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Johnson, Nicholas

Publications and source records attributed to Johnson, Nicholas.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Carbon sequestration and nitrogen and phosphorus accumulation in a freshwater, estuarine marsh: Effects of microtopography and nutrient loads

Wetlands are widely recognized as nutrient sinks for their ability to remove nutrients in runoff and retain them in soils. This is a valuable service, especially in agricultural watersheds, making nutrient removal one of the main goals in many wetland creation and restoration projects. However, incorporating nutrient management considerations requires site-level assessments, the scale at which wetland creation and restoration occur. Here we studied how carbon (C) sequestration, and nitrogen (N) and phosphorus (P) accumulation vary at different microtopographic levels and locations within a freshwater, estuarine marsh on the coast of Lake Erie. We further explored links between C sequestration, and N and P accumulation in recent years, and orthophosphate ($PO_{4}^{3-}$), ammonium ($NH_{4}^{+}$), and nitrate ($NO_{3}^{-}$) concentrations and loads. The rates of C sequestration and N accumulation were relatively lower at spots of intermediate depth and locations closer to the wetland’s main channel. P accumulation was highest at deep spots but did not differ among locations based on distance from the channel. Empirical models showed that nitrate load is the most important variable explaining the variability in C, N, and P sequestration/accumulation (r2 = 0.57, 0.61, and 0.32, respectively) and that the relationship between inorganic nutrient loads and accumulation was negative. Our findings suggest that including microtopographic relief features in wetland creation and design, especially deeper spots, is critical to enhancing wetland ecosystems’ C, N, and P sinking capacity. Also, that upstream nitrate management should be a priority to increase benefits from C sequestration and long-term N and P accumulation.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2021

A Projected Network Model of Online Disinformation Cascades

Within the past half-decade, it has become overwhelmingly clear that suppressing the spread of deliberate false and misleading information is of the utmost importance for protecting democratic institutions. Disinformation has been found to come from both foreign and domestic actors, but the effects from either can be disastrous. From the simple encouragement of unwarranted distrust to conspiracy theories promoting violence, the results of disinformation have put the functionality of American democracy under direct threat. Present scientific challenges posed by this problem include detecting disinformation, quantifying its potential impact, and preventing its amplification. We present a model on which we can experiment with possible strategies toward the third challenge: the prevention of amplification. This is a social contagion network model, which is decomposed into layers to represent physical, ''offline'', interactions as well as virtual interactions on a social media platform. Along with the topological modifications to the standard contagion model, we use state-transition rules designed specifically for disinformation, and distinguish between contagious and non-contagious infected nodes. We use this framework to explore the effect of grassroots social movements on the size of disinformation cascades by simulating these cascades in scenarios where a proportion of the agents remove themselves from the social platform. We also test the efficacy of strategies that could be implemented at the administrative level by the online platform to minimize such spread. These top-down strategies include banning agents who disseminate false information, or providing corrective information to individuals exposed to false information to decrease their probability of believing it. We find an abrupt transition to smaller cascades when a critical number of random agents are removed from the platform, as well as steady decreases in the size of cascades with increasingly more convincing corrective information. Finally, we compare simulated cascades on this framework with real cascades of disinformation recorded on Whatsapp surrounding the 2019 Indian election. We find a set of hyperparameter values that produces a distribution of cascades matching the scaling exponent of the distribution of actual cascades recorded in the dataset. We acknowledge the available future directions for improving the performance of the framework and validation methods, as well as ways to extend the model to capture additional features of social contagion.

42 ENGINEERING↗
DOE OSTI2020

Distributed Aerodynamic Control using Active Trailing-Edge Flaps for Large Wind Turbines

This work presents a numerical framework to investigate distributed aerodynamic control devices for application in large wind turbines. Tool capabilities were extended to facilitate multiple aerodynamic polar tables. The airfoil aerodynamics characteristics were automatically determined, and blade-pitch, generator-torque, and trailing-edge-flap controllers were tuned in-the-loop according to a specific blade design. This automated workflow allows analysis and optimization of trailing-edge flaps, enabling codesign studies. Results targeted reductions of root-flap-bending moment derivatives. The applied trailing-edge-flap control reduced the standard deviation of root-flap-bending moments by more than 6% and benefit related parameters, e.g., reduce blade-tip deflections, by up to 8%. Because of varying thrust distributions along the blade span, different flap designs have nonlinear characteristics in terms of the control objective and show best performance when located at the radial position with maximum thrust. In general, larger flaps provide a greater influence to reduce the target control objective.

17 WIND ENERGY↗
DOE OSTI2020

Investigation of the nacelle blockage effect for a downwind turbine

As downwind turbines garner increasing research interest, nacelle blockage becomes an important consideration. This paper examines nacelle blockage effects under a variety of Reynolds numbers, turbulence intensities, and nacelle geometries and proposes a computationally inexpensive nacelle blockage engineering model. Results show minimal nacelle blockage impacts on rotor loading and performance, except for tall (low-aspect-ratio) nacelles, which can cause an increase in rotor C p of up to 0.6%. In all cases, nacelle blockage increases rotor loading and performance metrics, however little. Finally, (more expensive) geometry-resolved and (less expensive) body-force computational fluid dynamics modeling techniques are compared, and body-force models are found to need improvement to adequately model nacelle blockage.

17 WIND ENERGY↗
DOE OSTI2019

Verification of Floating Offshore Wind Linearization Functionality in OpenFAST

The wind engineering community relies on multiphysics engineering software to run nonlinear time-domain simulations, e.g., for design standards-based loads analysis. Although most physics involved in wind energy are nonlinear, linearization of the underlying nonlinear system equations is often advantageous to understand the system properties and exploit well-established methods and tools for analyzing linear systems. Previous work in this area has focused on the development of the new linearization functionality of the open-source engineering tool OpenFAST for floating offshore wind turbines, as well as the concepts and mathematical background needed to understand and apply it correctly. This paper focuses on the verification of this new linearization functionality, which is carried out by comparing results to previous stable versions of FAST. A nonlinear time-domain simulation for a floating offshore platform is also compared to the time-domain response of the linearized state-space model. The linearized results show good alignment between OpenFAST and previous versions of FAST, as well as with the time-domain simulations, thereby showing the accuracy of the new features in OpenFAST.

17 WIND ENERGY↗
DOE OSTI2019

Investigation of Innovative Rotor Concepts for the Big Adaptive Rotor Project

The Big Adaptive Rotor (BAR) project is funded by the U.S. Department of Energy (DOE) Wind Energy Technologies Office and has the objective of researching and developing innovative technologies that enable large land-based rotors that have the potential to produce a 10% increase in CF over current technology. The innovative technologies must have a reduction in specific power to at least 150 W/m2 at an International Electrotechnical Commission Class III wind site for a 5-megawatt (MW) or higher rating and maintain a competitive levelized cost of energy (LCOE) compared to current technology. The purpose of this document is to: 1. Identify and classify innovative BAR concepts, 2. Evaluate the concepts in terms of their potential to impact wind plant LCOE and other performance metrics of interest and identify science and engineering challenges that would limit the commercialization of these concepts, and 3. Quantitatively analyze and compare the BAR concepts.

17 WIND ENERGY↗
DOE OSTI2019

IEA Wind TCP: Results of IEA Wind TCP Workshop on a Grand Vision for Wind Energy Technology

This report provides workshop findings including the future landscape of the electricity system with over 50% of the electricity supply from wind energy, a review of key innovation opportunities in different topic areas that will be embodied in the wind power plant of the future (including innovations that both reduce levelized cost of energy and increase system value of wind energy), and identification of the key R&D challenges that must be addressed to realize the wind power plant of the future and the Grand Vision.

17 WIND ENERGY↗
DOE OSTI2017

Xyce

The Xyce Parallel Electronic Simulator simulates electronic circuit behavior in DC, AC, HB, MPDE and transient mode using standard analog (DAE) and/or device (PDE) device models including several age and radiation aware devices. It supports a variety of computing platforms (both serial and parallel) computers. Lastly, it uses a variety of modern solution algorithms dynamic parallel load-balancing and iterative solvers.

Thomquist, Heidi K.↗
DOE OSTI2013

Xyce(™) Parallel Electronic Simulator v.7.6

The Xyce Parallel Electronic Simulator simulates electronic circuit behavior in DC, AC, HB, MPDE and transient mode using standard analog (DAE) and/or device (PDE) device models including several age and radiation aware devices. It supports a variety of computing platforms (both serial and parallel) computers. Lastly, it uses a variety of modern solution algorithms dynamic parallel load-balancing and iterative solvers.! ! Xyce is primarily used to simulate the voltage and current behavior of a circuit network (a network of electronic devices connected via a conductive network). As a tool, it is mainly used for the design and analysis of electronic circuits.! ! Kirchoff's conservation laws are enforced over a network using modified nodal analysis. This results in a set of differential algebraic equations (DAEs). The resulting nonlinear problem is solved iteratively using a fully coupled Newton method, which in turn results in a linear system that is solved by either a standard sparse-direct solver or iteratively using Trilinos linear solver packages, also developed at Sandia National Laboratories.

Keiter, Eric↗
NASA NTRS2013

Orbital Debris: Past, Present, and Future

In the early days of spaceflight, the gBig Sky h theory was the near universally accepted paradigm for dealing with collisions of orbiting objects. This theory was also used during the early years of the aviation industry. Just as it did in aviation, the gBig Sky h theory breaks down as more and more objects accumulate in the environment. Fortunately, by the late 1970 fs some visionaries in NASA and the US Department of Defense (DoD) realized that trends in the orbital environment would inevitably lead to increased risks to operational spacecraft from collisions with other orbiting objects. The NASA Orbital Debris Program was established at and has been conducted at Johnson Space Center since 1979. At the start of 1979, fewer than 5000 objects were being tracked by the US Space Surveillance Network and very few attempts had been made to sample the environment for smaller sizes. Today, the number of tracked objects has quadrupled. Ground ]based and in situ measurements have statistically sampled the LEO environment over most sizes and mitigation guidelines and requirements are common among most space faring nations. NASA has been a leader, not only in defining the debris environment, but in promoting awareness of the issues in the US and internationally, and in providing leadership in developing policies to address the issue. This paper will discuss in broad terms the evolution of the NASA debris program from its beginnings to its present broad range of debris related research. The paper will discuss in some detail current research topics and will attempt to predict future research trends.

Stansbery, Gene↗
NASA NTRS2012

End-of-Mission Passivation: Successes and Challenges

The passivation of spacecraft and launch vehicle orbital stages at end-of-mission has been a principal space debris mitigation measure world-wide since the 1980 s. Space vehicle passivation includes the removal of stored energies, especially those associated with propulsion and electrical power systems. Prior to 2007 the breakup of non-functioning, non-passivated space vehicles was the major source of hazardous debris in Earth orbit. The United Nations and the Inter-Agency Space Debris Coordination Committee have both included passivation in their formal space debris mitigation guidelines. This often simple countermeasure has been adopted by many spacefaring countries and organizations and has undoubtedly prevented numerous major satellite breakups. For some existing space vehicle designs, passivation requires changes in hardware, software, and/or operational procedures. Questions about the permissible degree of passivation for both current and future space vehicles have arisen and are addressed herein. An important element to be considered is the potentially long period in which the space vehicle will remain in orbit, i.e., up to 25 years after mission termination in LEO and for centuries in orbits above LEO. Finally, the issue of passivation of space vehicles which have failed prematurely is addressed.

Johnson, Nicholas↗