High-fidelity System Model Research Articles

Adjoint-based limit cycle oscillation instability sensitivity and suppression

Dynamical systems often exhibit limit cycle oscillations (LCOs), self-sustaining oscillations of limited amplitude. LCOs can be supercritical or subcritical. The supercritical response is benign, while the subcritical response can be bi-stable and exhibit a hysteretic response. Subcritical responses can be avoided in design optimization by enforcing LCO stability. However, many high-fidelity system models are computationally expensive to evaluate. Thus, there is a need for an efficient computational approach that can model instability and handle hundreds or thousands of design variables. To address this need, we propose a simple metric to determine the LCO stability using a fitted bifurcation diagram slope. We develop an adjoint-based formula to efficiently compute the stability derivative with respect to many design variables. To evaluate the stability derivative, we only need to compute the time-spectral adjoint equation three times, regardless of the number of design variables. The proposed adjoint method is verified with finite differences, achieving a five-digit agreement between the two approaches. We consider a stability-constrained LCO parameter optimization problem using an analytic model to demonstrate that the optimizer can suppress the instability. We also consider a more realistic LCO speed and stability-constrained airfoil problem that minimizes the normalized mass and stiffness. The proposed method could be extended to optimization problems with a partial differential equation (PDE)-based model, opening the door to other applications where high-fidelity models are needed.

Review of different fault detection methods and their impact on pre‐emptive VSC‐HVDC dc protection performance

Multi-terminal voltage-sourced converters (VSC) high-voltage direct current (HVDC) transmission system is expected to play a vital role in future power systems. Compared with ac power transmission, dc transmission is more vulnerable to faults due to low dc-side impedances and sensitive power electronics in the converters. Dc protection issues must be tackled before any multi-terminal VSC-HVDC grid can be built. The multi-terminal VSC-HVDC system is studied in detail using switching models for two-level converters, detailed equivalent models for the modular multi-level converters, detailed hybrid circuit breaker switching models and frequency-dependent phase models for dc cables. Using such high-fidelity system models, a systematic study of HVDC fault protection methodologies in more detail than previous studies is conducted. This is the first comprehensive study that includes pre-emptive circuit breaker operation. The results presented in this study underline the benefits of such a detailed treatment of the breaker, and of considering it as part of a fast power electronics system rather than isolated dc equipment. The study identifies the best existing fault detection method and tests it extensively. In order to further improve post-fault system recovery response, which is a key but often neglected part of previous studies, a novel bump-less transfer control has been implemented in the converters.

High fidelity system modeling for high quality image reconstruction in clinical CT.

Today, while many researchers focus on the improvement of the regularization term in IR algorithms, they pay less concern to the improvement of the fidelity term. In this paper, we hypothesize that improving the fidelity term will further improve IR image quality in low-dose scanning, which typically causes more noise. The purpose of this paper is to systematically test and examine the role of high-fidelity system models using raw data in the performance of iterative image reconstruction approach minimizing energy functional. We first isolated the fidelity term and analyzed the importance of using focal spot area modeling, flying focal spot location modeling, and active detector area modeling as opposed to just flying focal spot motion. We then compared images using different permutations of all three factors. Next, we tested the ability of the fidelity terms to retain signals upon application of the regularization term with all three factors. We then compared the differences between images generated by the proposed method and Filtered-Back-Projection. Lastly, we compared images of low-dose in vivo data using Filtered-Back-Projection, Iterative Reconstruction in Image Space, and the proposed method using raw data. The initial comparison of difference maps of images constructed showed that the focal spot area model and the active detector area model also have significant impacts on the quality of images produced. Upon application of the regularization term, images generated using all three factors were able to substantially decrease model mismatch error, artifacts, and noise. When the images generated by the proposed method were tested, conspicuity greatly increased, noise standard deviation decreased by 90% in homogeneous regions, and resolution also greatly improved. In conclusion, the improvement of the fidelity term to model clinical scanners is essential to generating higher quality images in low-dose imaging.

Airborne Electro-Mechanical Actuator Test Stand for Development of Prognostic Health Management Systems


 
 
 With the advent of the next generation of aerospace systems equipped with fly-by-wire controls, electromechanical actuators (EMA) are quickly becoming components critical to safety of aerospace vehicles. Being relatively new to the field, however, EMA lack the knowledge base compared to what is accumulated for the more traditional actuator types, especially when it comes to fault detection and prognosis. Scarcity of health monitoring data from fielded systems and prohibitive costs of carrying out real flight tests create the need to build high-fidelity system models and design affordable yet realistic experimental setups. The objective of this work is to build an EMA test stand that, unlike current laboratory stands typically weighing in excess of one metric ton, is portable enough to be easily placed aboard a wide variety of aircraft. This stand, named the FLEA (for Flyable Electromechanical Actuator test stand), allows testing EMA fault detection and prognosis technologies in flight environment, thus substantially increasing their technology readiness level – all without the expense of dedicated flights, as the stand is designed to function as a non-intrusive secondary payload. No aircraft modifications are required and data can be collected during any available flight opportunity: pilot currency flights, ferry flights, or flights dedicated to other experiments. The stand is currently equipped with a prototype version of NASA Ames developed prognostic health management system with models aimed at detecting and tracking several fault types. At this point the team has completed test flights of the stand on US Air Force C-17 aircraft and US Army UH-60 helicopters and more experiments, both laboratory and airborne, are planned for the coming months.
 
 

High-fidelity system modeling of advanced nuclear energy systems approaching a zero-nuclear-waste limit

This paper will present a high-fidelity system analysis of advanced nuclear energy systems approaching a zero-nuclear-waste limit. The focus will be on achievable performance characteristics. The applied high-fidelity system modeling approach will be discussed with illustrations using obtained physics simulation results. The near-term deployable dedicated small 14 MeV-neutron waste incineration back-end clean-up facilities are envisioned as advanced incinerators that are used in combination with thermal reactors (Westinghouse AP1000 and Generation IV VHTR are considered in the present study) to minimize the remaining long-lived radioactive species. These 14 MeV transmuters are viewed as small facilities designed to reduce waste inventories prior to reprocessing stages minimizing spent fuel activity levels and repository requirements. It is envisioned that under sustainable nuclear energy scenario small clean-up facilities will eventually evolve into self-consistent complexes producing electricity and serving as energy sources for industrial applications. The paper discusses physics features of these advanced systems and challenges of their conclusive evaluations.

Design Optimization of a Space Launch Vehicle Using a Genetic Algorithm

This paper describes an effort to optimize the design of an entire space launch vehicle to lowEarth (circular) orbit, consisting of multiple stages using a genetic algorithm with the goal of minimizing vehicle weight and ultimately vehicle cost. The entire launch vehicle system is analyzed using various multistage configurations to reach low Earth orbit. Specifically, threeand four-stage solid propellant vehicles have been analyzed. The vehicle performance modeling requires that analysis from four separate disciplines be integrated into the design optimization process. The disciplines of propulsion characteristics, aerodynamics,mass properties, andflight dynamics have been integrated to produce a high-fidelity systemmodel of the entire vehicle. In addition, the systemmodel has been validated using the existing launch vehicle data. The cost model is mass based and uses extensive historical data to produce a cost estimating relationship for a solid propellant vehicle. For the design optimization, the goal is for the genetic algorithm tominimize the differences between thedesired andactual orbital parameters. This ensures that thepayload achieves the desired orbit. One final goal is to minimize the overall vehicle mass, thus minimizing the system cost per launch. This paper will represent the first effort of its kind to minimize the solid propellant launch vehicle cost at the preliminary design level using a genetic algorithm.