was prepared for manufacturing. An operating procedure was established to outline the complicated steps of its assembly. Finally, it provides future aerodynamics teams with a solid foundation upon which improvements can be made.

Soft materials such as rubber, elastomers, and soft biological tissues mechanically deform at large strain isochorically for all time, or during their initial transient (when a pore fluid, typically incompressible such as water, does not have time to flow out of the deforming polymer or soft tissue porous skeleton). Simulating these large isochoric deformations computationally, such as with the Finite Element Method (FEM), requires higher order (typically quadratic) interpolation functions and/or enhancements through hybrid/mixed methods to maintain stability. Lower order (linear) finite elements with hybrid/mixed formulation may not perform stably for all mechanical loading scenarios involving large isochoric deformations, whereas quadratic finite elements with or without hybrid/mixed formulation typically perform stably, especially when large bending or folding deformations are being simulated. For topology-optimization design of soft robotics, for instance, the FEM solid mechanics solver must run efficiently and stably. Stability is ensured by the higher order finite element formulation (with possible enhancement), but efficiency for higher order FEM remains a challenge. Thus, this paper addresses efficiency from the perspective of computer science algorithms and programming. The proposed efficient algorithm utilizes the Portable, Extensible Toolkit for Scientific Computation (PETSc), along with the libCEED library for efficient compiler optimized tensor-product-basis computation to demonstrate an efficient nonlinear solution algorithm. For preconditioning, a scalable p-multigrid method is presented whereby a hierarchy of levels is constructed. In contrast to classical geometric multigrid, also known as h-multigrid, each level in p-multigrid is related to a different approximation polynomial order, p, instead of the element size, h. A Chebyshev polynomial smoother is used on each multigrid level. Algebraic MultiGrid (AMG) is then applied to the assembled Q 1 (linear) coarse mesh on the nodes of the quadratic Q 2 (quadratic) mesh. This allows low storage that can be efficiently used to accelerate the convergence to solution. For a Neo-Hookean hyperelastic problem, we examine a residual and matrix-free Jacobian formulation of a tri-quadratic hexahedral finite element with enhancement. Efficiency estimates on AVX-2 architecture based on CPU time are provided as a comparison to similar simulation (and mesh) of isochoric large deformation hyperelasticity as applied to soft materials conducted with the commercially-available FEM software program ABAQUS. The particular problem in consideration is the simulation of an assistive device in the form of finger-bending in 3D.

In this paper, a generalized finite element method (GFEM) with local gradient smoothed approximation (LGS-GFEM) using triangular meshes is proposed. The displacement field function of LGS-GFEM consists of the finite element shape function and the node displacement function. In order to obtain the nodal displacement function, the second order Taylor expansion is considered. The derivative term in Taylor expansion is obtained by using gradient smoothed technique in a smoothed domain. The displacement in smoothed operation is interpolated by polynomial basis function and radial basis function. Two kinds of integration schemes are considered, i.e. LGS-GFEM-I and LGS-GFEM-II respectively. The smoothed composite shape function of LGS-GFEM retains the ideal Kronecker property of the finite element shape function. Besides, the proposed LGS-GFEM has some other important properties such as no extra DOFs, linear independent, etc. The superiority of LGS-GFEM including high accuracy, rapid error convergence and temporal stability, is demonstrated by two representative numerical examples of static and free vibration, and compared with the classical finite element of triangular (FEM-T3) and quadrilateral (FEM-Q4) elements.

Morbid obesity is a widespread problem in the united states. The rate of falls and injures is much higher for obese individuals when compared to individuals of normative weight. Obesity can also impose excessive load to the hip and knee joints during the sit-to stand transition, which is conducive to osteoarthritis and increased sedentariety. In order to help individuals with the aforementioned problems, a sit to stand assistive device can be created that compensates for the subject’s lack of propulsive force. This device can reduce the chances of falling while trying to stand and help professionals when aiding these individuals in the healthcare setting. Users can interact with the propulsive loading provided by the device, which can be customizable to their needs. This product is one of the few sit to stand passive, assistive devices allowing the individual to use lower body strength and very minimal upper body strength. The objectives of our device are to be adjustable, help individuals stand without falling forward, be relatively inexpensive, easy to use, and safe for a hospital environment. Structural and stability analysis of the device is here presented, to assess the safety of its use.

This paper discusses the design and testing of a mast system constructed to collect ticks from rugged terrain in remote areas via a mobile robot. The goal of this project is to develop a robotic tick collection mast that can be used to collect ticks in diverse tick habitats, such as shrubs, weeds, and grass of various heights, and in different environmental conditions. The project focuses on locating the ideal design of the mast system, including materials and dimensions, types of mounting fixtures, and a center of gravity that ensures maximum tick collection without causing robot instability. The design was tested in the field and through simulations. The optimum design uses a mast system with two fixtures, a drag mast and a flag mast, made from t-slotted aluminum. The drag mast holds strips lower to the ground to collect ticks from the short grass, and the flag mast holds strips in the air, along the side of the robot, to collect ticks from taller grass and shrubs. The mast system design does not impact the robot’s ability to traverse challenging terrain, cause the robot to lose traction, frequently become permanently entangled in the brush, or have any other detrimental effect on robot mobility. The drag and flag mounts are two separate fixtures that are removable and adjustable along the horizontal and vertical axes to accommodate various terrain types. The strip attachment points are also adjustable horizontally to allow the rearrangement of strips for optimum tick collection in different tick habitats. The strength and stability of the mast system were evaluated using stress and center of gravity simulations. The design’s performance was also analyzed in the field using three types of tests: flagging and dragging, traversing uneven ground, and ascending and descending hills. The flagging and dragging test ran the robot along the side of taller foliage to test for tangling or breakage. The uneven ground test examined the robot’s performance over the ground that was pitted and dimpled to assess stability. The angled ground test studied the robot’s ability to ascend and descend hills to ensure it did not tip in any direction. A robotic solution with a mast system and flagging and dragging mounts protects humans from being infected with potential illnesses from bugs and insects carrying disease pathogens, injuries from traveling across rough ground, and harm due to exposure to the elements. Productivity is also increased, as a robot can remain in the field for an extended period. The mast system design captures ticks despite adverse conditions, is easily interchangeable, and does not hinder tick collection.

Linear infinite dimensional systems are described by a closed, densely defined linear operator that generates a continuous semigroup of bounded operators on a general Hilbert space of states and are controlled via a finite number of actuators and sensors. Many distributed applications are included in this formulation, such as large flexible aerospace structures, adaptive optics, diffusion reactions, smart electric power grids, and quantum information systems. In this paper, we focus on infinite dimensional linear systems for which a fixed gain linear infinite or finite dimensional controller is already in place. We augment this controller with a direct adaptive controller that will maintain stability of the full closed loop system even when the fixed gain controller fails to do so. We prove that the transmission zeros of the combined system are the original open loop transmission zeros, and the point spectrum of the controller alone. Therefore, the combined plant plus controller is Almost Strictly Dissipative (ASD) if and only if the original open loop system is minimum phase, and the fixed gain controller alone is exponentially stable. This result is true whether the fixed gain controller is finite or infinite dimensional. In particular this guarantees that a controller for an infinite dimensional plant based on a reduced -order approximation can be stabilized by augmentation with direct adaptive control to mitigate risks. These results are illustrated by application to direct adaptive control of general linear diffusion systems on a Hilbert space that are described by self-adjoint operators with compact resolvent.

It is necessary to keep the closed-loop system stable in data-driven feedback tuning. A widely-used strategy is using stability criterions as the constraint while parameter updating. In this strategy, the conservatism of the stability constraint has great influence on the achievable convergence performance. In this paper, a less conservative stability constraint is proposed to improve the convergence rate of data-driven feedback tuning methods. Specifically, the proposed stability constraint is developed based on small gain theorem (SGT). The conservatism is reduced through extension of SGT and further reduced using the properties of H ∞ norm. Besides, an unbiased data-driven estimation method of H ∞ norm is employed to estimate the proposed stability constraint accurately. Simulations are conducted to test the performance of the proposed stability constraint. The results demonstrate that the proposed stability constraint is less conservative and contributes to higher convergence rate.

In this paper, two different types of model-based trajectory control schemes are designed and compared for the control of robotic manipulators. First, two PD-based control schemes and one sliding-mode control scheme are designed, where Lyapunov stability theorem is used as a mathematical design tool. Then, the performances of the PD-based control schemes are compared to those of the sliding-mode control schemes with realistic computer simulations. The global asymptotic stability and the boundedness of all internal signals of the designed control schemes are shown with Lyapunov stability theorem.

Semiactive suspension system provides comfort and precise support for any type of driving in vehicles. Their main feature consists in the modification of the damping coefficient by applying an external control. Commonly, these suspensions work with non-linear dampers, such as magnetorheological, electrorheological, pneumatic, dry friction, among others; which generate a discontinuous behavior of force, causing an annoying noise known as “chattering”; however, this can be deleted by the correct application of the control technique. So, control strategy selection is a key task in the modeling of dynamic behavior and to describe the variation of characteristics, as well as to achieve the best vehicle’s driving experience in terms of comfort, performance, reliability, stability, and safety. This article shows three advanced control techniques used to design a semi-active vehicle suspension taking the quarter car as the model. From the review of the state of the art, relevant works and authors on the subject are reported. After, the application of the control techniques is shown together with the results obtained, specially, the performance of the system is carried out by means of computer simulations in the Matlab/Simulink virtual environment, accompanied by near-reality disturbances to verify the effectiveness of this study.

Stall flutter of long blades influences the operation safety of the large steam turbines in off-design conditions. As angles of attack are typically high, a partial or complete separation of the flow from the blade surface occurs. The prediction of stall flutter of turbine blades is a crucial task in the design and development of modern turbomachinery units and reliable design tools are necessary. In this work, aerodynamic stability of a linear turbine blade cascade is tested experimentally at high angle of attack +15°, Ma = 0.2 and the reduced frequency of 0.38. Controlled flutter testing has been performed in a travelling wave mode approach for the torsion with the motion amplitude of 0.5°. In addition, ANSYS CFX with SST k-ω turbulent model is used for URANS simulations of a full-scale computational domain. A separation bubble formed on suction surface near the leading edge has been found in CFD results for each blade. Excellent agreement between the experimental and numerical results in stability maps has been achieved for this case under investigation. This is encouraging and both experimental and numerical techniques will be tested further.

In this paper, the authors represent the robustness capabilities of their previously implemented parallel feedforward compensator configuration, applicable to nonminimum-phase (NMP) systems. In the previous papers, it was shown how a paralleled feedforward compensator can stabilize a general purpose NMP system with ease. However, the robustness analysis was postponed for a later study. Thus, herein, the robustness capabilities of the proposed configuration in the presence of noise and disturbance are investigated. It will be shown that the main idea behind the stability of the parallel feedforward compensation with derivative (PFCD) methodology comes handy when trying to test for the robustness purposes. At the end, a simulation example is brought to represent the ballparks of the methodology.

In this paper, inspired by the geometric inversion transformation, a novel transformation of the z-plane root locus to a pseudo s-plane is proposed. In the z-plane, the stability of a discrete closed-loop system (a sampled-data control system) requires that all the system poles lie within the unit circle. In root locus analysis, the unit circle region seems congested, compared to the stability region of a continuous system, which is the left half of the s-plane. In the case of fast sampling, the poles of a discrete system can really be in a small neighborhood, thus making the control implementation difficult. The geometric transformation developed in this work helps widen or enlarge the space for the system poles and preserves most of the features of z-plane root loci, including marginal stability and root loci branching off at vertical angles. The usefulness of the new transformation in design of discrete control systems is demonstrated in a numerical example.

Heavy duty gas turbines are usually equipped with hydrodynamic bearings which are either lemon-bore or tilting pad type. Baker Hughes legacy gas turbines use these two types of bearings, and its selection is based on 1) considering pros & cons from Rotor dynamics, 2) bearing performance, 3) bearing housing stiffness, 4) vibration detection & control. Non-contact probes are used to monitor the vibrations of rotor. Majority of legacy gas turbines are not equipped with these probes. Due to this fact, over the years it resulted in non-detection of dynamics & vibration issue, which caused frequent bearing replacement. As the increase in industry demand to apply and measure vibrations using non-contact probes on bearings, an effort was made by Baker Hughes to implement these on existing fleet units. Also, in order to increase rotor dynamics stability of low-pressure rotor, to improve bearing life and performance, effort was made to replace lemon-bore bearings with tilting pad. This paper demonstrates efforts made to design the titling pad which would fit within envelop of already available bearing housing. Bearing/shaft clearance, bearing performance, modification of bearing retainer clearances are the mandatory tasks which would be dealt in this study. The swap of bearing type, and its effect on whole gas turbine rotor dynamic stability, checking the frequency crossovers with Campbell diagram would also be dealt in this paper. This paper also focuses on assessment on oil passage routing, temperature & proximity probe instrumentation routing design. Re-design is performed by analyzing various configuration, assessing different sensitivity studies & validation of modified bearing housing from structural integrity, ultimate load capability, & split plane oil leakage retention and its comparison with baseline are most important aspects of finalization of this change, which will be showcased in this paper. Instrumentation routing was a critical task when the considering bearing replacement from lemon-bore to tilting pad. As lemon-bore type bearings just have an elliptical inner surface, it’s quite easy to install the thermocouples into a simple hole. But as replacement has tilting pads, the challenge is to instrument the pads without effecting their movement and functionality. Such best practices are also dealt in this paper. Comparison of tilting-pad with lemon-bore, considering the fixed shaft diameter, the retainer outer diameter of tilting pad is higher than lemon-bore. This effect has a change in bearing seat on bearing housing, thereby reducing the effective stiffness of the housing, and the reduced split plane surface. To tackle this situation, several sensitivities were executed, by re-modifying the bolts and bolt holes on the existing housing, without modifying the housing envelop.

In this work, the nonlinear dynamic response of a shallow spherical cap subjected to a time-dependent pressure load is analyzed. The Novozhilov’s nonlinear thin shell theory is used to express the strain-displacement relations, geometric imperfections are considered. Using the Rayleigh-Ritz method, the displacement fields are expanded using a mixed series: Legendre polynomials are considered in the azimuthal direction and harmonic functions in the circumferential one. The dynamic model is derived by using the Lagrange equations. The response of a homogeneous shell, made of isotropic material, clamped at its ends, and subjected to an external pressure load is investigated: using the continuation software AUTO, the bifurcation analysis of equilibrium points and periodic responses has been performed. The model is validated by means of comparison with the existing results in literature for spherical shells having a circular base, in particular with models developed through the Marguerre’s theory.

Ground vehicle traffic jams are a serious issue in today’s society. Despite advances in traffic flow management in recent years, predicting traffic jams is still a challenge. Recently, novel techniques have been developed in complex systems theory to enable forecasting emergent behaviors in dynamical systems. Forecasting methods have been developed based on exploiting the phenomenon of critical slowing down, which occurs in dynamical systems near certain types of bifurcations and phase transitions. Herein, we explore recently developed tools of tipping point forecasting in complex systems, namely early warning indicators and bifurcation forecasting methods, and investigate their application to predict traffic jams on roads. The measurements required for forecasting are recorded dynamical features of the system such as headways between cars in traffic or density of cars on road. Forecasting approaches are applied to simulated and experimental traffic flow conditions. Results show that one can successfully predict proximity to the critical point of congestion as well as traffic dynamics after this critical point using the proposed approaches. The methodologies presented can be used to analyze stability of traffic models and address challenges related to the complexity of traffic dynamics.

In this paper, the semi-analytical method of implicit discrete maps is employed to investigate the nonlinear dynamical behavior of a nonlinear spring pendulum. The implicit discrete maps are developed through the midpoint scheme of the corresponding differential equations of a nonlinear spring pendulum system. Using discrete mapping structures, different periodic motions are obtained for the bifurcation trees. With varying excitation amplitude, a bifurcation tree of period-1 motion to chaos is achieved through the bifurcation tree of period-1 to period-2 motions. The corresponding stability and bifurcations are studied through eigenvalue analysis. Finally, numerical illustrations of periodic motions are obtained numerically and analytically.

In this paper, periodic motions in an autonomous system with a discontinuous vector field are discussed. The periodic motions are obtained by constructing a set of algebraic equations based on motion mapping structures. The stability of periodic motions is investigated through eigenvalue analysis. The grazing bifurcations are presented by varying the spring stiffness. Once the grazing bifurcation occurs, periodic motions switches from the old motion to a new one. Numerical simulations are conducted for motion illustrations. The parameter study helps one understand autonomous discontinuous dynamical systems.